Patent Publication Number: US-2022233803-A1

Title: Heat and moisture exchanger for patient interface

Description:
1 CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/855,109, filed May 31, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in Patent Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     2 BACKGROUND OF THE TECHNOLOGY 
     2.1 Field of the Technology 
     The present technology relates to one or more of the screening, diagnosis, monitoring, treatment, prevention and amelioration of respiratory-related disorders. The present technology also relates to medical devices or apparatus, and their use. 
     2.2 Description of the Related Art 
     2.2.1 Human Respiratory System and its Disorders 
     The respiratory system of the body facilitates gas exchange. The nose and mouth form the entrance to the airways of a patient. 
     The airways include a series of branching tubes, which become narrower, shorter and more numerous as they penetrate deeper into the lung. The prime function of the lung is gas exchange, allowing oxygen to move from the inhaled air into the venous blood and carbon dioxide to move in the opposite direction. The trachea divides into right and left main bronchi, which further divide eventually into terminal bronchioles. The bronchi make up the conducting airways, and do not take part in gas exchange. Further divisions of the airways lead to the respiratory bronchioles, and eventually to the alveoli. The alveolated region of the lung is where the gas exchange takes place, and is referred to as the respiratory zone. See “Respiratory Physiology”, by John B. West, Lippincott Williams &amp; Wilkins, 9th edition published 2012. 
     A range of respiratory disorders exist. Certain disorders may be characterised by particular events, e.g. apneas, hypopneas, and hyperpneas. 
     Examples of respiratory disorders include Obstructive Sleep Apnea (OSA), Cheyne-Stokes Respiration (CSR), respiratory insufficiency, Obesity Hyperventilation Syndrome (OHS), Chronic Obstructive Pulmonary Disease (COPD), Neuromuscular Disease (NMD) and Chest wall disorders. 
     Obstructive Sleep Apnea (OSA), a form of Sleep Disordered Breathing (SDB), is characterised by events including occlusion or obstruction of the upper air passage during sleep. It results from a combination of an abnormally small upper airway and the normal loss of muscle tone in the region of the tongue, soft palate and posterior oropharyngeal wall during sleep. The condition causes the affected patient to stop breathing for periods typically of 30 to 120 seconds in duration, sometimes 200 to 300 times per night. It often causes excessive daytime somnolence, and it may cause cardiovascular disease and brain damage. The syndrome is a common disorder, particularly in middle aged overweight males, although a person affected may have no awareness of the problem. See U.S. Pat. No. 4,944,310 (Sullivan). 
     Cheyne-Stokes Respiration (CSR) is another form of sleep disordered breathing. CSR is a disorder of a patient&#39;s respiratory controller in which there are rhythmic alternating periods of waxing and waning ventilation known as CSR cycles. CSR is characterised by repetitive de-oxygenation and re-oxygenation of the arterial blood. It is possible that CSR is harmful because of the repetitive hypoxia. In some patients CSR is associated with repetitive arousal from sleep, which causes severe sleep disruption, increased sympathetic activity, and increased afterload. See U.S. Pat. No. 6,532,959 (Berthon-Jones). 
     Respiratory failure is an umbrella term for respiratory disorders in which the lungs are unable to inspire sufficient oxygen or exhale sufficient CO 2  to meet the patient&#39;s needs. Respiratory failure may encompass some or all of the following disorders. 
     A patient with respiratory insufficiency (a form of respiratory failure) may experience abnormal shortness of breath on exercise. 
     Obesity Hyperventilation Syndrome (OHS) is defined as the combination of severe obesity and awake chronic hypercapnia, in the absence of other known causes for hypoventilation. Symptoms include dyspnea, morning headache and excessive daytime sleepiness. 
     Chronic Obstructive Pulmonary Disease (COPD) encompasses any of a group of lower airway diseases that have certain characteristics in common. These include increased resistance to air movement, extended expiratory phase of respiration, and loss of the normal elasticity of the lung. Examples of COPD are emphysema and chronic bronchitis. COPD is caused by chronic tobacco smoking (primary risk factor), occupational exposures, air pollution and genetic factors. Symptoms include: dyspnea on exertion, chronic cough and sputum production. 
     Neuromuscular Disease (NMD) is a broad term that encompasses many diseases and ailments that impair the functioning of the muscles either directly via intrinsic muscle pathology, or indirectly via nerve pathology. Some NMD patients are characterised by progressive muscular impairment leading to loss of ambulation, being wheelchair-bound, swallowing difficulties, respiratory muscle weakness and, eventually, death from respiratory failure. Neuromuscular disorders can be divided into rapidly progressive and slowly progressive: (i) Rapidly progressive disorders: Characterised by muscle impairment that worsens over months and results in death within a few years (e.g. Amyotrophic lateral sclerosis (ALS) and Duchenne muscular dystrophy (DMD) in teenagers); (ii) Variable or slowly progressive disorders: Characterised by muscle impairment that worsens over years and only mildly reduces life expectancy (e.g. Limb girdle, Facioscapulohumeral and Myotonic muscular dystrophy). Symptoms of respiratory failure in NMD include: increasing generalised weakness, dysphagia, dyspnea on exertion and at rest, fatigue, sleepiness, morning headache, and difficulties with concentration and mood changes. 
     Chest wall disorders are a group of thoracic deformities that result in inefficient coupling between the respiratory muscles and the thoracic cage. The disorders are usually characterised by a restrictive defect and share the potential of long term hypercapnic respiratory failure. Scoliosis and/or kyphoscoliosis may cause severe respiratory failure. Symptoms of respiratory failure include: dyspnea on exertion, peripheral oedema, orthopnea, repeated chest infections, morning headaches, fatigue, poor sleep quality and loss of appetite. 
     A range of therapies have been used to treat or ameliorate such conditions. Furthermore, otherwise healthy individuals may take advantage of such therapies to prevent respiratory disorders from arising. However, these have a number of shortcomings. 
     2.2.2 Therapies 
     Various respiratory therapies, such as Continuous Positive Airway Pressure (CPAP) therapy, Non-invasive ventilation (NIV), Invasive ventilation (IV), and High Flow Therapy (HFT) have been used to treat one or more of the above respiratory disorders. 
     2.2.2.1 Respiratory Pressure Therapies 
     Respiratory pressure therapy is the application of a supply of air to an entrance to the airways at a controlled target pressure that is nominally positive with respect to atmosphere throughout the patient&#39;s breathing cycle (in contrast to negative pressure therapies such as the tank ventilator or cuirass). 
     Continuous Positive Airway Pressure (CPAP) therapy has been used to treat Obstructive Sleep Apnea (OSA). The mechanism of action is that continuous positive airway pressure acts as a pneumatic splint and may prevent upper airway occlusion, such as by pushing the soft palate and tongue forward and away from the posterior oropharyngeal wall. Treatment of OSA by CPAP therapy may be voluntary, and hence patients may elect not to comply with therapy if they find devices used to provide such therapy one or more of: uncomfortable, difficult to use, expensive and aesthetically unappealing. 
     Non-invasive ventilation (NIV) provides ventilatory support to a patient through the upper airways to assist the patient breathing and/or maintain adequate oxygen levels in the body by doing some or all of the work of breathing. The ventilatory support is provided via a non-invasive patient interface. NIV has been used to treat CSR and respiratory failure, in forms such as OHS, COPD, NMD and Chest Wall disorders. In some forms, the comfort and effectiveness of these therapies may be improved. 
     Invasive ventilation (IV) provides ventilatory support to patients that are no longer able to effectively breathe themselves and may be provided using a tracheostomy tube. In some forms, the comfort and effectiveness of these therapies may be improved. 
     2.2.2.2 Flow Therapies 
     Not all respiratory therapies aim to deliver a prescribed therapeutic pressure. Some respiratory therapies aim to deliver a prescribed respiratory volume, by delivering an inspiratory flow rate profile over a targeted duration, possibly superimposed on a positive baseline pressure. In other cases, the interface to the patient&#39;s airways is ‘open’ (unsealed) and the respiratory therapy may only supplement the patient&#39;s own spontaneous breathing with a flow of conditioned or enriched gas. In one example, High Flow therapy (HFT) is the provision of a continuous, heated, humidified flow of air to an entrance to the airway through an unsealed or open patient interface at a “treatment flow rate” that is held approximately constant throughout the respiratory cycle. The treatment flow rate is nominally set to exceed the patient&#39;s peak inspiratory flow rate. HFT has been used to treat OSA, CSR, respiratory failure, COPD, and other respiratory disorders. One mechanism of action is that the high flow rate of air at the airway entrance improves ventilation efficiency by flushing, or washing out, expired CO 2  from the patient&#39;s anatomical deadspace. Hence, HFT is thus sometimes referred to as a deadspace therapy (DST). Other benefits may include the elevated warmth and humidification (possibly of benefit in secretion management) and the potential for modest elevation of airway pressures. As an alternative to constant flow rate, the treatment flow rate may follow a profile that varies over the respiratory cycle. 
     Another form of flow therapy is long-term oxygen therapy (LTOT) or supplemental oxygen therapy. Doctors may prescribe a continuous flow of oxygen enriched gas at a specified oxygen concentration (from 21%, the oxygen fraction in ambient air, to 100%) at a specified flow rate (e.g., 1 litre per minute (LPM), 2 LPM, 3 LPM, etc.) to be delivered to the patient&#39;s airway. 
     2.2.2.3 Supplementary Oxygen 
     For certain patients, oxygen therapy may be combined with a respiratory pressure therapy or HFT by adding supplementary oxygen to the pressurised flow of air. When oxygen is added to respiratory pressure therapy, this is referred to as RPT with supplementary oxygen. When oxygen is added to HFT, the resulting therapy is referred to as HFT with supplementary oxygen. 
     2.2.3 Respiratory Therapy Systems 
     These respiratory therapies may be provided by a respiratory therapy system or device. Such systems and devices may also be used to screen, diagnose, or monitor a condition without treating it. 
     A respiratory therapy system may comprise a Respiratory Pressure Therapy Device (RPT device), an air circuit, a humidifier, a patient interface, an oxygen source, and data management. 
     Another form of therapy system is a mandibular repositioning device. 
     2.2.3.1 Patient Interface 
     A patient interface may be used to interface respiratory equipment to its wearer, for example by providing a flow of air to an entrance to the airways. The flow of air may be provided via a mask to the nose and/or mouth, a tube to the mouth or a tracheostomy tube to the trachea of a patient. Depending upon the therapy to be applied, the patient interface may form a seal, e.g., with a region of the patient&#39;s face, to facilitate the delivery of gas at a pressure at sufficient variance with ambient pressure to effect therapy, e.g., at a positive pressure of about 10 cmH 2 O relative to ambient pressure. For other forms of therapy, such as the delivery of oxygen, the patient interface may not include a seal sufficient to facilitate delivery to the airways of a supply of gas at a positive pressure of about 10 cmH 2 O. For flow therapies such as nasal HFT, the patient interface is configured to insufflate the nares but specifically to avoid a complete seal. One example of such a patient interface is a nasal cannula. 
     Certain other mask systems may be functionally unsuitable for the present field. For example, purely ornamental masks may be unable to maintain a suitable pressure. Mask systems used for underwater swimming or diving may be configured to guard against ingress of water from an external higher pressure, but not to maintain air internally at a higher pressure than ambient. 
     Certain masks may be clinically unfavourable for the present technology e.g. if they block airflow via the nose and only allow it via the mouth. 
     Certain masks may be uncomfortable or impractical for the present technology if they require a patient to insert a portion of a mask structure in their mouth to create and maintain a seal via their lips. 
     Certain masks may be impractical for use while sleeping, e.g. for sleeping while lying on one&#39;s side in bed with a head on a pillow. 
     The design of a patient interface presents a number of challenges. The face has a complex three-dimensional shape. The size and shape of noses and heads varies considerably between individuals. Since the head includes bone, cartilage and soft tissue, different regions of the face respond differently to mechanical forces. The jaw or mandible may move relative to other bones of the skull. The whole head may move during the course of a period of respiratory therapy. 
     As a consequence of these challenges, some masks suffer from being one or more of obtrusive, aesthetically undesirable, costly, poorly fitting, difficult to use, and uncomfortable especially when worn for long periods of time or when a patient is unfamiliar with a system. Wrongly sized masks can give rise to reduced compliance, reduced comfort and poorer patient outcomes. Masks designed solely for aviators, masks designed as part of personal protection equipment (e.g. filter masks), SCUBA masks, or for the administration of anaesthetics may be tolerable for their original application, but nevertheless such masks may be undesirably uncomfortable to be worn for extended periods of time, e.g., several hours. This discomfort may lead to a reduction in patient compliance with therapy. This is even more so if the mask is to be worn during sleep. 
     CPAP therapy is highly effective to treat certain respiratory disorders, provided patients comply with therapy. If a mask is uncomfortable, or difficult to use a patient may not comply with therapy. Since it is often recommended that a patient regularly wash their mask, if a mask is difficult to clean (e.g., difficult to assemble or disassemble), patients may not clean their mask and this may impact on patient compliance. 
     While a mask for other applications (e.g. aviators) may not be suitable for use in treating sleep disordered breathing, a mask designed for use in treating sleep disordered breathing may be suitable for other applications. 
     For these reasons, patient interfaces for delivery of CPAP during sleep form a distinct field. 
     2.2.3.1.1 Seal-Forming Structure 
     Patient interfaces may include a seal-forming structure. Since it is in direct contact with the patient&#39;s face, the shape and configuration of the seal-forming structure can have a direct impact the effectiveness and comfort of the patient interface. 
     A patient interface may be partly characterised according to the design intent of where the seal-forming structure is to engage with the face in use. In one form of patient interface, a seal-forming structure may comprise a first sub-portion to form a seal around the left naris and a second sub-portion to form a seal around the right naris. In one form of patient interface, a seal-forming structure may comprise a single element that surrounds both nares in use. Such single element may be designed to for example overlay an upper lip region and a nasal bridge region of a face. In one form of patient interface a seal-forming structure may comprise an element that surrounds a mouth region in use, e.g. by forming a seal on a lower lip region of a face. In one form of patient interface, a seal-forming structure may comprise a single element that surrounds both nares and a mouth region in use. These different types of patient interfaces may be known by a variety of names by their manufacturer including nasal masks, full-face masks, nasal pillows, nasal puffs and oro-nasal masks. 
     A seal-forming structure that may be effective in one region of a patient&#39;s face may be inappropriate in another region, e.g. because of the different shape, structure, variability and sensitivity regions of the patient&#39;s face. For example, a seal on swimming goggles that overlays a patient&#39;s forehead may not be appropriate to use on a patient&#39;s nose. 
     Certain seal-forming structures may be designed for mass manufacture such that one design fit and be comfortable and effective for a wide range of different face shapes and sizes. To the extent to which there is a mismatch between the shape of the patient&#39;s face, and the seal-forming structure of the mass-manufactured patient interface, one or both must adapt in order for a seal to form. 
     One type of seal-forming structure extends around the periphery of the patient interface, and is intended to seal against the patient&#39;s face when force is applied to the patient interface with the seal-forming structure in confronting engagement with the patient&#39;s face. The seal-forming structure may include an air or fluid filled cushion, or a moulded or formed surface of a resilient seal element made of an elastomer such as a rubber. With this type of seal-forming structure, if the fit is not adequate, there will be gaps between the seal-forming structure and the face, and additional force will be required to force the patient interface against the face in order to achieve a seal. 
     Another type of seal-forming structure incorporates a flap seal of thin material positioned about the periphery of the mask so as to provide a self-sealing action against the face of the patient when positive pressure is applied within the mask. Like the previous style of seal forming portion, if the match between the face and the mask is not good, additional force may be required to achieve a seal, or the mask may leak. Furthermore, if the shape of the seal-forming structure does not match that of the patient, it may crease or buckle in use, giving rise to leaks. 
     Another type of seal-forming structure may comprise a friction-fit element, e.g. for insertion into a naris, however some patients find these uncomfortable. 
     Another form of seal-forming structure may use adhesive to achieve a seal. Some patients may find it inconvenient to constantly apply and remove an adhesive to their face. 
     A range of patient interface seal-forming structure technologies are disclosed in the following patent applications, assigned to ResMed Limited: WO 1998/004,310; WO 2006/074513; WO 2010/135785. 
     One form of nasal pillow is found in the Adam Circuit manufactured by Puritan Bennett. Another nasal pillow, or nasal puff is the subject of U.S. Pat. No. 4,782,832 (Trimble et al.), assigned to Puritan-Bennett Corporation. 
     ResMed Limited has manufactured the following products that incorporate nasal pillows: SWIFT™ nasal pillows mask, SWIFT™ II nasal pillows mask, SWIFT™ LT nasal pillows mask, SWIFT™ FX nasal pillows mask and MIRAGE LIBERTY™ full-face mask. The following patent applications, assigned to ResMed Limited, describe examples of nasal pillows masks: International Patent Application WO2004/073,778 (describing amongst other things aspects of the ResMed Limited SWIFT™ nasal pillows), US Patent Application 2009/0044808 (describing amongst other things aspects of the ResMed Limited SWIFT™ LT nasal pillows); International Patent Applications WO 2005/063328 and WO 2006/130903 (describing amongst other things aspects of the ResMed Limited MIRAGE LIBERTY™ full-face mask); International Patent Application WO 2009/052560 (describing amongst other things aspects of the ResMed Limited SWIFT™ FX nasal pillows). 
     2.2.3.1.2 Positioning and Stabilising 
     A seal-forming structure of a patient interface used for positive air pressure therapy is subject to the corresponding force of the air pressure to disrupt a seal. Thus a variety of techniques have been used to position the seal-forming structure, and to maintain it in sealing relation with the appropriate portion of the face. 
     One technique is the use of adhesives. See for example US Patent Application Publication No. US 2010/0000534. However, the use of adhesives may be uncomfortable for some. 
     Another technique is the use of one or more straps and/or stabilising harnesses. Many such harnesses suffer from being one or more of ill-fitting, bulky, uncomfortable and awkward to use. 
     2.2.3.2 Respiratory Pressure Therapy (RPT) Device 
     A respiratory pressure therapy (RPT) device may be used individually or as part of a system to deliver one or more of a number of therapies described above, such as by operating the device to generate a flow of air for delivery to an interface to the airways. The flow of air may be pressure-controlled (for respiratory pressure therapies) or flow-controlled (for flow therapies such as HFT). Thus RPT devices may also act as flow therapy devices. Examples of RPT devices include a CPAP device and a ventilator. 
     Air pressure generators are known in a range of applications, e.g. industrial-scale ventilation systems. However, air pressure generators for medical applications have particular requirements not fulfilled by more generalised air pressure generators, such as the reliability, size and weight requirements of medical devices. In addition, even devices designed for medical treatment may suffer from shortcomings, pertaining to one or more of: comfort, noise, ease of use, efficacy, size, weight, manufacturability, cost, and reliability. 
     An example of the special requirements of certain RPT devices is acoustic noise. 
     
       
         
           
               
            
               
                   
               
               
                 Table of noise output levels of prior RPT devices 
               
               
                 (one specimen only, measured using test method specified 
               
               
                 in ISO 3744 in CPAP mode at 10 cmH 2 O). 
               
            
           
           
               
               
               
            
               
                   
                 A-weighted sound 
                 Year 
               
               
                 RPT Device name 
                 pressure level dB(A) 
                 (approx.) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 C-Series Tango ™ 
                 31.9 
                 2007 
               
               
                 C-Series Tango ™ with Humidifier 
                 33.1 
                 2007 
               
               
                 S8 Escape ™ II 
                 30.5 
                 2005 
               
               
                 S8 Escape ™ II with H4i ™ Humidifier 
                 31.1 
                 2005 
               
               
                 S9 AutoSet ™ 
                 26.5 
                 2010 
               
               
                 S9 AutoSet ™ with H5i Humidifier 
                 28.6 
                 2010 
               
               
                   
               
            
           
         
       
     
     One known RPT device used for treating sleep disordered breathing is the S9 Sleep Therapy System, manufactured by ResMed Limited. Another example of an RPT device is a ventilator. Ventilators such as the ResMed Stellar™ Series of Adult and Paediatric Ventilators may provide support for invasive and non-invasive non-dependent ventilation for a range of patients for treating a number of conditions such as but not limited to NMD, OHS and COPD. 
     The ResMed Elisée™ 150 ventilator and ResMed VS III™ ventilator may provide support for invasive and non-invasive dependent ventilation suitable for adult or paediatric patients for treating a number of conditions. These ventilators provide volumetric and barometric ventilation modes with a single or double limb circuit. RPT devices typically comprise a pressure generator, such as a motor-driven blower or a compressed gas reservoir, and are configured to supply a flow of air to the airway of a patient. In some cases, the flow of air may be supplied to the airway of the patient at positive pressure. The outlet of the RPT device is connected via an air circuit to a patient interface such as those described above. 
     The designer of a device may be presented with an infinite number of choices to make Design criteria often conflict, meaning that certain design choices are far from routine or inevitable. Furthermore, the comfort and efficacy of certain aspects may be highly sensitive to small, subtle changes in one or more parameters. 
     2.2.3.3 Air Circuit 
     An air circuit is a conduit or a tube constructed and arranged to allow, in use, a flow of air to travel between two components of a respiratory therapy system such as the RPT device and the patient interface. In some cases, there may be separate limbs of the air circuit for inhalation and exhalation. In other cases, a single limb air circuit is used for both inhalation and exhalation. 
     2.2.3.4 Humidifier 
     Delivery of a flow of air without humidification may cause drying of airways. The use of a humidifier with an RPT device and the patient interface produces humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. In addition in cooler climates, warm air applied generally to the face area in and about the patient interface is more comfortable than cold air. Humidifiers therefore often have the capacity to heat the flow of air was well as humidifying it. 
     A range of artificial humidification devices and systems are known, however they may not fulfil the specialised requirements of a medical humidifier. 
     Medical humidifiers are used to increase humidity and/or temperature of the flow of air in relation to ambient air when required, typically where the patient may be asleep or resting (e.g. at a hospital). A medical humidifier for bedside placement may be small A medical humidifier may be configured to only humidify and/or heat the flow of air delivered to the patient without humidifying and/or heating the patient&#39;s surroundings. Room-based systems (e.g. a sauna, an air conditioner, or an evaporative cooler), for example, may also humidify air that is breathed in by the patient, however those systems would also humidify and/or heat the entire room, which may cause discomfort to the occupants. Furthermore medical humidifiers may have more stringent safety constraints than industrial humidifiers 
     While a number of medical humidifiers are known, they can suffer from one or more shortcomings. Some medical humidifiers may provide inadequate humidification, some are difficult or inconvenient to use by patients. 
     2.2.3.5 Oxygen Source 
     Experts in this field have recognized that exercise for respiratory failure patients provides long term benefits that slow the progression of the disease, improve quality of life and extend patient longevity. Most stationary forms of exercise like tread mills and stationary bicycles, however, are too strenuous for these patients. As a result, the need for mobility has long been recognized. Until recently, this mobility has been facilitated by the use of small compressed oxygen tanks or cylinders mounted on a cart with dolly wheels. The disadvantage of these tanks is that they contain a finite amount of oxygen and are heavy, weighing about 50 pounds when mounted. 
     Oxygen concentrators have been in use for about 50 years to supply oxygen for respiratory therapy. Traditional oxygen concentrators have been bulky and heavy making ordinary ambulatory activities with them difficult and impractical. Recently, companies that manufacture large stationary oxygen concentrators began developing portable oxygen concentrators (POCs). The advantage of POCs is that they can produce a theoretically endless supply of oxygen. In order to make these devices small for mobility, the various systems necessary for the production of oxygen enriched gas are condensed. POCs seek to utilize their produced oxygen as efficiently as possible, in order to minimise weight, size, and power consumption. This may be achieved by delivering the oxygen as series of pulses or “boli”, each bolus timed to coincide with the start of inspiration. This therapy mode is known as pulsed or demand (oxygen) delivery (POD), in contrast with traditional continuous flow delivery more suited to stationary oxygen concentrators. 
     2.2.3.6 Data Management 
     There may be clinical reasons to obtain data to determine whether the patient prescribed with respiratory therapy has been “compliant”, e.g. that the patient has used their RPT device according to one or more “compliance rules”. One example of a compliance rule for CPAP therapy is that a patient, in order to be deemed compliant, is required to use the RPT device for at least four hours a night for at least 21 of 30 consecutive days. In order to determine a patient&#39;s compliance, a provider of the RPT device, such as a health care provider, may manually obtain data describing the patient&#39;s therapy using the RPT device, calculate the usage over a predetermined time period, and compare with the compliance rule. Once the health care provider has determined that the patient has used their RPT device according to the compliance rule, the health care provider may notify a third party that the patient is compliant. 
     There may be other aspects of a patient&#39;s therapy that would benefit from communication of therapy data to a third party or external system. 
     Existing processes to communicate and manage such data can be one or more of costly, time-consuming, and error-prone. 
     2.2.3.7 Mandibular Repositioning 
     A mandibular repositioning device (MRD) or mandibular advancement device (MAD) is one of the treatment options for sleep apnea and snoring. It is an adjustable oral appliance available from a dentist or other supplier that holds the lower jaw (mandible) in a forward position during sleep. The MRD is a removable device that a patient inserts into their mouth prior to going to sleep and removes following sleep. Thus, the MRD is not designed to be worn all of the time. The MRD may be custom made or produced in a standard form and includes a bite impression portion designed to allow fitting to a patient&#39;s teeth. This mechanical protrusion of the lower jaw expands the space behind the tongue, puts tension on the pharyngeal walls to reduce collapse of the airway and diminishes palate vibration. 
     In certain examples a mandibular advancement device may comprise an upper splint that is intended to engage with or fit over teeth on the upper jaw or maxilla and a lower splint that is intended to engage with or fit over teeth on the upper jaw or mandible. The upper and lower splints are connected together laterally via a pair of connecting rods. The pair of connecting rods are fixed symmetrically on the upper splint and on the lower splint. 
     In such a design the length of the connecting rods is selected such that when the MRD is placed in a patient&#39;s mouth the mandible is held in an advanced position. The length of the connecting rods may be adjusted to change the level of protrusion of the mandible. A dentist may determine a level of protrusion for the mandible that will determine the length of the connecting rods. 
     Some MRDs are structured to push the mandible forward relative to the maxilla while other MADs, such as the ResMed Narval CC™ MRD are designed to retain the mandible in a forward position. This device also reduces or minimises dental and temporo-mandibular joint (™ J) side effects. Thus, it is configured to minimises or prevent any movement of one or more of the teeth. 
     2.2.3.8 Vent Technologies 
     Some forms of treatment systems may include a vent to allow the washout of exhaled carbon dioxide. The vent may allow a flow of gas from an interior space of a patient interface, e.g., the plenum chamber, to an exterior of the patient interface, e.g., to ambient. 
     The vent may comprise an orifice and gas may flow through the orifice in use of the mask. Many such vents are noisy. Others may become blocked in use and thus provide insufficient washout. Some vents may be disruptive of the sleep of a bed partner  1100  of the patient  1000 , e.g. through noise or focused airflow. 
     ResMed Limited has developed a number of improved mask vent technologies. See International Patent Application Publication No. WO 1998/034665; International Patent Application Publication No. WO 2000/078381; U.S. Pat. No. 6,581,594; US Patent Application Publication No. US 2009/0050156; US Patent Application Publication No. 2009/0044808. 
     
       
         
           
               
            
               
                   
               
               
                 Table of noise of prior masks (ISO 17510-2: 
               
               
                 2007, 10 cmH 2 O pressure at 1 m) 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 A-weighted 
                 A-weighted 
                   
               
               
                   
                   
                 sound power 
                 sound 
               
               
                   
                 Mask 
                 level dB(A) 
                 pressure dB(A) 
                 Year 
               
               
                 Mask name 
                 type 
                 (uncertainty) 
                 (uncertainty) 
                 (approx.) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Glue-on (*) 
                 nasal 
                 50.9 
                 42.9 
                 1981 
               
               
                 ResCare 
                 nasal 
                 31.5 
                 23.5 
                 1993 
               
               
                 standard (*) 
               
               
                 ResMed 
                 nasal 
                 29.5 
                 21.5 
                 1998 
               
               
                 Mirage ™ (*) 
               
               
                 ResMed 
                 nasal 
                 36 (3) 
                 28 (3) 
                 2000 
               
               
                 UltraMirage ™ 
               
               
                 ResMed Mirage 
                 nasal 
                 32 (3) 
                 24 (3) 
                 2002 
               
               
                 Activa ™ 
               
               
                 ResMed Mirage 
                 nasal 
                 30 (3) 
                 22 (3) 
                 2008 
               
               
                 Micro ™ 
               
               
                 ResMed Mirage ™ 
                 nasal 
                 29 (3) 
                 22 (3) 
                 2008 
               
               
                 SoftGel 
               
               
                 ResMed 
                 nasal 
                 26 (3) 
                 18 (3) 
                 2010 
               
               
                 Mirage ™ FX 
               
               
                 ResMed 
                 nasal 
                 37   
                 29   
                 2004 
               
               
                 Mirage Swift ™ 
                 pillows 
               
               
                 (*) 
               
               
                 ResMed 
                 nasal 
                 28 (3) 
                 20 (3) 
                 2005 
               
               
                 Mirage Swift ™ 
                 pillows 
               
               
                 II 
               
               
                 ResMed 
                 nasal 
                 25 (3) 
                 17 (3) 
                 2008 
               
               
                 Mirage Swift ™ 
                 pillows 
               
               
                 LT 
               
               
                 ResMed AirFit 
                 nasal 
                 21 (3) 
                 13 (3) 
                 2014 
               
               
                 P10 
                 pillows 
               
               
                   
               
               
                 ((*) one specimen only, measured using test method specified in ISO 3744 in CPAP mode at 10 cmH 2 O) 
               
            
           
         
       
     
     Sound pressure values of a variety of objects are listed below 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                 A-weighted sound 
                   
               
               
                 Object 
                 pressure dB(A) 
                 Notes 
               
               
                   
               
             
            
               
                 Vacuum cleaner: Nilfisk 
                 68 
                 ISO 3744 at 1 m 
               
               
                 Walter Broadly Litter 
                   
                 distance 
               
               
                 Hog: B+ Grade 
               
               
                 Conversational speech 
                 60 
                 1 m distance 
               
               
                 Average home 
                 50 
               
               
                 Quiet library 
                 40 
               
               
                 Quiet bedroom at night 
                 30 
               
               
                 Background in TV studio 
                 20 
               
               
                   
               
            
           
         
       
     
     2.2.4 Screening, Diagnosis, and Monitoring Systems 
     Polysomnography (PSG) is a conventional system for diagnosis and monitoring of cardio-pulmonary disorders, and typically involves expert clinical staff to apply the system. PSG typically involves the placement of 15 to 20 contact sensors on a patient in order to record various bodily signals such as electroencephalography (EEG), electrocardiography (ECG), electrooculograpy (EOG), electromyography (EMG), etc. PSG for sleep disordered breathing has involved two nights of observation of a patient in a clinic, one night of pure diagnosis and a second night of titration of treatment parameters by a clinician. PSG is therefore expensive and inconvenient. In particular it is unsuitable for home screening/diagnosis/monitoring of sleep disordered breathing. 
     Screening and diagnosis generally describe the identification of a condition from its signs and symptoms. Screening typically gives a true/false result indicating whether or not a patient&#39;s SDB is severe enough to warrant further investigation, while diagnosis may result in clinically actionable information. Screening and diagnosis tend to be one-off processes, whereas monitoring the progress of a condition can continue indefinitely. Some screening/diagnosis systems are suitable only for screening/diagnosis, whereas some may also be used for monitoring. 
     Clinical experts may be able to screen, diagnose, or monitor patients adequately based on visual observation of PSG signals. However, there are circumstances where a clinical expert may not be available, or a clinical expert may not be affordable. Different clinical experts may disagree on a patient&#39;s condition. In addition, a given clinical expert may apply a different standard at different times. 
     3 BRIEF SUMMARY OF THE TECHNOLOGY 
     The present technology is directed towards providing medical devices used in the screening, diagnosis, monitoring, amelioration, treatment, or prevention of respiratory disorders having one or more of improved comfort, cost, efficacy, ease of use and manufacturability. 
     A first aspect of the present technology relates to apparatus used in the screening, diagnosis, monitoring, amelioration, treatment or prevention of a respiratory disorder. 
     Another aspect of the present technology relates to methods used in the screening, diagnosis, monitoring, amelioration, treatment or prevention of a respiratory disorder. 
     An aspect of certain forms of the present technology is to provide methods and/or apparatus that improve the compliance of patients with respiratory therapy. 
     Another aspect of the present technology is directed to a patient interface comprising: a plenum chamber; a seal-forming structure; and a positioning and stabilising structure. A vent structure may also be included. 
     A patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH 2 O above ambient air pressure; a seal-forming structure constructed and arranged to seal with a region of the patient&#39;s face surrounding an entrance to the patient&#39;s airways; a positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head; and a vent structure to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient. The patient interface may also be configured to allow the patient to breath from ambient through their mouth in the absence of a flow of pressurised air through the plenum chamber inlet port, or the patient interface is configured to leave the patient&#39;s mouth uncovered. 
     A patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH 2 O above ambient air pressure, said plenum chamber including a plenum chamber inlet port sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient; a seal-forming structure constructed and arranged to seal with a region of the patient&#39;s face surrounding an entrance to the patient&#39;s airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least the patient&#39;s nares, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient&#39;s respiratory cycle in use; a positioning and stabilising structure to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head, the positioning and stabilising structure comprising a tie, the tie being constructed and arranged so that at least a portion overlies a region of the patient&#39;s head superior to an otobasion superior of the patient&#39;s head in use; and a vent structure to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use; wherein the patient interface is configured to allow the patient to breath from ambient through their mouth in the absence of a flow of pressurised air through the plenum chamber inlet port, or the patient interface is configured to leave the patient&#39;s mouth uncovered. 
     Another aspect of the present technology is directed to a patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure; a seal-forming structure constructed and arranged to seal with a region of the patient&#39;s face; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient; and a material configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure. Additionally, the patient interface may be configured to leave the patient&#39;s mouth uncovered, or if the seal-forming structure is configured to seal around the patient&#39;s nose and mouth, the patient interface is configured to allow the patient to breath from ambient in the absence of a flow of pressurised air. 
     Another aspect of the present technology is directed to a patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH2O above ambient air pressure by a flow of air at the therapeutic pressure for breathing by a patient; a seal-forming structure constructed and arranged to seal with a region of the patient&#39;s face at least partly surrounding an entrance to the patient&#39;s airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least the patient&#39;s nares, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient&#39;s respiratory cycle in use; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use; and a material configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure, the material being positioned in communication with the plenum chamber inlet port; wherein the patient interface is configured to leave the patient&#39;s mouth uncovered, or if the seal-forming structure is configured to seal around the patient&#39;s nose and mouth, the patient interface is configured to allow the patient to breath from ambient in the absence of a flow of pressurised air. 
     Another aspect of the present technology is directed to a patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH2O above ambient air pressure by a flow of air at the therapeutic pressure for breathing by a patient, the plenum chamber further comprising two plenum chamber connectors positioned on corresponding lateral sides of the plenum chamber and configured to receive the flow of air at the therapeutic pressure; a seal-forming structure connected to the plenum chamber, the seal-forming structure being constructed and arranged to seal with a region of the patient&#39;s face at least partly surrounding an entrance to the patient&#39;s airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least the patient&#39;s nares, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient&#39;s respiratory cycle in use; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head, the positioning and stabilising structure further comprising two conduits, each of the conduits being configured to be positioned on a corresponding lateral side of the patient&#39;s head in use and configured to connect to a corresponding one of the plenum chamber connectors; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use; and a material configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure, the material being positioned within the plenum chamber such that at least a portion of the flow of air at the therapeutic pressure entering the plenum chamber from the plenum chamber connectors passes through the material before entering the patient&#39;s airways; wherein the patient interface is configured to leave the patient&#39;s mouth uncovered, or if the seal-forming structure is configured to seal around the patient&#39;s nose and mouth, the patient interface is configured to allow the patient to breath from ambient in the absence of a flow of pressurised air. 
     In examples of any one of the aspects described in the preceding paragraphs: (a) the material may comprise foam or paper, (b) the material may comprise a salt applied to a surface of the material, (c) the material may be removable from the plenum chamber, (d) the plenum chamber may comprise a main frame and a removable frame that is removable from the main frame, (e) the material may be attached to the removable frame such that the removable frame and the material are removable from the main frame together, (f) the removable frame may comprise vent holes, (g) the vent holes may include two groups of vent holes, each of the groups of vent holes being positioned proximal to a corresponding lateral side of the removable frame, (h) the main frame may comprise the plenum chamber connectors, each of the plenum chamber connectors being positioned on a corresponding lateral side of the main frame, (i) the main frame may comprise vent holes, (j) the vent holes may include two groups of vent holes, each of the groups of vent holes being positioned proximal to a corresponding lateral side of the main frame, (k) the removable frame may comprise the plenum chamber connectors, each of the plenum chamber connectors being positioned on a corresponding lateral side of the removable frame, (l) two supports may extend from the removable frame, the material being attached to the supports such that the material is spaced from the removable frame and a void is at least partly formed by the removable frame, the material, and the supports, (m) the plenum chamber connectors may be pneumatically connected to the void to provide the flow of air at the therapeutic pressure to the void, (n) the vent holes may be pneumatically connected to the void to allow air in the void to pass through the vent holes to ambient, (o) the material may comprise a posterior surface that faces the patient&#39;s face in use, the posterior surface being positively curved to avoid contact with the patient&#39;s face during use, (p) the seal-forming structure may be configured to leave the patient&#39;s mouth uncovered and the seal-forming structure may comprise nasal pillows or a nasal cradle, (q) the seal-forming structure may be configured to seal around the patient&#39;s nose and mouth and the seal-forming structure may comprise a nasal hole or two nasal holes that are configured to pneumatically communicate with the patient&#39;s nares in use and a mouth hole that is configured to pneumatically communicate with the patient&#39;s mouth in use, (r) the material may be positioned within the plenum chamber so that the flow of air must pass through the material before entering the patient&#39;s airways, and/or (s) the material may be positioned within the plenum chamber so that gas exhaled by the patient must pass through the material before exiting to ambient through the vent holes. 
     Another aspect of the present technology is directed to a patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure; a seal-forming structure connected to the plenum chamber, the seal-forming structure being constructed and arranged to seal with a region of the patient&#39;s face; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient; and a material configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure, the material being attached to the plenum chamber such that the plenum chamber holds the material in a predetermined shape. 
     Another aspect of the present technology is directed to a patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH2O above ambient air pressure by a flow of air at the therapeutic pressure for breathing by a patient; a seal-forming structure connected to the plenum chamber, the seal-forming structure being constructed and arranged to seal with a region of the patient&#39;s face at least partly surrounding an entrance to the patient&#39;s airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least the patient&#39;s nares, the seal-forming structure being constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient&#39;s respiratory cycle in use; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient&#39;s head; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use; and a material configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure, the material being attached to the plenum chamber such that the plenum chamber holds the material in a predetermined shape, and the material being positioned within the plenum chamber such that at least a portion of the flow of air at the therapeutic pressure entering the plenum chamber passes through the material before entering the patient&#39;s airways; wherein the patient interface is configured to leave the patient&#39;s mouth uncovered, or if the seal-forming structure is configured to seal around the patient&#39;s nose and mouth, the patient interface is configured to allow the patient to breath from ambient in the absence of a flow of pressurised air. 
     In examples of any one of the aspects described in the preceding paragraphs: (a) the material may comprise foam or paper, (b) the material may comprise a salt applied to a surface of the material, (c) the material may be removable from the plenum chamber, (d) the plenum chamber may comprise a main frame and a removable frame that is removable from the main frame, (e) the material may be attached to the removable frame such that the removable frame and the material are removable from the main frame together, (f) the removable frame may comprise vent holes, (g) the vent holes may include two groups of vent holes, each of the groups of vent holes being positioned proximal to a corresponding lateral side of the removable frame, (h) the main frame may comprise the plenum chamber connectors, each of the plenum chamber connectors being positioned on a corresponding lateral side of the main frame, (i) the main frame may comprise vent holes, (j) the vent holes may include two groups of vent holes, each of the groups of vent holes being positioned proximal to a corresponding lateral side of the main frame, (k) the removable frame may comprise the plenum chamber connectors, each of the plenum chamber connectors being positioned on a corresponding lateral side of the removable frame, (l) two supports may extend from the removable frame, the material being attached to the supports such that the material is spaced from the removable frame and a void is at least partly formed by the removable frame, the material, and the supports, (m) the plenum chamber connectors may be pneumatically connected to the void to provide the flow of air at the therapeutic pressure to the void, (n) the vent holes may be pneumatically connected to the void to allow air in the void to pass through the vent holes to ambient, (o) the material may comprise a posterior surface that faces the patient&#39;s face in use, the posterior surface being positively curved to avoid contact with the patient&#39;s face during use, (p) the seal-forming structure may be configured to leave the patient&#39;s mouth uncovered and the seal-forming structure may comprise nasal pillows or a nasal cradle, (q) the seal-forming structure may be configured to seal around the patient&#39;s nose and mouth and the seal-forming structure may comprise a nasal hole or two nasal holes that are configured to pneumatically communicate with the patient&#39;s nares in use and a mouth hole that is configured to pneumatically communicate with the patient&#39;s mouth in use, (r) the material may be positioned within the plenum chamber so that the flow of air must pass through the material before entering the patient&#39;s airways, (s) the material may be positioned within the plenum chamber so that gas exhaled by the patient must pass through the material before exiting to ambient through the vent holes, (t) the plenum chamber may comprise two plenum chamber connectors positioned on corresponding lateral sides of the plenum chamber and configured to receive the flow of air at the therapeutic pressure, (u) the positioning and stabilising structure may comprise two conduits, each of the conduits being configured to be positioned on a corresponding lateral side of the patient&#39;s head in use and configured to connect to a corresponding one of the plenum chamber connectors, and/or (v) the material may be positioned within the plenum chamber such that at least a portion of the flow of air at the therapeutic pressure entering the plenum chamber from the plenum chamber connectors passes through the material before entering the patient&#39;s airways. 
     Another aspect of one form of the present technology is a respiratory pressure therapy (RPT) system that may include the patient interface of any one of the preceding aspects; a respiratory pressure therapy (RPT) device configured to generate the flow of air at the therapeutic pressure; and an air circuit configured to connect the RPT device and the patient interface to direct the flow of air at the therapeutic pressure from the RPT device to the patient interface, wherein the RPT system does not include a humidifier. 
     Another aspect of one form of the present technology is a patient interface that is moulded or otherwise constructed with a perimeter shape which is complementary to that of an intended wearer. 
     An aspect of one form of the present technology is a method of manufacturing apparatus. 
     An aspect of certain forms of the present technology is a medical device that is easy to use, e.g. by a person who does not have medical training, by a person who has limited dexterity, vision or by a person with limited experience in using this type of medical device. 
     An aspect of one form of the present technology is a portable RPT device that may be carried by a person, e.g., around the home of the person. 
     An aspect of one form of the present technology is a patient interface that may be washed in a home of a patient, e.g., in soapy water, without requiring specialised cleaning equipment. An aspect of one form of the present technology is a humidifier tank that may be washed in a home of a patient, e.g., in soapy water, without requiring specialised cleaning equipment. 
     The methods, systems, devices and apparatus described may be implemented so as to improve the functionality of a processor, such as a processor of a specific purpose computer, respiratory monitor and/or a respiratory therapy apparatus. Moreover, the described methods, systems, devices and apparatus can provide improvements in the technological field of automated management, monitoring and/or treatment of respiratory conditions, including, for example, sleep disordered breathing. 
     Of course, portions of the aspects may form sub-aspects of the present technology. Also, various ones of the sub-aspects and/or aspects may be combined in various manners and also constitute additional aspects or sub-aspects of the present technology. 
     Other features of the technology will be apparent from consideration of the information contained in the following detailed description, abstract, drawings and claims. 
    
    
     
       4 BRIEF DESCRIPTION OF THE DRAWINGS 
       The present technology is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements including: 
       4.1 Respiratory Therapy Systems 
         FIG. 1A  shows a system including a patient  1000  wearing a patient interface  3000 , in the form of nasal pillows, receiving a supply of air at positive pressure from an RPT device  4000 . Air from the RPT device  4000  is conditioned in a humidifier  5000 , and passes along an air circuit  4170  to the patient  1000 . A bed partner  1100  is also shown. The patient is sleeping in a supine sleeping position. 
         FIG. 1B  shows a system including a patient  1000  wearing a patient interface  3000 , in the form of a nasal mask, receiving a supply of air at positive pressure from an RPT device  4000 . Air from the RPT device is humidified in a humidifier  5000 , and passes along an air circuit  4170  to the patient  1000 . 
         FIG. 1C  shows a system including a patient  1000  wearing a patient interface  3000 , in the form of a full-face mask, receiving a supply of air at positive pressure from an RPT device  4000 . Air from the RPT device is humidified in a humidifier  5000 , and passes along an air circuit  4170  to the patient  1000 . The patient is sleeping in a side sleeping position. 
       4.2 Respiratory System and Facial Anatomy 
         FIG. 2A  shows an overview of a human respiratory system including the nasal and oral cavities, the larynx, vocal folds, oesophagus, trachea, bronchus, lung, alveolar sacs, heart and diaphragm. 
         FIG. 2B  shows a view of a human upper airway including the nasal cavity, nasal bone, lateral nasal cartilage, greater alar cartilage, nostril, lip superior, lip inferior, larynx, hard palate, soft palate, oropharynx, tongue, epiglottis, vocal folds, oesophagus and trachea. 
         FIG. 2C  is a front view of a face with several features of surface anatomy identified including the lip superior, upper vermilion, lower vermilion, lip inferior, mouth width, endocanthion, a nasal ala, nasolabial sulcus and cheilion. Also indicated are the directions superior, inferior, radially inward and radially outward. 
         FIG. 2D  is a side view of a head with several features of surface anatomy identified including glabella, sellion, pronasale, subnasale, lip superior, lip inferior, supramenton, nasal ridge, alar crest point, otobasion superior and otobasion inferior. Also indicated are the directions superior &amp; inferior, and anterior &amp; posterior. 
         FIG. 2E  is a further side view of a head. The approximate locations of the Frankfort horizontal and nasolabial angle are indicated. The coronal plane is also indicated. 
         FIG. 2F  shows a base view of a nose with several features identified including naso-labial sulcus, lip inferior, upper Vermilion, naris, subnasale, columella, pronasale, the major axis of a naris and the midsagittal plane. 
         FIG. 2G  shows a side view of the superficial features of a nose. 
         FIG. 2H  shows subcutaneal structures of the nose, including lateral cartilage, septum cartilage, greater alar cartilage, lesser alar cartilage, sesamoid cartilage, nasal bone, epidermis, adipose tissue, frontal process of the maxilla and fibrofatty tissue. 
         FIG. 2I  shows a medial dissection of a nose, approximately several millimeters from the midsagittal plane, amongst other things showing the septum cartilage and medial crus of greater alar cartilage. 
         FIG. 2J  shows a front view of the bones of a skull including the frontal, nasal and zygomatic bones. Nasal concha are indicated, as are the maxilla, and mandible. 
         FIG. 2K  shows a lateral view of a skull with the outline of the surface of a head, as well as several muscles. The following bones are shown: frontal, sphenoid, nasal, zygomatic, maxilla, mandible, parietal, temporal and occipital. The mental protuberance is indicated. The following muscles are shown: digastricus, masseter, sternocleidomastoid and trapezius. 
         FIG. 2L  shows an anterolateral view of a nose. 
       4.3 Patient Interface 
         FIG. 3A  shows a patient interface in the form of a nasal mask in accordance with one form of the present technology. 
         FIG. 3B  shows a schematic of a cross-section through a structure at a point. An outward normal at the point is indicated. The curvature at the point has a positive sign, and a relatively large magnitude when compared to the magnitude of the curvature shown in  FIG. 3C . 
         FIG. 3C  shows a schematic of a cross-section through a structure at a point. An outward normal at the point is indicated. The curvature at the point has a positive sign, and a relatively small magnitude when compared to the magnitude of the curvature shown in  FIG. 3B . 
         FIG. 3D  shows a schematic of a cross-section through a structure at a point. An outward normal at the point is indicated. The curvature at the point has a value of zero. 
         FIG. 3E  shows a schematic of a cross-section through a structure at a point. An outward normal at the point is indicated. The curvature at the point has a negative sign, and a relatively small magnitude when compared to the magnitude of the curvature shown in  FIG. 3F . 
         FIG. 3F  shows a schematic of a cross-section through a structure at a point. An outward normal at the point is indicated. The curvature at the point has a negative sign, and a relatively large magnitude when compared to the magnitude of the curvature shown in  FIG. 3E . 
         FIG. 3G  shows a cushion for a mask that includes two pillows. An exterior surface of the cushion is indicated. An edge of the surface is indicated. Dome and saddle regions are indicated. 
         FIG. 3H  shows a cushion for a mask. An exterior surface of the cushion is indicated. An edge of the surface is indicated. A path on the surface between points A and B is indicated. A straight line distance between A and B is indicated. Two saddle regions and a dome region are indicated. 
         FIG. 3I  shows the surface of a structure, with a one dimensional hole in the surface. The illustrated plane curve forms the boundary of a one dimensional hole. 
         FIG. 3J  shows a cross-section through the structure of  FIG. 3I . The illustrated surface bounds a two dimensional hole in the structure of  FIG. 3I . 
         FIG. 3K  shows a perspective view of the structure of  FIG. 3I , including the two dimensional hole and the one dimensional hole. Also shown is the surface that bounds a two dimensional hole in the structure of  FIG. 3I . 
         FIG. 3L  shows a mask having an inflatable bladder as a cushion. 
         FIG. 3M  shows a cross-section through the mask of  FIG. 3L , and shows the interior surface of the bladder. The interior surface bounds the two dimensional hole in the mask. 
         FIG. 3N  shows a further cross-section through the mask of  FIG. 3L . The interior surface is also indicated. 
         FIG. 3O  illustrates a left-hand rule. 
         FIG. 3P  illustrates a right-hand rule. 
         FIG. 3Q  shows a left ear, including the left ear helix. 
         FIG. 3R  shows a right ear, including the right ear helix. 
         FIG. 3S  shows a right-hand helix. 
         FIG. 3T  shows a view of a mask, including the sign of the torsion of the space curve defined by the edge of the sealing membrane in different regions of the mask. 
         FIG. 3U  shows a view of a plenum chamber  3200  showing a sagittal plane and a mid-contact plane. 
         FIG. 3V  shows a view of a posterior of the plenum chamber of  FIG. 3U . The direction of the view is normal to the mid-contact plane. The sagittal plane in  FIG. 3V  bisects the plenum chamber into left-hand and right-hand sides. 
         FIG. 3W  shows a cross-section through the plenum chamber of  FIG. 3V , the cross-section being taken at the sagittal plane shown in  FIG. 3V . A ‘mid-contact’ plane is shown. The mid-contact plane is perpendicular to the sagittal plane. The orientation of the mid-contact plane corresponds to the orientation of a chord  3240  which lies on the sagittal plane and just touches the cushion of the plenum chamber at two points on the sagittal plane: a superior point  3250  and an inferior point  3230 . Depending on the geometry of the cushion in this region, the mid-contact plane may be a tangent at both the superior and inferior points. 
         FIG. 3X  shows the plenum chamber  3200  of  FIG. 3U  in position for use on a face. The sagittal plane of the plenum chamber  3200  generally coincides with the midsagittal plane of the face when the plenum chamber is in position for use. The mid-contact plane corresponds generally to the ‘plane of the face’ when the plenum chamber is in position for use. In  FIG. 3X  the plenum chamber  3200  is that of a nasal mask, and the superior point  3250  sits approximately on the sellion, while the inferior point  3230  sits on the lip superior. 
         FIG. 3Y  shows a patient interface in the form of a nasal cannula in accordance with one form of the present technology. 
       4.4 RPT Device 
         FIG. 4A  shows an RPT device in accordance with one form of the present technology. 
         FIG. 4B  is a schematic diagram of the pneumatic path of an RPT device in accordance with one form of the present technology. The directions of upstream and downstream are indicated with reference to the blower and the patient interface. The blower is defined to be upstream of the patient interface and the patient interface is defined to be downstream of the blower, regardless of the actual flow direction at any particular moment. Items which are located within the pneumatic path between the blower and the patient interface are downstream of the blower and upstream of the patient interface. 
         FIG. 4C  is a schematic diagram of the electrical components of an RPT device in accordance with one form of the present technology. 
       4.5 Humidifier 
         FIG. 5A  shows an isometric view of a humidifier in accordance with one form of the present technology. 
         FIG. 5B  shows an isometric view of a humidifier in accordance with one form of the present technology, showing a humidifier reservoir  5110  removed from the humidifier reservoir dock  5130 . 
         FIG. 5C  shows a schematic of a humidifier in accordance with one form of the present technology. 
       4.6 Breathing Waveforms 
         FIG. 6  shows a model typical breath waveform of a person while sleeping. 
       4.7 HME and Patient Interface of the Present Technology 
         FIG. 7  is an anterolateral view from a superior position of a patient interface according to an example of the present technology worn by a patient. 
         FIG. 8  is an anterior view of a patient interface according to an example of the present technology worn by a patient. 
         FIG. 9  is a superior view of a patient interface according to an example of the present technology worn by a patient. 
         FIG. 10  is a lateral view of a patient interface according to an example of the present technology worn by a patient. 
         FIG. 11  is an anterolateral view from a superior position of a patient interface according to an example of the present technology worn by a patient. 
         FIG. 12  is an anterior view of a patient interface according to an example of the present technology. 
         FIG. 13  is an anterolateral view from a superior position of a patient interface according to an example of the present technology. 
         FIG. 14  is a posterior view of a patient interface according to an example of the present technology. 
         FIG. 15  is a superior view of a patient interface according to an example of the present technology. 
         FIG. 16  is an anterior perspective view of a seal-forming structure and a plenum chamber of a patient interface according to an example of the present technology. 
         FIG. 17  is a posterior perspective view of a seal-forming structure and a plenum chamber of a patient interface according to an example of the present technology. 
         FIG. 18  is a posterior view of a seal-forming structure and a plenum chamber of a patient interface according to an example of the present technology. 
         FIG. 19  is an anterior view of a seal-forming structure and a plenum chamber of a patient interface according to an example of the present technology. 
         FIG. 20  is a lateral view of a seal-forming structure and a plenum chamber of a patient interface according to an example of the present technology. 
         FIG. 21  is a cross-sectional view taken through line  21 - 21  of  FIG. 19  of a seal-forming structure, a plenum chamber, and a heat and moisture exchanger of a patient interface according to an example of the present technology. 
         FIG. 22  is a cross-sectional view taken through line  22 ,  23 - 22 ,  23  of  FIG. 20  of a seal-forming structure, a plenum chamber, and a heat and moisture exchanger of a patient interface according to an example of the present technology. 
         FIG. 23  is a cross-sectional view taken through line  22 ,  23 - 22 ,  23  of  FIG. 20  of a seal-forming structure, a plenum chamber, and a heat and moisture exchanger of a patient interface according to an example of the present technology. 
         FIG. 24  is an anterior perspective view of a seal-forming structure and a plenum chamber of a patient interface with a removable frame removed according to an example of the present technology. 
         FIG. 25  is an anterior view of a seal-forming structure and a plenum chamber of a patient interface with a removable frame and a heat and moisture exchanger removed according to an example of the present technology. 
         FIG. 26  is an exploded view of a seal-forming structure, a plenum chamber, and a heat and moisture exchanger of a patient interface according to an example of the present technology. 
         FIG. 27  is a posterior view of a plenum chamber and a heat and moisture exchanger of a patient interface with a seal-forming structure removed according to an example of the present technology. 
         FIG. 28  is a perspective view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 29  is a posterior view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 30  is another perspective view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 31  is a cross-sectional view taken through line  31 - 31  of  FIG. 29  of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 32  is an exploded view of a seal-forming structure, a plenum chamber, and a heat and moisture exchanger of a patient interface according to an example of the present technology. 
         FIG. 33  is a posterior view of a plenum chamber and a heat and moisture exchanger of a patient interface with a seal-forming structure removed according to an example of the present technology. 
         FIG. 34  is a perspective view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 35  is another perspective view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 36  is another perspective view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 37  is a posterior view of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 38  is a cross-sectional view taken through line  38 - 38  of  FIG. 27  of a removable frame and a heat and moisture exchanger for a patient interface according to an example of the present technology. 
         FIG. 39  is an anterior perspective view of a removable frame for a patient interface according to an example of the present technology. 
         FIG. 40  is an anterior view of a removable frame for a patient interface according to an example of the present technology. 
         FIG. 41  is a posterior perspective view of a removable frame for a patient interface according to an example of the present technology. 
         FIG. 42  is a posterior view of a removable frame for a patient interface according to an example of the present technology. 
         FIG. 43  is a detailed cross-sectional view of a patient interface according to an example of the present technology based on  FIG. 22  and showing airflow through the patient interface during breath pause. 
         FIG. 44  is a detailed cross-sectional view of a patient interface according to an example of the present technology based on  FIG. 22  and showing airflow through the patient interface during inhalation. 
         FIG. 45  is a detailed cross-sectional view of a patient interface according to an example of the present technology based on  FIG. 22  and showing airflow through the patient interface during breath pause. 
       4.8 Additional Patient Interface Configurations 
         FIG. 46  is an anterolateral view from a superior position of a patient interface according to an example of the present technology. 
         FIG. 47  is a perspective view of a patient interface according to another example of the present technology. 
     
    
    
     5 DETAILED DESCRIPTION OF EXAMPLES OF THE TECHNOLOGY 
     Before the present technology is described in further detail, it is to be understood that the technology is not limited to the particular examples described herein, which may vary. It is also to be understood that the terminology used in this disclosure is for the purpose of describing only the particular examples discussed herein, and is not intended to be limiting. 
     The following description is provided in relation to various examples which may share one or more common characteristics and/or features. It is to be understood that one or more features of any one example may be combinable with one or more features of another example or other examples. In addition, any single feature or combination of features in any of the examples may constitute a further example. 
     5.1 Therapy 
     In one form, the present technology comprises a method for treating a respiratory disorder comprising applying positive pressure to the entrance of the airways of a patient  1000 . 
     In certain examples of the present technology, a supply of air at positive pressure is provided to the nasal passages of the patient via one or both nares. 
     In certain examples of the present technology, mouth breathing is limited, restricted or prevented. 
     5.2 Respiratory Therapy Systems 
     In one form, the present technology comprises a respiratory therapy system for treating a respiratory disorder. The a respiratory therapy system may comprise an RPT device  4000  for supplying a flow of air to the patient  1000  via an air circuit  4170  and a patient interface  3000  or  3800 . 
     5.3 PATIENT INTERFACE 
     A non-invasive patient interface  3000  in accordance with one aspect of the present technology comprises the following functional aspects: a seal-forming structure  3100 , a plenum chamber  3200 , a positioning and stabilising structure  3300 , a vent  3400 , one form of connection port  3600  for connection to air circuit  4170 , and a forehead support  3700 . In some forms a functional aspect may be provided by one or more physical components. In some forms, one physical component may provide one or more functional aspects. In use the seal-forming structure  3100  is arranged to surround an entrance to the airways of the patient so as to maintain positive pressure at the entrance(s) to the airways of the patient  1000 . The sealed patient interface  3000  is therefore suitable for delivery of positive pressure therapy. 
     An unsealed patient interface  3800 , in the form of a nasal cannula, includes nasal prongs  3810   a ,  3810   b  which can deliver air to respective nares of the patient  1000  via respective orifices in their tips. Such nasal prongs do not generally form a seal with the inner or outer skin surface of the nares. The air to the nasal prongs may be delivered by one or more air supply lumens  3820   a ,  3820   b  that are coupled with the nasal cannula  3800 . The lumens  3820   a ,  3820   b  lead from the nasal cannula  3800  to a respiratory therapy device via an air circuit. The unsealed patient interface  3800  is particularly suitable for delivery of flow therapies, in which the RPT device generates the flow of air at controlled flow rates rather than controlled pressures. The “vent” at the unsealed patient interface  3800 , through which excess airflow escapes to ambient, is the passage between the end of the prongs  3810   a  and  3810   b  of the cannula  3800  via the patient&#39;s nares to atmosphere. 
     If a patient interface is unable to comfortably deliver a minimum level of positive pressure to the airways, the patient interface may be unsuitable for respiratory pressure therapy. 
     The patient interface  3000  in accordance with one form of the present technology is constructed and arranged to be able to provide a supply of air at a positive pressure of at least 6 cmH 2 O with respect to ambient. 
     The patient interface  3000  in accordance with one form of the present technology is constructed and arranged to be able to provide a supply of air at a positive pressure of at least 10 cmH 2 O with respect to ambient. 
     The patient interface  3000  in accordance with one form of the present technology is constructed and arranged to be able to provide a supply of air at a positive pressure of at least 20 cmH 2 O with respect to ambient. 
     5.3.1 Seal-Forming Structure 
     In one form of the present technology, a seal-forming structure  3100  provides a target seal-forming region, and may additionally provide a cushioning function. The target seal-forming region is a region on the seal-forming structure  3100  where sealing may occur. The region where sealing actually occurs—the actual sealing surface—may change within a given treatment session, from day to day, and from patient to patient, depending on a range of factors including for example, where the patient interface was placed on the face, tension in the positioning and stabilising structure and the shape of a patient&#39;s face. 
     In one form the target seal-forming region is located on an outside surface of the seal-forming structure  3100 . 
     In certain forms of the present technology, the seal-forming structure  3100  is constructed from a biocompatible material, e.g. silicone rubber. 
     A seal-forming structure  3100  in accordance with the present technology may be constructed from a soft, flexible, resilient material such as silicone. 
     In certain forms of the present technology, a system is provided comprising more than one a seal-forming structure  3100 , each being configured to correspond to a different size and/or shape range. For example the system may comprise one form of a seal-forming structure  3100  suitable for a large sized head, but not a small sized head and another suitable for a small sized head, but not a large sized head. 
     5.3.1.1 Sealing Mechanisms 
     In one form, the seal-forming structure includes a sealing flange utilizing a pressure assisted sealing mechanism. In use, the sealing flange can readily respond to a system positive pressure in the interior of the plenum chamber  3200  acting on its underside to urge it into tight sealing engagement with the face. The pressure assisted mechanism may act in conjunction with elastic tension in the positioning and stabilising structure. 
     In one form, the seal-forming structure  3100  comprises a sealing flange and a support flange. The sealing flange comprises a relatively thin member with a thickness of less than about 1 mm, for example about 0.25 mm to about 0.45 mm, which extends around the perimeter of the plenum chamber  3200 . Support flange may be relatively thicker than the sealing flange. The support flange is disposed between the sealing flange and the marginal edge of the plenum chamber  3200 , and extends at least part of the way around the perimeter. The support flange is or includes a spring-like element and functions to support the sealing flange from buckling in use. 
     In one form, the seal-forming structure may comprise a compression sealing portion or a gasket sealing portion. In use the compression sealing portion, or the gasket sealing portion is constructed and arranged to be in compression, e.g. as a result of elastic tension in the positioning and stabilising structure. 
     In one form, the seal-forming structure comprises a tension portion. In use, the tension portion is held in tension, e.g. by adjacent regions of the sealing flange. 
     In one form, the seal-forming structure comprises a region having a tacky or adhesive surface. 
     In certain forms of the present technology, a seal-forming structure may comprise one or more of a pressure-assisted sealing flange, a compression sealing portion, a gasket sealing portion, a tension portion, and a portion having a tacky or adhesive surface. 
     5.3.1.2 Nose Bridge or Nose Ridge Region 
     In one form, the non-invasive patient interface  3000  comprises a seal-forming structure that forms a seal in use on a nose bridge region or on a nose-ridge region of the patient&#39;s face. 
     In one form, the seal-forming structure includes a saddle-shaped region constructed to form a seal in use on a nose bridge region or on a nose-ridge region of the patient&#39;s face. 
     5.3.1.3 Upper Lip Region 
     In one form, the non-invasive patient interface  3000  comprises a seal-forming structure that forms a seal in use on an upper lip region (that is, the lip superior) of the patient&#39;s face. 
     In one form, the seal-forming structure includes a saddle-shaped region constructed to form a seal in use on an upper lip region of the patient&#39;s face. 
     5.3.1.4 Chin-Region 
     In one form the non-invasive patient interface  3000  comprises a seal-forming structure that forms a seal in use on a chin-region of the patient&#39;s face. 
     In one form, the seal-forming structure includes a saddle-shaped region constructed to form a seal in use on a chin-region of the patient&#39;s face. 
     5.3.1.5 Forehead Region 
     In one form, the seal-forming structure that forms a seal in use on a forehead region of the patient&#39;s face. In such a form, the plenum chamber may cover the eyes in use. 
     5.3.1.6 Nasal Pillows 
     In one form the seal-forming structure of the non-invasive patient interface  3000  comprises a pair of nasal puffs, or nasal pillows, each nasal puff or nasal pillow being constructed and arranged to form a seal with a respective naris of the nose of a patient. 
     Nasal pillows in accordance with an aspect of the present technology include: a frusto-cone, at least a portion of which forms a seal on an underside of the patient&#39;s nose, a stalk, a flexible region on the underside of the frusto-cone and connecting the frusto-cone to the stalk. In addition, the structure to which the nasal pillow of the present technology is connected includes a flexible region adjacent the base of the stalk. The flexible regions can act in concert to facilitate a universal joint structure that is accommodating of relative movement both displacement and angular of the frusto-cone and the structure to which the nasal pillow is connected. For example, the frusto-cone may be axially displaced towards the structure to which the stalk is connected. 
     5.3.1.7 Nasal Cradle 
       FIGS. 7-15  show a seal-forming structure  3100  according to an example of the present technology. The examples of seal-forming structure  3100  may be considered nasal cradle cushions and are intended to provide a flow of pressurised gas to the patient&#39;s nares by sealing against at least the underside of the patient&#39;s nose. The exemplary seal-forming structures  3100  will engage the patient&#39;s face below the bridge of the nose and some examples, depending on the size and shape of the patient&#39;s nose, may engage the patient&#39;s nose below the pronasale. The exemplary seal-forming structures  3100  will also engage the patient&#39;s face at least above the upper vermillion Thus, the exemplary seal-forming structures  3100  may seal against the patient&#39;s lip superior in use. Furthermore, the patient&#39;s mouth may remain uncovered by the seal-forming structure  3100  of the depicted examples such that the patient may breathe freely, i.e., directly to atmosphere, without interference from the seal-forming structure  3100 . 
     Examples of a nasal cradle cushion, e.g., the exemplary seal-forming structures disclosed herein, may include a superior saddle or concave region that has positive curvature across the cushion. Also, a nasal cradle cushion may be understood to have a single target seal forming region or surface, whereas a pillows cushion may have two target seal forming regions (one for each naris). Cradle cushions may also have a posterior wall that contacts the patient&#39;s lip superior and an upper, central, surface contacts the underside of the patient&#39;s nose. These two surfaces on the patient&#39;s face may form a nasolabial angle between them (see  FIG. 2E ). A cradle cushion may be shaped to have a nasolabial angle within the range of 90 degrees to 120 degrees. 
     Furthermore, the exemplary seal-forming structures  3100  may also be shaped and dimensioned such that no portion of the seal-forming structure  3100  enters into the patient&#39;s nares during use. 
     5.3.2 Plenum Chamber 
     The plenum chamber  3200  has a perimeter that is shaped to be complementary to the surface contour of the face of an average person in the region where a seal will form in use. In use, a marginal edge of the plenum chamber  3200  is positioned in close proximity to an adjacent surface of the face. Actual contact with the face is provided by the seal-forming structure  3100 . The seal-forming structure  3100  may extend in use about the entire perimeter of the plenum chamber  3200 . In some forms, the plenum chamber  3200  and the seal-forming structure  3100  are formed from a single homogeneous piece of material. 
     In certain forms of the present technology, the plenum chamber  3200  does not cover the eyes of the patient in use. In other words, the eyes are outside the pressurised volume defined by the plenum chamber. Such forms tend to be less obtrusive and/or more comfortable for the wearer, which can improve compliance with therapy. 
     In certain forms of the present technology, the plenum chamber  3200  is constructed from a transparent material, e.g. a transparent polycarbonate. The use of a transparent material can reduce the obtrusiveness of the patient interface, and help improve compliance with therapy. The use of a transparent material can aid a clinician to observe how the patient interface is located and functioning. 
     In certain forms of the present technology, the plenum chamber  3200  is constructed from a translucent material. The use of a translucent material can reduce the obtrusiveness of the patient interface, and help improve compliance with therapy. 
     Each plenum chamber connector  3214  in these examples may include a notch  3216  that is connected to a clip projection of a clip with a snap-fit. Each plenum chamber connector  3214  may be split to form a slot  3220  to allow deformation within the clip during connection. Each clip may be joined to a clip overmold, which is an intermediate component between the end  3314  of the positioning and stabilising structure  3300 . Surrounding the ends  3314  of the positioning and stabilising structure  3300  may be a lip that seals with the plenum chamber connector  3214 . The lip may surround the entire opening at the end  3314  of the positioning and stabilising structure  3300  to ensure a complete and gas tight seal against the plenum chamber connector  3214  so that the entire flow of pressurised gas passing through the positioning and stabilising structure  3300  reaches the patient via the plenum chamber  3200  and the seal-forming structure  3100 . The plenum chamber connector  3214  may also include a chamfered edge  3218  that provides a smooth transition into the end  3314  of the positioning and stabilising structure  3300 . 
     In the examples of  FIGS. 7-23 , the seal-forming structure  3100  may be constructed from a relatively resilient material, such as an elastomer like silicone as described above, while the plenum chamber  3200  may be constructed from a relatively rigid material, such as polycarbonate. In such an example, the seal-forming structure  3100  may be molded onto the plenum chamber  3200 , and this molded connection may be permanent. Alternatively, the seal-forming structure  3100  and the plenum chamber  3200  may be formed separately so as to be removably attachable to one another, e.g., by the patient for cleaning or replacement of the seal-forming structure  3100 . In other examples, the seal-forming structure  3100  and the plenum chamber  3200  may each be constructed from a relatively resilient material, such as an elastomer like silicone, and in further examples both components may be molded to form a single, homogeneous piece of the relatively resilient material. 
     5.3.3 Heat and Moisture Exchanger (HME) 
       FIGS. 16-23  show examples of a seal-forming structure  3100  and a plenum chamber  3200 , as well as a heat and moisture exchanger (HME), according to the present technology. The HME may be in the form of an HME material  3900 . The HME material  3900  may be positioned within the plenum chamber  3200  as can be seen in the cross-sectional views. 
     The plenum chamber  3200  may be a two-part structure that includes a main frame  3202  and a removable frame  3204  that is removable from the main frame  3202 . The HME material  3900  may be attached to the removable frame  3204  so that the HME material  3900  can be removed from the plenum chamber  3200  with the removable frame  3204 , as will be described further below.  FIG. 24  shows the removable frame  3204  removed from the main frame  3202  while the HME material  3900  remains to depict the position of the HME material  3900  within the plenum chamber  3200 .  FIG. 25  shows the removable frame  3204  and the HME material  3900  removed from the main frame  3202  to depict the interior of the seal-forming structure  3100  and the plenum chamber  3200 . 
       FIGS. 22 and 26-31  show a first variation of the HME material  3900 .  FIGS. 23 and 32-38  show a second variation of the HME material  3900 . The first variation of the HME material  3900  is relatively thinner, i.e., the dimension between an anterior surface  3906  and a posterior surface  3904  of the HME material  3900 , than the second variation. The cross-sectional views of  FIGS. 22 and 23  show that each variation of the HME material  3900  has a constant thickness, i.e., the dimension between the anterior surface  3906  and the posterior surface  3904 , from lateral surface  3912  to lateral surface  3914 . However, it should be understood that the thickness dimension can vary at different points along the HME material  3900  between lateral surfaces  3912 ,  3914  as needed to fit within the plenum chamber  3200 . 
     Additionally, in each variation of the HME material  3900  it can be seen that the height, i.e., the dimension between a superior surface  3908  and an inferior surface  3910 , is lowest at the medial or central portion  3916  of the HME material  3900  and increases in each lateral direction towards a respective one of the lateral surfaces  3912 ,  3914 . Likewise, a wall  3226  of the removable frame  3204  can be seen with a height that is lowest at a medial portion  3203  and that increase toward respective lateral portions  3205 . Furthermore, the main frame anterior hole  3206 , where the removable frame  3204  removably attaches to the main frame  3202 , may also be shaped and dimensioned to correspond to the wall  3226  of the removable frame. Also, the HME material  3900  may have a height at various points along its length such that it can fit through the main frame anterior hole  3206  when the removable frame  3204  removably attaches to the main frame  3202 . The HME material  3900  may also be a material resilient such that the HME material  3900  can deform, as needed, when inserted into the main frame  3202 . 
     It should be understood that the height dimension of the wall  3226  of the removable frame  3204  and the HME material  3900  can vary at different points between lateral portions  3205  and lateral surfaces  3912 ,  3914 , respectively, as needed to fit within the plenum chamber  3200 . For example, the HME material  3900  may have a constant height between lateral surfaces  3912 ,  3914 . 
     The first variation of the HME material  3900  includes a notched portion  3902  at each lateral side of the posterior surface  3904 , while the second variation of the HME material  3900  does not have notched portions  3902 . However, it should be understood that the relatively thinner HME material  3900  could have no notched portions  3902  and the relatively thicker HME material  3900  could have notched portions  3902 . The notched portions  3902  may be included to provide more clearance for the seal-forming structure  3100  and the plenum chamber  3200  when the HME material  3900  is installed in the plenum chamber  3200 . Also, a relatively thinner material, as in the first variation, may provide more clearance from the seal-forming structure  3100  and the patient&#39;s face. 
       FIGS. 39-42  show the removable frame  3204  in isolation. The vents  3400  are shown, which may be formed by two groups of vent holes, each positioned proximal to a corresponding lateral end of the removable frame  3204 . The removable frame  3204  may also include supports  3210 , and the HME material  3900  may be attached to these supports  3210 . 
     U.S. Publication No. 2016/0175552 A1 discloses positioning an HME within a nasal mask, such as the RESMED® AirFit™ N20, or a full-face mask, such as the RESMED® AirFit™ F20. Due to the configuration of these patient interfaces, there may be adequate space within such patient interfaces for an HME of sufficient size to provide a desired level of performance (i.e., heat and moisture exchange). However, more compact patient interfaces such as nasal cradle masks, like the RESMED® AirFit™ N30i, nasal pillows masks, like the RESMED® AirFit™ P30i or the RESMED® AirFit™ P10, and ultra-compact full-face masks, like the RESMED® AirFit™ F30, may have less volume in the plenum chamber  3200  and as such the size, shape, position, and orientation of the HME may be chosen to provide a desired level of performance (i.e., heat and moisture exchange) in a sufficiently compact form-factor. While the HME material  3900  and associated features are described in the context of a nasal cradle seal-forming structure  3100 , it should be understood that the concepts described below may be similarly applicable to nasal pillows patient interfaces and to ultra-compact full-face patient interfaces, e.g., as described below in section 5.3.11. Additionally, while the HME material  3900  and associated features are described in the context of a positioning and stabilising structure  3300  that includes conduits to provide air to the patient&#39;s airways, these HME concepts may be similarly applicable to patient interfaces without such conduits and that use a tube connected to the plenum chamber  3200  separately from the positioning and stabilising structure  3300 . 
     For example, it may be advantageous for the HME to be at least one of sized, shaped, positioned, and oriented within the plenum chamber  3200  to avoid contact with the patient&#39;s face, e.g., the nose and/or lips, when the patient interface  3000  is worn by the patient. Additionally, while the HME may be constructed and arranged to avoid contact with the patient&#39;s face during use, it may be advantageous to include a large enough HME material  3900  to provide a desired level of performance (i.e., heat and moisture exchange). If the HME material  3900  is too small, then it may not be capable of adsorbing a sufficient quantity of moisture and heat from exhaled air to heat and humidify incoming air. It should be understand that in the case of a salted foam, such as examples of the HME material  3900  disclosed herein, the salt content may be more determinative of HME performance, i.e., the ability to adsorb and desorb moisture to and from the air in the plenum chamber  3200 , than the size, e.g., volume, of the HME material  3900  itself. Powered humidification, i.e., a water tub that may be heated to vaporize water into the incoming flow of pressurized air after the blower  4142  and before reaching the patient via the air circuit  4170  as described in  FIGS. 5A and 5B , may be capable of providing approximately 22 mg/L of water vapor per unit volume of air. The HME material  3900  of the present technology may be configured to provide at least 7 mg/L water vapor per unit volume of air, or at least 14 mg/L of water vapor per unit volume of air, or at least 25 mg/L of water vapor per unit volume of air. At the same time, it may also be advantageous to ensure that a balance is struck with impedance because adding the HME material  3900  to the interior of the plenum chamber  3200  may restrict incoming and outgoing airflow. For example, a large portion of HME material  3900  may increase performance, but may also increase impedance. 
     The HME material  3900  according to the examples disclosed herein includes a posterior surface  3904 , i.e., the surface that faces the patient&#39;s face during use, that is concave or shaped so as to be curved positively so that the posterior surface  3904  is spaced away from the patient&#39;s face in use, even when the seal-forming structure  3100  is compressed by tension from the positioning and stabilising structure  3300 . This shape may provide an increased surface area for the posterior surface  3904  which may enhance performance by providing more surface area to interact with the patient&#39;s exhaled air to adsorb moisture and desorb moisture to the incoming flow of air. This shape may also provide additional clearance for the patient&#39;s facial features, i.e., nose and/or lips. HME material  3900  constructed from foam or paper may be shaped with this curvature of the posterior surface  3904 . The anterior surface  3906  of the HME material  3900  may also be curved negatively, i.e., convex, at a magnitude that allows the HME material  3900  to have a desired thickness, whether constant or varied, as discussed above. 
     By curving the HME material  3900  as described, the HME material  3900  may follow the general contour of the cradle-type seal-forming structure  3100 , e.g., as shown in  FIGS. 22 and 23 , which may also avoid contact with the seal-forming structure  3100  as well. It should be understood that some degree of contact between the HME material  3900  and the seal-forming structure  3100  may be permissible when the seal-forming structure  3100  is compressed against the patient&#39;s face during use, e.g., so long as the HME material  3900  is sufficiently deformable so as not to cause discomfort for the patient and so long as the HME material  3900  is blocked from contacting the patient&#39;s skin by the seal-forming structure  3100 . Additionally, the thinner HME material  3900  of the first variation shown in  FIG. 22  compared with the thicker HME material  3900  of the second variation shown in  FIG. 23  shows that the HME material  3900  may extend into the volume bounded by the seal-forming structure  3100 , as in the second variation, or it may not, as in the first variation. 
     The curved shape of the HME material  3900  may be achieved by cutting the HME material  3900  into the curved shape from a bulk quantity of foam (e.g., open cell foam) or paper (e.g., corrugated layers of paper), or by deforming the HME material  3900  into a curved shape and retaining that shape by attaching the HME material  3900  to a relatively rigid structure, such as the removable frame  3204 . The removable frame  3204  may be constructed from the same material as the main frame  3202 , e.g., when the plenum chamber  3200  is constructed from polycarbonate or a similarly rigid material. Alternatively, the removable frame  3204  may be constructed from a material that is different from that of the main frame  3202 , e.g., where the main frame  3202  is constructed in one homogeneous piece of relatively resilient material such as an elastomer like silicone. The removable frame  3204  may be constructed from a material that is sufficiently flexible to permit removable attachment to the main frame  3202 , while being sufficiently supportive so as to maintain the shape and position of the HME material  3900  within the plenum chamber  3200 . 
     The HME material  3900  may be attached to the removable frame  3204  at supports  3210  that extend from the wall  3226 . The removable frame  3204 , including the wall  3226  and the supports  3210 , may also have a concave or positively curved shape on the side that faces the patient during use to correspond to and/or impart the desired curvature onto the HME material  3900 . The supports  3210  may extend laterally across the removable frame  3204 , along the wall  3226 , and between lateral portions  3205  such that when the HME material  3900  is attached to the supports  3210  a void  3212  is formed by the wall  3226 , the supports  3210 , and the anterior surface  3906  of the HME material  3900 . The void  3212  may be a passageway for incoming air from the conduits via the plenum chamber connectors  3214  to reach the anterior surface  3906  of the HME material  3900  before traveling to the patient&#39;s airways. The void  3212  may also be a passageway for air to exit to ambient via the vents  3400  after passing through the HME material  3900 . 
     The supports  3210  are shown not extending to the full width of the wall  3226  such that the lateral portions  3205  extend beyond the supports  3210  in lateral directions. Also, the supports  3210  are shown being approximately the same width, but each support  3210  may have a different width. There may also be more than two supports  3210 , as necessary to support the HME material  3900   
     When the removable frame  3204  is attached to the HME material  3900 , e.g., with an adhesive, these components may be removed from the main frame  3202  and the rest of the patient interface  3000  for replacement of the removable frame  3204  and the HME material  3900  and/or cleaning of the patient interface  3000 . If the HME material  3900  has a salt applied to enhance its capacity to adsorb moisture, washing the HME material  3900  may wash away the salt. However, periodic cleaning of the seal-forming structure  3100  that contacts the patient&#39;s skin is also recommended. Thus, removing the HME material  3900 , e.g., along with the removable frame  3204 , may allow for cleaning of the seal-forming structure  3100  without deteriorating performance of the HME material  3900 . 
     Also, the HME material  3900  may be replaced as a single unit with the removable frame  3204  after a prescribed interval because performance of the HME material  3900  may degrade after a period of time and the HME material  3900  may become soiled. Thus, rather than cleaning the HME material  3900 , it may be preferable and more cost-effective for the patient to replace it. 
     The removable frame  3204  may be attached to the main frame  3202  within a main frame anterior hole  3206 . The removable frame  3204  may include a inner connecting surface  3222  that contacts the main frame  3202  at the perimeter of the main frame anterior hole  3206 . The removable frame  3204  may be removably connected to the main frame  3202  with a friction fit or a snap fit. Close tolerances between the main frame  3202  and the removable frame  3204  may minimize leak between these components. A lip seal, e.g., formed from silicone, may be overmolded to one or more of the removable frame  3204  and the main frame  3202  to seal between these components and minimize leak. The removable frame  3204  may also include an outer grip surface  3224  so that the patient can grasp the removable frame  3204  to install and remove the removable frame  3204  from the main frame  3202 . The removable frame  3204  may include clips or pull/push tabs to provide a more positive or user-friendly experience in assembly and removal, e.g., in the form of audible and/or tactile feedback when the components are engaged or disengaged. 
     An alternative to the curved shape of the HME material  3900  may be a rectangular HME material  3900  that spans the plenum chamber  3200 , e.g., from one plenum chamber connector  3214  to the other plenum chamber connector  3214 . This straight path is shorter than a curved path spanning the same two points, thereby providing a relatively shorter distance to fit the HME material  3900  and, accordingly, there may be a smaller surface area for heat and moisture exchange, as well as increased risk of contact with the patient&#39;s nose and/or mouth, thereby causing comfort issues. 
     The HME material  3900  may separate the interior of the plenum chamber  3200  from the patient to form the void  3212 , and as can be seen in  FIGS. 22 and 23  the anterior surface  3906  of the HME material  3900  is positioned so as to be at least partially facing the conduit connectors  3214  to allow incoming airflow to impinge upon the anterior surface  3906  of the HME material  3900 . The main frame  3202  of the plenum chamber  3200  may include a main frame posterior hole  3208  into which the HME material  3900  may extend. The HME material  3900  may occupy all or substantially all of the main frame posterior hole  3208  such that all or substantially all of the exhaled air from the patient, which contains heat and moisture, will contact the HME material  3900  at its posterior surface  3904  before passing through the HME material  3900 . All or substantially all of the pressurized air entering the plenum chamber  3200  via the plenum chamber connectors  3214  may similarly be forced to pass through the HME material  3900 , with the exception of some amount of air that passes through the vents  3400  directly to atmosphere, to take up heat and moisture before passing through the seal-forming structure  3100  for breathing by the patient. There may be no gaps or channels around the HME material  3900  so that the humid/heated air from the patient does not to bypass the HME material  3900  before exiting to ambient via the vents  3400 . Alternatively, there may be gaps between the perimeter of the HME material  3900  and the perimeter of the main frame posterior hole  3208  that permit air traveling to and from the patient to bypass the HME material  3900 , which may improve washout of carbon dioxide. 
     Foam may be used for the HME material  3900  and may be relatively easier to deform into the curved shape than a corrugated paper structure, which may be relatively more rigid. The increased flexibility of foam, as compared to a corrugated paper structure, may also perform better at enclosing the spaces around the perimeter of the main frame posterior hole  3208  so that air cannot flow around the HME material  3900 . Additionally, an open-cell foam may allow air to flow in any direction through the foam&#39;s cells, while a corrugated paper structure may only permit flow through the corrugations. 
     The location of the vents  3400  may also impact HME performance. There is a risk of incoming airflow, that is relatively dry and cool, taking up moisture and heat from the HME material  3900  and exiting to ambient via the vents  3400  with the heat and moisture instead of being inhaled by the patient. For example, vents  3400  positioned medially may permit such an undesirable effect because air may enter the plenum chamber  3200  from the plenum chamber connectors  3214 , then pass over the anterior surface  3906  of the HME material  3900 , and then pass through the vents  3400  to ambient. Thus, the heated and humidified air may never reach the patient. 
     By positioning the vents  3400  at lateral portions  3205  of the wall  3226  of the removable frame  3204  but not at the medial portion  3203 , as shown in the depicted examples, the incoming airflow is permitted to have less contact with the anterior surface  3906  of the HME material  3900  before reaching the vents  3400 , which in turn mitigates the amount of heat and humidity extracted from the HME material  3900  and taken directly to ambient without reaching the patient&#39;s airways. By allowing the incoming flow of air to bypass the HME material  3900 , the portion thereof that escapes directly to ambient is less able to receive heat and moisture from the HME material  3900  and take the heat and moisture directly to ambient. Also, by positioning the vents  3400  relatively laterally outward, the vents  3400  may also be proximal to the plenum chamber connectors  3214 , which function as the inlet ports for the incoming flow of air. Furthermore, as can be seen in  FIGS. 22 and 23  the anterior surface  3906  of the HME material  3900  is positioned so as to be at least partially facing the vents  3400  to allow outgoing airflow passing through the HME material  3900  to reach the vents  3400  and then atmosphere. 
     While the depicted examples show the vents  3400  on the removable frame  3204  of the plenum chamber  3200 , the vents  3400  may be positioned further laterally outward and on the main frame  3202 , e.g., proximal to the plenum chamber connectors  3214 . Alternatively, the main frame  3202  may include vents  3400  in addition to the removable frame  3204 . 
       FIGS. 43-45  depict the flow of air through the patient interface  3000  according to an example of the present technology during three different stages of breathing, which shows how positioning the vents  3400  laterally outward minimizes airflow over the HME material  3900 . The regions with the heaviest hatching show relatively high air velocity. The regions with the lightest hatching show air velocity that is relatively low. 
     In  FIG. 43 , which is during breath pause, the lightest hatched regions around the HME material  3900  and in the void  3212  adjacent the HME material  3900  indicate relatively little airflow around the HME material  3900 . Thus, moisture lost from the HME material  3900  to directly to ambient may be reduced or minimized During breath pause, a relatively large amount of the incoming airflow from the lateral ends  3314  of the conduits may pass directly to ambient via the vents  3400 , as indicated by the regions of higher air velocity at the vents  3400 . 
       FIG. 44  shows the inhalation phase. In this image, a relatively lightly hatched region adjacent the anterior surface  3906  of the HME material  3900  indicates an even distribution of air flow velocity across the anterior surface  3906 , which in turn provides consistent desorption of moisture from the HME material  3900  to the incoming flow of air for inhalation by the patient. 
       FIG. 45  shows the exhalation phase. This image indicates that the location of vents  3400  may maximize the contact area for exhaled air to pass through the HME material  3900  for adsorption of moisture before passing to ambient through the vents  3400 . 
     By positioning the vents  3400  laterally outwards, as compared with vents  3400  positioned medially, testing showed that performance can be improved by a factor of approximately 2. For example, the HME material  3900  was able to provide approximately 6-7 mg/L of water vapor per unit volume of air when the vents  3400  were positioned medially. When the vents  3400  are positioned laterally performance of the HME material  3900  was improved to approximately 14-15 mg/L of water vapor per unit volume of air. 
     Additionally, by placing the vents  3400  laterally outward as shown in the examples, vent flow remains sufficient despite the increased impedance of the HME material  3900 , as compared to a similar patient interface  3000  without the HME material  3900  positioned therein, such that the number of vent holes and/or the vent hole size does not need to be increased. 
     A patient interface  3000  that includes the HME material  3900  may be included in a system that does not include a humidifier, e.g., as described in section 5.6 below, because the HME material  3900  operates to heat and/or humidify the flow of air directed to the patient. 
     5.3.4 Positioning and Stabilising Structure 
     The seal-forming structure  3100  of the patient interface  3000  of the present technology may be held in sealing position in use by the positioning and stabilising structure  3300 . 
     In one form the positioning and stabilising structure  3300  provides a retention force at least sufficient to overcome the effect of the positive pressure in the plenum chamber  3200  to lift off the face. 
     In one form the positioning and stabilising structure  3300  provides a retention force to overcome the effect of the gravitational force on the patient interface  3000 . 
     In one form the positioning and stabilising structure  3300  provides a retention force as a safety margin to overcome the potential effect of disrupting forces on the patient interface  3000 , such as from tube drag, or accidental interference with the patient interface. 
     In one form of the present technology, a positioning and stabilising structure  3300  is provided that is configured in a manner consistent with being worn by a patient while sleeping. In one example the positioning and stabilising structure  3300  has a low profile, or cross-sectional thickness, to reduce the perceived or actual bulk of the apparatus. In one example, the positioning and stabilising structure  3300  comprises at least one strap having a rectangular cross-section. In one example the positioning and stabilising structure  3300  comprises at least one flat strap. 
     In one form of the present technology, a positioning and stabilising structure  3300  is provided that is configured so as not to be too large and bulky to prevent the patient from lying in a supine sleeping position with a back region of the patient&#39;s head on a pillow. 
     In one form of the present technology, a positioning and stabilising structure  3300  is provided that is configured so as not to be too large and bulky to prevent the patient from lying in a side sleeping position with a side region of the patient&#39;s head on a pillow. 
     In one form of the present technology, a positioning and stabilising structure  3300  is provided with a decoupling portion located between an anterior portion of the positioning and stabilising structure  3300 , and a posterior portion of the positioning and stabilising structure  3300 . The decoupling portion does not resist compression and may be, e.g. a flexible or floppy strap. The decoupling portion is constructed and arranged so that when the patient lies with their head on a pillow, the presence of the decoupling portion prevents a force on the posterior portion from being transmitted along the positioning and stabilising structure  3300  and disrupting the seal. 
     In one form of the present technology, a positioning and stabilising structure  3300  comprises a strap constructed from a laminate of a fabric patient-contacting layer, a foam inner layer and a fabric outer layer. In one form, the foam is porous to allow moisture, (e.g., sweat), to pass through the strap. In one form, the fabric outer layer comprises loop material to engage with a hook material portion. 
     In certain forms of the present technology, a positioning and stabilising structure  3300  comprises a strap that is extensible, e.g. resiliently extensible. For example the strap may be configured in use to be in tension, and to direct a force to draw a seal-forming structure into sealing contact with a portion of a patient&#39;s face. In an example the strap may be configured as a tie. 
     In one form of the present technology, the positioning and stabilising structure comprises a first tie, the first tie being constructed and arranged so that in use at least a portion of an inferior edge thereof passes superior to an otobasion superior of the patient&#39;s head and overlays a portion of a parietal bone without overlaying the occipital bone. 
     In one form of the present technology suitable for a nasal-only mask or for a full-face mask, the positioning and stabilising structure includes a second tie, the second tie being constructed and arranged so that in use at least a portion of a superior edge thereof passes inferior to an otobasion inferior of the patient&#39;s head and overlays or lies inferior to the occipital bone of the patient&#39;s head. 
     In one form of the present technology suitable for a nasal-only mask or for a full-face mask, the positioning and stabilising structure includes a third tie that is constructed and arranged to interconnect the first tie and the second tie to reduce a tendency of the first tie and the second tie to move apart from one another. 
     In certain forms of the present technology, a positioning and stabilising structure  3300  comprises a strap that is bendable and e.g. non-rigid. An advantage of this aspect is that the strap is more comfortable for a patient to lie upon while the patient is sleeping. 
     In certain forms of the present technology, a positioning and stabilising structure  3300  comprises a strap constructed to be breathable to allow moisture vapour to be transmitted through the strap, 
     In certain forms of the present technology, a system is provided comprising more than one positioning and stabilizing structure  3300 , each being configured to provide a retaining force to correspond to a different size and/or shape range. For example the system may comprise one form of positioning and stabilizing structure  3300  suitable for a large sized head, but not a small sized head, and another. suitable for a small sized head, but not a large sized head. 
       FIGS. 7-15  depict examples of the present technology, including a positioning and stabilising structure  3300 . In these examples, the positioning and stabilising structure  3300  includes lateral portions  3302  and superior portions  3304  in the form of conduits that direct a flow pressurised gas from a hub  3306  to ends  3314 . The positioning and stabilising structure  3300  may be arranged such that the hub  3306  and the decoupling structure  3500  are positioned superior to the patient&#39;s head in use. As described below, the decoupling structure  3500  may be rotatable within the hub  3306  and when the patient is wearing the patient interface  3000 , e.g., during therapy, the location of the hub  3306  and the decoupling structure  3500  superior to the patient&#39;s head allows the patient to move more freely without becoming entangled with the air circuit  4170 . 
     The positioning and stabilising structure  3300  may be constructed of silicone. For example, the lateral portions  3302 , the superior portions  3304 , the hub  3306 , and the lateral ends  3314  may able constructed or molded from a single piece of silicone. 
     The superior portions  3304  of the positioning and stabilising structure  3300  have ridges and valleys (or concertina sections) that allow the superior portions  3304  to conform to the shape of the corresponding portion of the patient&#39;s head in use. The ridges and valleys of the superior portions  3304  allow the superior portions  3304  to be extended and contracted along the longitudinal axis to accommodate larger or smaller heads. The ridges and valleys of the superior portions  3304  allow the superior portions  3304  to be flexed to different radii of curvature to accommodate patient heads of different shapes and sizes. 
     The lateral portions  3302  portions of the positioning and stabilising structure  3300  may not be formed with the ridges and valleys of the superior portions  3304 . Therefore, the lateral portions  3302  may be less extensible and flexible than the superior portions  3304 , which may be advantageous because there is less variability in the shape and size of the lateral sides of a patient&#39;s head. 
     The ends  3314  may connect to respective plenum chamber connectors  3214 . As described above, the plenum chamber connectors  3214  receive the flow of pressurised gas from the positioning and stabilising structure  3300 , which passes through the plenum chamber  3200 , through the seal-forming structure  3100 , and on to the patient&#39;s airways. As described above, the ends  3314  may include clip overmolds and clips that facilitate connection of the ends  3314  to plenum chamber connectors  3214 . 
     The lateral portions  3302  may also each include a tab  3308  that receives a posterior strap end portion  3311  of a posterior strap  3310 . The posterior strap  3310  may be length-adjustable, e.g., with a hook and loop material arrangement whereby one of the posterior strap end portion  3311  and the remainder of the posterior strap  3310  includes hook material on its exterior while the other includes loop material on its exterior. The length adjustability of the posterior strap  3310  allows tension on the lateral portions  3302  to be increased to pull the seal-forming structure  3100  into sealing engagement with the patient&#39;s face at a desired amount of pressure (i.e., sufficiently tight to avoid leaks while not so tight as to cause discomfort). 
     The lateral portions  3302  may also be provided with sleeves  3312  that cushion the patient&#39;s face against the lateral portions  3302 . The sleeves  3312  may be constructed of a breathable textile material that has a soft feel. 
     5.3.5 Vent 
     In one form, the patient interface  3000  includes a vent  3400  constructed and arranged to allow for the washout of exhaled gases, e.g. carbon dioxide. 
     In certain forms the vent  3400  is configured to allow a continuous vent flow from an interior of the plenum chamber  3200  to ambient whilst the pressure within the plenum chamber is positive with respect to ambient. The vent  3400  is configured such that the vent flow rate has a magnitude sufficient to reduce rebreathing of exhaled CO 2  by the patient while maintaining the therapeutic pressure in the plenum chamber in use. 
     One form of vent  3400  in accordance with the present technology comprises a plurality of holes, for example, about 20 to about 80 holes, or about 40 to about 60 holes, or about 45 to about 55 holes. 
     The vent  3400  may be located in the plenum chamber  3200 . The plenum chamber vent  3400  may comprise a plurality of holes, as described above. The holes of the plenum chamber vent  3400  may be divided into two groups spaced apart laterally. The axis of the flow path through each of the holes of the plenum chamber vent  3400  may be parallel such that cross-flow is avoided to prevent generation of additional noise. The vent holes may be circular. 
     The holes of the plenum chamber vent  3400  may decrease in radius from the interior of the plenum chamber  3200  to the exterior. Each vent hole is provided with a draft angle. Each hole has a smaller diameter at its anterior end than at its posterior end. The draft angle means that the holes do not have a small cross section across the entire chassis thickness, which helps to provide effective carbon dioxide wash out at high levels of humidification. Additionally, a larger draft angle may result in a plenum chamber  3200  that is easier to manufacture, especially when the plenum chamber  3200  is formed from an injection moulded plastics material. The draft angle enables relatively thick vent pins to be used in the mould and easier ejection. 
     The holes of the plenum chamber vent  3400  may be provided in two sets towards the middle of the plenum chamber  3200  and the sets may be symmetrical across the centreline of the plenum chamber  3200 . Providing a pattern of multiple vent holes may reduce noise and diffuse the flow concentration. 
     The holes of the plenum chamber vent  3400  may be placed at an optimum distance away from the centreline of the plenum chamber  3200 . Placing the holes of the plenum chamber vent  3400  towards the centreline may advantageously reduce the chance that the vent holes are blocked when the patient is sleeping on their side. However, placing the vent holes too close to the middle of the plenum chamber  3200  may result in excessive weakening of the plenum chamber  3200  at the centre, especially since the cross-section of the plenum chamber  3200  in the depicted examples is smallest at the centre due to the overall shape of the plenum chamber  3200 . The location of the holes of the plenum chamber vent  3400  may avoid hole blockage during side sleep while leaving the middle section of the chassis sufficiently strong. 
     The size of each vent hole and the number of vent holes may be optimised to achieve a balance between noise reduction while achieving the necessary carbon dioxide washout, even at extreme humidification. In the depicted examples, the vent holes of the plenum chamber vent  3400  may not provide the total amount of venting for the system. The decoupling structure  3500  may include a decoupling structure vent  3402 . The decoupling structure vent  3402  may include one hole or a plurality of holes through the decoupling structure  3500 . The decoupling structure vent  3402  may function to bleed off excess pressure generated by the RPT device  4000  before reaching the patient, while the plenum chamber vent  3400  may function to washout carbon dioxide exhaled by the patient during therapy. 
     5.3.6 Decoupling Structure(s) 
     In one form the patient interface  3000  includes at least one decoupling structure, for example, a swivel or a ball and socket. 
     The hub  3306 , described above, is connected to a decoupling structure  3500 , which is a rotatable elbow in these examples. The decoupling structure  3500  may be rotatable 360° within the hub  3306  in use. The decoupling structure  3500  may be removable from the hub  3306  by manually depressing buttons  3504  to release catches (not shown) from within the hub  3306 . 
     The decoupling structure  3500  may also include a swivel  3502  that allows for rotatable connection to an air circuit  4170 . 
     The rotatability of the decoupling structure  3500 , the decoupling structure  3500  being in the form of an elbow, and the rotatability of the swivel  3502  on the decoupling structure  3500  may all increased the degrees of freedom, which in turn reduce tube drag and torque on the patient interface  3000  caused by the connection to the air circuit  4170 . 
     5.3.7 Connection Port 
     Connection port  3600  allows for connection to the air circuit  4170 . 
     5.3.8 Forehead Support 
     In one form, the patient interface  3000  includes a forehead support  3700 . In other forms, the patient interface  3000  does not include a forehead support. 
     5.3.9 Anti-Asphyxia Valve 
     In one form, the patient interface  3000  includes an anti-asphyxia valve. 
     5.3.10 Ports 
     In one form of the present technology, a patient interface  3000  includes one or more ports that allow access to the volume within the plenum chamber  3200 . In one form this allows a clinician to supply supplementary oxygen. In one form, this allows for the direct measurement of a property of gases within the plenum chamber  3200 , such as the pressure. 
     5.3.11 Patient Interface Configurations 
     5.3.11.1 Tube-Up, Nasal Pillows Patient Interface 
       FIG. 46  depicts an example of a tube-up, nasal pillows patient interface. Similar to the example shown in  FIGS. 7-15 , the tube-up aspect may be understood to describe conduits of the positioning and stabilising structure  3300  that pass along corresponding lateral sides of the patient&#39;s head between the corresponding eye and ear to be connected to the air circuit. The nasal pillows aspect may be understood to describe the sealing and face contacting arrangement in which the seal-forming structure  3100  is shaped and dimensioned to contact and seal against the patient&#39;s face around the inferior periphery of each of the patient&#39;s nares. The seal-forming structure  3100  may leave the mouth uncovered while contacting the ala and the columella, and possibly the patient&#39;s upper lip. 
     The HME features described above in section 5.3.3 may be incorporated into a patient interface  3000  of this configuration. 
     5.3.11.2 Tube-Up, Ultra-Compact Full-Face Patient Interface 
       FIG. 47  shows a patient interface  3000  that may be understood to be an ultra-compact full-face arrangement. The sealing arrangement of the seal-forming structure  3100  may be understood to be an ultra-compact full-face or oro-nasal arrangement. Full-face may be understood to mean that the patient&#39;s nose and mouth are sealed from atmosphere by the seal-forming structure  3100 . Ultra-compact may be understood to mean that the seal-forming structure  3100  does not engage the patient&#39;s face above the bridge of the nose or above the pronasale. In an ultra-compact full-face arrangement, at least a portion of the patient&#39;s pronasale may remain uncovered. As will be described below, the seal-forming structure  3100  may have an opening that corresponds to the patient&#39;s mouth. The seal-forming structure  3100  may have another opening that corresponds to the patient&#39;s nose, and that opening may be further divided into separate openings for each naris. Also, this example of patient interface  3000  may not include a forehead support. 
     The positioning and stabilising structure  3300  in this example comprises a pair of headgear tubes  3340 , i.e., conduits. This arrangement may also be understood to be a tube-up system in that the headgear tubes  3340  are in pneumatic communication with the air circuit  4170  above or superior to the patient&#39;s head so that the air circuit  4170  avoids overlaying the patient&#39;s face during use, which may be bothersome. 
     The pair of headgear tubes  3340  are connected to each other at their superior ends and are each configured to lie against superior and lateral surfaces of the patient&#39;s head in use. Each of the headgear tubes  3340  may be configured to lie between a corresponding eye and ear of the patient in use. The inferior end of each headgear tube  3340  is configured to fluidly connect to the plenum chamber  3200 . In this example, the inferior end of each headgear tube  3340  connects to a headgear tube connector  3344 , i.e., conduit connector, configured to connect to the shell or main frame  3202  of the plenum chamber  3200 . The positioning and stabilising structure  3300  comprises a conduit headgear inlet  3390  at the junction of the two headgear tubes  3340 . The conduit headgear inlet  3390  is configured to receive a pressurised flow of gas, for example via an elbow comprising a connection port  3600 , and allow the flow of gas into the headgear tubes  3340 . The headgear tubes  3340  supply the pressurised flow of gas to the plenum chamber  3200 . 
     The positioning and stabilising structure  3300  may comprise one or more straps in addition to the headgear tubes  3340 . In this example the positioning and stabilising structure  3300  comprises a pair of upper straps  3322  and a pair of lower straps  3320 . The posterior ends of the upper straps  3322  and lower straps  3320  are joined together. The junction between the upper straps  3322  and lower strap  3320  is configured to lie against a posterior surface of the patient&#39;s head in use, providing an anchor for the upper strap  3322  and lower straps  3320 . Anterior ends of the upper straps  3322  connect to the headgear tubes  3340 . In this example each headgear tube  3340  comprises a tab  3342  having an opening through which a respective upper strap  3322  can be passed through and then looped back and secured onto itself to secure the upper headgear strap  3322  to the headgear tube  3340 . The positioning and stabilising structure  3300  also comprises a lower strap clip  3326  provided to the anterior end of each of the lower straps  3320 . Each of the lower strap clips  3326  is configured to connect to a lower connection point  3325  on the plenum chamber  3200 . In this example, the lower strap clips  3326  are secured magnetically to the lower connection points  3325 . In some examples, there is also a mechanical engagement between the lower strap clips  3326  and the lower connection points  3325 . 
     The headgear tube connectors  3344  may be configured to allow the patient to breathe ambient air in the absence of pressure within the plenum chamber  3200 . Each headgear tube connector  3344  may comprise an anti-asphyxia valve (AAV). The AAV in each headgear tube connector  3344  may be configured to open in the absence of pressure within the plenum chamber  3200  in order to allow a flow of air between the interior of the plenum chamber  3200  and ambient. Each AAV may be biased into a configuration which blocks the flow of air from the interior of the plenum chamber  3200  into a respective headgear tube  3340  but allows for the exchange of air between the plenum chamber  3200  and ambient. When the headgear tubes  3340  are pressurised, the AAV in each headgear tube connector  3344  may prevent the exchange of air between the interior of the plenum chamber  3200  and ambient but allow for a flow of air from the respective headgear tube  3340  into the plenum chamber  3200  for breathing by the patient. 
     The plenum chamber  3200  shown in  FIG. 47  comprises a vent  3400 . In this example, the vent  3400  comprises a plurality of holes. The vent  3400  may be formed on the removable frame  3204 . In these examples, the vent  3400  is provided to the main frame  3202 . In some examples of the present technology the patient interface  3000  comprises a diffuser configured to diffuse air flowing though the vent  3400 . The vent  3400  may be positioned centrally on the main frame  3202  to avoid being covered by the patient&#39;s bed or bed clothes during side sleeping. 
     The HME features described above in section 5.3.3 may be incorporated into a patient interface  3000  of this configuration. 
     5.4 RPT Device 
     An RPT device  4000  in accordance with one aspect of the present technology comprises mechanical, pneumatic, and/or electrical components and is configured to execute one or more algorithms  4300 , such as any of the methods, in whole or in part, described herein. The RPT device  4000  may be configured to generate a flow of air for delivery to a patient&#39;s airways, such as to treat one or more of the respiratory conditions described elsewhere in the present document. 
     In one form, the RPT device  4000  is constructed and arranged to be capable of delivering a flow of air in a range of −20 L/min to +150 L/min while maintaining a positive pressure of at least 6 cmH 2 O, or at least 10cmH 2 O, or at least 20 cmH 2 O. 
     The RPT device may have an external housing  4010 , formed in two parts, an upper portion  4012  and a lower portion  4014 . Furthermore, the external housing  4010  may include one or more panel(s)  4015 . The RPT device  4000  comprises a chassis  4016  that supports one or more internal components of the RPT device  4000 . The RPT device  4000  may include a handle  4018 . 
     The pneumatic path of the RPT device  4000  may comprise one or more air path items, e.g., an inlet air filter  4112 , an inlet muffler  4122 , a pressure generator  4140  capable of supplying air at positive pressure (e.g., a blower  4142 ), an outlet muffler  4124  and one or more transducers  4270 , such as pressure sensors  4272  and flow rate sensors  4274 . 
     One or more of the air path items may be located within a removable unitary structure which will be referred to as a pneumatic block  4020 . The pneumatic block  4020  may be located within the external housing  4010 . In one form a pneumatic block  4020  is supported by, or formed as part of the chassis  4016 . 
     The RPT device  4000  may have an electrical power supply  4210 , one or more input devices  4220 , a central controller  4230 , a therapy device controller  4240 , a pressure generator  4140 , one or more protection circuits  4250 , memory  4260 , transducers  4270 , data communication interface  4280  and one or more output devices  4290 . Electrical components  4200  may be mounted on a single Printed Circuit Board Assembly (PCBA)  4202 . In an alternative form, the RPT device  4000  may include more than one PCBA  4202 . 
     5.4.1 RPT Device Mechanical &amp; Pneumatic Components 
     An RPT device may comprise one or more of the following components in an integral unit. In an alternative form, one or more of the following components may be located as respective separate units. 
     5.4.1.1 Air Filter(s) 
     An RPT device in accordance with one form of the present technology may include an air filter  4110 , or a plurality of air filters  4110 . 
     In one form, an inlet air filter  4112  is located at the beginning of the pneumatic path upstream of a pressure generator  4140 . 
     In one form, an outlet air filter  4114 , for example an antibacterial filter, is located between an outlet of the pneumatic block  4020  and a patient interface  3000  or  3800 . 
     5.4.1.2 Muffler(s) 
     An RPT device in accordance with one form of the present technology may include a muffler  4120 , or a plurality of mufflers  4120 . 
     In one form of the present technology, an inlet muffler  4122  is located in the pneumatic path upstream of a pressure generator  4140 . 
     In one form of the present technology, an outlet muffler  4124  is located in the pneumatic path between the pressure generator  4140  and a patient interface  3000  or  3800 . 
     5.4.1.3 Pressure Generator 
     In one form of the present technology, a pressure generator  4140  for producing a flow, or a supply, of air at positive pressure is a controllable blower  4142 . For example the blower  4142  may include a brushless DC motor  4144  with one or more impellers. The impellers may be located in a volute. The blower may be capable of delivering a supply of air, for example at a rate of up to about 120 litres/minute, at a positive pressure in a range from about 4 cmH 2 O to about 20 cmH 2 O, or in other forms up to about 30 cmH 2 O when delivering respiratory pressure therapy. The blower may be as described in any one of the following patents or patent applications the contents of which are incorporated herein by reference in their entirety: U.S. Pat. Nos. 7,866,944; 8,638,014; 8,636,479; and PCT Patent Application Publication No. WO 2013/020167. 
     The pressure generator  4140  is under the control of the therapy device controller  4240 . 
     In other forms, a pressure generator  4140  may be a piston-driven pump, a pressure regulator connected to a high pressure source (e.g. compressed air reservoir), or a bellows. 
     5.4.1.4 Transducer(s) 
     Transducers may be internal of the RPT device, or external of the RPT device. External transducers may be located for example on or form part of the air circuit, e.g., the patient interface. External transducers may be in the form of non-contact sensors such as a Doppler radar movement sensor that transmit or transfer data to the RPT device. 
     In one form of the present technology, one or more transducers  4270  are located upstream and/or downstream of the pressure generator  4140 . The one or more transducers  4270  may be constructed and arranged to generate signals representing properties of the flow of air such as a flow rate, a pressure or a temperature at that point in the pneumatic path. 
     In one form of the present technology, one or more transducers  4270  may be located proximate to the patient interface  3000  or  3800 . 
     In one form, a signal from a transducer  4270  may be filtered, such as by low-pass, high-pass or band-pass filtering. 
     5.4.1.4.1 Flow Rate Sensor 
     A flow rate sensor  4274  in accordance with the present technology may be based on a differential pressure transducer, for example, an SDP600 Series differential pressure transducer from SENSIRION. 
     In one form, a signal generated by the flow rate sensor  4274  and representing a flow rate is received by the central controller  4230 . 
     5.4.1.4.2 Pressure Sensor 
     A pressure sensor  4272  in accordance with the present technology is located in fluid communication with the pneumatic path. An example of a suitable pressure sensor is a transducer from the HONEYWELL ASDX series. An alternative suitable pressure sensor is a transducer from the NPA Series from GENERAL ELECTRIC. 
     In one form, a signal generated by the pressure sensor  4272  is received by the central controller  4230 . 
     5.4.1.4.3 Motor Speed Transducer 
     In one form of the present technology a motor speed transducer  4276  is used to determine a rotational velocity of the motor  4144  and/or the blower  4142 . A motor speed signal from the motor speed transducer  4276  may be provided to the therapy device controller  4240 . The motor speed transducer  4276  may, for example, be a speed sensor, such as a Hall effect sensor. 
     5.4.1.5 Anti-Spill Back Valve 
     In one form of the present technology, an anti-spill back valve  4160  is located between the humidifier  5000  and the pneumatic block  4020 . The anti-spill back valve is constructed and arranged to reduce the risk that water will flow upstream from the humidifier  5000 , for example to the motor  4144 . 
     5.4.2 RPT Device Electrical Components 
     5.4.2.1 Power Supply 
     A power supply  4210  may be located internal or external of the external housing  4010  of the RPT device  4000 . 
     In one form of the present technology, power supply  4210  provides electrical power to the RPT device  4000  only. In another form of the present technology, power supply  4210  provides electrical power to both RPT device  4000  and humidifier  5000 . 
     5.4.2.2 Input Devices 
     In one form of the present technology, an RPT device  4000  includes one or more input devices  4220  in the form of buttons, switches or dials to allow a person to interact with the device. The buttons, switches or dials may be physical devices, or software devices accessible via a touch screen. The buttons, switches or dials may, in one form, be physically connected to the external housing  4010 , or may, in another form, be in wireless communication with a receiver that is in electrical connection to the central controller  4230 . 
     In one form, the input device  4220  may be constructed and arranged to allow a person to select a value and/or a menu option. 
     5.4.2.3 Central Controller 
     In one form of the present technology, the central controller  4230  is one or a plurality of processors suitable to control an RPT device  4000 . 
     Suitable processors may include an x86 INTEL processor, a processor based on ARM® Cortex®-M processor from ARM Holdings such as an STM32 series microcontroller from ST MICROELECTRONIC. In certain alternative forms of the present technology, a 32-bit RISC CPU, such as an STR9 series microcontroller from ST MICROELECTRONICS or a 16-bit RISC CPU such as a processor from the MSP430 family of microcontrollers, manufactured by TEXAS INSTRUMENTS may also be suitable. 
     In one form of the present technology, the central controller  4230  is a dedicated electronic circuit. 
     In one form, the central controller  4230  is an application-specific integrated circuit. In another form, the central controller  4230  comprises discrete electronic components. 
     The central controller  4230  may be configured to receive input signal(s) from one or more transducers  4270 , one or more input devices  4220 , and the humidifier  5000 . 
     The central controller  4230  may be configured to provide output signal(s) to one or more of an output device  4290 , a therapy device controller  4240 , a data communication interface  4280 , and the humidifier  5000 . 
     In some forms of the present technology, the central controller  4230  is configured to implement the one or more methodologies described herein, such as the one or more algorithms  4300  expressed as computer programs stored in a non-transitory computer readable storage medium, such as memory  4260 . In some forms of the present technology, the central controller  4230  may be integrated with an RPT device  4000 . However, in some forms of the present technology, some methodologies may be performed by a remotely located device. For example, the remotely located device may determine control settings for a ventilator or detect respiratory related events by analysis of stored data such as from any of the sensors described herein. 
     5.4.2.4 Clock 
     The RPT device  4000  may include a clock  4232  that is connected to the central controller  4230 . 
     5.4.2.5 Therapy Device Controller 
     In one form of the present technology, therapy device controller  4240  is a therapy control module  4330  that forms part of the algorithms  4300  executed by the central controller  4230 . 
     In one form of the present technology, therapy device controller  4240  is a dedicated motor control integrated circuit. For example, in one form a MC33035 brushless DC motor controller, manufactured by ONSEMI is used. 
     5.4.2.6 Protection Circuits 
     The one or more protection circuits  4250  in accordance with the present technology may comprise an electrical protection circuit, a temperature and/or pressure safety circuit. 
     5.4.2.7 Memory 
     In accordance with one form of the present technology the RPT device  4000  includes memory  4260 , e.g., non-volatile memory. In some forms, memory  4260  may include battery powered static RAM. In some forms, memory  4260  may include volatile RAM. 
     Memory  4260  may be located on the PCBA  4202 . Memory  4260  may be in the form of EEPROM, or NAND flash. 
     Additionally or alternatively, RPT device  4000  includes a removable form of memory  4260 , for example a memory card made in accordance with the Secure Digital (SD) standard. 
     In one form of the present technology, the memory  4260  acts as a non-transitory computer readable storage medium on which is stored computer program instructions expressing the one or more methodologies described herein, such as the one or more algorithms  4300 . 
     5.4.2.8 Data Communication Systems 
     In one form of the present technology, a data communication interface  4280  is provided, and is connected to the central controller  4230 . Data communication interface  4280  may be connectable to a remote external communication network  4282  and/or a local external communication network  4284 . The remote external communication network  4282  may be connectable to a remote external device  4286 . The local external communication network  4284  may be connectable to a local external device  4288 . 
     In one form, data communication interface  4280  is part of the central controller  4230 . In another form, data communication interface  4280  is separate from the central controller  4230 , and may comprise an integrated circuit or a processor. 
     In one form, remote external communication network  4282  is the Internet. The data communication interface  4280  may use wired communication (e.g. via Ethernet, or optical fibre) or a wireless protocol (e.g. CDMA, GSM, LTE) to connect to the Internet. 
     In one form, local external communication network  4284  utilises one or more communication standards, such as Bluetooth, or a consumer infrared protocol. 
     In one form, remote external device  4286  is one or more computers, for example a cluster of networked computers. In one form, remote external device  4286  may be virtual computers, rather than physical computers. In either case, such a remote external device  4286  may be accessible to an appropriately authorised person such as a clinician. 
     The local external device  4288  may be a personal computer, mobile phone, tablet or remote control. 
     5.4.2.9 Output Devices Including Optional Display, Alarms 
     An output device  4290  in accordance with the present technology may take the form of one or more of a visual, audio and haptic unit. A visual display may be a Liquid Crystal Display (LCD) or Light Emitting Diode (LED) display. 
     5.4.2.9.1 Display Driver 
     A display driver  4292  receives as an input the characters, symbols, or images intended for display on the display  4294 , and converts them to commands that cause the display  4294  to display those characters, symbols, or images. 
     5.4.2.9.2 Display 
     A display  4294  is configured to visually display characters, symbols, or images in response to commands received from the display driver  4292 . For example, the display  4294  may be an eight-segment display, in which case the display driver  4292  converts each character or symbol, such as the figure “0”, to eight logical signals indicating whether the eight respective segments are to be activated to display a particular character or symbol. 
     5.5 Air Circuit 
     An air circuit  4170  in accordance with an aspect of the present technology is a conduit or a tube constructed and arranged to allow, in use, a flow of air to travel between two components such as RPT device  4000  and the patient interface  3000  or  3800 . 
     In particular, the air circuit  4170  may be in fluid connection with the outlet of the pneumatic block  4020  and the patient interface. The air circuit may be referred to as an air delivery tube. In some cases there may be separate limbs of the circuit for inhalation and exhalation. In other cases a single limb is used. 
     In some forms, the air circuit  4170  may comprise one or more heating elements configured to heat air in the air circuit, for example to maintain or raise the temperature of the air. The heating element may be in a form of a heated wire circuit, and may comprise one or more transducers, such as temperature sensors. In one form, the heated wire circuit may be helically wound around the axis of the air circuit  4170 . The heating element may be in communication with a controller such as a central controller  4230 . One example of an air circuit  4170  comprising a heated wire circuit is described in U.S. Pat. No. 8,733,349, which is incorporated herewithin in its entirety by reference. 
     5.5.1 Supplementary Gas Delivery 
     In one form of the present technology, supplementary gas, e.g. oxygen,  4180  is delivered to one or more points in the pneumatic path, such as upstream of the pneumatic block  4020 , to the air circuit  4170 , and/or to the patient interface  3000  or  3800 . 
     5.6 Humidifier 
     5.6.1 Humidifier Overview 
     In one form of the present technology there is provided a humidifier  5000  (e.g. as shown in  FIG. 5A ) to change the absolute humidity of air or gas for delivery to a patient relative to ambient air. Typically, the humidifier  5000  is used to increase the absolute humidity and increase the temperature of the flow of air (relative to ambient air) before delivery to the patient&#39;s airways. 
     The humidifier  5000  may comprise a humidifier reservoir  5110 , a humidifier inlet  5002  to receive a flow of air, and a humidifier outlet  5004  to deliver a humidified flow of air. In some forms, as shown in  FIG. 5A  and  FIG. 5B , an inlet and an outlet of the humidifier reservoir  5110  may be the humidifier inlet  5002  and the humidifier outlet  5004  respectively. The humidifier  5000  may further comprise a humidifier base  5006 , which may be adapted to receive the humidifier reservoir  5110  and comprise a heating element  5240 . 
     5.6.2 Humidifier Components 
     5.6.2.1 Water Reservoir 
     According to one arrangement, the humidifier  5000  may comprise a water reservoir  5110  configured to hold, or retain, a volume of liquid (e.g. water) to be evaporated for humidification of the flow of air. The water reservoir  5110  may be configured to hold a predetermined maximum volume of water in order to provide adequate humidification for at least the duration of a respiratory therapy session, such as one evening of sleep. Typically, the reservoir  5110  is configured to hold several hundred millilitres of water, e.g. 300 millilitres (ml), 325 ml, 350 ml or 400 ml. In other forms, the humidifier  5000  may be configured to receive a supply of water from an external water source such as a building&#39;s water supply system. 
     According to one aspect, the water reservoir  5110  is configured to add humidity to a flow of air from the RPT device  4000  as the flow of air travels therethrough. In one form, the water reservoir  5110  may be configured to encourage the flow of air to travel in a tortuous path through the reservoir  5110  while in contact with the volume of water therein. 
     According to one form, the reservoir  5110  may be removable from the humidifier  5000 , for example in a lateral direction as shown in  FIG. 5A  and  FIG. 5B . 
     The reservoir  5110  may also be configured to discourage egress of liquid therefrom, such as when the reservoir  5110  is displaced and/or rotated from its normal, working orientation, such as through any apertures and/or in between its sub-components. As the flow of air to be humidified by the humidifier  5000  is typically pressurised, the reservoir  5110  may also be configured to prevent losses in pneumatic pressure through leak and/or flow impedance. 
     5.6.2.2 Conductive Portion 
     According to one arrangement, the reservoir  5110  comprises a conductive portion  5120  configured to allow efficient transfer of heat from the heating element  5240  to the volume of liquid in the reservoir  5110 . In one form, the conductive portion  5120  may be arranged as a plate, although other shapes may also be suitable. All or a part of the conductive portion  5120  may be made of a thermally conductive material such as aluminium (e.g. approximately 2 mm thick, such as 1 mm, 1.5 mm, 2.5 mm or 3 mm), another heat conducting metal or some plastics. In some cases, suitable heat conductivity may be achieved with less conductive materials of suitable geometry. 
     5.6.2.3 Humidifier Reservoir Dock 
     In one form, the humidifier  5000  may comprise a humidifier reservoir dock  5130  (as shown in  FIG. 5B ) configured to receive the humidifier reservoir  5110 . In some arrangements, the humidifier reservoir dock  5130  may comprise a locking feature such as a locking lever  5135  configured to retain the reservoir  5110  in the humidifier reservoir dock  5130 . 
     5.6.2.4 Water Level Indicator 
     The humidifier reservoir  5110  may comprise a water level indicator  5150  as shown in  FIG. 5A-5B . In some forms, the water level indicator  5150  may provide one or more indications to a user such as the patient  1000  or a care giver regarding a quantity of the volume of water in the humidifier reservoir  5110 . The one or more indications provided by the water level indicator  5150  may include an indication of a maximum, predetermined volume of water, any portions thereof, such as 25%, 50% or 75% or volumes such as 200 ml, 300 ml or 400 ml. 
     5.6.2.5 Humidifier Transducer(s) 
     The humidifier  5000  may comprise one or more humidifier transducers (sensors)  5210  instead of, or in addition to, transducers  4270  described above. Humidifier transducers  5210  may include one or more of an air pressure sensor  5212 , an air flow rate transducer  5214 , a temperature sensor  5216 , or a humidity sensor  5218  as shown in  FIG. 5C . A humidifier transducer  5210  may produce one or more output signals which may be communicated to a controller such as the central controller  4230  and/or the humidifier controller  5250 . In some forms, a humidifier transducer may be located externally to the humidifier  5000  (such as in the air circuit  4170 ) while communicating the output signal to the controller. 
     5.6.2.5.1 Pressure Transducer 
     One or more pressure transducers  5212  may be provided to the humidifier  5000  in addition to, or instead of, a pressure sensor  4272  provided in the RPT device  4000 . 
     5.6.2.5.2 Flow Rate Transducer 
     One or more flow rate transducers  5214  may be provided to the humidifier  5000  in addition to, or instead of, a flow rate sensor  4274  provided in the RPT device  4000 . 
     5.6.2.5.3 Temperature Transducer 
     The humidifier  5000  may comprise one or more temperature transducers  5216 . The one or more temperature transducers  5216  may be configured to measure one or more temperatures such as of the heating element  5240  and/or of the flow of air downstream of the humidifier outlet  5004 . In some forms, the humidifier  5000  may further comprise a temperature sensor  5216  to detect the temperature of the ambient air. 
     5.6.2.5.4 Humidity Transducer 
     In one form, the humidifier  5000  may comprise one or more humidity sensors  5218  to detect a humidity of a gas, such as the ambient air. The humidity sensor  5218  may be placed towards the humidifier outlet  5004  in some forms to measure a humidity of the gas delivered from the humidifier  5000 . The humidity sensor may be an absolute humidity sensor or a relative humidity sensor. 
     5.6.2.6 Heating Element 
     A heating element  5240  may be provided to the humidifier  5000  in some cases to provide a heat input to one or more of the volume of water in the humidifier reservoir  5110  and/or to the flow of air. The heating element  5240  may comprise a heat generating component such as an electrically resistive heating track. One suitable example of a heating element  5240  is a layered heating element such as one described in the PCT Patent Application Publication No. WO 2012/171072, which is incorporated herewith by reference in its entirety. 
     In some forms, the heating element  5240  may be provided in the humidifier base  5006  where heat may be provided to the humidifier reservoir  5110  primarily by conduction as shown in  FIG. 5B . 
     5.6.2.7 Humidifier Controller 
     According to one arrangement of the present technology, a humidifier  5000  may comprise a humidifier controller  5250  as shown in  FIG. 5C . In one form, the humidifier controller  5250  may be a part of the central controller  4230 . In another form, the humidifier controller  5250  may be a separate controller, which may be in communication with the central controller  4230 . 
     In one form, the humidifier controller  5250  may receive as inputs measures of properties (such as temperature, humidity, pressure and/or flow rate), for example of the flow of air, the water in the reservoir  5110  and/or the humidifier  5000 . The humidifier controller  5250  may also be configured to execute or implement humidifier algorithms and/or deliver one or more output signals. 
     As shown in  FIG. 5C , the humidifier controller  5250  may comprise one or more controllers, such as a central humidifier controller  5251 , a heated air circuit controller  5254  configured to control the temperature of a heated air circuit  4171  and/or a heating element controller  5252  configured to control the temperature of a heating element  5240 . 
     5.7 Breathing Waveforms 
       FIG. 6  shows a model typical breath waveform of a person while sleeping. The horizontal axis is time, and the vertical axis is respiratory flow rate. While the parameter values may vary, a typical breath may have the following approximate values: tidal volume Vt 0.5 L, inhalation time Ti 1.6 s, peak inspiratory flow rate Qpeak 0.4 L/s, exhalation time Te 2.4 s, peak expiratory flow rate Qpeak −0.5 L/s. The total duration of the breath, Ttot, is about 4 s. The person typically breathes at a rate of about 15 breaths per minute (BPM), with Ventilation Vent about 7.5 L/min. A typical duty cycle, the ratio of Ti to Ttot, is about 40%. 
     5.8 Glossary 
     For the purposes of the present technology disclosure, in certain forms of the present technology, one or more of the following definitions may apply. In other forms of the present technology, alternative definitions may apply. 
     5.8.1 General 
     Air: In certain forms of the present technology, air may be taken to mean atmospheric air, and in other forms of the present technology air may be taken to mean some other combination of breathable gases, e.g. atmospheric air enriched with oxygen. 
     Ambient: In certain forms of the present technology, the term ambient will be taken to mean (i) external of the treatment system or patient, and (ii) immediately surrounding the treatment system or patient. 
     For example, ambient humidity with respect to a humidifier may be the humidity of air immediately surrounding the humidifier, e.g. the humidity in the room where a patient is sleeping. Such ambient humidity may be different to the humidity outside the room where a patient is sleeping. 
     In another example, ambient pressure may be the pressure immediately surrounding or external to the body. 
     In certain forms, ambient (e.g., acoustic) noise may be considered to be the background noise level in the room where a patient is located, other than for example, noise generated by an RPT device or emanating from a mask or patient interface. Ambient noise may be generated by sources outside the room. 
     Automatic Positive Airway Pressure (APAP) therapy: CPAP therapy in which the treatment pressure is automatically adjustable, e.g. from breath to breath, between minimum and maximum limits, depending on the presence or absence of indications of SDB events. 
     Continuous Positive Airway Pressure (CPAP) therapy: Respiratory pressure therapy in which the treatment pressure is approximately constant through a respiratory cycle of a patient. In some forms, the pressure at the entrance to the airways will be slightly higher during exhalation, and slightly lower during inhalation. In some forms, the pressure will vary between different respiratory cycles of the patient, for example, being increased in response to detection of indications of partial upper airway obstruction, and decreased in the absence of indications of partial upper airway obstruction. 
     Flow rate: The volume (or mass) of air delivered per unit time. Flow rate may refer to an instantaneous quantity. In some cases, a reference to flow rate will be a reference to a scalar quantity, namely a quantity having magnitude only. In other cases, a reference to flow rate will be a reference to a vector quantity, namely a quantity having both magnitude and direction. Flow rate may be given the symbol Q. ‘Flow rate’ is sometimes shortened to simply ‘flow’ or ‘airflow’. 
     In the example of patient respiration, a flow rate may be nominally positive for the inspiratory portion of a breathing cycle of a patient, and hence negative for the expiratory portion of the breathing cycle of a patient. Device flow rate, Qd, is the flow rate of air leaving the RPT device. Total flow rate, Qt, is the flow rate of air and any supplementary gas reaching the patient interface via the air circuit. Vent flow rate, Qv, is the flow rate of air leaving a vent to allow washout of exhaled gases. Leak flow rate, Ql, is the flow rate of leak from a patient interface system or elsewhere. Respiratory flow rate, Qr, is the flow rate of air that is received into the patient&#39;s respiratory system. 
     Flow therapy: Respiratory therapy comprising the delivery of a flow of air to an entrance to the airways at a controlled flow rate referred to as the treatment flow rate that is typically positive throughout the patient&#39;s breathing cycle. 
     Humidifier: The word humidifier will be taken to mean a humidifying apparatus constructed and arranged, or configured with a physical structure to be capable of providing a therapeutically beneficial amount of water (H 2 O) vapour to a flow of air to ameliorate a medical respiratory condition of a patient. 
     Leak: The word leak will be taken to be an unintended flow of air. In one example, leak may occur as the result of an incomplete seal between a mask and a patient&#39;s face. In another example leak may occur in a swivel elbow to the ambient. 
     Noise, conducted (acoustic): Conducted noise in the present document refers to noise which is carried to the patient by the pneumatic path, such as the air circuit and the patient interface as well as the air therein. In one form, conducted noise may be quantified by measuring sound pressure levels at the end of an air circuit. 
     Noise, radiated (acoustic): Radiated noise in the present document refers to noise which is carried to the patient by the ambient air. In one form, radiated noise may be quantified by measuring sound power/pressure levels of the object in question according to ISO 3744. 
     Noise, vent (acoustic): Vent noise in the present document refers to noise which is generated by the flow of air through any vents such as vent holes of the patient interface. 
     Patient: A person, whether or not they are suffering from a respiratory condition. 
     Pressure: Force per unit area. Pressure may be expressed in a range of units, including cmH 2 O, g-f/cm 2  and hectopascal. 1 cmH 2 O is equal to 1 g-f/cm 2  and is approximately 0.98 hectopascal (1 hectopascal=100 Pa=100 N/m 2 =1 millibar 0.001 atm). In this specification, unless otherwise stated, pressure is given in units of cmH 2 O. 
     The pressure in the patient interface is given the symbol Pm, while the treatment pressure, which represents a target value to be achieved by the interface pressure Pm at the current instant of time, is given the symbol Pt. 
     Respiratory Pressure Therapy (RPT): The application of a supply of air to an entrance to the airways at a treatment pressure that is typically positive with respect to atmosphere. 
     Ventilator: A mechanical device that provides pressure support to a patient to perform some or all of the work of breathing. 
     5.8.1.1 Materials 
     Silicone or Silicone Elastomer: A synthetic rubber. In this specification, a reference to silicone is a reference to liquid silicone rubber (LSR) or a compression moulded silicone rubber (CMSR). One form of commercially available LSR is SILASTIC (included in the range of products sold under this trademark), manufactured by Dow Corning. Another manufacturer of LSR is Wacker. Unless otherwise specified to the contrary, an exemplary form of LSR has a Shore A (or Type A) indentation hardness in the range of about 35 to about 45 as measured using ASTM D2240. 
     Polycarbonate: a thermoplastic polymer of Bisphenol-A Carbonate. 
     5.8.1.2 Mechanical Properties 
     Resilience: Ability of a material to absorb energy when deformed elastically and to release the energy upon unloading. 
     Resilient: Will release substantially all of the energy when unloaded. Includes e.g. certain silicones, and thermoplastic elastomers. 
     Hardness: The ability of a material per se to resist deformation (e.g. described by a Young&#39;s Modulus, or an indentation hardness scale measured on a standardised sample size).
         ‘Soft’ materials may include silicone or thermo-plastic elastomer (TPE), and may, e.g. readily deform under finger pressure.   ‘Hard’ materials may include polycarbonate, polypropylene, steel or aluminium, and may not e.g. readily deform under finger pressure.       

     Stiffness (or rigidity) of a structure or component: The ability of the structure or component to resist deformation in response to an applied load. The load may be a force or a moment, e.g. compression, tension, bending or torsion. The structure or component may offer different resistances in different directions. The inverse of stiffness is flexibility. 
     Floppy structure or component: A structure or component that will change shape, e.g. bend, when caused to support its own weight, within a relatively short period of time such as 1 second. 
     Rigid structure or component: A structure or component that will not substantially change shape when subject to the loads typically encountered in use. An example of such a use may be setting up and maintaining a patient interface in sealing relationship with an entrance to a patient&#39;s airways, e.g. at a load of approximately 20 to 30 cmH 2 O pressure. 
     As an example, an I-beam may comprise a different bending stiffness (resistance to a bending load) in a first direction in comparison to a second, orthogonal direction. In another example, a structure or component may be floppy in a first direction and rigid in a second direction. 
     5.8.2 Respiratory Cycle 
     Apnea: According to some definitions, an apnea is said to have occurred when flow falls below a predetermined threshold for a duration, e.g. 10 seconds. An obstructive apnea will be said to have occurred when, despite patient effort, some obstruction of the airway does not allow air to flow. A central apnea will be said to have occurred when an apnea is detected that is due to a reduction in breathing effort, or the absence of breathing effort, despite the airway being patent. A mixed apnea occurs when a reduction or absence of breathing effort coincides with an obstructed airway. 
     Breathing rate: The rate of spontaneous respiration of a patient, usually measured in breaths per minute. 
     Duty cycle: The ratio of inhalation time, Ti to total breath time, Ttot. 
     Effort (breathing): The work done by a spontaneously breathing person attempting to breathe. 
     Expiratory portion of a breathing cycle: The period from the start of expiratory flow to the start of inspiratory flow. 
     Flow limitation: Flow limitation will be taken to be the state of affairs in a patient&#39;s respiration where an increase in effort by the patient does not give rise to a corresponding increase in flow. Where flow limitation occurs during an inspiratory portion of the breathing cycle it may be described as inspiratory flow limitation. Where flow limitation occurs during an expiratory portion of the breathing cycle it may be described as expiratory flow limitation. 
     Types of flow limited inspiratory waveforms: 
     (i) Flattened: Having a rise followed by a relatively flat portion, followed by a fall. 
     (ii) M-shaped: Having two local peaks, one at the leading edge, and one at the trailing edge, and a relatively flat portion between the two peaks. 
     (iii) Chair-shaped: Having a single local peak, the peak being at the leading edge, followed by a relatively flat portion. 
     (iv) Reverse-chair shaped: Having a relatively flat portion followed by single local peak, the peak being at the trailing edge. 
     Hypopnea: According to some definitions, a hypopnea is taken to be a reduction in flow, but not a cessation of flow. In one form, a hypopnea may be said to have occurred when there is a reduction in flow below a threshold rate for a duration. A central hypopnea will be said to have occurred when a hypopnea is detected that is due to a reduction in breathing effort. In one form in adults, either of the following may be regarded as being hypopneas:
         (i) a 30% reduction in patient breathing for at least 10 seconds plus an associated 4% desaturation; or   (ii) a reduction in patient breathing (but less than 50%) for at least 10 seconds, with an associated desaturation of at least 3% or an arousal.       

     Hyperpnea: An increase in flow to a level higher than normal. 
     Inspiratory portion of a breathing cycle: The period from the start of inspiratory flow to the start of expiratory flow will be taken to be the inspiratory portion of a breathing cycle. 
     Patency (airway): The degree of the airway being open, or the extent to which the airway is open. A patent airway is open. Airway patency may be quantified, for example with a value of one (1) being patent, and a value of zero (0), being closed (obstructed). 
     Positive End-Expiratory Pressure (PEEP): The pressure above atmosphere in the lungs that exists at the end of expiration. 
     Peak flow rate (Qpeak): The maximum value of flow rate during the inspiratory portion of the respiratory flow waveform. 
     Respiratory flow rate, patient airflow rate, respiratory airflow rate (Qr): These terms may be understood to refer to the RPT device&#39;s estimate of respiratory flow rate, as opposed to “true respiratory flow rate” or “true respiratory flow rate”, which is the actual respiratory flow rate experienced by the patient, usually expressed in litres per minute. 
     Tidal volume (Vt): The volume of air inhaled or exhaled during normal breathing, when extra effort is not applied. In principle the inspiratory volume Vi (the volume of air inhaled) is equal to the expiratory volume Ve (the volume of air exhaled), and therefore a single tidal volume Vt may be defined as equal to either quantity. In practice the tidal volume Vt is estimated as some combination, e.g. the mean, of the inspiratory volume Vi and the expiratory volume Ve. 
     (inhalation) Time (Ti): The duration of the inspiratory portion of the respiratory flow rate waveform. 
     (exhalation) Time (Te): The duration of the expiratory portion of the respiratory flow rate waveform. 
     (total) Time (Ttot): The total duration between the start of one inspiratory portion of a respiratory flow rate waveform and the start of the following inspiratory portion of the respiratory flow rate waveform. 
     Typical recent ventilation: The value of ventilation around which recent values of ventilation Vent over some predetermined timescale tend to cluster, that is, a measure of the central tendency of the recent values of ventilation. 
     Upper airway obstruction (UAO): includes both partial and total upper airway obstruction. This may be associated with a state of flow limitation, in which the flow rate increases only slightly or may even decrease as the pressure difference across the upper airway increases (Starling resistor behaviour). 
     Ventilation (Vent): A measure of a rate of gas being exchanged by the patient&#39;s respiratory system. Measures of ventilation may include one or both of inspiratory and expiratory flow, per unit time. When expressed as a volume per minute, this quantity is often referred to as “minute ventilation”. Minute ventilation is sometimes given simply as a volume, understood to be the volume per minute. 
     5.8.3 Ventilation 
     Adaptive Servo-Ventilator (ASV): A servo-ventilator that has a changeable, rather than fixed target ventilation. The changeable target ventilation may be learned from some characteristic of the patient, for example, a respiratory characteristic of the patient. 
     Backup rate: A parameter of a ventilator that establishes the minimum breathing rate (typically in number of breaths per minute) that the ventilator will deliver to the patient, if not triggered by spontaneous respiratory effort. 
     Cycled: The termination of a ventilator&#39;s inspiratory phase. When a ventilator delivers a breath to a spontaneously breathing patient, at the end of the inspiratory portion of the breathing cycle, the ventilator is said to be cycled to stop delivering the breath. 
     Expiratory positive airway pressure (EPAP): a base pressure, to which a pressure varying within the breath is added to produce the desired interface pressure which the ventilator will attempt to achieve at a given time. 
     End expiratory pressure (EEP): Desired interface pressure which the ventilator will attempt to achieve at the end of the expiratory portion of the breath. If the pressure waveform template Π(Φ) is zero-valued at the end of expiration, i.e. Π(Φ)=0 when Φ=1, the EEP is equal to the EPAP. 
     Inspiratory positive airway pressure (IPAP): Maximum desired interface pressure which the ventilator will attempt to achieve during the inspiratory portion of the breath. 
     Pressure support: A number that is indicative of the increase in pressure during ventilator inspiration over that during ventilator expiration, and generally means the difference in pressure between the maximum value during inspiration and the base pressure (e.g., PS=IPAP−EPAP). In some contexts pressure support means the difference which the ventilator aims to achieve, rather than what it actually achieves. 
     Servo-ventilator: A ventilator that measures patient ventilation, has a target ventilation, and which adjusts the level of pressure support to bring the patient ventilation towards the target ventilation. 
     Spontaneous/Timed (S/T): A mode of a ventilator or other device that attempts to detect the initiation of a breath of a spontaneously breathing patient. If however, the device is unable to detect a breath within a predetermined period of time, the device will automatically initiate delivery of the breath. 
     Swing: Equivalent term to pressure support. 
     Triggered: When a ventilator delivers a breath of air to a spontaneously breathing patient, it is said to be triggered to do so at the initiation of the respiratory portion of the breathing cycle by the patient&#39;s efforts. 
     5.8.4 Anatomy 
     5.8.4.1 Anatomy of the Face 
     Ala: the external outer wall or “wing” of each nostril (plural: alar) 
     Alare: The most lateral point on the nasal ala. 
     Alar curvature (or alar crest) point: The most posterior point in the curved base line of each ala, found in the crease formed by the union of the ala with the cheek. 
     Auricle: The whole external visible part of the ear. 
     (nose) Bony framework: The bony framework of the nose comprises the nasal bones, the frontal process of the maxillae and the nasal part of the frontal bone. 
     (nose) Cartilaginous framework: The cartilaginous framework of the nose comprises the septal, lateral, major and minor cartilages. 
     Columella: the strip of skin that separates the nares and which runs from the pronasale to the upper lip. 
     Columella angle: The angle between the line drawn through the midpoint of the nostril aperture and a line drawn perpendicular to the Frankfort horizontal while intersecting subnasale. 
     Frankfort horizontal plane: A line extending from the most inferior point of the orbital margin to the left tragion. The tragion is the deepest point in the notch superior to the tragus of the auricle. 
     Glabella: Located on the soft tissue, the most prominent point in the midsagittal plane of the forehead. 
     Lateral nasal cartilage: A generally triangular plate of cartilage. Its superior margin is attached to the nasal bone and frontal process of the maxilla, and its inferior margin is connected to the greater alar cartilage. 
     Greater alar cartilage: A plate of cartilage lying below the lateral nasal cartilage. It is curved around the anterior part of the naris. Its posterior end is connected to the frontal process of the maxilla by a tough fibrous membrane containing three or four minor cartilages of the ala. 
     Nares (Nostrils): Approximately ellipsoidal apertures forming the entrance to the nasal cavity. The singular form of nares is naris (nostril). The nares are separated by the nasal septum. 
     Naso-labial sulcus or Naso-labial fold: The skin fold or groove that runs from each side of the nose to the corners of the mouth, separating the cheeks from the upper lip. 
     Naso-labial angle: The angle between the columella and the upper lip, while intersecting subnasale. 
     Otobasion inferior: The lowest point of attachment of the auricle to the skin of the face. 
     Otobasion superior: The highest point of attachment of the auricle to the skin of the face. 
     Pronasale: the most protruded point or tip of the nose, which can be identified in lateral view of the rest of the portion of the head. 
     Philtrum: the midline groove that runs from lower border of the nasal septum to the top of the lip in the upper lip region. 
     Pogonion: Located on the soft tissue, the most anterior midpoint of the chin. 
     Ridge (nasal): The nasal ridge is the midline prominence of the nose, extending from the Sellion to the Pronasale. 
     Sagittal plane: A vertical plane that passes from anterior (front) to posterior (rear). The midsagittal plane is a sagittal plane that divides the body into right and left halves. 
     Sellion: Located on the soft tissue, the most concave point overlying the area of the frontonasal suture. 
     Septal cartilage (nasal): The nasal septal cartilage forms part of the septum and divides the front part of the nasal cavity. 
     Subalare: The point at the lower margin of the alar base, where the alar base joins with the skin of the superior (upper) lip. 
     Subnasal point: Located on the soft tissue, the point at which the columella merges with the upper lip in the midsagittal plane. 
     Supramenton: The point of greatest concavity in the midline of the lower lip between labrale inferius and soft tissue pogonion 
     5.8.4.2 Anatomy of the Skull 
     Frontal bone: The frontal bone includes a large vertical portion, the squama frontalis, corresponding to the region known as the forehead. 
     Mandible: The mandible forms the lower jaw. The mental protuberance is the bony protuberance of the jaw that forms the chin. 
     Maxilla: The maxilla forms the upper jaw and is located above the mandible and below the orbits. The frontal process of the maxilla projects upwards by the side of the nose, and forms part of its lateral boundary. 
     Nasal bones: The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, the “bridge” of the nose. 
     Nasion: The intersection of the frontal bone and the two nasal bones, a depressed area directly between the eyes and superior to the bridge of the nose. 
     Occipital bone: The occipital bone is situated at the back and lower part of the cranium. It includes an oval aperture, the foramen magnum, through which the cranial cavity communicates with the vertebral canal. The curved plate behind the foramen magnum is the squama occipitalis. 
     Orbit: The bony cavity in the skull to contain the eyeball. 
     Parietal bones: The parietal bones are the bones that, when joined together, form the roof and sides of the cranium. 
     Temporal bones: The temporal bones are situated on the bases and sides of the skull, and support that part of the face known as the temple. 
     Zygomatic bones: The face includes two zygomatic bones, located in the upper and lateral parts of the face and forming the prominence of the cheek. 
     5.8.4.3 Anatomy of the Respiratory System 
     Diaphragm: A sheet of muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity, containing the heart, lungs and ribs, from the abdominal cavity. As the diaphragm contracts the volume of the thoracic cavity increases and air is drawn into the lungs. 
     Larynx: The larynx, or voice box houses the vocal folds and connects the inferior part of the pharynx (hypopharynx) with the trachea. 
     Lungs: The organs of respiration in humans. The conducting zone of the lungs contains the trachea, the bronchi, the bronchioles, and the terminal bronchioles. The respiratory zone contains the respiratory bronchioles, the alveolar ducts, and the alveoli. 
     Nasal cavity: The nasal cavity (or nasal fossa) is a large air filled space above and behind the nose in the middle of the face. The nasal cavity is divided in two by a vertical fin called the nasal septum. On the sides of the nasal cavity are three horizontal outgrowths called nasal conchae (singular “concha”) or turbinates. To the front of the nasal cavity is the nose, while the back blends, via the choanae, into the nasopharynx. 
     Pharynx: The part of the throat situated immediately inferior to (below) the nasal cavity, and superior to the oesophagus and larynx. The pharynx is conventionally divided into three sections: the nasopharynx (epipharynx) (the nasal part of the pharynx), the oropharynx (mesopharynx) (the oral part of the pharynx), and the laryngopharynx (hypopharynx). 
     5.8.5 Patient Interface 
     Anti-asphyxia valve (AAV): The component or sub-assembly of a mask system that, by opening to atmosphere in a failsafe manner, reduces the risk of excessive CO 2  rebreathing by a patient. 
     Elbow: An elbow is an example of a structure that directs an axis of flow of air travelling therethrough to change direction through an angle. In one form, the angle may be approximately 90 degrees. In another form, the angle may be more, or less than 90 degrees. The elbow may have an approximately circular cross-section. In another form the elbow may have an oval or a rectangular cross-section. In certain forms an elbow may be rotatable with respect to a mating component, e.g. about 360 degrees. In certain forms an elbow may be removable from a mating component, e.g. via a snap connection. In certain forms, an elbow may be assembled to a mating component via a one-time snap during manufacture, but not removable by a patient. 
     Frame: Frame will be taken to mean a mask structure that bears the load of tension between two or more points of connection with a headgear. A mask frame may be a non-airtight load bearing structure in the mask. However, some forms of mask frame may also be air-tight. 
     Headgear: Headgear will be taken to mean a form of positioning and stabilizing structure designed for use on a head. For example the headgear may comprise a collection of one or more struts, ties and stiffeners configured to locate and retain a patient interface in position on a patient&#39;s face for delivery of respiratory therapy. Some ties are formed of a soft, flexible, elastic material such as a laminated composite of foam and fabric. 
     Membrane: Membrane will be taken to mean a typically thin element that has, preferably, substantially no resistance to bending, but has resistance to being stretched. 
     Plenum chamber: a mask plenum chamber will be taken to mean a portion of a patient interface having walls at least partially enclosing a volume of space, the volume having air therein pressurised above atmospheric pressure in use. A shell may form part of the walls of a mask plenum chamber. 
     Seal: May be a noun form (“a seal”) which refers to a structure, or a verb form (“to seal”) which refers to the effect. Two elements may be constructed and/or arranged to ‘seal’ or to effect ‘sealing’ therebetween without requiring a separate ‘seal’ element per se. 
     Shell: A shell will be taken to mean a curved, relatively thin structure having bending, tensile and compressive stiffness. For example, a curved structural wall of a mask may be a shell. In some forms, a shell may be faceted. In some forms a shell may be airtight. In some forms a shell may not be airtight. 
     Stiffener: A stiffener will be taken to mean a structural component designed to increase the bending resistance of another component in at least one direction. 
     Strut: A strut will be taken to be a structural component designed to increase the compression resistance of another component in at least one direction. 
     Swivel (noun): A subassembly of components configured to rotate about a common axis, preferably independently, preferably under low torque. In one form, the swivel may be constructed to rotate through an angle of at least 360 degrees. In another form, the swivel may be constructed to rotate through an angle less than 360 degrees. When used in the context of an air delivery conduit, the sub-assembly of components preferably comprises a matched pair of cylindrical conduits. There may be little or no leak flow of air from the swivel in use. 
     Tie (noun): A structure designed to resist tension. 
     Vent: (noun): A structure that allows a flow of air from an interior of the mask, or conduit, to ambient air for clinically effective washout of exhaled gases. For example, a clinically effective washout may involve a flow rate of about 10 litres per minute to about 100 litres per minute, depending on the mask design and treatment pressure. 
     5.8.6 Shape of Structures 
     Products in accordance with the present technology may comprise one or more three-dimensional mechanical structures, for example a mask cushion or an impeller. The three-dimensional structures may be bounded by two-dimensional surfaces. These surfaces may be distinguished using a label to describe an associated surface orientation, location, function, or some other characteristic. For example a structure may comprise one or more of an anterior surface, a posterior surface, an interior surface and an exterior surface. In another example, a seal-forming structure may comprise a face-contacting (e.g. outer) surface, and a separate non-face-contacting (e.g. underside or inner) surface. In another example, a structure may comprise a first surface and a second surface. 
     To facilitate describing the shape of the three-dimensional structures and the surfaces, we first consider a cross-section through a surface of the structure at a point, p. See  FIG. 3B  to  FIG. 3F , which illustrate examples of cross-sections at point p on a surface, and the resulting plane curves.  FIGS. 3B to 3F  also illustrate an outward normal vector at p. The outward normal vector at p points away from the surface. In some examples we describe the surface from the point of view of an imaginary small person standing upright on the surface. 
     5.8.6.1 Curvature in One Dimension 
     The curvature of a plane curve at p may be described as having a sign (e.g. positive, negative) and a magnitude (e.g.  1 /radius of a circle that just touches the curve at p). 
     Positive curvature: If the curve at p turns towards the outward normal, the curvature at that point will be taken to be positive (if the imaginary small person leaves the point p they must walk uphill). See  FIG. 3B  (relatively large positive curvature compared to  FIG. 3C ) and  FIG. 3C  (relatively small positive curvature compared to  FIG. 3B ). Such curves are often referred to as concave. 
     Zero curvature: If the curve at p is a straight line, the curvature will be taken to be zero (if the imaginary small person leaves the point p, they can walk on a level, neither up nor down). See  FIG. 3D . 
     Negative curvature: If the curve at p turns away from the outward normal, the curvature in that direction at that point will be taken to be negative (if the imaginary small person leaves the point p they must walk downhill). See  FIG. 3E  (relatively small negative curvature compared to  FIG. 3F ) and  FIG. 3F  (relatively large negative curvature compared to  FIG. 3E ). Such curves are often referred to as convex. 
     5.8.6.2 Curvature of Two Dimensional Surfaces 
     A description of the shape at a given point on a two-dimensional surface in accordance with the present technology may include multiple normal cross-sections. The multiple cross-sections may cut the surface in a plane that includes the outward normal (a “normal plane”), and each cross-section may be taken in a different direction. Each cross-section results in a plane curve with a corresponding curvature. The different curvatures at that point may have the same sign, or a different sign. Each of the curvatures at that point has a magnitude, e.g. relatively small. The plane curves in  FIGS. 3B to 3F  could be examples of such multiple cross-sections at a particular point. 
     Principal curvatures and directions: The directions of the normal planes where the curvature of the curve takes its maximum and minimum values are called the principal directions. In the examples of  FIG. 3B  to  FIG. 3F , the maximum curvature occurs in  FIG. 3B , and the minimum occurs in  FIG. 3F , hence  FIG. 3B  and  FIG. 3F  are cross sections in the principal directions. The principal curvatures at p are the curvatures in the principal directions. 
     Region of a surface: A connected set of points on a surface. The set of points in a region may have similar characteristics, e.g. curvatures or signs. 
     Saddle region: A region where at each point, the principal curvatures have opposite signs, that is, one is positive, and the other is negative (depending on the direction to which the imaginary person turns, they may walk uphill or downhill). 
     Dome region: A region where at each point the principal curvatures have the same sign, e.g. both positive (a “concave dome”) or both negative (a “convex dome”). 
     Cylindrical region: A region where one principal curvature is zero (or, for example, zero within manufacturing tolerances) and the other principal curvature is non-zero. 
     Planar region: A region of a surface where both of the principal curvatures are zero (or, for example, zero within manufacturing tolerances). 
     Edge of a surface: A boundary or limit of a surface or region. 
     Path: In certain forms of the present technology, ‘path’ will be taken to mean a path in the mathematical-topological sense, e.g. a continuous space curve from f(0) to f(1) on a surface. In certain forms of the present technology, a ‘path’ may be described as a route or course, including e.g. a set of points on a surface. (The path for the imaginary person is where they walk on the surface, and is analogous to a garden path). 
     Path length: In certain forms of the present technology, ‘path length’ will be taken to mean the distance along the surface from f(0) to f(1), that is, the distance along the path on the surface. There may be more than one path between two points on a surface and such paths may have different path lengths. (The path length for the imaginary person would be the distance they have to walk on the surface along the path). 
     Straight-line distance: The straight-line distance is the distance between two points on a surface, but without regard to the surface. On planar regions, there would be a path on the surface having the same path length as the straight-line distance between two points on the surface. On non-planar surfaces, there may be no paths having the same path length as the straight-line distance between two points. (For the imaginary person, the straight-line distance would correspond to the distance ‘as the crow flies’.) 
     5.8.6.3 Space Curves 
     Space curves: Unlike a plane curve, a space curve does not necessarily lie in any particular plane. A space curve may be closed, that is, having no endpoints. A space curve may be considered to be a one-dimensional piece of three-dimensional space. An imaginary person walking on a strand of the DNA helix walks along a space curve. A typical human left ear comprises a helix, which is a left-hand helix, see  FIG. 3Q . A typical human right ear comprises a helix, which is a right-hand helix, see  FIG. 3R .  FIG. 3S  shows a right-hand helix. The edge of a structure, e.g. the edge of a membrane or impeller, may follow a space curve. In general, a space curve may be described by a curvature and a torsion at each point on the space curve. Torsion is a measure of how the curve turns out of a plane. Torsion has a sign and a magnitude. The torsion at a point on a space curve may be characterised with reference to the tangent, normal and binormal vectors at that point. 
     Tangent unit vector (or unit tangent vector): For each point on a curve, a vector at the point specifies a direction from that point, as well as a magnitude. A tangent unit vector is a unit vector pointing in the same direction as the curve at that point. If an imaginary person were flying along the curve and fell off her vehicle at a particular point, the direction of the tangent vector is the direction she would be travelling. 
     Unit normal vector: As the imaginary person moves along the curve, this tangent vector itself changes. The unit vector pointing in the same direction that the tangent vector is changing is called the unit principal normal vector. It is perpendicular to the tangent vector. 
     Binormal unit vector: The binormal unit vector is perpendicular to both the tangent vector and the principal normal vector. Its direction may be determined by a right-hand rule (see e.g.  FIG. 3P ), or alternatively by a left-hand rule ( FIG. 3O ). 
     Osculating plane: The plane containing the unit tangent vector and the unit principal normal vector. See  FIGS. 3O and 3P . 
     Torsion of a space curve: The torsion at a point of a space curve is the magnitude of the rate of change of the binormal unit vector at that point. It measures how much the curve deviates from the osculating plane. A space curve which lies in a plane has zero torsion. A space curve which deviates a relatively small amount from the osculating plane will have a relatively small magnitude of torsion (e.g. a gently sloping helical path). A space curve which deviates a relatively large amount from the osculating plane will have a relatively large magnitude of torsion (e.g. a steeply sloping helical path). With reference to  FIG. 3S , since T2&gt;T1, the magnitude of the torsion near the top coils of the helix of  FIG. 3S  is greater than the magnitude of the torsion of the bottom coils of the helix of  FIG. 3S   
     With reference to the right-hand rule of  FIG. 3P , a space curve turning towards the direction of the right-hand binormal may be considered as having a right-hand positive torsion (e.g. a right-hand helix as shown in  FIG. 3S ). A space curve turning away from the direction of the right-hand binormal may be considered as having a right-hand negative torsion (e.g. a left-hand helix). 
     Equivalently, and with reference to a left-hand rule (see  FIG. 3O ), a space curve turning towards the direction of the left-hand binormal may be considered as having a left-hand positive torsion (e.g. a left-hand helix). Hence left-hand positive is equivalent to right-hand negative. See  FIG. 3T . 
     5.8.6.4 Holes 
     A surface may have a one-dimensional hole, e.g. a hole bounded by a plane curve or by a space curve. Thin structures (e.g. a membrane) with a hole, may be described as having a one-dimensional hole. See for example the one dimensional hole in the surface of structure shown in  FIG. 3I , bounded by a plane curve. 
     A structure may have a two-dimensional hole, e.g. a hole bounded by a surface. For example, an inflatable tyre has a two dimensional hole bounded by the interior surface of the tyre. In another example, a bladder with a cavity for air or gel could have a two-dimensional hole. See for example the cushion of  FIG. 3L  and the example cross-sections therethrough in  FIG. 3M  and  FIG. 3N , with the interior surface bounding a two dimensional hole indicated. In a yet another example, a conduit may comprise a one-dimension hole (e.g. at its entrance or at its exit), and a two-dimension hole bounded by the inside surface of the conduit. See also the two dimensional hole through the structure shown in  FIG. 3K , bounded by a surface as shown. 
     5.9 Other Remarks 
     Unless the context clearly dictates otherwise and where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit, between the upper and lower limit of that range, and any other stated or intervening value in that stated range is encompassed within the technology. The upper and lower limits of these intervening ranges, which may be independently included in the intervening ranges, are also encompassed within the technology, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the technology. 
     Furthermore, where a value or values are stated herein as being implemented as part of the technology, it is understood that such values may be approximated, unless otherwise stated, and such values may be utilized to any suitable significant digit to the extent that a practical technical implementation may permit or require it. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this technology belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present technology, a limited number of the exemplary methods and materials are described herein. 
     When a particular material is identified as being used to construct a component, obvious alternative materials with similar properties may be used as a substitute. Furthermore, unless specified to the contrary, any and all components herein described are understood to be capable of being manufactured and, as such, may be manufactured together or separately. 
     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include their plural equivalents, unless the context clearly dictates otherwise. 
     All publications mentioned herein are incorporated herein by reference in their entirety to disclose and describe the methods and/or materials which are the subject of those publications. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present technology is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates, which may need to be independently confirmed. 
     The terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. 
     The subject headings used in the detailed description are included only for the ease of reference of the reader and should not be used to limit the subject matter found throughout the disclosure or the claims. The subject headings should not be used in construing the scope of the claims or the claim limitations. 
     Although the technology herein has been described with reference to particular examples, it is to be understood that these examples are merely illustrative of the principles and applications of the technology. In some instances, the terminology and symbols may imply specific details that are not required to practice the technology. For example, although the terms “first” and “second” may be used, unless otherwise specified, they are not intended to indicate any order but may be utilised to distinguish between distinct elements. Furthermore, although process steps in the methodologies may be described or illustrated in an order, such an ordering is not required. Those skilled in the art will recognize that such ordering may be modified and/or aspects thereof may be conducted concurrently or even synchronously. 
     It is therefore to be understood that numerous modifications may be made to the illustrative examples and that other arrangements may be devised without departing from the spirit and scope of the technology. 
     
       
         
           
               
             
               
                   
               
               
                 5.10 REFERENCE SIGNS LIST 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 patient 
                 1000 
               
               
                   
                 bed partner 
                 1100 
               
               
                   
                 patient interface 
                 3000 
               
               
                   
                 seal-forming structure 
                 3100 
               
               
                   
                 connecting portion 
                 3102 
               
               
                   
                 plenum chamber 
                 3200 
               
               
                   
                 main frame 
                 3202 
               
               
                   
                 medial portion 
                 3203 
               
               
                   
                 removable frame 
                 3204 
               
               
                   
                 lateral portion 
                 3205 
               
               
                   
                 main frame anterior hole 
                 3206 
               
               
                   
                 main frame posterior hole 
                 3208 
               
               
                   
                 support 
                 3210 
               
               
                   
                 void 
                 3212 
               
               
                   
                 plenum chamber connector 
                 3214 
               
               
                   
                 notch 
                 3216 
               
               
                   
                 chamfered edge 
                 3218 
               
               
                   
                 slot 
                 3220 
               
               
                   
                 inner connecting surface 
                 3222 
               
               
                   
                 outer grip surface 
                 3224 
               
               
                   
                 wall 
                 3226 
               
               
                   
                 inferior point 
                 3230 
               
               
                   
                 chord 
                 3240 
               
               
                   
                 superior point 
                 3250 
               
               
                   
                 positioning and stabilising structure 
                 3300 
               
               
                   
                 lateral portion 
                 3302 
               
               
                   
                 superior portion 
                 3304 
               
               
                   
                 hub 
                 3306 
               
               
                   
                 tab 
                 3308 
               
               
                   
                 posterior strap 
                 3310 
               
               
                   
                 end portion 
                 3311 
               
               
                   
                 sleeve 
                 3312 
               
               
                   
                 lateral end 
                 3314 
               
               
                   
                 lower strap 
                 3320 
               
               
                   
                 upper strap 
                 3322 
               
               
                   
                 connection point 
                 3325 
               
               
                   
                 clip 
                 3326 
               
               
                   
                 headgear tube 
                 3340 
               
               
                   
                 tab 
                 3342 
               
               
                   
                 headgear tube connector 
                 3344 
               
               
                   
                 conduit headgear inlet 
                 3390 
               
               
                   
                 vent 
                 3400 
               
               
                   
                 decoupling structure vent 
                 3402 
               
               
                   
                 decoupling structure 
                 3500 
               
               
                   
                 swivel 
                 3502 
               
               
                   
                 button 
                 3504 
               
               
                   
                 connection port 
                 3600 
               
               
                   
                 forehead support 
                 3700 
               
               
                   
                 cannula 
                 3800 
               
               
                   
                 heat and moisture exchanger (HME) material 
                 3900 
               
               
                   
                 notched portion 
                 3902 
               
               
                   
                 posterior surface 
                 3904 
               
               
                   
                 anterior surface 
                 3906 
               
               
                   
                 superior surface 
                 3908 
               
               
                   
                 inferior surface 
                 3910 
               
               
                   
                 lateral surface 
                 3912 
               
               
                   
                 lateral surface 
                 3914 
               
               
                   
                 medial portion 
                 3916 
               
               
                   
                 RPT device 
                 4000 
               
               
                   
                 external housing 
                 4010 
               
               
                   
                 upper portion 
                 4012 
               
               
                   
                 lower portion 
                 4014 
               
               
                   
                 panel 
                 4015 
               
               
                   
                 chassis 
                 4016 
               
               
                   
                 handle 
                 4018 
               
               
                   
                 pneumatic block 
                 4020 
               
               
                   
                 air filter 
                 4110 
               
               
                   
                 inlet air filter 
                 4112 
               
               
                   
                 outlet air filter 
                 4114 
               
               
                   
                 muffler 
                 4120 
               
               
                   
                 inlet muffler 
                 4122 
               
               
                   
                 outlet muffler 
                 4124 
               
               
                   
                 pressure generator 
                 4140 
               
               
                   
                 blower 
                 4142 
               
               
                   
                 motor 
                 4144 
               
               
                   
                 anti - spill back valve 
                 4160 
               
               
                   
                 air circuit 
                 4170 
               
               
                   
                 electrical components 
                 4200 
               
               
                   
                 PCBA 
                 4202 
               
               
                   
                 power supply 
                 4210 
               
               
                   
                 input device 
                 4220 
               
               
                   
                 central controller 
                 4230 
               
               
                   
                 clock 
                 4232 
               
               
                   
                 therapy device controller 
                 4240 
               
               
                   
                 protection circuits 
                 4250 
               
               
                   
                 memory 
                 4260 
               
               
                   
                 transducer 
                 4270 
               
               
                   
                 pressure sensor 
                 4272 
               
               
                   
                 flow rate sensor 
                 4274 
               
               
                   
                 motor speed transducer 
                 4276 
               
               
                   
                 data communication interface 
                 4280 
               
               
                   
                 remote external communication network 
                 4282 
               
               
                   
                 local external communication network 
                 4284 
               
               
                   
                 remote external device 
                 4286 
               
               
                   
                 local external device 
                 4288 
               
               
                   
                 output device 
                 4290 
               
               
                   
                 output device 
                 4290 
               
               
                   
                 display driver 
                 4292 
               
               
                   
                 display 
                 4294 
               
               
                   
                 algorithms 
                 4300 
               
               
                   
                 therapy control module 
                 4330 
               
               
                   
                 humidifier 
                 5000 
               
               
                   
                 humidifier inlet 
                 5002 
               
               
                   
                 humidifier outlet 
                 5004 
               
               
                   
                 humidifier base 
                 5006 
               
               
                   
                 reservoir 
                 5110 
               
               
                   
                 conductive portion 
                 5120 
               
               
                   
                 humidifier reservoir dock 
                 5130 
               
               
                   
                 locking lever 
                 5135 
               
               
                   
                 water level indicator 
                 5150 
               
               
                   
                 humidifier transducer 
                 5210 
               
               
                   
                 air pressure sensor 
                 5212 
               
               
                   
                 air flow rate transducer 
                 5214 
               
               
                   
                 temperature sensor 
                 5216 
               
               
                   
                 humidity sensor 
                 5218 
               
               
                   
                 heating element 
                 5240 
               
               
                   
                 humidifier controller 
                 5250 
               
               
                   
                 central humidifier controller 
                 5251 
               
               
                   
                 heating element controller 
                 5252 
               
               
                   
                 air circuit controller 
                 5254 
               
               
                   
                 nasal prongs 
                  3810a 
               
               
                   
                 nasal prongs 
                  3810b 
               
               
                   
                 air supply lumens 
                  3820a 
               
               
                   
                 air supply lumens 
                  3820b