Patent Publication Number: US-2023149649-A1

Title: Single flow and pressure activated aav

Description:
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. 
     1 CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/960,982, filed Jan. 14, 2020, which is incorporated herein by reference in its entirety. 
    
    
     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’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’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’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’s airways is ‘open’ (unsealed) and the respiratory therapy may only supplement the patient’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’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’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’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’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’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’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’s face may be inappropriate in another region, e.g. because of the different shape, structure, variability and sensitivity regions of the patient’s face. For example, a seal on swimming goggles that overlays a patient’s forehead may not be appropriate to use on a patient’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’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’s face when force is applied to the patient interface with the seal-forming structure in confronting engagement with the patient’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 2006/074,513; WO 2010/135,785. 
     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. 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), U.S. Pat. Application 2009/0044808 (describing amongst other things aspects of the ResMed Limited SWIFT™ LT nasal pillows); International Patent Applications WO 2005/063,328 and WO 2006/130,903 (describing amongst other things aspects of the ResMed Limited MIRAGE LIBERTY™ full-face mask); International Patent Application WO 2009/052,560 (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 U.S. Pat. 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). 
     
       
         
           
               
               
               
             
               
                 RPT Device name 
                 A-weighted sound pressure level dB(A) 
                 Year (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’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’s compliance, a provider of the RPT device, such as a health care provider, may manually obtain data describing the patient’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’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’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’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 (TMJ) 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 focussed airflow. 
     ResMed Limited has developed a number of improved mask vent technologies. See International Patent Application Publication No. WO 1998/034,665; International Patent Application Publication No. WO 2000/078,381; U.S. Pat. No. 6,581,594; U.S. Pat. Application Publication No. US 2009/0050156; U.S. Pat. Application Publication No. 2009/0044808. 
     Table of noise of prior masks (ISO 17510-2:2007, 10 cmH 2 O pressure at 1 m) 
     
       
         
           
               
               
               
               
               
             
               
                 Mask name 
                 Mask type 
                 A-weighted sound power level dB(A) (uncertainty) 
                 A-weighted sound pressure dB(A) (uncertainty) 
                 Year (approx.) 
               
             
            
               
                 Glue-on ( ∗ ) 
                 nasal 
                 50.9 
                 42.9 
                 
                   1981 
                 
               
               
                 ResCare standard ( ∗ ) 
                 nasal 
                 31.5 
                 23.5 
                 
                   1993 
                 
               
               
                 ResMed Mirage™ ( ∗ ) 
                 nasal 
                 29.5 
                 21.5 
                 
                   1998 
                 
               
               
                 ResMed UltraMirage™ 
                 nasal 
                 36 (3) 
                 28 (3) 
                 
                   2000 
                 
               
               
                 ResMed Mirage Activa™ 
                 nasal 
                 32 (3) 
                 24 (3) 
                 
                   2002 
                 
               
               
                 ResMed Mirage Micro™ 
                 nasal 
                 30 (3) 
                 22 (3) 
                 
                   2008 
                 
               
               
                 ResMed Mirage™ SoftGel 
                 nasal 
                 29 (3) 
                 22 (3) 
                 
                   2008 
                 
               
               
                 ResMed Mirage™ FX 
                 nasal 
                 26 (3) 
                 18 (3) 
                 
                   2010 
                 
               
               
                 ResMed Mirage Swift™ ( ∗ ) 
                 nasal pillows 
                 37 
                 29 
                 
                   2004 
                 
               
               
                 ResMed Mirage Swift™ II 
                 nasal pillows 
                 28 (3) 
                 20 (3) 
                 
                   2005 
                 
               
               
                 ResMed Mirage Swift™ LT 
                 nasal pillows 
                 25 (3) 
                 17 (3) 
                 
                   2008 
                 
               
               
                 ResMed AirFit P10 
                 nasal pillows 
                 21 (3) 
                 13 (3) 
                 
                   2014 
                 
               
            
           
         
       
     
     (* 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 
     
       
         
           
               
               
               
             
               
                 Object 
                 A-weighted sound pressure dB(A) 
                 Notes 
               
             
            
               
                 Vacuum cleaner: Nilfisk Walter Broadly Litter Hog: B+ Grade 
                 68 
                 ISO  3744  at 1 m distance 
               
               
                 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’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’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. 
     An aspect of the present technology is directed to a patient interface that may comprise: a plenum chamber; a seal-forming structure; and a positioning and stabilising structure. The patient interface may further comprise a vent structure. The patient interface may further be configured to leave the patient’s mouth uncovered, or if the seal-forming structure is configured to seal around the patient’s nose and mouth, the patient interface may be further configured to allow the patient to breath from ambient in the absence of a flow of pressurised air through the plenum chamber inlet port. 
     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 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’s face surrounding an entrance to the patient’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 an entrance to the patient’s nares, the seal-forming structure constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient’s respiratory cycle in use; a positioning and stabilising structure configured to hold the seal-forming structure in a therapeutically effective position on the patient’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’s head superior to an otobasion superior of the patient’s head in use; and 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; wherein the patient interface is configured to leave the patient’s mouth uncovered, or if the seal-forming structure is configured to seal around the patient’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 through the plenum chamber inlet port. 
     An aspect of the present technology relates to a patient interface including an anti-asphyxia valve (AAV) configured to allow the patient to breathe in ambient air and exhale if pressurized gas is not of sufficient magnitude or not delivered. 
     Another aspect of the present technology relates to a patient interface to deliver a flow of air at a positive pressure with respect to ambient air pressure to an entrance to the patient’s airways including at least the entrance of a patient’s nares while the patient is sleeping, to ameliorate sleep disordered breathing. The patient interface includes a seal-forming structure constructed and arranged to form a seal with a region of a patient’s face surrounding the entrance to the patient’s airways, a shell, and an AAV provided to the shell. The shell and the seal-forming structure form at least a portion of a plenum chamber pressurizable to a therapeutic pressure. The shell includes a first port into the plenum chamber, the first port configured to allow air to flow between the plenum chamber and ambient. The shell includes a passageway including a second port into the plenum chamber, the passageway configured to communicate with the flow of air at positive pressure. The AAV is configured to regulate flow through the first port and the second port to (1) provide a flow path for pressurized air when pressure in the plenum chamber is above a predetermined magnitude and (2) provide a breathable flow path when pressure in the plenum chamber is below the predetermined magnitude or not delivered. The AAV includes a flap portion structured and arranged to regulate flow through the first port and the second port. The flap portion is biased or pre-loaded to an activated position when pressure in the plenum chamber is below the predetermined magnitude or not delivered to uncover the first port so that a breathable flow of gas is allowed to pass along the breathable flow path that extends through the first port, and the flap portion is configured to cover the second port in the activated position so that exhaled air from the patient is prevented from entering the passageway. The flap portion is deflected to a de-activated position when pressure in the plenum chamber is above the predetermined magnitude to uncover the second port so that the flow of air at positive pressure is allowed to pass along the flow path for pressurized air that extends through the second port, and the flap portion is configured to cover the first port in the de-activated position to maintain the therapeutic pressure in the plenum chamber in use. 
     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 patient interface that may be washed in a home of a patient, e.g., in soapy water, without requiring specialised cleaning equipment. 
     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.  1 A  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.  1 B  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.  1 C  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.  2 A  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.  2 B  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.  2 C  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.  2 D  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.  2 E  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.  2 F  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.  2 G  shows a side view of the superficial features of a nose. 
         FIG.  2 H  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.  2 I  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.  2 J  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.  2 K  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.  2 L  shows an anterolateral view of a nose. 
       4.3 Patient Interface 
         FIG.  3 A  shows a patient interface in the form of a nasal mask in accordance with one form of the present technology. 
         FIG.  3 B  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.  3 C . 
         FIG.  3 C  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.  3 B . 
         FIG.  3 D  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.  3 E  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.  3 F . 
         FIG.  3 F  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.  3 E . 
         FIG.  3 G  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.  3 H  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.  3 I  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.  3 J  shows a cross-section through the structure of  FIG.  3 I . The illustrated surface bounds a two dimensional hole in the structure of  FIG.  3 I . 
         FIG.  3 K  shows a perspective view of the structure of  FIG.  31   , 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.  3 I . 
         FIG.  3 L  shows a mask having an inflatable bladder as a cushion. 
         FIG.  3 M  shows a cross-section through the mask of  FIG.  3 L , and shows the interior surface of the bladder. The interior surface bounds the two dimensional hole in the mask. 
         FIG.  3 N  shows a further cross-section through the mask of  FIG.  3 L . The interior surface is also indicated. 
         FIG.  3 O  illustrates a left-hand rule. 
         FIG.  3 P  illustrates a right-hand rule. 
         FIG.  3 Q  shows a left ear, including the left ear helix. 
         FIG.  3 R  shows a right ear, including the right ear helix. 
         FIG.  3 S  shows a right-hand helix. 
         FIG.  3 T  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.  3 U  shows a view of a plenum chamber  3200  showing a sagittal plane and a mid-contact plane. 
         FIG.  3 V  shows a view of a posterior of the plenum chamber of  FIG.  3 U . The direction of the view is normal to the mid-contact plane. The sagittal plane in  FIG.  3 V  bisects the plenum chamber into left-hand and right-hand sides. 
         FIG.  3 W  shows a cross-section through the plenum chamber of  FIG.  3 V , the cross-section being taken at the sagittal plane shown in  FIG.  3 V . 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  3210  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  3220  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.  3 X  shows the plenum chamber  3200  of  FIG.  3 U  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.  3 X  the plenum chamber  3200  is that of a nasal mask, and the superior point  3220  sits approximately on the sellion, while the inferior point  3230  sits on the lip superior. 
         FIG.  4    shows a patient interface in accordance with another form of the present technology. 
       4.4 Aav Arrangement 
         FIG.  5    is a rear perspective view of a patient interface including an AAV arrangement in an activated position according to an example of the present technology. 
         FIG.  6    is an enlarged view of a portion of  FIG.  5   . 
         FIG.  7    is a cross-sectional view showing the AAV arrangement in an activated position according to an example of the present technology. 
         FIG.  8    is another cross-sectional view showing the AAV arrangement in an activated position according to an example of the present technology. 
         FIG.  9    is a rear perspective view of a patient interface including an AAV arrangement in a de-activated position according to an example of the present technology. 
         FIG.  10    is an enlarged view of a portion of  FIG.  9   . 
         FIG.  11    is a cross-sectional view showing the AAV arrangement in a de-activated position according to an 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 , e.g., see  FIGS.  1 A to  1 C . 
     5.3 Patient Interface 
     As shown in  FIG.  3 A , 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. 
       FIG.  4    shows a patient interface  3000  in accordance with another form of the present technology. The patient interface  3000  in this example also comprises a positioning and stabilising structure  3300  to hold the plenum chamber  3200  in sealing position on the patient’s face in use. The positioning and stabilising structure  3300  in this example comprises a pair of headgear tubes  3340 . 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’s head in use. Each of the headgear tubes  3340  may be configured to lie between and eye and an 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  configured to connect to the shell  3205  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 hollow interiors of 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  3310  and a pair of lower straps  3320 . The posterior ends of the upper straps  3310  and lower straps  3320  are joined together. The junction between the upper straps  3310  and lower strap  3320  is configured to lie against a posterior surface of the patient’s head in use, providing an anchor for the upper strap  3310  and lower straps  3320 . Anterior ends of the upper straps  3310  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  3310  can be passed through and then looped back and secured onto itself to secure the upper headgear strap  3310  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 clip  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 . 
     In some examples of the present technology, the plenum chamber  3200  is at least partially formed by the shell  3205  and the seal-forming structure  3100 . The plenum chamber  3200  may comprise a cushion module or cushion assembly, for example. The shell  3205  may function as a chassis for the seal-forming structure  3100 . 
     The exemplary patient interface  3000  in  FIG.  4    is an oronasal patient interface. That is, the patient interface  3000  is configured to seal around both the patient’s nasal airways and oral airway. In some examples the patient interface  3000  comprises separate seals around each of the nasal airways and oral airway. The patient interface  3000  may comprise a plenum chamber  3200  having a nasal portion and an oral portion. The seal forming structure may be configured to surround the nasal airways at the nasal portion and to seal around the patient’s mouth at the oral portion. As such, the seal-forming structure  3100  may also be considered to have a nasal portion and an oral portion, the nasal portions and oral portions of the seal-forming structure comprising those parts that seal around the patient’s nasal airways and mouth respectively. 
     In an example, the seal-forming structure  3100  at the nasal portion does not lie over a nose bridge region or nose ridge region of the patient’s face and instead seals against inferior surfaces of the patient’s nose. The nasal portion may seal against the lip superior, the ala and the anterior surface of the pronasale and/or the inferior surface of the pronasale. The actual sealing locations may differ between patients. The nasal portion may also be configured to contact and/or seal to a region of the patient’s face between the ala and the nasolabial sulcus and at the lateral portions of the lip superior proximate the nasolabial sulcus. 
     The seal-forming structure  3100  of the oral portion may be configured to form a seal to a periphery of the patient’s mouth in use. The oral portion may be configured to form a seal to the patient’s face at the lip superior, nasolabial sulcus, cheeks, lip inferior, supramenton, for example. 
     The seal-forming structure  3100  may have one or more holes therein such that the flow of air at a therapeutic pressure is delivered to the patient’s nares and to the patient’s mouth via the one or more holes. The seal-forming structure may define an oral hole and one or more nasal holes to deliver the flow of air to the patient. In an example, the plenum chamber  3200  comprises a seal-forming structure  3100  comprising an oral hole and two nasal holes. Each of the nasal holes may be positioned on the plenum chamber  3200  to be substantially aligned with a nare of the patient in order to deliver a flow of air thereto in use. 
     Further examples and details of the oronasal patient interface of  FIG.  4    are described in PCT Application No. PCT/AU2019/050278, filed Mar. 28, 2019, which is incorporated herein by reference in its entirety. 
     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’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 springlike 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’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’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’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’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’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’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’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’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. 
     5.3.2 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. 
     5.3.3 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’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’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’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’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’s head and overlays or lies inferior to the occipital bone of the patient’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. 
     5.3.4 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 . Alternatively, the vent  3400  is located in a decoupling structure, e.g., a swivel. 
     5.3.5 AAV Arrangement 
       FIGS.  5  to  11    show an anti-asphyxia valve (AAV) arrangement  3500  according to an example of the present technology. 
     The AAV arrangement  3500  is configured and arranged to allow the patient to breathe in ambient air and exhale through a first port or opening  3510  (e.g., see  FIGS.  5  to  7   ) if pressurized gas is not of sufficient magnitude or not delivered. In addition, the AAV arrangement  3500  is configured and arranged to prevent CO 2  from entering the headgear tubes  3340  via a second port or opening  3520  (e.g., see  FIGS.  8  to  11   ) if pressurized gas is not of sufficient magnitude or not delivered. 
     In the illustrated example, the patient interface  3000  is an oronasal patient interface, e.g., such as the type shown in  FIG.  4   . However, it should be appreciated that aspects of the present technology may be adapted for use with other suitable interface types. 
     In illustrated example, the shell or chassis  3205  of the oronasal patient interface comprises two inlet ports  3240  provided to lateral sides of the shell  3205  (see  FIG.  8   ). The inlet ports  3240  in this example are configured to connect to respective ones of the headgear tubes  3340 . In an example, the inlet ports  3240  may receive combined headgear and conduit connection assemblies (e.g., headgear tube connector  3344  as shown in  FIG.  4   ) in order to provide multiple functions such as supply of the flow of air and headgear attachment points. 
     In the illustrated example, the posterior side of shell  3205  comprises a plurality of walls that form a passageway  3530  that communicates the two inlet ports  3240  with the plenum chamber  3200 . As illustrated, an inferior wall of the passageway  3530  includes the second port or opening  3520  into the plenum chamber  3200 . In use, a pressurized flow of gas passes through the hollow interiors of the headgear tubes  3340 , through respective inlet ports  3240  into the passageway  3530 , and through the passageway  3530  to the second port  3520  into the plenum chamber  3200 . Thus, the passageway  3530  forms a single channel or passage into the plenum chamber  3200 . 
     In the illustrated example, the first port or opening  3510  is provided to the shell  3205  inferior to the passageway  3530  and second port  3520  thereof, e.g., see  FIGS.  5 ,  7 ,  9 , and  11   . As illustrated, the first port  3510  includes an axis A1 that is transverse, e.g., perpendicular, to an axis A2 of the second port  3520  (see  FIG.  7   ). As described below, such port arrangement allows the anti-asphyxia valve (AAV)  3550  of the AAV arrangement to deflect or pivot about 90° to regulate air flow through the first and second ports  3510 ,  3520 , i.e., flap portion  3555  of the AAV  3550  includes a working angle or opening radius of about 90°. 
     In the illustrated example, the first port  3510  is provided centrally with respect to the plenum chamber  3200 , e.g., so that that the AAV arrangement is located where it is most effective for reducing CO 2 , i.e., aligned approximately with the patient’s mouth which provides a large portion of inhaled/exhaled gas. Also, such positioning of the first port  3510  is less prone to being blocked during side sleeping. 
     The AAV  3550  includes a single-flap arrangement structured and arranged to selectively cover the first and second ports  3510 ,  3520  in use. As illustrated, the AAV  3550  includes a retaining portion  3552  and a flap portion  3555  that is movably connected, e.g., hingedly connected, to the retaining portion  3552  which allows the flap portion  3555  to pivot relative to the retaining portion  3552 . 
     In an example, the AAV  3550  may comprise a one-piece construction of a relatively flexible, elastic material, e.g., silicone or other thermoplastic elastomer. In another example, the AAV  3550  may comprise a combination of materials, e.g., elastic and plastic materials. 
     The AAV  3550  may be removably or permanently secured to the shell  3205  in any suitable manner, e.g., interference fit assembly. For example, as shown in  FIG.  7   , the retaining portion  3552  may be secured within an opening  3207  provided to the shell  3205  between the first and second ports  3510 ,  3520 . As illustrated, the retaining portion  3552  includes an enlarged head  3553  at its free end to prevent withdrawal of the retaining portion  3552  from the shell  3205 . In an alternative example, one or more portions of the AAV  3550  may be formed in one piece with the shell  3205 , e.g., by over-molding or insert-molding. For example, the AAV  3550  (e.g., comprising silicone) may be overmolded to the shell  3205  (e.g., comprising polycarbonate). 
     In the illustrated example, the flap portion  3555  of the AAV  3550  and the first and second ports  3510 ,  3520  in the shell  3205  include a rectangular shape. However, it should be appreciated the flap portion  3555  of the AAV  3550  and the first and second ports  3510 ,  3520  in the shell  3205  may have other suitable shapes, e.g., non-circular and circular shapes. 
     The single AAV  3550  with a single flap and one-piece construction provides minimal parts to limit the overall number of parts for the patient interface, provides less manufacturing processes, reduces the cost of the patient interface, provides a design that is less sensitive to dimensional tolerances, provides a more durable and robust design during use and when cleaning, and/or provides a smaller profile that accommodates less space and improves aesthetics. 
     The flap portion  3555  of the AAV  3550  is biased or pre-loaded relative to the retaining portion  3552  into engagement with edges forming the second port  3520  which defines a stop for the flap portion  3555 , e.g., see  FIGS.  5  to  8   . This arrangement allows the flap portion  3555  to remain in an activated position when pressurized gas is not of sufficient magnitude or not delivered. As illustrated, the flap portion  3555  of the AAV  3550  is structured to cover the entirety of the second port  3520  in the activated position. The flap portion  3555  of the AAV  3550  is also structured to cover the entirety of the first port  3510  in the de-activated position. 
     The AAV  3550  is supported by the shell  3205  adjacent the first and second ports  3510 ,  3520 . The flap portion  3555  is movable towards and away from the first and second ports  3510 ,  3520  to selectively cover or close the first and second ports  3510 ,  3520 , e.g., depending on the presence of pressurized gas. That is, the AAV arrangement  3500  is structured and arranged to regulate flow through the first and second ports  3510 ,  3520  to (1) provide a flow path for pressurized gas when pressure in the patient interface is above a predetermined magnitude and (2) provide a breathable flow path when pressure in the patient interface is below a predetermined magnitude or not delivered. 
     As shown in  FIGS.  5  to  8   , when pressure in the patient interface is below a predetermined magnitude or not delivered (e.g., when the RPT device is not operating), the flap portion  3555  of the AAV  3550  assumes an activated position so that air may pass through the first port  3510 . That is, pressurized gas is not delivered to the patient interface or is not of sufficient magnitude which allows the flap portion  3555  to be biased or deflected upwardly away from the first port  3510  and into engagement with edges forming the second port  3520 , i.e., first port  3510  is kept open by the spring force of the flap portion  3555 . The pressure difference between an interior of the patient interface, e.g., the plenum chamber  3200 , and an exterior of the patient interface, e.g., atmosphere, is not large enough to deflect the flap portion  3555  towards the first port  3510 . As a result, air may pass through the first port  3510  to allow the patient to breathe in ambient air and exhale. Moreover, the flap portion  3555  covers or occludes the second port  3520  so that CO 2  rich exhaled air is prevented from entering the headgear tubes  3340  via the passageway  3530 . 
     As shown in  FIGS.  9  to  11   , when pressure in the patient interface is above a predetermined magnitude (e.g., when the RPT device is operating), the flap portion  3555  of the AAV  3550  assumes a de-activated position so that air may pass through the second port  3520 . That is, the flow of gas passing through the passageway  3530  to the second port  3520  is of sufficient magnitude to overcome the spring force of the the flap portion  3555  and deflect the flap portion  3555  downwardly away from the second port  3520  and towards the first port  3510 , i.e., the flow of air against the flap portion  3555  moves the flap portion  3555  towards the first port  3510 . The pressure difference between an interior of the patient interface, e.g., the plenum chamber  3200 , and an exterior of the patient interface, e.g., atmosphere, is large enough to further deflect and maintain the flap portion  3555  in engagement with edges forming the first port  3510 , i.e., the de-activated position. The pressure gradient when the RPT device is operating maintains the flap portion  3555  in the de-activated position. As a result, air may pass through the second port  3520  to allow delivery of pressurized gas to the patient. Moreover, the flap portion  3555  covers or occludes the first port  3510  to maintain the therapeutic pressure in the plenum chamber  3200  in use. 
     The above-described AAV  3550  provides an arrangement that is decoupled from the headgear tubes  3340 , i.e., the AAV  3550  provides a separate and distinct structure from the connection of the headgear tubes  3340  to the inlet ports  3240  of the shell  3205 . As a result, the connection of the headgear tubes  3340  to the inlet ports  3240  of the shell  3205  may comprise a simple construction, e.g., headgear tubes  3340  may plug into respective ones of the inlet ports  3240  of the shell  3205  and such connection may be aided by magnets to facilitate alignment and retention of the headgear tubes  3340  to the shell  3205 . 
     Also, the above-described AAV  3550  provides an arrangement that is flow and pressure activated, i.e., both flow and pressure activates or deflects the flap portion  3555  from the activated position that occludes the second port  3520  to the de-activated position that occludes the first port  3510 . 
     While not shown, it should be appreciated that a vent may be provided to the shell  3205  for gas washout, e.g., vent provided to the shell  3205  inferior to and spaced apart from the first port  3510 . 
     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. 
     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 . 
     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. 
     5.4 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 . 
     In particular, the air circuit  4170  may be in fluid connection with the outlet of the pneumatic block of the RPT device  4000  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. One example of an air circuit  4170  comprising a heated wire circuit is described in United States Patent 8,733,349, which is incorporated herewithin in its entirety by reference. 
     5.4.1 Supplementary Gas Delivery 
     In one form of the present technology, supplementary gas, e.g. oxygen, may be delivered to one or more points in the pneumatic path, such as upstream of the pneumatic block, to the air circuit  4170 , and/or to the patient interface  3000 . 
     5.5 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. 
     5.5.1 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’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’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’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. 
     5.5.1.1 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. 
     5.5.1.2 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’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’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. 
     5.5.2 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’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’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’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. 
     5.5.3 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’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’s efforts. 
     5.5.4 Anatomy 
     5.5.4.1 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. 
     Subnasalpoint: 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 
     5.5.4.2 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. 
     5.5.4.3 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). 
     5.5.5 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’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. 
     5.5.6 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.  3 B  to  FIG.  3 F , which illustrate examples of cross-sections at point p on a surface, and the resulting plane curves.  FIGS.  3 B to  3 F  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. 
     5.5.6.1 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. ⅟radius 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.  3 B  (relatively large positive curvature compared to  FIG.  3 C ) and  FIG.  3 C  (relatively small positive curvature compared to  FIG.  3 B ). 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.  3 D . 
     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.  3 E  (relatively small negative curvature compared to  FIG.  3 F ) and  FIG.  3 F  (relatively large negative curvature compared to  FIG.  3 E ). Such curves are often referred to as convex. 
     5.5.6.2 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.  3 B to  3 F  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.  3 B  to  FIG.  3 F , the maximum curvature occurs in  FIG.  3 B , and the minimum occurs in  FIG.  3 F , hence  FIG.  3 B  and  FIG.  3 F  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’.) 
     5.5.6.3 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.  3 Q . A typical human right ear comprises a helix, which is a right-hand helix, see  FIG.  3 R .  FIG.  3 S  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.  3 P ), or alternatively by a left-hand rule ( FIG.  3 O ). 
     Osculating plane: The plane containing the unit tangent vector and the unit principal normal vector. See  FIGS.  3 O and  3 P . 
     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.  3 S , since T2&gt;T1, the magnitude of the torsion near the top coils of the helix of  FIG.  3 S  is greater than the magnitude of the torsion of the bottom coils of the helix of  FIG.  3 S   
     With reference to the right-hand rule of  FIG.  3 P , 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.  3 S ). 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.  3 O ), 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.  3 T . 
     5.5.6.4 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.  3 I , 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.  3 L  and the example cross-sections therethrough in  FIG.  3 M  and  FIG.  3 N , 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.  3 K , bounded by a surface as shown. 
     5.6 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. 
     5.7 Reference Signs List 
     
       
         
           
               
               
             
               
                 Feature Item 
                 Number 
               
             
            
               
                 patient 
                 
                   1000 
                 
               
               
                 bed partner 
                 
                   1100 
                 
               
               
                 patient interface 
                 
                   3000 
                 
               
               
                 seal - forming structure 
                 
                   3100 
                 
               
               
                 plenum chamber 
                 
                   3200 
                 
               
               
                 shell 
                 
                   3205 
                 
               
               
                 opening 
                 
                   3207 
                 
               
               
                 inlet port 
                 
                   3240 
                 
               
               
                 positioning and stabilising structure 
                 
                   3300 
                 
               
               
                 upper strap 
                 
                   3310 
                 
               
               
                 lower strap 
                 
                   3320 
                 
               
               
                 connection point 
                 
                   3325 
                 
               
               
                 clip 
                 
                   3326 
                 
               
               
                 headgear tube 
                 
                   3340 
                 
               
               
                 tab 
                 
                   3342 
                 
               
               
                 headgear tube connector 
                 
                   3344 
                 
               
               
                 conduit headgear inlet 
                 
                   3390 
                 
               
               
                 vent 
                 
                   3400 
                 
               
               
                 AAV arrangement 
                 
                   3500 
                 
               
               
                 first port 
                 
                   3510 
                 
               
               
                 second port 
                 
                   3520 
                 
               
               
                 passageway 
                 
                   3530 
                 
               
               
                 AAV 
                 
                   3550 
                 
               
               
                 retaining portion 
                 
                   3552 
                 
               
               
                 enlarged head 
                 
                   3553 
                 
               
               
                 flap portion 
                 
                   3555 
                 
               
               
                 connection port 
                 
                   3600 
                 
               
               
                 forehead support 
                 
                   3700 
                 
               
               
                 RPT device 
                 
                   4000 
                 
               
               
                 air circuit 
                 
                   4170 
                 
               
               
                 humidifier 
                 
                   5000