Abstract:
In accordance with the present invention, there is provided a mask for achieving positive pressure mechanical ventilation (inclusive of CPAP, ventilator support, critical care ventilation, emergency applications), and a method for a operating a ventilation system including such mask. The mask of the present invention includes a piloted exhalation valve that is used to achieve the target pressures/flows to the patient. The pilot for the valve may be pneumatic and driven from the gas supply tubing from the ventilator. The pilot may also be a preset pressure derived in the mask, a separate pneumatic line from the ventilator, or an electro-mechanical control. Additionally, the valve can be implemented with a diaphragm or with a flapper.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/499,950 entitled VENTILATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE filed Jun. 22, 2011, and U.S. Provisional Patent Application Ser. No. 61/512,750 entitled VENTILATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE AND METHOD OF VENTILATING A PATIENT USING THE SAME filed Jul. 28, 2011 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods for controlling delivery of a pressurized flow of breathable gas to a patient and, more particularly, to a ventilation mask such as a nasal mask, nasal prongs mask or nasal pillows mask for use in critical care ventilation, respiratory insufficiency or PAP (Positive Airway Pressure) therapy and incorporating a piloted exhalation valve inside the mask. 
     2. Description of the Related Art 
     As is known in the medical arts, mechanical ventilators comprise medical devices that either perform or supplement breathing for patients. Early ventilators, such as the “iron lung”, created negative pressure around the patient&#39;s chest to cause a flow of ambient air through the patient&#39;s nose and/or mouth into their lungs. However, the vast majority of contemporary ventilators instead use positive pressure to deliver gas to the patient&#39;s lungs via a patient circuit between the ventilator and the patient. The patient circuit typically consists of one or two large bore tubes (e.g., 22 mm ID for adults; 15 mm ID for pediatric) that interface to the ventilator on one end and a patient mask on the other end. Most often, the patient mask is not provided as part of the ventilator system, and a wide variety of patient masks can be used with any ventilator. The interfaces between the ventilator, patient circuit and patient masks are standardized as generic 15 mm/22 mm conical connectors, the size and shape of which are specified by regulatory bodies to assure interoperability. 
     Current ventilators are designed to support either single limb or dual limb patient circuits. Ventilators using single limb patient circuit are most typically used for less acute clinical requirements, such as treatment of obstructive sleep apnea or respiratory insufficiency. Ventilators using dual limb patient circuits are most typically used for critical care applications. 
     Single limb patient circuits are used only to carry gas flow from the ventilator to the patient and patient mask, and require a patient mask with vent holes. The pressure/flow characteristics of the vent holes in the mask are maintained according to standards that assure interoperability of masks with a multitude of ventilators that follow the standard. When utilizing single limb circuits, the patient inspires fresh gas from the patient circuit, and expires CO2-enriched gas, which is purged from the system through the vent holes in the mask and partially breathed down the tube to the ventilator and re-breathed during the next breath. This constant purging of flow through vent holes in the mask when using single-limb circuits provides several disadvantages: 1) it requires the ventilator to provide significantly more flow than the patient requires, adding cost/complexity to the ventilator and requiring larger tubing; 2) the constant flow through the vent holes creates noise, which has proven to be a significant detriment to patients with sleep apnea that are trying to sleep with the mask, and also to their sleep partners; 3) the additional flow coming into proximity of the patient&#39;s nose and then exiting the system often causes dryness in the patient, which often drives the need for adding humidification to the system; and 4) patient-expired CO2 flows partially out of the vent holes in the mask and partially into the patient circuit tubing, requiring a minimum flow through the tubing at all times in order to flush the CO2. To address the problem of undesirable flow of patient-expired CO2 back into the patient circuit tubing, currently known CPAP systems typically have a minimum-required pressure of 4 cmH2O whenever the patient is wearing the mask, which produces significant discomfort, claustrophobia and/or feeling of suffocation to early CPAP users and leads to a high (approximately 50%) non-compliance rate with CPAP therapy. 
     When utilizing dual limb circuits, the patient inspires fresh gas from one limb (the “inspiratory limb”) of the patient circuit and expires CO2-enriched gas from the second limb (the “expiratory limb”) of the patient circuit. Both limbs of the dual limb patient circuit are connected together in a “Y” proximal to the patient to allow a single 15 mm or 22 mm conical connection to the patient mask. 
     In the patient circuits described above, the ventilator pressurizes the gas to be delivered to the patient inside the ventilator to the intended patient pressure, and then delivers that pressure to the patient through the patient circuit. Very small pressure drops develop through the patient circuit, typically around 1 cmH2O, due to gas flow though the small amount of resistance created by the 22 mm or 15 mm ID tubing. Some ventilators compensate for this small pressure either by mathematical algorithms, or by sensing the tubing pressure more proximal to the patient. 
     Ventilators that utilize a dual limb patient circuit typically include an exhalation valve at the end of the expiratory limb proximal to the ventilator. The ventilator controls the exhalation valve, closes it during inspiration, and opens it during exhalation. Less sophisticated ventilators have binary control of the exhalation valve, in that they can control it to be either open or closed. More sophisticated ventilators are able to control the exhalation valve in an analog fashion, allowing them to control the pressure within the patient circuit by incrementally opening or closing the valve. Valves that support this incremental control are referred to as active exhalation valves. In existing ventilation systems, active exhalation valves are most typically implemented physically within the ventilator, and the remaining few ventilation systems with active exhalation valves locate the active exhalation valve within the patient circuit proximal to the ventilator. Active exhalation valves inside ventilators are typically actuated via an electromagnetic coil in the valve, whereas active exhalation valves in the patient circuit are typically pneumatically piloted from the ventilator. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a mask for achieving positive pressure mechanical ventilation (inclusive of PAP, ventilatory support, critical care ventilation, emergency applications), and a method for a operating a ventilation system including such mask. The mask may include a pressure sensing modality proximal to the patient connection. Such pressure sensing modality may be a pneumatic port with tubing that allows transmission of the patient pressure back to the ventilator for measurement, or may include a transducer within the mask. The pressure sensing port, if included in the mask, is used in the system to allow pressure sensing for achieving and/or monitoring the therapeutic pressures. Alternately or additionally, the mask may include a flow sensing modality located therewithin for achieving and/or monitoring the patient and/or therapeutic flows. 
     The mask of the present invention also includes a piloted exhalation valve that is used to achieve the target pressures/flows to the patient. In the preferred embodiment, the pilot for the valve is pneumatic and driven from the gas supply tubing from the ventilator. The pilot can also be a preset pressure derived in the mask, a separate pneumatic line from the ventilator, or an electro-mechanical control. In accordance with the present invention, the valve can be implemented with a diaphragm or with a flapper. 
     One of the primary benefits attendant to including the valve inside the mask is that it provides a path for patient-expired CO2 to exit the system without the need for a dual-limb patient circuit, and without the disadvantages associated with traditional single-limb patient circuits. For instance, in applications treating patients with sleep apnea, having the valve inside the mask allows patients to fall asleep while wearing the mask without the treatment pressure turned on, thereby preventing patient discomfort typically experienced with falling asleep while breathing at a positive pressure. In accordance with the present invention, the sensing described above may be used to sense a predetermined event, such as a set time, the detection of an event indicating patient airway obstruction, or the detection of a patient falling asleep, and start the positive airway pressure therapy upon sensing any such event, unlike existing devices which attempt to alleviate patient discomfort by starting at a lower pressure level (typically 4 cmH2O) and ramping the pressure up to a therapeutic level over a period of time. Additionally, having a valve inside the mask mitigates the need to have vent holes within the patient mask (a typical feature of mask used for sleep apnea) coincident with a purge flow to bleed patient expired CO2 from the system. Alleviating the mask vent holes and associated extra flow of gas through the mask helps reduce noise generated by the mask, reduce CO2 re-breathing, reduce patient nose dryness cause by excess gas flowing past the patient, and reduce flow requirements of the ventilator. Yet another benefit of the mask without vent holes and having the valve inside the same is that because there is not a constant flow through the mask and out of any vent holes, a heat moisture exchanger can also be incorporated into the mask, allowing a simple method of providing heated and humidified gas to the patient. 
     Another benefit for having the valve inside the mask is that it allows for a significant reduction in the tubing size, as it supports the ventilator delivering higher pressures than the patient&#39;s therapeutic pressure. In this regard, pressure from the ventilator is significantly higher than the patient&#39;s therapeutic pressure. Pressure sensing can be implemented inside the mask near the patient interface port(s), facilitating the ventilator to have a means to servo control pressure at the patient interface port(s). Having higher pressure from the ventilator and an active exhalation valve in the mask allows for the tubing size to be significantly smaller (e.g. 1-9 mm ID) compared to conventional ventilators (22 mm ID for adults/15 mm ID for pediatric). One obvious benefit of smaller tubing is that it provides less bulk for patient and/or caregivers to manage. For today&#39;s smallest ventilators, the bulk of the tubing is as significant as the bulk of the ventilator. Another benefit of the smaller tubing is that is allows for more convenient ways of affixing the mask to the patient. For instance, the tubing can go around the patient&#39;s ears to hold the mask to the face, instead of requiring straps (typically called “headgear”) to affix the mask to the face. Along these lines, the discomfort, complication, and non-discrete look of the headgear is another significant factor leading to the high non-compliance rate for CPAP therapy. Another benefit to the smaller tubing is that the mask can become smaller because it does not need to interface with the large tubing. Indeed, large masks are another significant factor leading to the high non-compliance rate for CPAP therapy since, in addition to being non-discrete, they often cause claustrophobia. 
     The present invention is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
         FIG. 1  is an isometric view of a nasal pillows mask constructed in accordance with a first embodiment of the present invention and including an integrated diaphragm-implementation piloted exhalation valve; 
         FIG. 2  is a front elevational view of the nasal pillows mask shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the nasal pillows mask shown in  FIG. 2 ; 
         FIG. 4  is an exploded, cross-sectional front view of the nasal pillows mask shown in  FIG. 3 ; 
         FIG. 5  is a front elevational view of a nasal pillows mask constructed in accordance with a second embodiment of the present invention and including an integrated flapper-implementation exhalation valve; 
         FIG. 6  is a top plan view of the nasal pillows mask shown in  FIG. 5 ; 
         FIG. 7  is a bottom plan view of the nasal pillows mask shown in  FIG. 5 ; 
         FIG. 8  is a cross-sectional, isometric view of the nasal pillows mask shown in  FIG. 5 ; and 
         FIG. 9  is a cross-sectional view of the nasal pillows mask shown in  FIG. 5 . 
     
    
    
     Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating various embodiments of the present invention only, and not for purposes of limiting the same,  FIGS. 1-4  depict a ventilation mask  10  constructed in accordance with a first embodiment of the present invention. The mask is depicted as a nasal prongs mask, however those skilled in the art will recognize that other ventilation masks are contemplated herein such as nasal pillows masks, nasal masks and oronasal masks and for purposes of this application the term mask and/or ventilation mask will include all such mask structures. Additionally, for purposes of this application, the term “direct nasal interface mask” will be deemed to encompass those masks which are configured to facilitate the direct introduction of therapeutic fluid pressure into the nostrils of a patent, such masks including, but not being limited to, nasal pillows masks, nasal prongs masks, and nasal cradle masks. The mask  10  includes an integrated, diaphragm-implemented, piloted exhalation valve  12 , the structural and functional attributes of which will be described in more detail below. 
     As seen in  FIGS. 1-4 , the mask  10  comprises a housing  14  which defines first and second fluid flow passages  16 ,  18 . As best seen in  FIGS. 3 and 4 , the flow passages  16 ,  18  are formed within the housing  14  to have substantially identical shapes or contours. Although illustrated with a pair of flow passages,  16 ,  18 , those skilled in the art will recognize that a single flow passage is additionally contemplated herein. In the mask  10 , one end of each of the flow passages  16 ,  18  is defined by a respective one of an identically configured pair of generally cylindrical, tubular protrusions  20   a ,  20   b  of the housing  14 . The opposite end of each of the flow passages  16 ,  18  is defined by a respective one of an identically configured pair of connector ports  22   a ,  22   b  of the housing  14 . As seen in  FIGS. 3 and 4 , the connector ports  22   a ,  22   b  are each sized and configured to accommodate the advancement of a distal end portion of a tubular fluid line  24  therein. As is apparent from  FIG. 3 , the operative engagement of a fluid line  24  to each of the connector ports  22   a ,  22   b  effectively places such fluid lines  24  into fluid communication with respective ones of the flow passages  16 ,  18 . In the housing  14 , the spacing between the protrusions  20   a ,  20   b  is selected to facilitate the general alignment thereof with the nostrils of an adult patient when the mask  10  is worn by such patient. 
     In the mask  10 , the flow passages  16 ,  18  are preferably not fluidly isolated from each other. Rather, as also seen in  FIGS. 3 and 4 , the housing  14  may define an optional cross passage  26  which extends between the protrusions  20   a ,  20   b  thereof, and effectively places the flow passages  16 ,  18  into fluid communication with each other. The cross passage  26  is further placed into fluid communication with ambient air by an optional vent port  28  which is fluidly coupled thereto. The vent port  28  is defined by and extends axially through a generally cylindrical boss  30  of the housing  14  which protrudes upwardly between the protrusions  20   a ,  20   b  thereof. 
     The housing  14  of the mask  10  further defines an internal valve chamber  32  which fluidly communicates with the cross passage  26 . As further seen in  FIGS. 3 and 4 , disposed at the junction between the cross passage  26  and valve chamber  32  is a tubular projection  34  of the housing  14 . The projection  34  defines an annular distal rim or seating surface  36  which is used in the operation of the valve  12  in manner which will be described in more detail below. The projection  34  protrudes into the valve chamber  32 , and defines the conduit which places the valve chamber  32  into fluid communication with the cross passage  26 . 
     As best seen in  FIGS. 3 and 4 , the valve chamber  32  is defined in large measure by a valve wall  38  of the housing  14  which is generally oriented between the flow passages  16 ,  18  thereof and, when viewed from the perspective shown in  FIGS. 1-4 , is disposed below the cross passage  26 . As is also apparent from  FIGS. 1 and 2 , the valve wall  38  has a perforated construction, thus facilitating the fluid communication between the valve chamber  32  partially defined thereby and ambient air. 
     In the mask  10 , the end of the valve chamber  32  disposed furthest from the cross passage  26  is enclosed by a valve cap  40  which may be removably attached or permanently attached to the distal portion or rim of the valve wall  38  in the manner best seen in  FIG. 3 . The valve cap  40  includes a pilot port  42  which, when the valve cap  40  is coupled to the valve wall  38 , is placed into fluid communication with the valve chamber  32 . The pilot port  42  is partially defined by and extends axially through a generally cylindrical connector  44  of the valve cap  40 . As best seen in  FIGS. 3 and 4 , one end of the pilot port  42  is disposed within a generally planar base surface  46  defined by the valve cap  40 . In addition to the base surface  46 , the valve cap  40  defines a continuous shoulder  48  which, from the perspective shown in  FIG. 4 , is elevated above the base surface  46 . This embodiment shows a pneumatically piloted diaphragm; it is additionally contemplated that the valve  12  can be driven in an electromechanical manner (e.g., with an electromagnet instead of using the above mentioned pilot port  42 ). 
     The mask  10  of the present invention further comprises a diaphragm  50  which resides within the valve chamber  32 . Although various configurations of diaphragms  50  are contemplated herein, as is also best seen in  FIGS. 3 and 4 , the diaphragm  50  has an enlarged, central main body portion  52 , and a peripheral flange portion  54  which is integrally connected to and circumvents the main body portion  52 . The flange portion  54  includes an arcuately contoured central region which is oriented between the distal region thereof and the main body portion  52 , and defines a continuous, generally concave channel  56 . The diaphragm  50  is preferably fabricated from a suitable resilient material. 
     In the mask  10 , the distal region of the flange portion  54  of the diaphragm  50  which is disposed outward of the arcuate central region thereof is normally captured between the valve cap  40  and the valve wall  38  when the valve cap  40  is operatively engaged to the valve wall  38 . More particularly, as seen in  FIG. 3 , the distal region of the flange portion  54  is compressed and thus captured between the shoulder  48  of the valve cap  40  and a lip portion  58  of the valve wall  38  which protrudes inwardly from the inner surface thereof. The diaphragm  50  is preferably sized such that when the distal region of the flange portion  54  thereof is captured between the shoulder  48  and lip portion  58  in the aforementioned manner, the arcuate central region of the flange portion  54  is disposed directly adjacent the inner peripheral surface of the lip portion  58 . Additionally, the channel  56  defined by the flange portion  54  is directed toward and thus faces the base surface  46  of the valve cap  40 . 
     In the mask  10 , the diaphragm  50  effectively segregates the valve chamber  32  into a patient side or region  32   a , and a pilot side or region  32   b . More particularly, due to the aforementioned manner in which the diaphragm  50  is captured between the valve cap  40  and the valve wall  38 , the patient and pilot regions  32   a ,  32   b  of the valve chamber  32  are separated from each other by the diaphragm  50 , and are of differing volumes. Along these lines, the fluid conduit defined by the projection  34  communicates directly with the patient region  32   a  of the valve chamber  32 , while the pilot port  42  defined by the connector  44  communicates directly with the pilot region  32   b  of the valve chamber  32 . 
     The diaphragm  50  (and hence the valve  12 ) is selectively moveable between an open position (shown in  FIG. 3 ) and a closed position. Importantly, in either of its open or closed positions, the diaphragm  50  is not seated directly against the base surface  46  of the valve cap  40 . Rather, a gap is normally maintained therebetween. As seen in  FIG. 3 , the width of such gap when the diaphragm  50  is in its open position is generally equal to the fixed distance separating the base surface  46  of the valve cap  40  from the shoulder  48  thereof. When the diaphragm  50  is in its open position, it is also disposed in spaced relation to the projection  34  of the housing  14 , and in particular the seating surface  36  defined thereby. As such, when the diaphragm  50  is in its open position, fluid is able to freely pass between the flow passages  16 ,  18  and ambient air via the cross passage  26 , the flow conduit defined by the projection  34 , and the perforated openings within the valve wall  38  partially defining the valve chamber  32 . 
     The diaphragm  50  may be resiliently deformable from its open position (to which it may be normally biased) to its closed position. It is an important feature of the present invention that the diaphragm  50  is normally biased in its open position which provides a fail safe to allow a patient to inhale ambient air through the valve and exhale ambient air through the valve even during any ventilator malfunction. 
     When moved or actuated to the closed position, the main body portion  52  of the diaphragm  50  is firmly seated against the seating surface  36  defined by the projection  34 , thus effectively blocking fluid communication between the cross passage  26  (and hence the flow passages  16 ,  18 ) and the valve chamber  32 . More particularly, when viewed from the perspective shown in  FIG. 3 , the peripheral region of the top surface of the main body portion  52  is seated against the seating surface  36 , with a central region of the top surface of the main body portion  52  protruding slightly into the interior of the projection  34 , i.e., the fluid conduit defined by the projection  34 . 
     As is apparent from the foregoing description, in the mask  10 , the valve  12  thereof is collectively defined by the projection  34 , valve wall  38 , valve cap  40  and diaphragm  50 . Additionally, in the mask  10 , it is contemplated that the valve  12  will be piloted, with the movement of the diaphragm  50  to the closed position as described above being facilitated by the introduction of positive fluid pressure into the gap normally defined between the diaphragm  50  and the base surface  46  via the pilot port  42 , i.e., into the pilot region  32   b  of the valve chamber  32 . In this regard, it is contemplated that during the use of the mask  10  by a patient, a pilot fluid line (not shown) from a ventilator will be coupled to the connector  44 . It is also contemplated that during the inspiratory phase of the breathing cycle of the patient wearing the mask  10 , the fluid pressure level introduced into the pilot region  32   b  of the valve chamber  32  via the pilot port  42  will be sufficient to facilitate the movement of the diaphragm  50  to its closed position. Conversely, during the expiratory phase of the breathing cycle of the patient wearing the mask  10 , it is contemplated that the discontinuation of the fluid flow through the pilot port  42 , coupled with the resiliency of the diaphragm  50 , a biasing spring (not shown) operatively coupled to the main body portion  52  of the diaphragm  50 , and/or positive pressure applied to the main body portion  52  of the diaphragm  50 , will facilitate the movement of the diaphragm  50  back to the open position. As will be recognized, the movement of the diaphragm  50  to the open position allows the air exhaled from the patient to be vented to ambient air after entering the patient region  32   a  of the valve chamber  32  via the perforated openings of the valve wall  38  communicating with the valve chamber  32 . 
     As will be recognized, based upon the application of pilot pressure, the diaphragm  50  travels from a fully open position through a partially open position to a fully closed position. In this regard, the diaphragm  50  will be partially open or partially closed during exhalation to maintain desired ventilation therapy. Additionally, a positive airway pressure can be controlled with any expiratory flow value by modulating the pilot pressure within the pilot region  32   b  of the valve chamber  32  and hence the position of the diaphragm  50 . Further, when pilot pressure is discontinued to the diaphragm, the diaphragm  50  moves to an open position wherein the patient can inhale and exhale through the mask with minimal restriction and with minimal carbon dioxide retention within the mask  10 . This is an important feature of the present invention which allows a patient to wear the mask  10  without ventilation therapy being applied to the mask such that the mask  10  is comfortable to wear and can be worn without carbon dioxide buildup. This feature is highly advantageous for the treatment of obstructive sleep apnea where patients complain of discomfort with ventilation therapy due to mask and pressure discomfort. When it is detected that a patient requires sleep apnea therapy, the ventilation therapy can be started (i.e., in an obstructive sleep apnea situation). 
     In this regard, the present invention contemplates a method of ventilation utilizing a mask wherein patient inhalation and patient exhalation is facilitated through the mask to ambient air when the ventilator is not delivering a therapeutic level of pressure. For instance, additional valving in the mask may be implemented for this purpose. Since the mask does not facilitate CO2 buildup, the ventilator can remain off while the mask is worn by the patient and ventilation therapy can be initiated upon sensing or detecting a patient requirement, such as sleep apnea therapy, by conventional sensors incorporated into the mask and ventilator. In this regard, conventional ventilators can be readily modified via conventional software changes to allow the mask to be worn without supplying pressure to the mask unless and until a patient requirement is sensed and subsequently communicated to the ventilator to provide necessary ventilation to the patient. Such modification may additionally require the use of a conventional check valve to ensure that patient exhalation is facilitated through the exhalation valve on the mask and not back into the ventilator delivery circuit. 
     As indicated above, in the embodiment shown in  FIGS. 1-4 , the diaphragm  50  is pneumatically piloted, with the position thereof being regulated by selectively modulating the pilot pressure within the pilot region  32   b  of the valve chamber  32 . However, it is contemplated that alternative modalities, such as an electromagnetic actuator, can be used to drive the valve  12 . For example, as also indicated above, in an alternative embodiment, the valve  12  may be driven in an electromechanical manner through the use of an electromagnet instead of using the above-described pilot port  42 . 
     As indicated above, in the mask  10 , the valve cap  40  is releasably attached to the valve wall  38  of the housing  14 . As a result, the selective detachment of the valve cap  40  from the housing  14  allows for the removal of the diaphragm  50  from within the valve chamber  32  as permits the periodic cleaning or disinfection thereof. In addition, the detachment of the valve cap  40  from the valve wall  38  of the housing  14  also permits access to and the cleaning or disinfection of the interior surfaces of the valve chamber  32 . Port  28  provides a means for pressure measurement inside the mask, and thus may optionally serve as a pressure sensing port. 
     Referring now to  FIGS. 5-9 , there is shown a nasal pillows mask  100  constructed in accordance with a second embodiment of the present invention. The mask  100  includes an integrated, flapper-implemented exhalation valve  112 , the structural and functional attributes of which will be described in more detail below. 
     As seen in  FIGS. 5-9 , the mask  100  comprises a housing  114  which defines first and second fluid flow passages  116 ,  118 . As seen in  FIGS. 8 and 9 , the flow passages  116 ,  118  are formed within the housing  114  to have substantially identical shapes or contours. As with the first embodiment of this invention, a single flow passage is additionally expressly contemplated herein. In the mask  100 , one end of each of the flow passages  116 ,  118  is defined by a respective one of an identically configured pair of generally cylindrical, tubular protrusions  120   a ,  120   b  of the housing  114 . The opposite end of each of the flow passages  116 ,  118  is defined by a respective one of an identically configured pair of connector ports  122   a ,  122   b  of the housing  114 . The connector ports  122   a ,  122   b  are each sized and configured to accommodate the advancement and frictional retention of a distal end portion of a tubular fluid line  124  therein. As most apparent from  FIGS. 8 and 9 , the operative engagement of a fluid line  124  to each of the connector portions  122   a ,  122   b  effectively places such fluid lines  124  into fluid communication with respective ones of the flow passages  116 ,  118 . In the housing  114 , the spacing between the protrusions  120   a ,  120   b  is selected to facilitate the general alignment thereof with the nostrils of an adult patient when the mask  100  is worn by such patient. 
     In the mask  100 , the flow passages  116 ,  118  are not fluidly isolated from each other. Rather, as seen in  FIGS. 8 and 9 , the housing  114  further defines an optional cross passage  126  which extends between the protrusions  120   a ,  120   b  thereof, and effectively places the flow passages  116 ,  118  into fluid communication with each other. The cross passage  126  is further placed into communication with ambient air by an identically configured pair of vent ports  128  which are fluidly coupled thereto. The vent ports  128 , which are disposed in side-by-side, spaced relation to each other, are formed within the housing  14  between the protrusions  120   a ,  120   b  thereof and, when viewed from the perspective shown in  FIG. 9 , face downwardly in a direction opposite that of the open distal ends of the protrusions  120   a ,  120   b.    
     As is best seen in  FIGS. 8 and 9 , the protrusions  120   a ,  120   b  are preferably formed as separate and distinct components or sections of the housing  114  which, when mated to the remainder thereof, facilitate the formation of an identically configured pair of arcuate, semi-circular shoulders  130   a ,  130   b . The shoulders  130   a ,  130   b  defined by the housing  114  are located within the interiors of respective ones of the protrusions  120   a ,  120   b  thereof. More particularly, each shoulder  130   a ,  130   b  is formed in close proximity to that end of the corresponding protrusion  120   a ,  120   b  disposed furthest from the open distal end thereof. The use of the shoulders  130   a ,  130   b  will be described in more detail below. 
     In the mask  100 , the cross passage  126  is partially defined by one or more valve projections  132   a ,  132   b  of the housing  114  which are integrally connected to respective ones of the protrusions  120   a ,  120   b , and protrude generally perpendicularly from the inner surfaces thereof in opposed relation to each other. As seen in  FIGS. 6, 8, and 9 , the valve projections  132   a ,  132   b  are not sized to completely span or cover those portions of the flow passages  116 ,  118  defined by the protrusions  120   a ,  120   b . Rather, each of the valve projections  132   a ,  132   b  is formed to define an arcuate peripheral edge segment, and sized such that the arcuate peripheral edge segment thereof is separated or spaced from the inner surface of the corresponding protrusion  120   a ,  120   b  by a gap which is of a prescribed width. Further, as seen in  FIGS. 6 and 8 , each of the valve projections  132   a ,  132   b  preferably includes a plurality of flow openings  134  disposed therein in a generally circular pattern. The flow openings  134  of the valve projections  132   a ,  132   b  each fluidly communicate with the cross passage  126 , and are used for purposed which will also be described in more detail below. 
     The mask  100  of the present invention further comprises a flapper, which is preferably segregated into an identically configured pair of flapper segments  136   a ,  136   b . The flapper segments  136   a ,  136   b  are each preferably fabricated from a suitable, resilient material. As seen in  FIGS. 8 and 9 , the flapper segments  136   a ,  136   b  reside within the interiors of respective ones of the protrusions  120   a ,  120   b . Additionally, when viewed from the perspective shown in  FIGS. 8 and 9 , an inner end portion of each of the flapper segments  136   a ,  136   b  is firmly secured to the housing  114  as a result of being captured between prescribed components or sections thereof. However, those portions of the flapper segments  136   a ,  136   b  not rigidly secured to the housing  114  are free to resiliently move relative thereto, in a manner which will be described in more detail below. 
     The flapper segments  136   a ,  136   b  (and hence the valve  112 ) are selectively moveable between a closed position (shown in  FIGS. 8 and 9 ) and an open position. When the flapper segments  136   a ,  136   b  are each in the open position, that portion of the peripheral edge thereof not secured to the housing  114  (i.e., not captured between separate sections of the housing  114 ) is normally seated against a corresponding one of the shoulders  130   a ,  130   b . As a result, any fluid (e.g., air exhaled from the nose of a patient wearing the mask) flowing into the flow passages  116 ,  118  via the open distal ends of the protrusions  120   a ,  120   b  is vented to ambient air via the cross passage  126  and vent ports  128 . In this regard, such fluid is able to enter the cross passage  126  through the gaps defined between the valve projections  132   a ,  132   b  and inner surfaces of the corresponding protrusions  120   a ,  120   b.    
     The flapper segments  136   a ,  136   b  may be resiliently deformable from the open position described above (to which they are normally biased) to the closed position shown in  FIGS. 8 and 9 . More particularly, when moved or actuated to the closed position, those portions of the flapper segments  136   a ,  136   b  not secured to the housing  114  are effectively placed into sealed contact with peripheral portions of respective ones of the valve projections  132   a ,  132   b  in a manner substantially covering or obstructing the opposed ends of the cross passage  126  fluidly communicating the flow passages  116 ,  118 . However, even when the flapper segments  136   a ,  136   b  are in the closed position, some measure of fluid may still be vented from the flow passages  116 ,  118  to ambient air by entering the cross passage  126  via the flow openings  134  included in each of the valve projections  132   a ,  132   b.    
     As is apparent from the foregoing description, in the mask  100 , the valve  112  thereof is collectively defined by the shoulders  130   a ,  130   b , valve projections  132   a ,  132   b , and flapper segments  136   a ,  136   b  of the flapper. Additionally, in the mask  100 , it is contemplated that the flapper segments  136   a ,  136   b  will normally be biased to the open position. In this regard, it is contemplated that during the inspiratory phase of the breathing cycle of a patient using the mask  100 , positive fluid pressure introduced into the flow passages  116 ,  118  by a ventilator fluidly coupled thereto via the fluid lines  124  will act against the flapper segments  136   a ,  136   b  in a manner facilitating the movement of such flapper segments  136   a ,  136   b  from their normally open position, to the closed position shown in  FIGS. 8 and 9 . As a result, fluid is able to flow freely through the flow passages  116 ,  118  into the patient&#39;s nostrils, and is substantially prevented from being vented to ambient air via the cross passage  126 , except for a small portion of flow that passes through flow openings  134 . This small flow through flow openings  134  provides for a means to bleed off pressure and therefore more easily control the valve. 
     Conversely, during the expiratory phase of the breathing cycle of the patient wearing the mask  100 , it is contemplated that a reduction in the fluid pressure level introduced into the flow passages  116 ,  118  from the fluid lines  124  to below a prescribed level will allow the flapper segments  136   a ,  136   b  to resiliently return to their normal, open positions engaging respective ones of the shoulders  130   a ,  130   b . When the flapper segments  136   a ,  136   b  return to their open positions, air exhaled from the patient&#39;s nostrils during the expiratory phase of the patient&#39;s breathing circuit is vented to ambient air via the cross passage  126  and vent ports  128 . In this regard, though the movement of the flapper segments  136   a ,  136   b  to the open positions effectively blocks those portions of the flow passages  116 ,  118 , air exhaled from the patient is able to flow through the gaps defined between the valve projections  132   a ,  132   b  and the inner surfaces of the protrusions  120   a ,  120   b , and hence into the opposed open ends of the cross passage  126 . 
     Advantageously, the mask  100  constructed in accordance with the present invention has a total flow requirement which is much lower in comparison to that of a traditional vented PAP mask. This provides the mask  100  with several advantages, including: reduced flow from the ventilator, and thus the ability to use smaller tubes; a reduction in the conducted noise from the ventilator to ambient air through the open vent ports  128  in the mask  100 ; a reduction in oxygen consumption when required with the PAP therapy due to lower flow requirements; and a reduction in water consumption of a humidifier due to lower flow requirements. 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.