Abstract:
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 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. The mask of the present invention may further include a heat and moisture exchanger (HME) which is integrated therein.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. application Ser. No. 13/411,348 entitled VENTILATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE filed Mar. 2, 2012, which 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, the disclosures of which are incorporated herein by reference. 
    
    
     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 full face mask, nasal mask, nasal prongs mask or nasal pillows mask for use in critical care ventilation, respiratory insufficiency or OSA (obstructive sleep apnea) with CPAP (Continuous 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., from 22 mm ID for adults to 8 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 conical connectors, the size and shape of which are specified by regulatory bodies (e.g., ISO 5356-1 or similar standards). 
     Current ventilators are designed to support either “vented” or “leak” circuits, or “non-vented” or “non-leak” circuits. In vented circuits, the mask or patient interface is provided with an intentional leak, usually in the form of a plurality of vent openings. Ventilators using this configuration are most typically used for less acute clinical requirements, such as the treatment of obstructive sleep apnea or respiratory insufficiency. In non-vented circuits, the patient interface is usually not provided with vent openings. Non-vented circuits can have single limb or dual limb patient circuits, and an exhalation valve. Ventilators using non-vented patient circuits are most typically used for critical care applications. 
     Vented 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 openings. When utilizing vented circuits, the patient inspires fresh gas from the patient circuit, and expires CO2-enriched gas, which is purged from the system through the vent openings in the mask. This constant purging of flow through vent openings 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 openings creates and conducts noise, which has proven to be a significant detriment to patients with sleep apnea that are trying to sleep while wearing the mask; 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 and minimize the re-breathing of exhaled 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 cm H2O whenever the patient is wearing the mask, which often 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 non-vented 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 conical connection to the patient mask. When utilizing non-vented single limb circuits, an expiratory valve is placed along the circuit, usually proximal to the patient. During the inhalation phase, the exhalation valve is closed to the ambient and the patient inspires fresh gas from the single limb of the patient circuit. During the exhalation phase, the patient expires CO2-enriched gas from the exhalation valve that is open to ambient. The single limb and exhalation valve are usually connected to each other and to the patient mask with conical connections. 
     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 cm H2O, due to gas flow though the small amount of resistance created by the tubing. Some ventilators compensate for this small pressure drop 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, while ventilators that utilize a single limb, non-vented patient circuit typically include an exhalation valve at the end of the single limb proximal to the patient as indicated above. Exhalation valves can have fixed or adjustable PEEP (positive expiratory end pressure), typically in single limb configurations, or can be controlled by 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 patient. 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 through a separate pressure source such a secondary blower, or through a proportional valve modulating the pressure delivered by the main pressure source. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a mask (e.g., a nasal pillows mask) for achieving positive pressure mechanical ventilation (inclusive of CPAP, ventilatory support, critical care ventilation, emergency applications), and a method for a operating a ventilation system including such mask. The mask preferably includes 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 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 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 is preferably implemented with a diaphragm. 
     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 a single-limb patient circuit, such as high functional dead space. For instance, in applications treating patients with sleep apnea, having the valve inside the mask allows patients to wear the mask while the treatment pressure is turned off without risk of re-breathing excessive CO2. 
     Another benefit for having the valve inside the mask is that it allows for a significant reduction in the required flow generated by the ventilator for ventilating the patient since a continuous vented flow for CO2 washout is not required. Lower flow in turn allows for the tubing size to be significantly smaller (e.g., 2-9 mm ID) compared to conventional ventilators (22 mm ID for adults; 8 mm ID for pediatric). However, this configuration requires higher pressures than the patient&#39;s therapeutic pressure to be delivered by the ventilator. In this regard, pressure from the ventilator is significantly higher than the patient&#39;s therapeutic pressure, though the total pneumatic power delivered is still smaller than that delivered by a low pressure, high flow ventilator used in conjunction with a vented patient circuit and interface. 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. Yet another benefit is that smaller tubing more conveniently routed substantially reduces what is typically referred to as “tube drag” which is the force that the tube applies to the mask, displacing it from the patient&#39;s face. This force has to be counterbalanced by headgear tension, and the mask movements must be mitigated with cushion designs that have great compliance. The reduction in tube drag in accordance with the present invention allows for minimal headgear design (virtually none), reduced headgear tension for better patient comfort, and reduced cushion compliance that results in a smaller, more discrete cushion. 
     The mask of the present invention may further include a heat and moisture exchanger (HME) which is integrated therein. The HME can fully or at least partially replace a humidifier (cold or heated pass-over; active or passive) which may otherwise be included in the ventilation system employing the use of the mask. The HME is positioned within the mask so as to be able to intercept the flow delivered from a flow generator to the patient in order to humidify it, and further to intercept the exhaled flow of the patient in order to capture humidity and heat for the next breath. The HME can also be used as a structural member of the mask, adding a cushioning effect and simplifying the design of the cushion thereof. 
     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 top perspective view of a nasal pillows mask constructed in accordance with one embodiment of the present invention and including an integrated diaphragm-based piloted exhalation valve; 
         FIG. 2  is an exploded view of the nasal pillows mask shown in  FIG. 1 ; 
         FIG. 3  is a partial cross-sectional view of the nasal pillows mask shown in  FIG. 1  taken along lines  3 - 3  thereof, and depicting the valve pilot lumen extending through the cushion of the mask; 
         FIG. 4  is a partial cross-sectional view of the nasal pillows mask shown in  FIG. 1  taken along lines  4 - 4  thereof, and depicting the pressure sensing lumen extending through the cushion of the mask; 
         FIG. 5  is a cross-sectional view of the nasal pillows mask shown in  FIG. 1  taken along lines  5 - 5  thereof; 
         FIG. 6  is a top perspective view of cushion of the nasal pillows mask shown in  FIG. 1 ; 
         FIG. 7  is a top perspective view of exhalation valve of the nasal pillows mask shown in  FIG. 1 ; 
         FIG. 8  is a bottom perspective view of exhalation valve shown in  FIG. 7 ; 
         FIG. 9  is a cross-sectional view of exhalation valve shown in  FIGS. 7 and 8 ; 
         FIG. 10  is a cross-sectional view similar to  FIG. 5 , but depicting a variant of the nasal pillows mask shown in  FIG. 1  wherein an HME is integrated into the cushion thereof; 
         FIGS. 11A, 11B and 11C  are a series of graphs which provide visual representations corresponding to exemplary performance characteristics of the exhalation valve subassembly of any nasal pillows mask constructed in accordance with the present invention; 
         FIG. 12  is a schematic representation of an exemplary ventilation system wherein a tri-lumen tube is used to facilitate the operative interface between any nasal pillows mask constructed in accordance with the present invention a flow generating device; 
         FIG. 13  is a schematic representation of an exemplary ventilation system wherein a bi-lumen tube is used to facilitate the operative interface between any nasal pillows mask constructed in accordance with the present invention and a flow generating device; 
         FIG. 14  is a side-elevational view of any nasal pillows mask constructed in accordance with the present invention as cooperatively engagement in an exemplary manner to a patient through the use of a headgear assembly; 
         FIG. 15  is top perspective view of a nasal pillows mask constructed in accordance with another embodiment of the present invention and including an integrated diaphragm-based piloted exhalation valve; 
         FIG. 16  is an exploded view of the nasal pillows mask shown in  FIG. 15 ; 
         FIG. 17  is a partial cross-sectional view of the nasal pillows mask shown in  FIG. 15  taken along lines  17 - 17  thereof, and depicting the valve pilot lumen extending through the cushion of the mask; 
         FIG. 18  is a partial cross-sectional view of the nasal pillows mask shown in  FIG. 15  taken along lines  18 - 18  thereof, and depicting the pressure sensing lumen extending through the cushion of the mask; 
         FIG. 19  is a cross-sectional view of the nasal pillows mask shown in  FIG. 15  taken along lines  19 - 19  thereof; 
         FIG. 20  is a top perspective view of cushion of the nasal pillows mask shown in  FIG. 15 ; 
         FIG. 21  is a front elevational view of the exhalation valve subassembly for the nasal pillows mask shown in  FIG. 15 ; 
         FIG. 22  is a front exploded view of the exhalation valve subassembly shown in  FIG. 21 , depicting the exhalation valve and the shield plate thereof; 
         FIG. 23  is a rear exploded view of the exhalation valve subassembly shown in  FIG. 21 , depicting the exhalation valve and the shield plate thereof; 
         FIG. 24  is a cross-sectional view of the exhalation valve subassembly shown in  FIG. 21  taken along lines  24 - 24  thereof; 
         FIG. 25  is a cross-sectional view of the exhalation valve subassembly shown in  FIG. 21  taken along lines  25 - 25  thereof; 
         FIG. 26  is a an exploded view of the nasal pillows mask shown in  FIG. 15  in a partially assembled state prior to the attachment of the frame member to the cushion, and depicting the separation of the strike plate of the exhalation valve subassembly from the exhalation valve thereof which is positioned within the cushion; 
         FIG. 27  is a cross-sectional view similar to  FIG. 19 , but depicting a variant of the nasal pillows mask shown in  FIG. 15  wherein an HME is integrated into the cushion thereof; and 
         FIG. 28  is a cross-sectional view similar to  FIGS. 17 and 18 , but depicting a variant of the nasal pillows mask shown in  FIG. 15  wherein an HME is integrated into the cushion thereof. 
     
    
    
     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  (e.g., a nasal pillows mask) constructed in accordance with the present invention. Though the mask  10  is depicted as a nasal pillows mask, those skilled in the art will recognize that other ventilation masks are contemplated herein, such as nasal prongs masks, nasal masks, fill face masks and oronasal masks. As such, for purposes of this application, the term mask and/or ventilation mask is intended to encompass all such mask structures. 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 shown in  FIGS. 1-5 , the mask  10  comprises a housing or cushion  14 . The cushion  14 , which is preferably fabricated from a silicone elastomer having a Shore A hardness in the range of from about 20 to 60 and preferably about 40, is formed as a single, unitary component, and is shown individually in  FIG. 6 . The cushion  14  includes a main body portion  16  which defines a first outer end surface  18  and an opposed second outer end surface  20 . The main body portion  16  further defines an interior fluid chamber  22  which is of a prescribed volume. In addition to the main body portion  16 , the cushion  14  includes an identically configured pair of hollow pillow portions  24  which protrude from the main body portion  16  in a common direction and in a prescribed spatial relationship relative to each other. More particularly, in the cushion  14 , the spacing between the pillow portions  24  is selected to facilitate the general alignment thereof with the nostrils of an adult patient when the mask  10  is worn by such patient. As seen in  FIGS. 3 and 4 , each of the pillow portions  24  fluidly communicates with the fluid chamber  22 . 
     As shown in  FIG. 2 , the main body portion  16  of the cushion  14  includes an enlarged, circularly configured valve opening  26  which is in direct fluid communication with the fluid chamber  22 . The valve opening  26  is positioned in generally opposed relation to the pillow portions  24  of the cushion  14 , and is circumscribed by an annular valve seat  27  also defined by the main body portion  16 . As also shown in  FIG. 2 , the main body portion  16  further defines opposed first and second inner end surfaces  28 ,  30  which protrude outwardly from the periphery of the valve opening  26 , and are diametrically opposed relative thereto so as to be spaced by an interval of approximately 180°. The valve opening  26 , valve seat  27 , and first and second inner end surfaces  28 ,  30  are adapted to accommodate the exhalation valve  12  of the mask  10  in a manner which will be described in more detail below. 
     As shown  FIGS. 3-6 , the main body portion  16  of the cushion  14  further defines first and second gas delivery lumens  32 ,  34  which extend from respective ones of the first and second outer end surfaces  18 ,  20  into fluid communication with the fluid chamber  22 . Additionally, a pressure sensing lumen  36  defined by the main body portion extends from the first outer end surface  18  into fluid communication with the fluid chamber  22 . The main body portion  16  further defines a valve pilot lumen  38  which extends between the second outer end surface  20  and the second inner end surface  30 . The use of the first and second gas delivery lumens  32 ,  34 , the pressure sensing lumen  36 , and the valve pilot lumen  38  will also be discussed in more detail below. Those of ordinary skill in the art will recognize that the gas delivery lumens  32 ,  34 , may be substituted with a single gas delivery lumen and/or positioned within the cushion  14  in orientations other than those depicted in  FIG. 6 . For example, the gas delivery lumen(s) of the cushion  14  may be positioned frontally, pointing upwardly, pointing downwardly, etc. rather than extending laterally as shown in  FIG. 6 . 
     Referring now to  FIGS. 2-5 and 7-9 , the exhalation valve  12  of the mask  10  is made of three (3) parts or components, and more particularly a seat member  40 , a cap member  42 , and a diaphragm  44  which is operatively captured between the seat and cap members  40 ,  42 . The seat and cap members  40 ,  42  are each preferably fabricated from a plastic material, with the diaphragm  44  preferably being fabricated from an elastomer having a Shore A hardness in the range of from about 20-40. 
     As is most easily seen in  FIGS. 2, 7 and 9 , the seat member  40  includes a tubular, generally cylindrical wall portion  46  which defines a distal, annular outer rim  48  and an opposed annular inner seating surface  49 . As shown in  FIG. 9 , the diameter of the outer rim  48  exceeds that of the seating surface  49 . Along these lines, the inner surface of the wall portion  46  is not of a uniform inner diameter, but rather is segregated into first and second inner surface sections which are of differing inner diameters, and separated by an annular shoulder  51 . In addition to the wall portion  46 , the seat member  40  includes an annular flange portion  50  which protrudes radially from that end of the wall portion  46  opposite the outer rim  48 . As shown in  FIGS. 2 and 7 , the flange portion  50  includes a plurality of exhaust vents  52  which are located about the periphery thereof in a prescribed arrangement and spacing relative to each other. Additionally, as is apparent from  FIG. 9 , the seat member  40  is formed such that each of the exhaust vents  52  normally fluidly communicates with the bore or fluid conduit defined by the wall portion  46 . 
     The cap member  42  of the exhaust valve  12  comprises a circularly configured base portion  54  which defines an inner surface  56  and an opposed outer surface  58 . In addition to the base portion  54 , the cap member  42  includes an annular flange portion  60  which circumvents and protrudes generally perpendicularly relative to the inner surface  56  of the base portion  60 . The flange portion  60  defines a distal annular shoulder  62 . As shown in  FIG. 9 , the shoulder  62  and inner surface  56  extend along respective ones of a spaced, generally parallel pair of planes. Further, as shown in  FIG. 8 , formed in the outer surface  58  of the base portion  54  is an elongate groove  64  which extends diametrically across the outer surface  58 . The use of the groove  64  will be described in more detail below. The seat and cap members  40 ,  42 , when attached to each other in the fully assembled exhalation valve  12 , collectively define an interior valve chamber  59  of the exhalation valve  12 . More particularly, such valve chamber  59  is generally located between the inner surface  56  defined by the base portion  54  of the cap member  42  and the seating surface  49  defined by the wall portion  46  of the seat member  40 . 
     The diaphragm  44  of the exhalation valve  12 , which resides within the valve chamber  59 , has a circularly configured, central body portion  66 , and a peripheral flange portion  68  which is integrally connected to and circumvents the body portion  66 . The body portion  66  includes an annular lip  72  which circumvents and protrudes upwardly from one side or face thereof. The flange portion  68  includes an arcuately contoured primary region and a distal region which protrudes radially from the primary region. As such, the primary region of the flange portion  68  extends between the distal region thereof and the body portion  66 , and defines a continuous, generally concave channel  70 . 
     In the exhalation valve  12 , the flange portion  68  of the diaphragm  44  is operatively captured between the flange portions  50 ,  60  of the seat and cap members  40 ,  42 . More particularly, the annular distal region of the flange portion  68  is compressed (and thus captured) between the shoulder  62  defined by the flange portion  60  of the cap member  42 , and a complimentary annular shoulder  53  which is defined by the flange portion  50  of the seat member  40  proximate the exhaust vents  52 . The orientation of the diaphragm  44  within the valve chamber  59  when captured between the seat and cap members  40 ,  42  is such that the channel  70  defined by the arcuately contoured primary region of the flange portion  68  is directed toward or faces the seating surface  49  defined by the wall portion  46  of the seat member  40 . 
     The diaphragm  44  (and hence the exhalation valve  12 ) is selectively moveable between an open position (shown in  FIGS. 3-5 and 9 ) and a closed position. When in its normal, open position, the diaphragm  44  is in a relaxed, unbiased state. Importantly, in either of its open or closed positions, the diaphragm  44  is not normally seated directly against the inner surface  56  defined by the base portion  54  of the cap member  42 . Rather, a gap is normally maintained between the body portion  66  of the diaphragm  44  and the inner surface  56  of the base portion  54 . The width of such gap when the diaphragm  44  is in its open position is generally equal to the fixed distance separating the inner surface  56  of the base portion  54  from the shoulder  62  of the flange portion  60 . Further, when the diaphragm  44  is in its open position, the body portion  66 , and in particular the lip  72  protruding therefrom, is itself disposed in spaced relation to the seating surface  49  defined by the wall portion  46  of the seat member  40 . As such, when the diaphragm  44  is in its open position, fluid is able to freely pass through the fluid conduit defined by the wall portion  46 , between the seating surface  49  and diaphragm  44 , and through the exhaust vents  52  to ambient air. As shown in  FIGS. 3, 8 and 9 , the flange portion  60  of the cap member  42  is further provided with a pilot port  74  which extends therethrough and, in the fully assembled exhalation valve  12 , fluidly communicates with that portion of the valve chamber  59  disposed between the body portion  66  of the diaphragm  44  and the inner surface  56  of the base portion  54 . The use of the pilot port  74  will also be described in more detail below. 
     As will be discussed in more detail below, in the exhalation valve  12 , the diaphragm  44  is resiliently deformable from its open position (to which it may be normally biased) to its closed position. An important feature of the present invention is that the diaphragm  44  is normally biased to its open position which provides a failsafe to allow a patient to inhale ambient air through the exhalation valve  12  and exhale ambient air therethrough (via the exhaust vents  52 ) during any ventilator malfunction or when the mask  10  is worn without the therapy being delivered by the ventilator. When the diaphragm  44  is moved or actuated to its closed position, the lip  72  of the body portion  66  is firmly seated against the seating surface  49  defined by the wall portion  46  of the seat member  40 . The seating of the lip  72  against the seating surface  49  effectively blocks fluid communication between the fluid conduit defined by the wall portion  46  and the valve chamber  59  (and hence the exhaust vents  52  which fluidly communicate with the valve chamber  59 ). 
     In the mask  10 , the cooperative engagement between the exhalation valve  12  and the cushion  14  is facilitated by the advancement of the wall portion  46  of the seat member  40  into the valve opening  26  defined by the cushion  14 . As best seen in  FIG. 5 , such advancement is limited by the ultimate abutment or engagement of a beveled seating surface  76  defined by the flange portion  50  of the seat member  40  against the complimentary valve seat  27  of the cushion  14  circumventing the valve opening  26 . Upon the engagement of the seating surface  76  to the valve seat  27 , the fluid chamber  22  of the cushion  14  fluidly communicates with the fluid conduit defined by the wall portion  46  of the seat member  40 . As will be recognized, if the diaphragm  44  resides in its normal, open position, the fluid chamber  22  is further placed into fluid communication with the valve chamber  59  via the fluid conduit defined by the wall portion  46 , neither end of which is blocked or obstructed by virtue of the gap defined between the lip  72  of the diaphragm  44  and the seating surface  49  of the wall portion  46 . 
     When the exhalation valve  12  is operatively coupled to the cushion  14 , in addition to the valve seat  27  being seated against the seating surface  76 , the first and second inner end surfaces  28 ,  30  of the cushion  14  are seated against respective, diametrically opposed sections of the flange portion  68  defined by the cap member  42 . As best seen in  FIGS. 3 and 4 , the orientation of the exhalation valve  12  relative to the cushion  14  is such that the end of the valve pilot lumen  38  extending to the second inner end surface  30  is aligned and fluidly communicates with the pilot port  74  within the flange portion  60 . As such, in the mask  10 , the valve pilot lumen  38  is in continuous, fluid communication with that portion of the valve chamber  59  defined between the inner surface  56  of the base portion  54  and the body portion  66  of the diaphragm  44 . 
     To assist in maintaining the cooperative engagement between the exhalation valve  12  and the cushion  14 , the mask  10  is further preferably provided with an elongate frame member  78 . The frame member  78  has a generally V-shaped configuration, with a central portion thereof being accommodated by and secured within the complimentary groove  64  formed in the outer surface  58  defined by the base portion  54  of the cap member  42 . As shown in  FIGS. 3 and 4 , the opposed end portions of the frame members  78  are cooperatively engaged to respective ones of the first and second outer end surfaces  18 ,  20  of the cushion  14 . More particularly, as shown in  FIG. 2 , the frame member  78  includes an identically configured pair of first and second connectors  80 ,  82  which extend from respective ones of the opposed end portions thereof. An inner portion of the first connector  80  is advanced into and frictionally retained within the first gas delivery lumen  32  of the cushion  14 . Similarly, an inner portion of the second connector  82  is advanced into and frictionally retained within the second gas delivery lumen  34  of the cushion  14 . In addition to the inner portions advanced into respective ones of the first and second gas delivery lumens  32 ,  34 , the first and second connectors  80 ,  82  of the frame member  78  each further include an outer portion which, as will be described in more detail below, is adapted to be advanced into and frictionally retained within a corresponding lumen of a respective one of a pair of bi-lumen tubes fluidly coupled to the mask  10 . 
     As shown in  FIGS. 3 and 4 , the frame member  78  further includes a tubular, cylindrically configured pressure port  84  which is disposed adjacent the first connector  80 . The pressure port  84  is aligned and fluidly communicates with the pressure sensing lumen  36  of the cushion  14 . Similarly, the frame member  78  is also provided with a tubular, cylindrically configured pilot port  86  which is disposed adjacent the second connector  82 . The pilot port  86  is aligned and fluidly communicates with the valve pilot lumen  38  of the cushion  14 . As will also be discussed in more detail below, the pressure and pilot ports  84 ,  86  of the frame member  78  are adapted to be advanced into and frictionally maintained within corresponding lumens of respective ones of the aforementioned pair of bi-lumen tubes which are fluidly connected to the mask  10  within a ventilation system incorporating the same. The receipt of the frame member  78  within the groove  64  of the cap member  42  ensures that the cushion  14 , the exhalation valve  12  and the frame member  78  are properly aligned, and prevents relative movement therebetween. Though not shown, it is contemplated that in one potential variation of the mask  10 , the cushion  14  may be formed so as not to include the valve pilot lumen  38 . Rather, a suitable valve pilot lumen would be formed directly within the frame member  78  so as to extend therein between the pilot port  86  thereof and the pilot port  74  of the exhalation valve  12 . 
     In the mask  10 , the exhalation valve  12  is piloted, with the movement of the diaphragm  44  to the closed position described above being facilitated by the introduction of positive fluid pressure into the valve chamber  59 . More particularly, it is contemplated that during the inspiratory phase of the breathing cycle of a patient wearing the mask  10 , the valve pilot lumen  38  will be pressurized by a pilot line fluidly coupled to the pilot port  86 , with pilot pressure being introduced into that portion of the valve chamber  59  normally defined between the body portion  66  of the diaphragm  44  and the inner surface  56  defined by the base portion  54  of the cap member  42  via the pilot port  74  extending through the flange portion  60  of the cap member  42 . The fluid pressure level introduced into the aforementioned region of the valve chamber  59  via the pilot port  74  will be sufficient to facilitate the movement of the diaphragm  44  to its closed position described above. 
     Conversely, during the expiratory phase of the breathing cycle of the patient wearing the mask  10 , it is contemplated that the discontinuation or modulation of the fluid pressure through the valve pilot lumen  38  and hence into the aforementioned region of the valve chamber  59  via the pilot port  74 , coupled with the resiliency of the diaphragm  44  and/or positive pressure applied to the body portion  66  thereof, will facilitate the movement of the diaphragm  44  back to the open position or to a partially open position. In this regard, positive pressure as may be used to facilitate the movement of the diaphragm  44  to its open position may be provided by air which is exhaled from the patient during the expiratory phase of the breathing circuit and is applied to the body portion  66  via the pillows portions  24  of the cushion  14 , the fluid chamber  22 , and the fluid conduit defined by the wall portion of the seat member  40 . As will be recognized, the movement of the diaphragm  44  to the open position allows the air exhaled from the patient to be vented to ambient air after entering the valve chamber  59  via the exhaust vents  52  within the flange portion  50  of the seat member  40  which, as indicated above, fluidly communicate with the valve chamber  59 . 
     As will be recognized, based on the application of pilot pressure thereto, the diaphragm  44  travels from a fully open position through a partially open position to a fully closed position. In this regard, the diaphragm  44  will be partially open or partially closed during exhalation to maintain desired ventilation therapy. Further, when pilot pressure is discontinued to the diaphragm  44 , it moves to an open position wherein the patient can inhale and exhale through the mask  10  with minimal restriction and with minimal carbon dioxide retention therein. 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  10 , the aforementioned structural and functional features of the mask  10  making it more comfortable to wear, and further allowing it to be worn without carbon dioxide buildup. This feature is highly advantageous for the treatment of obstructive sleep apnea wherein 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). 
     To succinctly summarize the foregoing description of the structural and functional features of the mask  10 , during patient inhalation, the valve pilot lumen  38  is pressurized, which causes the diaphragm  44  to close against the seating surface  49 , thus effectively isolating the fluid chamber  22  of the mask  10  from the outside ambient air. The entire flow delivered from a flow generator fluidly coupled to the mask  10  is inhaled by the patient, assuming that unintentional leaks at the interface between the cushion  14  and the patient are discarded. This functionality differs from what typically occurs in a conventional CPAP mask, where venting to ambient air is constantly open, and an intentional leak flow is continuously expelled to ambient air. During patient exhalation, the pilot pressure introduced into the valve pilot lumen  38  is controlled so that the exhaled flow from the patient can be exhausted to ambient air through the exhalation valve  12  in the aforementioned manner. In this regard, the pilot pressure is “servoed” so that the position of the diaphragm  44  relative to the seating surface  49  is modulated, hence modulating the resistance of the exhalation valve  12  to the exhaled flow and effectively ensuring that the pressure in the fluid chamber  22  of the mask  10  is maintained at a prescribed therapeutic level throughout the entire length of the exhalation phase. When the valve pilot lumen  38  is not pressurized, the exhalation valve  12  is in a normally open state, with the diaphragm  44  being spaced from the seating surface  49  in the aforementioned manner, thus allowing the patient to spontaneously breathe in and out with minimal pressure drop (also referred to as back-pressure) in the order of less than about 2 cm H2O at 60 l/min. As a result, the patient can comfortably breathe while wearing the mask  10  and while therapy is not being administered to the patient. 
     Referring now to  FIGS. 11A, 11B and 11C , during use of the mask  10  by a patient, the functionality of the exhalation valve  12  can be characterized with three parameters. These are Pt which is the treatment pressure (i.e., the pressure in the mask  10  used to treat the patient; Pp which is the pilot pressure (i.e., the pressure used to pilot the diaphragm  44  in the exhalation valve  12 ); and Qv which is vented flow (i.e., flow that is exhausted from inside the exhalation valve  12  to ambient. These three particular parameters are labeled as Pt, Pp and Qv in  FIG. 9 . When the patient is ventilated, Pt is greater than zero, with the functionality of the exhalation valve  12  being described by the family of curves in the first and second quadrants of  FIG. 11A . In this regard, as apparent from  FIG. 11A , for any given Pt, it is evident that by increasing the pilot pressure Pp, the exhalation valve  12  will close and the vented flow will decrease. A decrease in the pilot pressure Pp will facilitate the opening of the valve  12 , thereby increasing vented flow. The vented flow will increase until the diaphragm  44  touches or contacts the inner surface  56  of the base portion  54  of the cap member  42 , and is thus not able to open further. Conversely, when the patient is not ventilated, the inspiratory phase can be described by the third and fourth quadrants. More particularly, Qv is negative and air enters the mask  10  through the valve  12 , with the pressure Pt in the mask  10  being less than or equal to zero. Pilot pressure Pp less than zero is not a configuration normally used during ventilation of the patient, but is depicted for a complete description of the functionality of the valve  12 . The family of curves shown in  FIG. 11A  can be described by a parametric equation. Further, the slope and asymptotes of the curves shown in  FIG. 11A  can be modified by, for example and not by way of limitation, changing the material used to fabricate the diaphragm  44 , changing the thickness of the diaphragm  44 , changing the area ratio between the pilot side and patient side of the diaphragm  44 , changing the clearance between the diaphragm  44  and the seating surface  49 , and/or changing the geometry of the exhaust vents  52 . 
     An alternative representation of the functional characteristics of the valve  12  can be described by graphs in which ΔP=Pt−Pp is shown. For example, the graph of  FIG. 11B  shows that for any given Pt, the vented flow can be modulated by changing ΔP. In this regard, ΔP can be interpreted as the physical position of the diaphragm  44 . Since the diaphragm  44  acts like a spring, the equation describing the relative position d of the diaphragm  44  from the seating surface  49  of the seat member  40  is k·d+Pt·At =Pp·Ap, where At is the area of the diaphragm  44  exposed to treatment pressure Pt and Ap is the area of the diaphragm  44  exposed to the pilot pressure Pp. A similar, alternative representation is provided in the graph of  FIG. 11C  which shows Pt on the x-axis and ΔP as the parameter. In this regard, for any given ΔP, the position d of the diaphragm  44  is determined, with the valve  12  thus being considered as a fixed opening valve. In this scenario Pt can be considered the driving pressure pushing air out of the valve  12 , with  FIG. 11C  further illustrating the highly non-linear behavior of the valve  12 . 
       FIG. 12  provides a schematic representation of an exemplary ventilation system  88  wherein a tri-lumen tube  90  is used to facilitate the fluid communication between the mask  10  and a blower or flow generator  92  of the system  88 . As represented in  FIG. 12 , one end of the tri-lumen tube  90  is fluidly connected to the flow generator  92 , with the opposite end thereof being fluidly connected to a Y-connector  94 . The three lumens defined by the tri-lumen tube  90  include a gas delivery lumen, a pressure sensing lumen, and a valve pilot lumen. The gas delivery lumen is provided with an inner diameter or ID in the range of from about 2 mm to 15 mm, and preferably about 4 mm to 10 mm. The pressure sensing and valve pilot lumens of the tri-lumen tube  90  are each preferably provided with an ID in the range of from about 0.5 mm to 2 mm. The outer diameter or OD of the tri-lumen tube  90  is preferably less than 17 mm, with the length thereof in the system  88  being about 2 m. The Y-connector  94  effectively bifurcates the tri-lumen tube  90  into the first and second bi-lumen tubes  96 ,  98 , each of which has a length of about 6 inches. The first bi-lumen tube  96  includes a gas delivery lumen having an ID in the same ranges described above in relation to the gas delivery lumen of the tri-lumen tube  90 . The gas delivery lumen of the first bi-lumen tube  96  is fluidly coupled to the outer portion of the first connector  80  of the frame member  78 . The remaining lumen of the first bi-lumen tube  96  is a pressure sensing lumen which has an ID in the same range described above in relation to the pressure sensing lumen of the tri-lumen tube  90 , and is fluidly coupled to the pressure port  84  of the frame member  78 . Similarly, the second bi-lumen tube  98  includes a gas delivery lumen having an ID in the same ranges described above in relation to the gas delivery lumen of the tri-lumen tube  90 . The gas delivery lumen of the second bi-lumen tube  98  is fluidly coupled to the outer portion of the second connector  82  of the frame member  78 . The remaining lumen of the second bi-lumen tube  98  is a valve pilot lumen which has an ID in the same range described above in relation to the valve pilot lumen of the tri-lumen tube  90 , and is fluidly coupled to the pilot port  86  of the frame member  78 . 
     In the system  88  shown in  FIG. 12 , the pilot pressure is generated at the flow generator  92 . In the prior art, a secondary blower or proportional valve that modulates the pressure from a main blower is used to generate a pressure to drive an expiratory valve. However, in the system  88  shown in  FIG. 12 , the outlet pressure of the flow generator  92  is used, with the flow generator  92  further being controlled during patient exhalation in order to have the correct pilot pressure for the exhalation valve  12 . This allows the system  88  to be inexpensive, not needing additional expensive components such as proportional valves or secondary blowers. 
       FIG. 13  provides a schematic representation of another exemplary ventilation system  100  wherein a bi-lumen tube  102  is used to facilitate the fluid communication between the mask  10  and the blower or flow generator  92  of the system  100 . As represented in  FIG. 13 , one end of the bi-lumen tube  102  is fluidly connected to the flow generator  92 , with the opposite end thereof being fluidly connected to the Y-connector  94 . The two lumens defined by the bi-lumen tube  102  include a gas delivery lumen and a pressure sensing lumen. The gas delivery lumen is provided with an inner diameter or ID in the range of from about 2 mm to 10 mm, and preferably about 4 mm to 7 mm. The pressure sensing lumen of the bi-lumen tube  102  is preferably provided with an ID in the range of from about 0.5 mm to 2 mm. The outer diameter or OD of the bi-lumen tube  90  is preferably less than 11 mm, with the length thereof being about 2 m. The Y-connector  94  effectively bifurcates the bi-lumen tube  102  into the first and second bi-lumen tubes  96 ,  98 , each of which has a length of about 6 inches. The first bi-lumen tube  96  includes a gas delivery lumen having an ID in the same ranges described above in relation to the gas delivery lumen of the bi-lumen tube  102 . The gas delivery lumen of the first bi-lumen tube  96  is fluidly coupled to the outer portion of the first connector  80  of the frame member  78 . The remaining lumen of the first bi-lumen tube  96  is a pressure sensing lumen which has an ID in the same range described above in relation to the pressure sensing lumen of the bi-lumen tube  102 , and is fluidly coupled to the pressure port  84  of the frame member  78 . Similarly, the second bi-lumen tube  98  includes a gas delivery lumen having an ID in the same ranges described above in relation to the gas delivery lumen of the bi-lumen tube  102 . The gas delivery lumen of the second bi-lumen tube  98  is fluidly coupled to the outer portion of the second connector  82  of the frame member  78 . The remaining lumen of the second bi-lumen tube  98  is a valve pilot lumen which has an ID in the same range described above in relation to the pressure sensing lumen of the bi-lumen tube  102 , and is fluidly coupled to the pilot port  86  of the frame member  78 . 
     In the system  100  shown in  FIG. 13 , the valve pilot lumen  38  is connected to the gas delivery air path at the Y-connector  94 . More particularly, the gas delivery lumen of the bi-lumen tube  102  is transitioned at the Y-connector  94  to the valve pilot lumen of the second bi-lumen tube  98 . As such, the pilot pressure will be proportional to the outlet pressure of the flow generator  92  minus the pressure drop along the bi-lumen tube  102 , which is proportional to delivered flow. This solution is useful when small diameter tubes are used in the system  100 , since such small diameter tubes require higher outlet pressure from the flow generator  92  for the same flow. In this regard, since the pressure at the outlet of the flow generator  92  would be excessive for piloting the exhalation valve  12 , a lower pressure along the circuit within the system  100  is used. In the system  100 , though it is easier to tap in at the Y-connector  94 , anywhere along the tube network is acceptable, depending on the pressure level of the flow generator  92  which is the pressure required by the patient circuit in order to deliver the therapeutic pressure and flow at the patient. 
     In each of the systems  88 ,  100 , it is contemplated that the control of the flow generator  92 , and hence the control of therapeutic pressure delivered to the patient wearing the mask  10 , may be governed by the data gathered from dual pressure sensors which take measurements at the mask  10  and the output of the flow generator  92 . As will be recognized, pressure sensing at the mask  10  is facilitated by the pressure sensing lumen  36  which, as indicated above, is formed within the cushion  14  and fluidly communicates with the fluid chamber  22  thereof. As also previously explained, one of the lumens of the first bi-lumen tube  96  in each of the systems  88 ,  100  is coupled to the pressure port  84  (and hence the pressure sensing lumen  36 ). As a result, the first bi-lumen tube  96 , Y-connector  94  and one of the tri-lumen or bi-lumen tubes  90 ,  102  collectively define a continuous pressure sensing fluid path between the mask  10  and a suitable pressure sensing modality located remotely therefrom. A more detailed discussion regarding the use of the dual pressure sensors to govern the delivery of therapeutic pressure to the patient is found in Applicant&#39;s co-pending U.S. application Ser. No. 13/411,257 entitled Dual Pressure Sensor Continuous Positive Airway Pressure (CPAP) Therapy filed Mar. 2, 2012, the entire disclosure of which is incorporated herein by reference. 
     Referring now to  FIG. 10 , there is shown a mask  10   a  which comprises a variant of the mask  10 . The sole distinction between the masks  10 ,  10   a  lies in the mask  10   a  including a heat and moisture exchanger or HME  104  which is positioned within the fluid chamber  22  of the cushion  14 . The HME  104  is operative to partially or completely replace a humidifier (cold or heated pass-over; active or passive) which would otherwise be fluidly coupled to the mask  10   a . This is possible because the average flow through the system envisioned to be used in conjunction with the mask  10   a  is about half of a prior art CPAP mask, due to the absence of any intentional leak in such system. 
     The HME  104  as a result of its positioning within the fluid chamber  22 , is able to intercept the flow delivered from the flow generator to the patient in order to humidify it, and is further able to capture humidity and heat from exhaled flow for the next breath. The pressure drop created by the HME  104  during exhalation (back-pressure) must be limited, in the order of less than 5 cm H2O at 60 l/min, and preferably lower than 2 cm H2O at 60 l/min. These parameters allow for a low back-pressure when the patient is wearing the mask  10   a  and no therapy is delivered to the patient. 
     It is contemplated that the HME  104  can be permanently assembled to the cushion  14 , or may alternatively be removable therefrom and thus washable and/or disposable. In this regard, the HME  104 , if removable from within the cushion  14 , could be replaced on a prescribed replacement cycle. Additionally, it is contemplated that the HME  104  can be used as an elastic member that adds elasticity to the cushion  14 . In this regard, part of the elasticity of the cushion  14  may be attributable to its silicone construction, and further be partly attributable to the compression and deflection of the HME  104  inside the cushion  14 . 
     Referring now to  FIGS. 15-19 , there is shown a ventilation mask  110  (e.g., a nasal pillows mask) constructed in accordance with another embodiment of the present invention. Like the mask  10  described above, the mask  110  includes an integrated, diaphragm-implemented, piloted exhalation valve  112 , the structural and functional attributes of which will be described in more detail below. 
     As shown in  FIGS. 15-19 , the mask  110  comprises a housing or cushion  114 . The cushion  114 , which is preferably fabricated from a silicone elastomer having a Shore A hardness in the range of from about 20 to 60 and preferably about 40, is formed as a single, unitary component, and is shown individually in  FIG. 20 . The cushion  114  includes a main body portion  116  which defines a first outer end surface  118  and an opposed second outer end surface  120 . The main body portion  116  further defines an interior fluid chamber  122  which is of a prescribed volume. In addition to the main body portion  116 , the cushion  14  includes an identically configured pair of hollow pillow portions  124  which protrude from the main body portion  116  in a common direction and in a prescribed spatial relationship relative to each other. More particularly, in the cushion  114 , the spacing between the pillow portions  124  is selected to facilitate the general alignment thereof with the nostrils of an adult patient when the mask  110  is worn by such patient. Each of the pillow portions  124  fluidly communicates with the fluid chamber  122 . 
     As shown in  FIG. 16 , the main body portion  116  of the cushion  114  includes an enlarged, circularly configured valve opening  126  which is in direct fluid communication with the fluid chamber  122 . The valve opening  126  is positioned in generally opposed relation to the pillow portions  124  of the cushion  114 . The valve opening  126  is adapted to accommodate an exhalation valve subassembly  111  of the mask  110  in a manner which will be described in more detail below. 
     The main body portion  116  of the cushion  114  further defines first and second gas delivery lumens  132 ,  134  which extend from respective ones of the first and second outer end surfaces  118 ,  120  into fluid communication with the fluid chamber  122 . Additionally, a pressure sensing lumen  136  defined by the main body portion  116  extends from the first outer end surface  118  into fluid communication with the fluid chamber  122 . The main body portion  116  further defines a valve pilot lumen  138  which extends from the second outer end surface  120  into fluid communication with the fluid chamber. Those of ordinary skill in the art will recognize that the gas delivery lumens  132 ,  134  may be substituted with a single gas delivery lumen and/or positioned within the cushion  114  in orientations other than those depicted in  FIG. 20 . For example, the gas delivery lumen(s) of the cushion  114  may be positioned frontally, pointing upwardly, pointing downwardly, etc. rather than extending laterally as shown in  FIG. 20 . The main body portion  116  of the cushion  114  further includes a mounting aperture  139  formed therein. As seen in  FIG. 18 , one end of the mounting aperture  139  communicates with the fluid chamber  122 , with the opposite simply terminating blindly within the main body portion  116 . The use of the first and second gas delivery lumens  132 ,  134 , the pressure sensing lumen  136 , the valve pilot lumen  138  and the mounting aperture  139  will be discussed in more detail below. 
     Referring now to  FIGS. 16-19 and 21-26 , the exhalation valve subassembly  111  of the mask  110  comprises the aforementioned exhalation valve  112  in combination with a shield plate  113 . The exhalation valve  112  of the mask  110  is itself made of three (3) parts or components, and more particularly a seat member  140 , a cap member  142 , and a diaphragm  144  which is operatively captured between the seat and cap members  140 ,  142 . The seat and cap members  140 ,  142  are each preferably fabricated from a plastic material, with the diaphragm  144  preferably being fabricated from an elastomer having a Shore A hardness in the range of from about 20-40. 
     The seat member  140  includes a tubular, generally cylindrical wall portion  146  which defines a distal, annular outer rim  148  and an opposed annular inner seating surface  149 . The wall portion further defines an outlet conduit  147  which extends between the outer rim  148  and seating surface  149 . In addition to the wall portion  146 , the seat member  140  includes an annular flange portion  150  which is integrally connected to the wall portion  146  by a series of spoke portions  151 . The spoke portions  151  extend to locations on the wall portion  146  proximate the seating surface  149 , with the flange portion  150  being positioned radially outward relative to the wall portion  146 . In the seat member  140 , the wall, flange and spoke portions  146 ,  150 ,  151  collectively define a plurality of exhaust vents  152  which are located about the periphery of the wall portion  146  in a prescribed arrangement and spacing relative to each other. The seat member  140  is formed such that each of the exhaust vents  152  normally fluidly communicates with the outlet conduit  147  defined by the wall portion  146 . 
     The cap member  142  of the exhalation valve  112  comprises a circularly configured base portion  154  which defines an inner surface  156 . In addition to the base portion  154 , the cap member  142  includes an annular flange portion  160  which circumvents and protrudes generally perpendicularly relative to the inner surface  156  of the base portion  154 . The cap member  142  further includes an identically configured pair of tube portions  162  which are integrally connected to the flange portion  160  in diametrically opposed relation to each other (i.e., approximately 180° apart). Each of the tube portions defines a lumen  164  extending therethrough and is used for reasons which will be discussed in more detail below. The seat and cap members  140 ,  142 , when attached to each other in the fully assembled exhalation valve  112 , collectively define an interior valve chamber of the exhalation valve  112 , such valve chamber generally being located between the inner surface  156  defined by the base portion  154  of the cap member  142  and the seating surface  149  defined by the wall portion  146  of the seat member  140 . 
     The diaphragm  144  of the exhalation valve  112 , which resides within the valve chamber, has a circularly configured, central body portion  166 , and a peripheral flange portion  168  which is integrally connected to and circumvents the body portion  166 . The flange portion  168  includes an arcuately contoured primary region and a distal region which protrudes radially from the primary region. As such, the primary region of the flange portion  168  extends between the distal region thereof and the body portion  166 , and defines a continuous, generally concave channel  170 . The body portion  166  of the diaphragm  144  may optionally be perforated, i.e., be provided with an array of small apertures which extend therethrough. 
     In the exhalation valve  112 , the flange portion  168  of the diaphragm  144  is operatively captured between complementary engagement surfaces defined by the flange portions  150 ,  160  of the seat and cap members  140 ,  142 . More particularly, the annular distal region of the flange portion  168  is compressed (and thus captured) between an annular shoulder defined by the flange portion  160  of the cap member  142 , and a complimentary annular shoulder which is defined by the flange portion  150  of the seat member  140  proximate the exhaust vents  152 . The orientation of the diaphragm  144  within the valve chamber when captured between the seat and cap members  140 ,  142  is such that the channel  170  defined by the arcuately contoured primary region of the flange portion  168  is directed toward or faces the seating surface  149  defined by the wall portion  146  of the seat member  140 . 
     The capture of the diaphragm  144  between the seat and cap members  140 ,  142  in the aforementioned manner results in the diaphragm  144  effectively segregating the valve chamber collectively defined by the seat and cap members  140 ,  142  into a pilot section  172  and an exhaust section  174 . The pilot section  172  of the valve chamber is located between the diaphragm  144  and the inner surface  156  of the base portion  154  of the cap member  142 . The exhaust section  174  of the valve chamber is located between the diaphragm  144  and both the exhaust vents  152  and the seating surface  149  of the wall portion  146  of the seat member  140 . In this regard, the outlet conduit  147  defined by the wall portion  146  fluidly communicates with the exhaust section  174  of the valve chamber. In addition, the lumens  164  of the tube portions  162  of the cap member  142  each fluidly communicate with the pilot section  172  of the valve chamber. 
     The diaphragm  144  (and hence the exhalation valve  112 ) is selectively moveable between an open position (shown in  FIGS. 17-19 and 24-25 ) and a closed position. When in its normal, open position, the diaphragm  144  is in a relaxed, unbiased state. Importantly, in either of its open or closed positions, the diaphragm  144  is not normally seated directly against the inner surface  156  defined by the base portion  154  of the cap member  142 . Rather, a gap is normally maintained between the body portion  166  of the diaphragm  144  and the inner surface  156  of the base portion  154 . The width of such gap when the diaphragm  144  is in its open position is generally equal to the fixed distance separating the inner surface  156  of the base portion  154  from the shoulder of the flange portion  160  which engages the distal region of the flange portion  168  of the diaphragm  144 . Further, when the diaphragm  144  is in its open position, the body portion  166  is itself disposed in spaced relation to the seating surface  149  defined by the wall portion  146  of the seat member  140 . As such, when the diaphragm  144  is in its open position, fluid is able to freely pass through the through the exhaust vents  152 , between the seating surface  149  and diaphragm  144 , and through the outlet conduit  147  defined by the wall portion  146  to ambient air. 
     In the exhalation valve  112 , the diaphragm  144  is resiliently deformable from its open position (to which it may be normally biased) to its closed position. An important feature of the present invention is that the diaphragm  144  is normally biased to its open position which provides a failsafe to allow a patient to inhale ambient air through the exhalation valve  112  and exhale ambient air therethrough (via the exhaust vents  52 ) during any ventilator malfunction or when the mask  110  is worn without the therapy being delivered by the ventilator. When the diaphragm  144  is moved or actuated to its closed position, the periphery of the body portion  166  is firmly seated against the seating surface  149  defined by the wall portion  146  of the seat member  140 . The seating of the body portion  166  of the diaphragm  144  against the seating surface  149  effectively blocks fluid communication between the outlet conduit  147  defined by the wall portion  146  and the exhaust section  174  of the valve chamber (and hence the exhaust vents  152  which fluidly communicate with the exhaust section  174 ). 
     In the mask  110 , the cooperative engagement between the exhalation valve  112  and the cushion  114  is facilitated by the advancement of the cap member  142  into the valve opening  126  defined by the cushion  114 . Subsequent to such advancement, one of the two tube portions  162  of the cap member  142  is partially advanced into and frictionally retained within the pilot lumen  138  of the cushion  114  in the manner shown in  FIG. 17 . As is apparent from  FIG. 17 , the advancement of one tube portion  162  of the cap member  142  into the pilot lumen  138  facilitates the placement of the pilot lumen  138  into fluid communication with the pilot section  172  of the valve chamber via the lumen  164  of the corresponding tube portion  162 . The remaining tube portion  162  of the cap member  142  (i.e., that tube portion  162  not advanced into the pilot lumen  138 ) is advanced into and frictionally retained within the above-described mounting aperture  139  in the manner shown in  FIG. 18 . Importantly, the resilient construction of the cushion  114 , and in particular the main body  116  thereof, allows for the cushion  114  to be bent, twisted or otherwise manipulated as is needed to facilitate the advancement of the tube portions  162  of the cap member  142  into respective ones of the pilot lumen  138  and mounting aperture  139  in the aforementioned manner. The advancement of the tube portions  162  into respective ones of the pilot lumen  138  and mounting aperture  139  causes the exhalation valve  112  to assume a position within the fluid chamber  122  of the cushion  114  as is best shown in  FIG. 26 . In this regard, the majority of the exhalation valve  112  resides within the fluid chamber  122 , with the exception of a small distal section of the wall portion  148  of the seat member  140  which protrudes from the valve opening  126  of the cushion  114 . 
     Due to the positioning of the majority of the exhalation valve  114  within the fluid chamber  122 , the exhaust section  174  of the valve chamber is placed into direct fluid communication with the fluid chamber  122  via the exhaust vents  152 . Thus, irrespective of whether the diaphragm  144  of the exhalation valve  112  is in its open or closed positions, the pilot lumen  138  of the cushion  114  is maintained in a constant state of fluid communication with the pilot section  172  of the valve chamber. Additionally, irrespective of whether the diaphragm  144  is in its open or closed positions, the fluid chamber  122  is maintained in a constant state of fluid communication with the exhaust section  174  of the valve chamber via the exhaust vents  152 . When the diaphragm  144  is in its open position, the fluid chamber  122  is further placed into fluid communication with both the outlet conduit  147  (and hence ambient air) via the open flow path defined between the body portion  166  of the diaphragm  144  and the seating surface  149  of the wall portion  146  of the seat member  140 . However, when the diaphragm  144  is moved to its closed position, the fluid communication between the fluid chamber  122  and outlet conduit  147  is effectively blocked by the sealed engagement of the body portion  166  of the diaphragm  144  against the seating surface  149  of the wall portion  146 . 
     As indicated above, in addition to the exhalation valve  112 , the exhalation valve subassembly  111  includes the shield plate  113 . The shield plate  113  has a generally oval, slightly arcuate profile, and includes a circularly configured opening  175  within the approximate center thereof. Additionally, formed within the peripheral side surface of the shield plate  113  is an elongate groove or channel  176 . In the mask  110 , the shield plate  113  is adapted to be advanced into the valve opening  126  subsequent to the cooperative engagement of the exhalation valve  112  to the cushion  114  in the aforementioned manner. More particularly, the advancement of the shield plate  113  into the valve opening  126  is facilitated in a manner wherein the wall portion  146  of the seat member  140  is advanced into and through the opening  175  of the shield plate  113 . In this regard, the wall portion  146  and the opening  175  have complimentary configurations, with the diameter of the opening  175  only slightly exceeding that of the outer diameter of the wall portion  148 . 
     Subsequent to the advancement of the wall portion  148  into the opening  175 , that peripheral edge or lip of the main body  116  of the cushion  114  defining the valve opening  126  is advanced into and firmly seated within the complimentary channel  176  formed in the peripheral side surface of the shield plate  113 . The receipt of such edge or lip of the cushion  114  into the channel  176  maintains the shield plate  113  in firm, frictional engagement to the cushion  114 . As is seen in  FIGS. 17 and 18 , the spatial relationship between the exhalation valve  112  and shield plate  113  when each is cooperatively engaged to the cushion  114  in the aforementioned manner is such that the distal section of the wall portion  146  which defines the outer rim  148  thereof protrudes slightly from the exterior surface of the shield plate  113 . 
     As will be recognized, the shield plate  113 , when cooperatively engaged to the cushion  114 , effectively encloses that portion of the fluid chamber  122  which would otherwise be directly accessible via the valve opening  126 . Importantly, by virtue of the attachment of the shield plate  113  to the main body  116  of the cushion  114 , virtually the entirety of the exhalation valve  112  is completely enclosed or contained within the fluid chamber  122  of the cushion  114 . As indicated above, only a small distal section of the wall portion  146  of the seat member  140  protrudes from the shield plate  113 , and in particular the opening  175  defined thereby. As a result, the exhaust vents  152  which facilitate the fluid communication between the fluid chamber  122  and the exhaust section  174  of the valve chamber, and between the fluid chamber  122  and the outlet conduit  147  (and hence ambient air) when the diaphragm  144  is in the open position, are effectively enclosed within the fluid chamber  122  as provides noise attenuation advantages which will be discussed in more detail below. 
     To assist in maintaining the cooperative engagement between the exhalation valve subassembly  111  and the cushion  114 , the mask  110  is further preferably provided with an elongate reinforcement frame member  178  which is attached to the cushion  114 . The frame member  178  has a generally U-shaped configuration, with a central portion thereof including a circularly configured opening  179  formed therein which is adapted to accommodate that aforementioned distal section of the wall portion  146  of the seat member  140  which protrudes from the shield plate  113 . In this regard, the diameter of the opening  179  is sized so as to only slightly exceed the outer diameter of the wall portion  146 . As seen in  FIG. 15 , the thickness of the central portion of the frame member  178  is such that when attached to cushion  114  subsequent to the advancement of the wall portion  146  into the complementary opening  179 , the outer rim  148  defined by the wall portion  146  is substantially flush or continuous with the exterior surface of the frame member  178 . 
     As shown in  FIGS. 17 and 18 , the opposed end portions of the frame member  178  are cooperatively engaged to respective ones of the first and second outer end surfaces  118 ,  120  of the cushion  114 . More particularly, the frame member  178  includes an identically configured pair of first and second connectors  180 ,  182  which are formed on respective ones of the opposed end portions thereof. An inner portion of the first connector  180  is advanced into and frictionally retained within the first gas delivery lumen  132  of the cushion  114 . Similarly, an inner portion of the second connector  182  is advanced into and frictionally retained within the second gas delivery lumen  134  of the cushion  114 . In addition to the inner portions advanced into respective ones of the first and second gas delivery lumens  132 ,  134 , the first and second connectors  180 ,  182  of the frame member  178  each further include an outer portion which is adapted to be advanced into and frictionally retained within a corresponding lumen of a respective one of a pair of bi-lumen tubes fluidly coupled to the mask  110 , in the same manner as described in detail above in relation to the mask  10 . 
     The frame member  178  further includes a tubular, cylindrically configured pressure port  184  which is disposed adjacent the first connector  180 . The pressure port  184  is aligned and fluidly communicates with the pressure sensing lumen  136  of the cushion  114 . Similarly, the frame member  178  is also provided with a tubular, cylindrically configured pilot port  186  which is disposed adjacent the second connector  182 . The pilot port  186  is aligned and fluidly communicates with the valve pilot lumen  138  of the cushion  114 . The pressure and pilot ports  184 ,  186  of the frame member  78  are adapted to be placed into fluid communication with corresponding lumens of respective ones of the aforementioned pair of bi-lumen tubes which are fluidly connected to the mask  110  within a ventilation system incorporating the same, also in the same manner as described in detail above in relation to the mask  10 . The receipt of the wall portion  146  of the seat member  140  into the opening  179  of the frame member  178  ensures that the cushion  114 , the exhalation valve subassembly  111  and the frame member  178  are properly aligned, and prevents relative movement therebetween. 
     In the mask  110 , the exhalation valve  112  is piloted, with the movement of the diaphragm  144  to the closed position described above being facilitated by the introduction of positive fluid pressure into the pilot section  172  of the valve chamber. More particularly, it is contemplated that during the inspiratory phase of the breathing cycle of a patient wearing the mask  110 , the valve pilot lumen  138  will be pressurized by a pilot line fluidly coupled to the pilot port  186 , with pilot pressure being introduced into that portion of the pilot section  172  of the valve chamber via the pilot lumen  138  and the lumen  164  of that tube portion  162  of the cap member  142  advanced into the pilot lumen  138 . The fluid pressure level introduced into the pilot section  172  of the valve chamber will be sufficient to facilitate the movement of the diaphragm  144  to its closed position described above. When the diaphragm  144  is in its closed position, fluid pressure introduced into the fluid chamber  122  via the gas delivery lumens  136 ,  138  is prevented from being exhausted to ambient air. In this regard, though such fluid may flow from the fluid chamber  122  into the exhaust section  174  of the valve chamber via the exhaust vents  152 , the engagement of the diaphragm  144  to the seating surface  149  defined by the wall portion  146  of the seat member  140  effectively blocks the flow of such fluid into the outlet conduit defined by the wall portion  146  and hence to ambient air. 
     Conversely, during the expiratory phase of the breathing cycle of the patient wearing the mask  110 , it is contemplated that the discontinuation or modulation of the fluid pressure through the valve pilot lumen  138  and hence into the pilot section  172  of the valve chamber, coupled with the resiliency of the diaphragm  144  and/or positive pressure applied to the body portion  166  thereof, will facilitate the movement of the diaphragm  144  back to the open position or to a partially open position. In this regard, positive pressure as may be used to facilitate the movement of the diaphragm  144  to its open position may be provided by air which is exhaled from the patient during the expiratory phase of the breathing circuit and is applied to the body portion  166  of the diaphragm  144  via the pillows portions  124  of the cushion  114 , the fluid chamber  122 , the exhaust vents  152 , and the exhaust section  174  of the valve chamber. As will be recognized, the movement of the diaphragm  144  to the open position allows the air exhaled from the patient into the fluid chamber  122  via the pillow portions  124  to be vented to ambient air after flowing from the fluid chamber  122  into the exhaust section  174  of the valve chamber via the exhaust vents  152 , and thereafter flowing from the exhaust section  174  between the diaphragm  144  and seating surface  149  of the wall portion  146  into the outlet conduit  147  also defined by the wall portion  146 . 
     As will be recognized, based on the application of pilot pressure thereto, the diaphragm  144  travels from a fully open position through a partially open position to a fully closed position. In this regard, the diaphragm  144  will be partially open or partially closed during exhalation to maintain desired ventilation therapy. Further, when pilot pressure is discontinued to the diaphragm  144 , it moves to an open position wherein the patient can inhale and exhale through the mask  110  with minimal restriction and with minimal carbon dioxide retention therein. This is an important feature of the present invention which allows a patient to wear the mask  110  without ventilation therapy being applied to the mask  110 , the aforementioned structural and functional features of the mask  110  making it more comfortable to wear, and further allowing it to be worn without carbon dioxide buildup. This feature is highly advantageous for the treatment of obstructive sleep apnea wherein 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). 
     To succinctly summarize the foregoing description of the structural and functional features of the mask  110 , during patient inhalation, the valve pilot lumen  138  is pressurized, which causes the diaphragm  144  to close against the seating surface  149 , thus effectively isolating the fluid chamber  122  of the mask  110  from the outside ambient air. The entire flow delivered from a flow generator fluidly coupled to the mask  110  is inhaled by the patient, assuming that unintentional leaks at the interface between the cushion  114  and the patient are discarded. This functionality differs from what typically occurs in a conventional CPAP mask, where venting to ambient air is constantly open, and an intentional leak flow is continuously expelled to ambient air. During patient exhalation, the pilot pressure introduced into the valve pilot lumen  138  is controlled so that the exhaled flow from the patient can be exhausted to ambient air through the exhalation valve  112  in the aforementioned manner. In this regard, the pilot pressure is “servoed” so that the position of the diaphragm  144  relative to the seating surface  149  is modulated, hence modulating the resistance of the exhalation valve  112  to the exhaled flow and effectively ensuring that the pressure in the fluid chamber  122  of the mask  110  is maintained at a prescribed therapeutic level throughout the entire length of the exhalation phase. When the valve pilot lumen  138  is not pressurized, the exhalation valve  112  is in a normally open state, with the diaphragm  144  being spaced from the seating surface  149  in the aforementioned manner, thus allowing the patient to spontaneously breathe in and out with minimal pressure drop (also referred to as back-pressure) in the order of less than about 2 cm H2O at 60 l/min. As a result, the patient can comfortably breathe while wearing the mask  110  and while therapy is not being administered to the patient. Importantly, the effective containment of the exhaust vents  152  within the fluid chamber  122  substantially mitigates or suppresses the noise generated by the mask  110  attributable to the flow of fluid into the exhaust section  174  of the valve chamber via the exhaust vents  152 . 
     Those of ordinary skill in the art will recognize that the functionality of the exhalation valve  112  during use of the mask  110  by a patient can be characterized with the same three parameters described above in relation to the mask  10  and shown in  FIGS. 11A, 11B and 11C . However, based on the structural features of the exhalation valve  112  in comparison to the exhalation valve  12 , the parameters Pt which is the treatment pressure (i.e., the pressure in the mask  110  used to treat the patient; Pp which is the pilot pressure (i.e., the pressure used to pilot the diaphragm  144  in the exhalation valve  112 ); and Qv which is vented flow (i.e., flow that is exhausted from inside the exhalation valve  112  to ambient are labeled in  FIG. 18  as Pt, Pp and Qv in the context of the exhalation valve  112 . As such, when the patient is ventilated, Pt is greater than zero, with the functionality of the exhalation valve  112  being described by the family of curves in the first and second quadrants of  FIG. 11A . In this regard, as apparent from  FIG. 11A , for any given Pt, it is evident that by increasing the pilot pressure Pp, the exhalation valve  112  will close and the vented flow will decrease. A decrease in the pilot pressure Pp will facilitate the opening of the exhalation valve  112 , thereby increasing vented flow. The vented flow will increase until the diaphragm  144  touches or contacts the inner surface  156  of the base portion  154  of the cap member  142 , and is thus not able to open further. Conversely, when the patient is not ventilated, the inspiratory phase can be described by the third and fourth quadrants. More particularly, Qv is negative and air enters the mask  110  through the exhalation valve  112 , with the pressure Pt in the mask  110  being less than or equal to zero. Pilot pressure Pp less than zero is not a configuration normally used during ventilation of the patient, but is depicted for a complete description of the functionality of the exhalation valve  112 . The family of curves shown in  FIG. 11A  can be described by a parametric equation. Further, the slope and asymptotes of the curves shown in  FIG. 11A  can be modified by, for example and not by way of limitation, changing the material used to fabricate the diaphragm  144 , changing the thickness of the diaphragm  144 , changing the area ratio between the side of the diaphragm  144  facing the pilot section  172  and the side facing the exhaust section  174 , changing the clearance between the diaphragm  144  and the seating surface  149 , and/or changing the geometry of the exhaust vents  152 . 
     As also discussed above in relation to the mask  10 , an alternative representation of the functional characteristics of the valve  112  can be described by graphs in which ΔP=Pt−Pp is shown. For example, the graph of  FIG. 11B  shows that for any given Pt, the vented flow can be modulated by changing ΔP. In this regard, ΔP can be interpreted as the physical position of the diaphragm  144 . Since the diaphragm  144  acts like a spring, the equation describing the relative position d of the diaphragm  144  from the seating surface  149  of the seat member  140  is k·d+Pt·At=Pp·Ap, where At is the area of the diaphragm  144  exposed to treatment pressure Pt and Ap is the area of the diaphragm  144  exposed to the pilot pressure Pp. A similar, alternative representation is provided in the graph of  FIG. 11C  which shows Pt on the x-axis and ΔP as the parameter. In this regard, for any given ΔP, the position d of the diaphragm  144  is determined, with the exhalation valve  112  thus being considered as a fixed opening valve. In this scenario Pt can be considered the driving pressure pushing air out of the exhalation valve  112 , with  FIG. 11C  further illustrating the highly non-linear behavior of the valve  112 . 
     The mask  110  may also be integrated into each of the above-described ventilation systems  88 ,  100  in substitution for the mask  10 . In this regard, as will be recognized by those of ordinary skill in the art, the first and second bi-lumen tubes  96 ,  98  of such ventilation systems  88 ,  100  would simply be cooperatively engaged to corresponding ones of the first and second connectors  180 ,  182 , pressure port  184  and pilot port  186  of the frame member  178  in the same manner described above regarding the engagement to the first and second connectors  80 ,  82 , pressure port  84  and pilot port  86  of the frame member  78 . 
     In the mask  110 , it is contemplated that exhalation vale subassembly  111 , and in particular the exhalation valve  112 , may be detached from the cushion  114  and removed from within the fluid chamber  122  as needed for periodic cleaning or replacement thereof. As will be recognized, such removal is facilitated by first detaching the shield plate  113  from the cushion  114  by removing the lip of the cushion  114  defining the valve opening  126  from within the channel  176  of the shield plate  113 . Thereafter, the exhalation valve  112  is simply grasped and pulled from within the fluid chamber  122 , the flexibility/resiliency of the cushion  114  allowing for the easy removal of the tube portions  162  of the cap member  142  from within respective ones of the pilot lumen  138  and mounting aperture  139 . The re-attachment of the exhalation valve subassembly  111  to the cushion  114  occurs in the reverse sequence, the exhalation valve  112  being advanced into the fluid chamber  122  and attached to the cushion  114  in the aforementioned manner prior to the attachment of the shield plate  113  to the cushion  114  in the aforementioned manner. 
     Referring now to  FIGS. 27 and 28 , there is shown a mask  110   a  which comprises a variant of the mask  110 . The sole distinction between the masks  110 ,  110   a  lies in the mask  110   a  including a heat and moisture exchanger or HME  204  which is positioned within the fluid chamber  122  of the cushion  114 . The HME  204  is operative to partially or completely replace a humidifier (cold or heated pass-over; active or passive) which would otherwise be fluidly coupled to the mask  110   a . This is possible because the average flow through the system envisioned to be used in conjunction with the mask  110   a  is about half of a prior art CPAP mask, due to the absence of any intentional leak in such system. 
     The HME  204 , as a result of its positioning within the fluid chamber  122 , is able to interact with the flow delivered from the flow generator to the patient in order to humidify it, and is further able to capture humidity and heat from exhaled flow for the next breath. The pressure drop created by the HME  204  during exhalation (back-pressure) must be limited, in the order of less than 5 cm H2O at 60 l/min, and preferably lower than 2 cm H2O at 60 l/min. These parameters allow for a low back-pressure when the patient is wearing the mask  110   a  and no therapy is delivered to the patient. 
     It is contemplated that the HME  204  can be permanently assembled to the cushion  114 , or may alternatively be removable therefrom and thus washable and/or disposable. In this regard, the HME  204 , if removable from within the cushion  114 , could be replaced on a prescribed replacement cycle. As will be recognized, the removal of the HME  204  from within the fluid chamber  122  would follow the detachment of the exhalation valve subassembly  111  from the cushion  114  in the manner described above. Similarly, the placement of the HME  204  back into the fluid chamber  122  would precede the reattachment of the exhalation valve subassembly  111  to the cushion  114  in the manner also described above. Additionally, it is contemplated that the HME  204  can be used as an elastic member that adds elasticity to the cushion  114 . In this regard, part of the elasticity of the cushion  114  may be attributable to its silicone construction, and further be partly attributable to the compression and deflection of the HME  204  inside the cushion  114 . 
     The integration of the exhalation valve  12 ,  112  into the cushion  14 ,  114  and in accordance with the present invention allows lower average flow compared to prior art CPAP masks. As indicated above, normal masks have a set of apertures called “vents” that create a continuous intentional leak during therapy. This intentional leak or vented flow is used to flush out the exhaled carbon dioxide that in conventional CPAP machines, with a standard ISO taper tube connecting the mask to the flow generator or blower, fills the tubing up almost completely with carbon dioxide during exhalation. The carbon dioxide accumulated in the tubing, if not flushed out through the vent flow, would be inhaled by the patient in the next breath, progressively increasing the carbon dioxide content in the inhaled gas at every breath. The structural/functional features of the exhalation valve  12 ,  112 , in conjunction with the use of small inner diameter, high pneumatic resistance tubes in the system in which the mask  10 ,  10   a ,  110 ,  110   a  is used, ensures that all the exhaled gas goes to ambient. As a result, a vent flow is not needed for flushing any trapped carbon dioxide out of the system. Further, during inspiration the exhalation valve  12 ,  112  can close, and the flow generator of the system needs to deliver only the patient flow, without the additional overhead of the intentional leak flow. In turn, the need for lower flow rates allows for the use of smaller tubes that have higher pneumatic resistance, without the need for the use of extremely powerful flow generators. The pneumatic power through the system can be kept comparable to those of traditional CPAP machines, though the pressure delivered by the flow generator will be higher and the flow lower. 
     The reduced average flow through the system in which the mask  10 ,  10   a ,  110 ,  110   a  is used means that less humidity will be removed from the system, as well as the patient. Conventional CPAP systems have to reintegrate the humidity vented by the intentional leak using a humidifier, with heated humidifiers being the industry standard. Active humidification introduces additional problems such as rain-out in the system tubing, which in turn requires heated tubes, and thus introducing more complexity and cost into the system. The envisioned system of the present invention, as not having any intentional leak flow, does not need to introduce additional humidity into the system. As indicated above, the HME  104 ,  204  can be introduced into the cushion  14 ,  114  of the mask  10   a ,  110   a  so that exhaled humidity can be trapped and used to humidify the air for the following breath. 
     In addition, a big portion of the noise of conventional CPAP systems is noise conducted from the flow generator through the tubing up to the mask and then radiated in the ambient through the vent openings. As previously explained, the system described above is closed to the ambient during inhalation which is the noisiest part of the therapy. The exhaled flow is also lower than the prior art so it can be diffused more efficiently, thus effectively decreasing the average exit speed and minimizing impingement noise of the exhaled flow on bed sheets, pillows, etc. 
     As also explained above, a patient can breathe spontaneously when the mask  10 ,  10   a ,  110 ,  100   a  is worn and not connected to the flow generator tubing, or when therapy is not administered. In this regard, there will be little back pressure and virtually no carbon dioxide re-breathing, due to the presence of the exhalation valve  12 ,  112  that is normally open and the inner diameters of the tubes integrated into the system. This enables a zero pressure start wherein the patient falls asleep wearing the mask  10 ,  10   a ,  110 ,  110   a  wherein the flow generator does not deliver any therapy. Prior art systems can only ramp from about 4 m H2O up to therapy pressure. A zero pressure start is more comfortable to patients that do not tolerate pressure. 
     As seen in  FIG. 14 , due to the reduced diameter of the various tubes (i.e., the tri-lumen tube  90  and bi-lumen tubes  96 ,  98 ,  102 ) integrated into the system  88 ,  100 , such tubes can be routed around the patient&#39;s ears similar to conventional O2 cannulas. More particularly, the tubing can go around the patient&#39;s ears to hold the mask  10 ,  10   a ,  110 ,  110   a  to the patient&#39;s face. This removes the “tube drag” problem described above since the tubes will not pull the mask  10 ,  10   a  away from the face of the patient when he or she moves. As previously explained, “tube drag” typically decreases mask stability on the patient and increases unintentional leak that annoys the patient. In the prior art, head gear tension is used to counter balance the tube drag, which leads to comfort issues. The tube routing of the present invention allows for lower head gear tension and a more comfortable therapy, especially for compliant patients that wear the mask  10 ,  10   a ,  110 ,  110   a  approximately eight hours every night. The reduction in tube drag in accordance with the present invention also allows for minimal headgear design (virtually none), reduced headgear tension for better patient comfort as indicated above, and reduced cushion compliance that results in a smaller, more discrete cushion  14 ,  114 . The tube dangling in front of the patient, also commonly referred to as the “elephant trunk” by patients, is a substantial psychological barrier to getting used to therapy. The tube routing shown in  FIG. 14 , in addition to making the mask  10 ,  10   a ,  110 ,  110   a  more stable upon the patient, avoids this barrier as well. Another benefit to the smaller tubing is that the mask  10 ,  10   a ,  110 ,  110   a  can become smaller because it does not need to interface with 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. 
     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.