Abstract:
In accordance with the present invention, there is provided a mask for achieving positive pressure mechanical ventilation (inclusive of CPAP, ventilator support, critical care ventilation, emergency applications), and a method for a operating a ventilation system including such mask. The mask of the present invention includes a piloted exhalation valve that is used to achieve the target pressures/flows to the patient. The pilot for the valve may be pneumatic and driven from the gas supply tubing from the ventilator. The pilot may also be a preset pressure derived in the mask, a separate pneumatic line from the ventilator, or an electro-mechanical control. 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 
       [0001]    The present invention is a continuation-in-part of U.S. patent application Ser. No. 13/411,407 entitled VENTILATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE filed Mar. 2, 2012, which is a continuation of U.S. patent 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 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    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. 
         [0005]    2. Description of the Related Art 
         [0006]    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). 
         [0007]    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. 
         [0008]    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 cmH2O 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. 
         [0009]    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. 
         [0010]    In the patient circuits described above, the ventilator pressurizes the gas to be delivered to the patient inside the ventilator to the intended patient pressure, and then delivers that pressure to the patient through the patient circuit. Very small pressure drops develop through the patient circuit, typically around 1 cmH2O, due to gas flow though the small amount of resistance created by the tubing. Some ventilators compensate for this small pressure drop either by mathematical algorithms, or by sensing the tubing pressure more proximal to the patient. 
         [0011]    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 
       [0012]    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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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 q cushioning effect and simplifying the design of the cushion thereof. 
         [0017]    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 
         [0018]    These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
           [0019]      FIG. 1  is top perspective view of a nasal pillows mask constructed in accordance with the present invention and including an integrated diaphragm-based piloted exhalation valve; 
           [0020]      FIG. 2  is an exploded view of the nasal pillows mask shown in  FIG. 1 ; 
           [0021]      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; 
           [0022]      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; 
           [0023]      FIG. 5  is a cross-sectional view of the nasal pillows mask shown in  FIG. 1  taken along lines  5 - 5  thereof; 
           [0024]      FIG. 6  is a top perspective view of cushion of the nasal pillows mask shown in  FIG. 1 ; 
           [0025]      FIG. 7  is a top perspective view of exhalation valve of the nasal pillows mask shown in  FIG. 1 ; 
           [0026]      FIG. 8  is a bottom perspective view of exhalation valve shown in  FIG. 7 ; 
           [0027]      FIG. 9  is a cross-sectional view of exhalation valve shown in  FIGS. 7 and 8 ; 
           [0028]      FIG. 10  is a cross-sectional view similar to  FIG. 5 , but depicting a variant of the nasal pillows mask wherein an HME is integrated into the cushion thereof; 
           [0029]      FIGS. 11A ,  11 B and  11 C are a series of graphs which provide visual representations corresponding to exemplary performance characteristics of the exhalation valve subassembly of the nasal pillows mask of the present invention; 
           [0030]      FIG. 12  is a schematic representation of an exemplary ventilation system wherein a tri-lumen tube is used to facilitate the operative interface between the nasal pillows mask and a flow generating device; 
           [0031]      FIG. 13  is a schematic representation of an exemplary ventilation system wherein a bi-lumen tube is used to facilitate the operative interface between the nasal pillows mask and a flow generating device; 
           [0032]      FIG. 14  is a side-elevational view of the nasal pillows mask of the present invention depicting an exemplary manner of facilitating the cooperative engagement thereof to a patient through the use of a headgear assembly; 
           [0033]      FIG. 15  is a front-elevational view of the nasal pillows mask of the present invention depicting an exemplary tri-lumen tube, Y-connector, and pair of bi-lumen tubes which are used to collectively facilitate the operative interface between the nasal pillows mask and a flow generating device in accordance with the schematic representation shown in  FIG. 12 ; 
           [0034]      FIG. 16  is an exploded view of the tri-lumen tube, Y-connector and bi-lumen tubes shown in  FIG. 15 ; 
           [0035]      FIG. 17  is a perspective view of the Y-connector shown in  FIGS. 15 and 16  in its disconnected state; 
           [0036]      FIG. 18  is a side-elevational view of the Y-connector shown in  FIGS. 15 ,  16  and  17  in its disconnected state; 
           [0037]      FIG. 19  is a cross-sectional view of one of the identically configured bi-lumen tubes taken along line  19 - 19  of  FIG. 16 ; 
           [0038]      FIG. 20  is a cross-sectional view of the tri-lumen tube taken along line  20 - 20  of  FIG. 16 ; 
           [0039]      FIG. 21  is a cross-sectional view of the tri-lumen tube, Y-connector and bi-lumen tubes shown in  FIG. 15  as operatively connected to each other; 
           [0040]      FIG. 22  is a cross-sectional view of one of the identically configured bi-lumen tubes shown in  FIGS. 15 and 16 , but further illustrating in more detail the generally elliptical profile of the gas delivery lumen thereof relative to the generally circular profile of a corresponding connector of the nasal pillows mask which is advanced therein; 
           [0041]      FIG. 23  is a cross-sectional view of one of the identically configured bi-lumen tubes which is similar to  FIG. 22 , but further illustrates the manner in which the receipt of a corresponding, generally circular connector of the nasal pillows mask or Y-connector into the generally elliptical gas delivery lumen facilitates the compression of a portion of the bi-lumen tube as effectively maintains the frictional and airtight engagement thereof with the nasal pillows mask; 
           [0042]      FIG. 24  is a perspective view of a segment of a quad-lumen tube which may be used an alternative to the tri-lumen tube shown in  FIGS. 15-16  and  19 - 20 ; and 
           [0043]      FIG. 25  is a cross-sectional view of the quad-lumen tube taken along line  25 - 25  of  FIG. 24 ; 
       
    
    
       [0044]    Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0045]    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. 
         [0046]    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 . 
         [0047]    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. 
         [0048]    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 . 
         [0049]    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. 
         [0050]    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 . 
         [0051]    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 . 
         [0052]    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 . 
         [0053]    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 . 
         [0054]    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. 
         [0055]    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 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 ). 
         [0056]    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 . 
         [0057]    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 . 
         [0058]    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. The first and second connectors  80 ,  82  each define opposed inner and outer portions which have generally cylindrical, tubular configurations. The 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, the inner portion of the second connector  82  is advanced into and frictionally retained within the second gas delivery lumen  34  of the cushion  14 . As will be described in more detail below, the outer portions of the first and second connectors  80 ,  82  of the frame member  78  are each 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 . 
         [0059]    As shown in  FIGS. 3 and 4 , the frame member  78  further includes a pressure port  84  which is disposed adjacent the first connector  80 . Like each of the first and second connectors  80 ,  82 , the pressure port  84  defines opposed inner and outer portions which each have a generally cylindrical, tubular configuration. The inner portion of the pressure port  84  is aligned and fluidly communicates with the pressure sensing lumen  36  of the cushion  14  subsequent to being advanced and frictionally maintained therein. The frame member  78  is also provided with a pilot port  86  which is disposed adjacent the second connector  82  and also defines opposed inner and outer portions which each have a generally cylindrical, tubular configuration. The inner portion of the pilot port  86  is aligned and fluidly communicates with the valve pilot lumen  38  of the cushion  14  subsequent to being advanced and frictionally maintained therein. As will also be discussed in more detail below, the outer portions of 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 . 
         [0060]    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. 
         [0061]    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 . 
         [0062]    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). 
         [0063]    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 601/min. As a result, the patient can comfortably breathe while wearing the mask  10  and while therapy is not being administered to the patient. 
         [0064]    Referring now to  FIGS. 11A ,  11 B and  11 C, 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 . 
         [0065]    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 . 
         [0066]      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 about 17 mm, with the length thereof in the system  88  being about 1.8 m or 6 ft. 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 24 inches. The first bi-lumen tube  96  includes a gas delivery lumen having an ID in the range of from about 1 mm to 10 mm, and preferably about 3 mm to 6 mm. 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 range of from about 1 mm to 10 mm, and preferably about 3 mm to 6 mm. 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 . 
         [0067]    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. 
         [0068]      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 about 11 mm, with the length thereof being about 1.8 m or 6 ft. 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 24 inches. The first bi-lumen tube  96  includes a gas delivery lumen having an ID in the range of from about 1 mm to 10 mm, and preferably about 3 mm to 6 mm. 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 range of from about 1 mm to 10 mm, and preferably about 3 mm to 6 mm. 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 . 
         [0069]    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. 
         [0070]    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. 
         [0071]    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. 
         [0072]    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 cmH2O at 601/min, and preferably lower than 2 cmH2O at 601/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. 
         [0073]    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 . 
         [0074]    The integration of the exhalation valve  12  into the cushion  14  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 , in conjunction with the use of small inner diameter, high pneumatic resistance tubes in the system in which the mask  10 ,  10   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  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. 
         [0075]    The reduced average flow through the system in which the mask  10 ,  10   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 prent invention, as not having any intentional leak flow, does not need to introduce additional humidity into the system. As indicated above, the HME  104  can be introduced into the cushion  14  of the mask  10   a  so that exhaled humidity can be trapped and used to humidify the air for the following breath. 
         [0076]    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. 
         [0077]    As also explained above, a patient can breathe spontaneously when the mask 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  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  wherein the flow generator does not deliver any therapy. Prior art systems can only ramp from about 4 mH2O up to therapy pressure. A zero pressure start is more comfortable to patients that do not tolerate pressure. 
         [0078]    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  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  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 . 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  more stable upon the patient, avoids this barrier as well. Another benefit to the smaller tubing is that the mask  10 ,  10   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. 
         [0079]    Referring now to  FIG. 15 , there is shown a front-elevational view of the nasal pillows mask  10 ,  10   a  of the present invention wherein an exemplary tri-lumen tube  90 , Y-connector  94 , and pair of bi-lumen tubes  96 ,  98  are used to collectively facilitate the operative interface between the nasal pillows mask  10 ,  10   a  and a flow generating device  92  in accordance with the schematic representation of the ventilation system  88  shown in  FIG. 12 . As indicated above, in the ventilation system  88 , the tri-lumen tube  90  is used to facilitate the fluid communication between the Y-connector  94  and the blower or flow generator  92  of the system  88 , with one end of the tri-lumen tube  90  being fluidly connected to the flow generator  92 , and the opposite end thereof being fluidly connected to the Y-connector  94 . 
         [0080]    As is best seen in  FIGS. 16 ,  20  and  21 , the tri-lumen tube  90  has a ribbon-like, generally elliptical or oval cross-sectional configuration, and defines three (3) lumens, along with a lengthwise cross-sectional axis A 1  and a widthwise cross-sectional axis A 2 , both of which are shown in  FIG. 20 . More particularly, the tri-lumen tube  90  includes a gas delivery lumen  104 , a pressure sensing lumen  106 , and a pilot lumen  108 . As best seen in  FIG. 20 , like the tri-lumen tube  90 , the gas delivery lumen  104  has a generally elliptical profile or cross-sectional configuration. Along these lines, the gas delivery lumen  104  is preferably formed so as to have a maximum length L in the range of from about 5 ft. to 10 ft., and preferably about 6 ft.; a maximum width W in the range of from about 8 mm to 13 mm; and a cross-sectional area equivalent to a circular lumen with a diameter of about 2 mm to 15 mm, and preferably about 4 mm to 10 mm, and most preferably about 8 mm. The pressure sensing and pilot lumens  106 ,  108  are disposed proximate respective ones of the ends of the gas delivery lumen  104  along the axis A 1 . However, both the pressure sensing and pilot lumens  106 ,  108  each have a generally circular cross-sectional configuration, as opposed to an elliptical cross-sectional configuration. As is further apparent from  FIGS. 16 ,  20  and  21 , the cross-sectional area of the gas delivery lumen  104  substantially exceeds that of each of the pressure sensing and pilot lumens  106 ,  108 , which are preferably identically sized to each other, and are each provided with an inner diameter or ID in the range of from about 0.5 mm to 2 mm. The tri-lumen tube  90  is preferably fabricated from a silicone, TPE or PVC material which has a Shore Hardness in the range of from about 50 A to 80 A, and thus possesses a prescribed level of resilience and flexibility. 
         [0081]    The structural features of the tri-lumen tube  90 , coupled with the material properties thereof, are selected to not only make it resiliently flexible, but to prevent either of the pressure sensing or pilot lumens  106 ,  108  from being collapsed by even an above-normal level of bending, twisting or other deflection of the tri-lumen tube  90 . In this regard, the elliptical cross-sectional configurations of the tri-lumen tube  90  and its gas delivery lumen  104 , coupled with the orientation of the pressure sensing and pilot lumens  106 ,  108  adjacent respective ones of the opposed ends thereof (along of the lengthwise cross-sectional axis A 1 ), imparts to the tri-lumen tube  90  a tendency to bend in a direction which is generally perpendicular to, rather than aligned with or parallel to the axis A 1  (similar to the bending of a ribbon). This manner of bending, which is generally along the axis A 2 , substantially reduces the susceptibility of the pressure sensing or pilot lumens  106 ,  108  to inadvertent collapse. Thus, even if the tri-lumen tube  90  is bent beyond that threshold which would typically be encountered during normal use of ventilation system  88  as could result in the collapse or blockage of the gas delivery lumen  104 , flow will typically still be maintained through both the pressure sensing and pilot lumens  106 ,  108 . This unobstructed fluid or pneumatic communication through the pressure sensing and pilot lumens  106 ,  108  provides a modality which, in concert with the control algorithms of the ventilation system  88 , may be used to facilitate not only the actuation of the exhalation valve  12  in a manner ensuring unhindered patient breathing through the mask  10 ,  10   a,  but also the sounding of an alarm within the ventilation system  88  and/or adjustment to other operational parameters thereof as are necessary to address the blockage or obstruction of the gas delivery lumen  104 . 
         [0082]    As indicated above, the structural attributes of the Y-connector  94 , which will be described in more detail below, effectively bifurcates the tri-lumen tube  90  into the first and second bi-lumen tubes  96 ,  98 , each of which is of a prescribed length. As best seen in  FIGS. 19 ,  22  and  23 , the first and second bi-lumen tubes  96 ,  98  are identically configured to each other, and each have a generally tear-drop shaped cross-sectional configuration defining an apex  118 . The first bi-lumen tube  96  includes a gas delivery lumen  110  and a pressure sensing lumen  112 . Similarly, the second bi-lumen to  98  includes a gas delivery lumen  114  and a pilot lumen  116 . 
         [0083]    In both the first and second bi-lumen tubes  96 ,  98 , the gas delivery lumens  110 ,  114  each have a generally elliptical cross-sectional configuration or profile, as is most easily seen in  FIG. 19 . The elliptical cross-sectional area is equivalent to that of a circular lumen having a diameter in the range of from about 1 mm to 10 mm, and preferably about 3 mm to 6 mm, and most preferably about 5 mm. However, the pressure sensing and pilot lumens  112 ,  116  each have a generally circular cross-sectional configuration or profile, with an inner diameter or ID in the range of from about 0.5 mm to 2 mm. Each bi-lumen tube  96 ,  98  is preferably fabricated from a silicone, TPE or PVC material which has a Shore Hardness in the range of from about 50 A to 80 A, and thus possesses a prescribed level of resilience and flexibility. The advantages attendant to forming each of the gas delivery lumens  110 ,  114  of the first and second bi-lumen tubes  96 ,  98  with a generally elliptical profile will be discussed more detail below. 
         [0084]    In the exemplary ventilation system  88 , the gas delivery lumen  110  of the first bi-lumen tube  96  is fluidly coupled to the generally cylindrical, tubular outer portion of the first connector  80  of the frame number  78 . The pressure sensing lumen  112  of the first bi-lumen tube  96  is itself fluidly coupled to the generally cylindrical, tubular outer portion of the pressure port  84  of the frame number  78  which, as indicated above, is disposed immediately adjacent the outer portion of the first connector  80 . As will be recognized, the pressure sensing lumen  112  is sized relative to the outer portion of the pressure port  84  such that the pressure port  84  is frictionally maintained within the first bi-lumen tube  96  once advanced into a corresponding end of the pressure sensing lumen  112  thereof. Similarly, the gas delivery lumen  110  is sized relative to the outer portion of the first connector  80  such that the first connector  80  is frictionally retained within the first bi-lumen tube  96  once advanced into a corresponding end of the gas delivery lumen  110 . 
         [0085]    Similar to the first bi-lumen tube  96 , the gas delivery lumen  114  of the second bi-lumen tube  98  is fluidly coupled to the generally cylindrical, tubular outer portion of the second connector  82  of the frame number  78 . The pilot lumen  116  of the second bi-lumen tube  98  is itself fluidly coupled to the generally cylindrical, tubular outer portion of the pilot port  86  of the frame number  78  which, as indicated above, is disposed immediately adjacent the outer portion of the second connector  80 . As will be recognized, the pilot lumen  116  is sized relative to the outer portion of the pilot port  86  such that the pilot port  86  is frictionally maintained within the second bi-lumen tube  98  once advanced into a corresponding end of the pilot lumen  116  thereof. Similarly, the gas delivery lumen  114  is sized relative to the outer portion of the second connector  82  such that the second connector  82  is frictionally retained within the second bi-lumen tube  98  once advanced into a corresponding end of the gas delivery lumen  114 . 
         [0086]    As seen in  FIG. 22 , whereas the outer portion of each of the first and second connectors  80 ,  82  has a generally circular cross-sectional configuration, the gas delivery lumens  110 ,  114  of the first and second bi-lumen tubes  96 ,  98  each have a generally elliptical cross-sectional configuration or profile as indicated above. In the ventilation system  88 , the relative orientations of the outer portions of the first connector  80  and pressure port  84  are the same as those of the outer portions of second connector  82  and pilot port  86 . Similarly, the relative orientations of the gas delivery and pressure sensing lumens  110 ,  112  of the first bi-lumen tube  96  are the same as the gas delivery and pilot lumens  114 ,  116  of the second bi-lumen tube  98 . In the ventilation system  88 , these relative orientations are specifically selected so as to achieve a prescribed offset between the axis of the outer portions of the first and second connectors  80 ,  82  and corresponding ones of the gas delivery lumens  110 ,  114  when the outer portions of the pressure and pilot ports  80 ,  82  are coaxially aligned with respective ones of the pressure sensing and pilot lumens  112 ,  116  of the first and second bi-lumen tubes  96 ,  98 . As a result of these offsets, the advancement of the outer portions of the first and second connectors  80 ,  82  into corresponding ends of respective ones of the gas delivery lumens  110 ,  114  facilitates the resilient deformation of each of the first and second bi-lumen tubes  96 ,  98  in a matter effectively compressing a web portion  120  thereof. As seen in  FIGS. 19 ,  22  and  23 , this web portion  120  is disposed between the gas delivery lumen  110 ,  114  and a corresponding one the pressure sensing and pilot lumens  112 ,  116 . Such compression of the web portion  120  effectively maintains a tight frictional engagement between the first and second bi-lumen tubes  96 ,  98  and the outer portions of respective ones of the first and second connectors  80 ,  82  which is less prone to leakage. 
         [0087]    The structural features of the first and second bi-lumen tubes  96 ,  98 , coupled with the material properties thereof, are selected to not only to provide resilient flexibility, but to prevent either of the pressure sensing or pilot lumens  112 ,  116  from being collapsed by even an above-normal level of bending, twisting or other deflection of the corresponding bi-lumen tube  96 ,  98 . In this regard, the elliptical or tear drop shaped cross-sectional configuration of each bi-lumen tube  96 ,  98 , coupled with the orientation of the corresponding pressure sensing or pilot lumen  112 ,  116  between the gas delivery lumen  110 ,  114  and the apex  118  thereof, substantially reduces the susceptibility of the pressure sensing or pilot lumens  112 ,  116  to inadvertent collapse. Thus, even if the first or second bi-lumen tube  96 ,  98  is bent beyond that threshold which would typically be encountered during normal use of ventilation system  88  as could result in the collapse or blockage of the corresponding gas delivery lumen  110 ,  114 , flow will typically still be maintained through both the pressure sensing and pilot lumens  112 ,  116 . As with the tri-lumen tube  90  described above, this unobstructed flow through the pressure sensing and pilot lumens  112 ,  116  provides a modality which, in concert with the control algorithms of the ventilation system  88 , may be used to facilitate not only the actuation of the exhalation valve  12  in a manner ensuring unhindered patient breathing through the mask  10 ,  10   a,  but also the sounding of an alarm within the ventilation system  88  and/or adjustment to other operational parameters thereof as are necessary to address the blockage or obstruction of either gas delivery lumen  110 ,  114 . 
         [0088]    Referring now to  FIGS. 15-18  and  21 , as indicated above, in the ventilation system  88  the Y-connector  94  facilitates the operative interface between the tri-lumen tube  90  and the first and second bi-lumen tubes  96 ,  98 . More particularly, the Y-connector  94  effectively divides the gas delivery lumen  104  of the tri-lumen tube  90  into the gas delivery lumens  110 ,  114  of the first and second bi-lumen tubes  96 ,  98 . 
         [0089]    Advantageously, the Y-connector  94  is adapted to allow for the selective disconnection or de-coupling of the tri-lumen tube  90  from the first and second bi-lumen tubes  96 ,  98  without disconnecting or separating either the tri-lumen tube  90  or either of the first and second bi-lumen tubes  96 ,  98  from the Y-connector  94 . In this regard, the Y-connector  94  comprises a male member  118  and a complimentary female member  120  which are releasably engageable to each other. The male member  118  includes a base portion  122  which has a generally elliptical or oval-shaped cross-sectional configuration. In this regard, the cross-sectional length and width dimensions of the base portion  122  along the lengthwise and widthwise major axes thereof are preferably equal or substantially equal to those of the tri-lumen tube  90 . 
         [0090]    In addition to the base portion  122 , the male member  118  includes first and second tube portions  124 ,  126  which protrude from respective ones of the opposed sides or faces of the base portion  122 . Like the base portion  122 , the first and second tube portions  124 ,  126  each have a generally elliptical or oval-shaped cross-sectional configuration. The height of the first tube portion  124  exceeds that of the second tube portion  126 . Additionally, the cross-sectional length and width dimensions of the second tube portion  126  are sized so as to slightly exceed those of the gas delivery lumen  104  of the tri-lumen tube  90 . Such relative sizing is selected such that the second tube portion  126  may be advanced into yet tightly frictionally maintained within one end of the gas delivery lumen  104 . The advancement of the second tube portion  126  into the gas delivery lumen  104  is typically limited by the abutment of the corresponding end of the tri-lumen tube  90  against that end or face of the base portion  122  having the second tube portion  126  protruding therefrom. As will be recognized, due to the maximum cross-sectional length and width dimensions of the base portion  122  preferably being equal or substantially equal to those of the tri-lumen tube  90 , the outer surface of the base portion  122  will be substantially flush or continuous with the outer surface of the tri-lumen tube  90  when the corresponding ends are abutted against each other in the aforementioned manner. In the male member  118 , the base portion  122  and the first and second tube portions  124 ,  126  collectively define a gas delivery lumen  128  which is most easily seen in  FIG. 21 . 
         [0091]    The male member  118  further comprises first and second pressure sensing ports  130 ,  132  which protrude from respective ones of the opposed sides or faces of the base portion  122 , and first and second pilot ports  134 ,  136  which also protrude from respective ones of the opposed sides or faces of the base portion  122 . In this regard, the first and second pressure sensing ports  130 ,  132  are coaxially aligned with each other, as are the first and second pilot ports  134 ,  136 . 
         [0092]    In the male member  118 , the first pressure sensing and pilot ports  130 ,  134  are identically configured to each other, with the second pressure sensing and pilot ports  132 ,  136  being identically configured to each other as well. Though the first and second pressure sensing ports  130 ,  132  and the first and second pilot ports  134 ,  136  each have tubular, generally cylindrical configurations with generally circular cross-sectional profiles, the outer diameters of the first pressure sensing and pilot ports  130 ,  134  exceed those of the second pressure sensing and pilot ports  132 ,  136 . As is best seen in  FIG. 21 , the first and second pressure sensing ports  130 ,  132  and the base portion  122  collectively define a pressure sensing lumen  138  of the male member  118 . Similarly, the first and second pilot lumens  134 ,  136  and the base portion  122  collectively define a pilot lumen  140  of the male member  118 . 
         [0093]    As is most apparent from  FIG. 16 , the first pressure sensing and pilot ports  130 ,  134  are oriented so as to be disposed adjacent respective ones of the opposed ends of the first tube portion  124  along the lengthwise cross-sectional axis thereof. Similarly, the second pressure sensing and pilot ports  132 ,  136  are positioned adjacent respective ones of the opposed ends of the second tube portion  126  along the lengthwise cross-sectional axis thereof. Importantly, the orientation of the second pressure sensing and pilot lumens  132 ,  136  relative to the second tube portion  126  is such that when the second tube portion  126  is coaxially aligned with the gas delivery lumen  104  of the tri-lumen tube  90 , the second pressure sensing and pilot ports  132 ,  136  will be coaxially aligned with respective ones of the pressure sensing and pilot lumens  106 ,  108  of the tri-lumen tube  90 . As such, when the second tube portion  126  is advanced into the gas delivery lumen  104  in the aforementioned manner, the second sensing port  132  will concurrently be advanced into one end of the pressure sensing lumen  106 , with the second pilot port  136  being concurrently advanced into one end of the pilot lumen  108 . Along these lines, the outer diameter dimensions of the second pressure sensing and pilot ports  132 , 136  are preferably sized relative to the inner diameter dimensions of the pressure sensing and pilot lumens  106 ,  108  such that the second pressure sensing and pilot ports  132 ,  136  are tightly frictionally retained within respective ones of the pressure sensing and pilot lumens  106 ,  108  upon being fully advanced therein. 
         [0094]    As is best seen in  FIGS. 16-18 , the male member  118  further includes an opposed, juxtaposed pair of locking tabs  142  which protrude from the base portion  122  and extend along the first tube portion  124 . More particularly, the locking tabs  142  are positioned on opposite sides of the first tube portion  124  so as to extend in generally perpendicular relation to the widthwise cross-sectional axis thereof. As will be described in more detail below, the locking tabs  142  are used to facilitate the releasable engagement of the male member  118  to the female member  120 . As will also be described in more detail below, the locking tabs  142  may be identically configured to each other, or may alternatively have dissimilar configurations for purposes of insuring that the male and female members  118 ,  120  are in prescribed orientations relative to each other as a precursor to being releasably engaged to each other. 
         [0095]    The female member  120  of the Y-connector  94  comprises a main body portion  144  which itself has a generally elliptical or oval-shaped cross-sectional configuration. In this regard, the cross-sectional length and width dimensions of the body portion  144  along the lengthwise and widthwise major axes thereof are preferably equal or substantially equal to those of the base portion  122  of the male member  118 , as well as the tri-lumen tube  90 . As best seen in  FIGS. 17 and 21 , the body portion  144  defines an elongate passage  146  which extends generally axially therein to one of the opposed sides or faces thereof. 
         [0096]    In addition to the body portion  144 , the female member  120  includes an identically configured pair of first and second gas delivery ports  148 ,  150  which protrude from a common side or face of the body portion  144 , and in particular that side opposite the side having the passage  146  extending thereto. The first and second gas delivery ports  148 ,  150  each have a tubular, generally cylindrical configuration with a generally circular cross-sectional profile. Additionally, the first and second gas delivery ports  148 ,  150  each fluidly communicate with one end of the passage  146 . 
         [0097]    The female member  120  further comprises a pressure sensing port  152  and a pilot port  154  which are identically configured to each other, and protrude from that side or face of the body portion  144  having the first and second gas delivery ports  148 ,  50  protruding therefrom. More particularly, the pressure sensing port  152  is disposed between the first gas delivery port  148  and one of the opposed ends of the body portion  144  along the lengthwise cross-sectional axis thereof. Similarly, the pilot port  154  is disposed between the second gas delivery port  150  and one of the opposed ends of the body portion  144  along the lengthwise cross-sectional axis thereof. The pressure sensing and pilot ports  152 ,  154  also each have a tubular, generally cylindrical configuration with a generally circular cross-sectional profile. As is best seen in  FIG. 21 , the pressure sensing port  152  and the body portion  144  collectively define a pressure sensing lumen  156  of the female member  120 . Similarly, the pilot port  154  and the body portion  144  collectively define a pilot lumen  158  of the female member  120 . 
         [0098]    The first gas delivery port  148  of the female member  120  is adapted to be advanced into one end of the gas delivery lumen  110  of the first bi-lumen tube  96 , with the second gas delivery port  150  being adapted for advancement into one end of the gas delivery lumen  114  of the second bi-lumen tube  98 . However, whereas the first and second gas delivery ports  148 ,  150  each have a generally circular cross-sectional configuration, the gas delivery lumens  110 ,  114  of the first and second bi-lumen tubes  96 ,  98  each have the generally elliptical cross-sectional configuration or profile as indicated above. In the ventilation system  88 , the relative orientations of the first gas delivery port  148  and the pressure sensing port  152  are the same as those of the first connector  80  and pressure port  84  of the mask  10 ,  10   a.  Similarly, the relative orientations of the second gas delivery port  150  and pilot port  154  are the same as those of the second connector  82  and pilot port  86  of the mask  10 ,  10   a.  As with the connection of the first and second bi-lumen tubes  96 ,  98  to the mask  10 ,  10   a  as explained above, these relative orientations are specifically selected so as to achieve a prescribed offset between the axes of the first and second gas delivery ports  148 ,  150  and corresponding ones of the gas delivery lumens  110 ,  114  when the pressure sensing port  152  is coaxially aligned with the pressure sensing lumen  112  of the first bi-lumen tube  96 , and the pilot port  154  is coaxially aligned with the pilot lumen  116  of the second bi-lumen tube  98 . As a result of these offsets, the advancement of the first and second gas delivery ports  148 ,  150  into corresponding ends of respective ones of the gas delivery lumens  110 ,  114  facilitates the resilient deformation of each of the first and second bi-lumen tubes  96 ,  98  in a manner effectively compressing the web portion  120  thereof. Such compression of the web portion  120  effectively maintains a tight frictional engagement between the first and second bi-lumen tubes  96 ,  98  and respective ones of the first and second gas delivery ports  148 ,  150  which is less prone to leakage. 
         [0099]    The advancement of the first gas delivery port  148  into the gas delivery lumen  110 , as well as the advancement of the second gas delivery port  150  into the gas delivery lumen  114 , is limited by the abutment of the corresponding ends of the first and second bi-lumen tubes  96 ,  98  against that end or face of the body portion  144  having the first and second gas delivery ports  148 ,  150 , as well as the pressure sensing and pilot ports  152 ,  154 , protruding therefrom. As will be recognized, when the first gas delivery port  148  is advanced into the gas delivery lumen  110  in the aforementioned manner, the pressure sensing port  152  is concurrently advanced into one end of the pressure sensing lumen  112 . Similarly, when the second gas delivery port  150  is advanced into the gas delivery lumen  114 , the pilot port  154  will concurrently be advanced into one end of the pilot lumen  116 . Along these lines, the outer diameter dimensions of the pressure sensing and pilot ports  152 ,  154  are preferably sized relative to the inner diameter dimensions of the pressure sensing and pilot lumens  112 ,  116  such that the pressure sensing and pilot ports  152 ,  154  are tightly frictionally retained within respective ones of the pressure sensing and pilot lumens  112 ,  116  upon being fully advanced therein. 
         [0100]    As best seen in  FIGS. 16 and 17 , the female member  120  further includes an opposed pair of retention tabs  160  which are formed in and extend partially along the body portion  144 . More particularly, the retention tabs  160  are positioned on opposite sides of the body portion  144  so as to extend in generally perpendicular relation to the widthwise cross-sectional axis thereof. As will also be described in more detail below, the retention tabs  160  are sized and configured to be releasably engageable to respective ones of the locking tabs  142  to facilitate the releasable engagement of the male member  118  to female member  120 . As with the locking tabs  142 , the retention tabs  160  may be identically configured to each other, or may alternatively have dissimilar configurations for purposes of ensuring that that the male and female members  118 ,  120  are in prescribed orientations relative to each other as a precursor to being releasably engaged to each other. 
         [0101]    In  FIGS. 16-18 , the male and female members  118 ,  120  of the Y-connector  124  are depicted in a disconnected or separated state. When the male member  118  is disconnected from the female member  120 , the gas delivery lumens  110 ,  114  of the first and second bi-lumen tubes  96 ,  98  each still fluidly communicate with the passage  146  of the female member  120  via respective ones of the first and second gas delivery ports  148 ,  150 . In addition, the pressure sensing lumen  112  of the first bi-lumen tube  96  still fluidly communicates with the pressure sensing lumen  156 , with the pilot lumen  116  of the second bi-lumen tube  98  still fluidly communicating with the pilot lumen  158 . Further, the gas delivery lumen  128  of the male member  118  still fluidly communicates with the gas delivery lumen  104  of the tri-lumen tube  90  via the second tube portion  126 , with the pressure sensing lumen  138  still fluidly communicating with the pressure sensing lumen  106  and the pilot lumen  140  still fluidly communicating with the pilot lumen  108 . 
         [0102]    The connection of the male and female members  118 ,  120  to each other is facilitated by initially advancing both the first tube portion  124  and the locking tabs  142  of the male member  118  into the passage  146  of the female member  120  which has a complementary shape adapted to accommodate both the first tube portion  124  and the locking tabs  142 . The orientation of the first pressure sensing and pilot ports  130 ,  134  of the male member  118  relative to the first tube portion  124  is such that the coaxial alignment of the first tube portion  124  with the passage  146  facilitates the concurrent coaxial alignment of the first pressure sensing port  130  with the pressure sensing lumen  156  and the coaxial alignment of the first pressure sensing port  134  with the pilot lumen  158 . Along these lines, the full advancement of the first tube portion  124  and locking tabs  142  into the passage  146  results in the concurrent advancement of the first pressure sensing port  130  into the pressure sensing lumen  156 , and the first pilot port  134  into the pilot lumen  158 . As is best seen in  FIG. 21 , those end portions of the pressure sensing and pilot lumens  156 ,  158  extending to the side or face of the body portion  144  having the open end of the passage  146  extending thereto are each slightly enlarged relative to the remainder thereof. These enlarged end portions of the pressure sensing and pilot lumens  156 ,  158  are adapted to accommodate respective ones of the first pressure sensing and pilot ports  130 ,  134  which, as indicated above, are slightly larger than corresponding ones of the second pressure sensing and pilot ports  132 ,  136 . 
         [0103]    As will be recognized, the full advancement of the first tube portion  124  and locking tabs  142  into the passage  146 , and advancement of the first pressure sensing and pilot ports  130 ,  134  into respective ones of the pressure sensing and pilot lumens  156 ,  158  is limited by the abutment of that side or face of the body portion  144  of the female member  120  opposite that having the first and second gas delivery ports  148 ,  150  protruding therefrom against that side or face of the base portion  120  of the male member  118  having the first tube portion  124  protruding therefrom. At the point of such abutment, the locking tabs  142  will releasably engage respective ones of the retention tabs  160  in a manner maintaining the male and female members  118 ,  120  in releasable engagement to each other. 
         [0104]    Each of the retention tabs  160  is sized and configured to be resiliently flexible. Based on the complementary shapes of the locking tabs  142  and retention tabs  160 , the application of compressive pressure to each of the retention tabs  160  when the male and female members  118 ,  120  are cooperatively engaged to each other facilitates the disengagement of the retention tabs  160  from respective ones of the locking tabs  142  as allows for the separation of the male and female members  118 ,  120  from each other. In the ventilation system  88 , it is important that the male and female members  118 ,  120  not be cross-connected as could result in the pressure sensing lumen  138  of the male member  118  being placed into fluid communication with the pilot lumen  150  of the female member  120 , and the pilot lumen  140  of the male member  118  being placed into fluid communication with the pressure sensing lumen  156  of the female member  120 . To prevent such occurrence, as indicated above, it is contemplated that the locking tabs  142  and/or the retention tabs  160  of each pair may be provided with dissimilar configurations as ensures that the male and female members  118 ,  120  can only be releasably connected to each other in one prescribed orientation relative to each other. As will be recognized, such orientation ensures that the pressure sensing lumens  138 ,  156  are properly placed into fluid communication with each other, as are the pilot lumens  140 ,  158 . 
         [0105]    As is best seen in  FIG. 21 , the full receipt of the first tube portion  124  of the male member  118  into the passage  146  of the female member  120  facilitates the placement of the gas delivery lumen  128  into fluid communication with a segment of the passage  146 , as well as each of the first and second gas delivery lumens  148 ,  150  which each fluidly communicate with the passage  146 . Similarly, the complete advancement of the first pressure sensing and pilot ports  130 ,  134  of the male member  118  into respective ones of the pressure sensing and pilot lumens  156 ,  158  of the female member  120  effectively places the pressure sensing and pilot lumens  138 ,  140  of the male member  118  into fluid communication with respective ones of the pressure sensing and pilot lumens  156 ,  158  of the female member  120 . Thus, when the male and female members  118 ,  120  of the Y-connector  94  are releasably connected to each other in the aforementioned manner, the gas delivery lumen  104  of the tri-lumen tube  90  is effectively bifurcated or divided, and thus placed into fluid communication with the gas delivery lumens  110 ,  114  of the first and second bi-lumen tubes  96 ,  98  via the gas delivery lumen  128  of the male member  118 , a segment of the passage  146  of the female member  120 , and each of the first and second gas delivery ports  148 ,  150  of the female member  120 . In addition, the pressure sensing lumen  106  of the tri-lumen tube  90  is placed into fluid communication with the pressure sensing lumen  112  of the first bi-lumen tube  96  by the pressure sensing lumen  138  of the male member  118  and the pressure sensing lumen  156  of the female member  120 . Similarly, the pilot lumen  108  of the tri-lumen tube  90  is placed into fluid communication with the pilot lumen  116  of the second bi-lumen tube  98  via the pilot lumen  140  of the male member  118  and the pilot lumen  158  of the female member  120 . Though not shown with particularity in  FIGS. 16-18  and  21 , it is contemplated that the Y-connector  94 , and in particular the male and female members  118 ,  120  thereof, may be outfitted with sealing members such as O-rings as needed to facilitate the formation of fluid-tight seals between the same when releasably connected to each other. 
         [0106]    Referring now to  FIGS. 24 and 25 , there is depicted a portion of a quad-lumen tube  190  which may be integrated into the ventilation system  88  as an alternative to the above-described tri-lumen tube  90 . The quad-lumen tube  190  has a generally circular, cross-sectional configuration, and defines four (4) lumens. More particularly, the quad-lumen tube  190  includes a gas delivery lumen  194 , a pressure sensing lumen  196 , a pilot lumen  198 , and an auxiliary lumen  200 . The pressure sensing, pilot and auxiliary lumens  196 ,  198 ,  200  are preferably disposed about the gas delivery lumen  194  in equidistantly spaced intervals of approximately 120°, and have identically dimensioned, generally circular cross-sectional configurations. 
         [0107]    As is apparent from  FIGS. 24 and 25 , the cross-sectional configuration of the quad-lumen tube  190  is not uniform along the entire length thereof. Rather, each of the opposed end portions of the quad-lumen tube  190  (one of which is shown in  FIG. 24 ) have cross-sectional configurations differing from that the central section or portion of the quad-lumen tube  190  extending between such end portions. More particularly, as seen in  FIG. 25 , the gas delivery lumen  194  defined by the central portion of the quad-lumen tube  190  extending between the end portions thereof does not have a generally circular cross-sectional configuration. Rather, portions of the quad-lumen tube  90  which accommodate respective ones of the pressure sensing, pilot and auxiliary lumens  196 ,  198 ,  200  protrude into the gas delivery lumen  194 , thus imparting a generally cloverleaf cross-sectional configuration thereto. In contrast, at each of the opposed end portions of the quad-lumen tube  190 , those portions of the quad-lumen tube  190  accommodating respective ones of the pressure sensing, pilot and auxiliary lumens  196 ,  198 ,  200  transition to the exterior of the gas delivery lumen  194 , thus resulting in such gas delivery lumen  194  assuming a generally circular cross-sectional configuration. Those of ordinary skill in the art will recognize that the quad-lumen tube  190  may be formed so as not to include the aforementioned alternatively configured end portions, the non-circular cross-sectional configuration of the central portion of the gas delivery lumen  194  thus being consistent throughout the entire length of the quad-lumen tube  190 . However, if the alternatively configured end portions are provided, it is contemplated that they may be formed through the implementation of a specialized extrusion process, or as separate parts which are glued or molded onto the aforementioned central portion of the quad-lumen tube  190 . In the quad-lumen tube  190 , it is also contemplated that the inner diameters of each of the pressure sensing, pilot and auxiliary lumens  196 ,  198 ,  200  will be similar to those of the pressure sensing and pilot lumens  106 ,  108  of the tri-lumen tube  90  as described above, with the cross-sectional area of the gas delivery lumen  194  being similar to that of the gas delivery lumen  104  of the tri-lumen tube  90  as also described above. 
         [0108]    As will be recognized by those of ordinary skill in the art, providing the gas delivery lumen  194  with a generally circular cross-sectional configuration or profile at each of the opposed end portions of the quad-lumen tube  190  makes it easier to couple or operatively interface the quad-lumen tube  190  to a Y-connector at one end thereof, and to a flow generator at the opposite end thereof. Along these lines, the circularly configured end portions of the gas delivery lumen  194  are more easily advanced over and frictionally retained upon a cylindrically configured port, as opposed to a port that would otherwise need to be provided in a non-standard configuration so as to be capable of advancement into the gas delivery lumen  194  having the shape shown in  FIG. 25 . In the quad-lumen tube  190 , the auxiliary lumen  200  may be used, for example, to route optical fibers or wires as could potentially be used to illuminate the Y-connector. Assuming such Y-connector has a two-piece, detachable construction as described above in relation to the Y-connector  94 , the illumination thereof would provide greater ease to a patient to effectuate the disconnection of the male and female members from each other at night, in darkness. 
         [0109]    The quad-lumen tube  190  is preferably fabricated from a silicone, TPE or PVC material which has a Shore Hardness in the range of from about 50 A to about 80 A, and thus possesses a prescribed level of resilience and flexibility. Further, as seen in  FIG. 24 , it is contemplated that the wall of the quad-lumen tube  190  defining or partially defining the gas delivery lumen  194  will have a spiral-shaped reinforcement ribbon  202  embedded therein. It is contemplated that such ribbon  202  will extend along the central portion of the quad-lumen tube  190 , but not into either of the alternatively configured end portions thereof. The ribbon  202  enhances the structural integrity of the quad-lumen tube  190 , thus making the gas delivery, pressure sensing and pilot lumens  194 ,  196 ,  198  less susceptible to collapse upon any excessive bending or compression of the quad-lumen tube  190 . It is also contemplated that the ribbon  202  could be substituted with a reinforcement braiding which is adapted to enhance the structural integrity of the quad-lumen tube  190  in the aforementioned manner. However, those or ordinary skill in the art will recognize that the quad-lumen tube  190  need necessarily have the ribbon  202  or other type of reinforcement braiding integrated therein. Along these lines, it is also contemplated that the quad-lumen tube  190  could further be alternatively configured such that the pressure sensing, pilot and auxiliary lumens  196 ,  198 ,  200  assume a generally helical profile along the length thereof, thus mimicking the effect of the effect of the ribbon  202 , and assisting in the prevention of the kinking or collapse of the quad-lumen tube  190 . Also, though not shown, it is contemplated that a reinforcement ribbon similar to the ribbon  202  or reinforcement braiding may be integrated into the above-described tri-lumen tube  90 . 
         [0110]    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.