Abstract:
The invention relates to a pressure responsive respiratory valve apparatus for enabling positive pressure from a source of pressure to be applied to a user&#39;s airway, and allowing ingress of a breathable gas from an inlet port into a user&#39;s airway during inhalation and egress of expired tidal volume of air from the user&#39;s respiratory system to an exhalation port during exhalation. The invention minimises rebreathing of expired gas and optimizes delivery of pressurised breathable gas by venting gas only during exhalation, as well as addressing important user considerations including minimizing noise, pressure swing, and size.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC §371 US National Phase application of serial number PCT/AU2011/001094 having an International Filing Date of 26 Aug. 2011 and claims priority under 35 USC §119(e) to U.S. Provisional Patent Application No. 61/392,954 filed 14 Oct. 2010. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates broadly to a respiratory valve apparatus. The present invention is in the field of positive pressure respiratory therapy such as nasal CPAP or positive pressure ventilation, as may be applied to a user through a user interface, such as a nasal mask, oronasal mask, nasal prongs or suitable invasive means, such as tracheostomy or endotracheal tube. 
     TECHNICAL BACKGROUND 
     Positive pressure respiratory therapies, such as continuous positive airway pressure (CPAP) therapy, and intermittent positive pressure ventilation therapy (IPPV), are commonly administered for the treatment of a wide range of respiratory conditions, including central and obstructive sleep apnea. (OSA), chronic obstructive pulmonary disease (COPD), restrictive respiratory insufficiency and acute respiratory failure (ARF). Acute and chronic life support systems also make use of positive 15 pressure respiratory therapies using volume or pressure controlled cycles. Typically, positive pressure delivered via an externally fitted interface such as a removable face mask, for example, would be used to provide intermittent positive pressure treatment of responsive conditions in a conscious user, that is, when the user is either awake or asleep, but otherwise fully arousable and able to sustain at least partial respiratory effort. Tracheostomy may be required for chronic life support involving, for example, total loss of respiratory muscle innervation. In temporarily anesthetised or otherwise unconscious users without sufficient self-supporting respiratory drive or those with unstable airways, ventilation would normally be accomplished with an invasive interface means, such as an endotracheal tube or pharyngeal mask, to ensure reliable connection of the pressure source and hence predictable ventilation. In general it is preferable to adopt the least invasive means to treat a specific condition in order to mitigate complications, complexity and cost of care. For this reason, the application of externally sealing face masks has become increasing popular in hospital settings where feasible. Examples include ventilation with full face or nasal masks for management of acute respiratory failure. Additional benefits of non-invasive positive pressure treatment methods include their improved applicability to use in a home setting where ease of use, comfort and efficacy are key factors in determining patient compliance with therapy. The ability to treat chronic disorders in the convenience of the home setting improves long-term health outcomes for users and relieves the burden on hospital resources. 
     Any gas pressure delivering means can be used as the source of positive pressure. In the prior art, a pressure source would typically comprise a gas flow generator and a gas flow circuit in the form of a flexible air delivery hose which connects the gas flow generator to a user interface. If a gas flow generator is made small enough it could be attached directly to, or integrated with, the user interface, and then it alone would comprise the positive pressure source. Where a. primary gas flow delivery tube is used it may include a. secondary tube or limb to recover or direct expired gas as may be used with anesthesia or ventilation. In the case of nasal 
     CPAP therapy, a source of breathable gas maintained at a substantially constant treatment pressure above atmospheric pressure over a breathing cycle, is applied to a user&#39;s airway via a user interface, such as a nasal mask, mouth mask, oronasal mask, nasal prongs or other externally fitted device. CPAP therapy is most commonly used to treat OSA in a home-care s setting, although it is also applicable to a variety of other respiratory conditions. CPAP therapy, particularly for home use, may also include limited pressure release modes, whereby treatment pressure is reduced during exhalation and restored to its previous preexhalation level at the end of expiration. This approach offers less resistance to expiratory effort and is intended to either enhance breathing comfort in the case of sleep apnea, or, to assist or support a user&#39;s own respiratory effort in the case of respiratory insufficiency. The latter case is often termed bi-level treatment to reflect the fact that distinct pressures are applied to a user&#39;s airway during a single breathing cycle. Pressures may also be varied as a function of time and flow, in which cases there may be a range of pressure applied during a single breathing cycle. To facilitate venting of expired tidal volume from a user&#39;s airway when using an externally fitted user interface, it typically will have incorporated into its structure a vent or vents of fixed dimension which are open to atmosphere. In this way, a breathable gas is able to leak from the vents continuously under the effect of the positive pressure applied to the user interface from the pressure source. The magnitude of this vented flow of breathable gas to atmosphere will be a function of the pressure within the user interface and vent configuration. Typically, designers will attempt to find a suitable compromise between vent size and number, such that sufficient gas is vented in order to limit the amount of exhaled carbon dioxide (CO2) which is rebreathed by the user, whilst keeping the magnitude of vented gas low enough not to require a significant increase in pressure source capacity to compensate for this operational leak. For example, at a user interface pressure of 10 cm of water, the vent flow rate may be in the order of 20 liters/min whereas at 20 cm of water it may be in the order 60 liters/min. These figures assume that there is no additional unintended leak, as may be attributable to a poorly fitting user interface. An absence of means to physically prevent reverse flow of expired gas back towards the pressure source means that some fraction of a user&#39;s exhaled air may accumulate within the pressure source and thereby be re-inhaled during subsequent inhalation or lung reinflation. Additionally, some fraction of the air provided by the pressure source is directed straight to atmosphere via the vents without entering to the user&#39;s airway since it must be applied to reducing the effective total dead space of the user including the interface and pressure source. It is apparent therefore that the lower the therapeutic pressure. the less effective is the venting of stale expired gas from a user&#39;s respiratory system. This last limitation in the current state of the art requires devices to specify a minimum operating pressure in order to provide a safe level of elimination of expired tidal volume and hence prevent rebreathing of CO2. Typically, this minimum pressure required from the pressure source is in the order of 4 cm of water. Prolonged use below this minimum pressure puts the user at risk of rebreathing a substantial proportion of their expired air and asphyxiation if both nose and mouth breathing routes are covered by a. well-fitting oronasal mask, for example. It is evident that the prior art exhibits further inherent limitations depending on operating circumstances. At high rates of breathing cycles, deep breathing, or a combination of both over a prolonged period, there will be increased levels of rebreathing of expired air and CO2, thereby increasing the user&#39;s effective airway dead space. which includes the interface and pressure source. Furthermore, the capacity of the pressure source must be increased to compensate for continuously vented flow which necessarily increases with pressure as described. Additionally, when humidification is required by the user, further capability must be factored into the design of the pressure source because some varying fraction of the humidified air is vented to atmosphere through the vents without entering the user&#39;s airway, thereby requiring both heating and storage of an additional volume of water that is not used to humidify a user&#39;s airway. Similarly, if required, flow of instilled supplementary therapeutic gases, for example oxygen, or other therapeutic substances must also be correspondingly increased to compensate for loss due to continuous venting of the breathable gas. 
     When breathable gas exits the vents of the user interface, it typically creates noise which may irritate the user or their bed partner. The acoustic magnitude of this vent noise is proportional to the rate of vent flow. It can be fluffier appreciated that exhaled gas combined with flow from the pressure source will exit the user interface with sufficient velocity and volume to increase the risk of spreading infectious particles if present in exhaled gas to the surrounding environment. This may pose a significant infection risk to health workers in a hospital setting and others in the vicinity. If the source of breathable gas fails to generate the prescribed minimum pressure, such as during a power failure, users fitted with a full face mask, such as one which covers both the nose and mouth, must also be fitted with an anti-asphyxia valve to ensure the user does not rebreathe a substantial part of their expired tidal volume which, in the absence of sufficient background pressure and corresponding flushing flow, will accumulate in the pressure source. A further application of the invention is bi-level therapy wherein, rather than administering a substantially constant positive pressure over a breathing cycle, pressure will be varied within a breathing cycle to assist natural breathing. As a general principle, pressure applied during lung filling or inspiration will be greater than that applied during lung emptying or expiration to facilitate gas movement into and out of a user&#39;s respiratory system. Transition from a higher to lower pressure is most often triggered by machine sensing of the user&#39;s breathing, or follows pre-set machine controlled breathing rates, pressures or volumes. Means of connecting a source of pressurised breathable gas to a user&#39;s facially accessible airway will involve a range of user interface devices similar to those described for CPAP therapy and corresponding methods of venting expired air from a user&#39;s respiratory system, that is, through a series of small vents often positioned in the interface itself. Such arrangements will suffer similar limitations to those described previously. Additionally, it can be appreciated that CO2 rebreathing may produce a more detrimental impact in bi-level therapy users due to the fact that these individuals typically exhibit a greater degree of respiratory impairment by virtue of background hypoxia, hypercapnia; more rapid breathing and perhaps exaggerated tidal volume, particularly during acute exacerbations and their prodrome. Since the pressure applied during exhalation is lower than that applied during inhalation, the ability to clear residual expired air from the system may be further compromised. A clinical compromise involves finding a suitable range of pressures to facilitate user comfort, adequate ventilation and adequate flushing of exhaled gas accumulated within the pressure source to minimize negative impacts related to CO2 rebreathing. In yet a further application, positive pressure therapies, particularly those involving ventilation, may also be administered via endotracheal tubes, laryngeal masks, tracheotomies or similar invasive means. In these circumstances, it is common to provide active venting wherein the venting of exhaled tidal volume from a user&#39;s airway is provided through an arrangement of inspiratory and/or expiratory valves placed in the breathing circuit and often under automated synchronised control from the pressure source. The prior art describes many circuit arrangements depending on the clinical requirements, including open and fully closed recirculating systems. In its simplest form an active exhalation valve will be present to direct exhaled gas to atmosphere while allowing fresh breathable gas to be supplied to the user&#39;s airway on cycling to an upper pressure. Such control means add mechanical and electrical complexity and increased risk of asphyxiation by rebreathing in the event of failure of valve actuation systems and components. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a respiratory valve apparatus for delivering a pressurised flow of breathable gas to the airway of a user, the respiratory valve apparatus comprising: a valve body including an inlet port for receiving the breathable gas, an outlet port for releasing the breathable gas to the user&#39;s airway during an inhalation phase and for receiving exhaled gas during an exhalation phase of the user&#39;s respiratory cycle, a breathable gas flow passage communicating between the inlet and outlet ports, an exhaust port for releasing the exhaled gas, and an exhaled gas flow passage communicating between the outlet and exhaust ports; a first valve means located in the breathable gas flow passage and being operable under the pressurised flow of breathable gas from the inlet port to open the breathable gas flow passage during the inhalation phase so as to permit flow of the breathable gas to the user, and to close the breathable gas flow passage during the exhalation phase; a second valve means located in the exhaled gas flow passage and including a flexible membrane which defines an internal cavity, the flexible membrane being operable under ambient pressure and during the inhalation phase to seal the exhaust port for its closure and during the exhalation phase to deflect to at least partly expose and open the exhaust port; an equilibrium passage disposed between an upstream side of the first valve means and the internal cavity of the second valve means and being operable under the pressurised flow of breathable gas from the inlet port to divert part of the breathable gas to the internal cavity of the flexible membrane via the equilibrium passage to at ambient pressure and during the inhalation phase maintain closure of the exhaust port. 
     Preferably the exhaust poll includes a plurality of substantially parallel slots spaced circumferentially around the valve body at the exhaled gas flow passage. More preferably the plurality of slots are staggered in length. Even more preferably the plurality of slots are each longitudinally tapered in thickness being progressively narrower in an upstream direction. 
     Preferably the flexible membrane is a flexible synthetic polymeric film of a thickness less than 0.1 mm. More preferably the synthetic polymeric film is polyethylene of a thickness less than 50 μm. Preferably the flexible membrane is a sock-like structure having an outer surface shaped substantially complementary to an inner cylindrical wall of the valve body against which it seals. More preferably the sock-like structure is tapered in shape with the flexible membrane becoming progressively smaller in circumference from its open end to its closed end. Alternatively the sock-like structure is tapered in shape with the flexible membrane becoming progressively larger in circumference from its open end to its closed end. Even more preferably the sock-like structure at or adjacent to its closed end includes an expanded annular sealing portion. 
     Preferably the valve body includes a humidification element coupled to the outlet port to capture moisture from the exhaled gas and transfer said moisture at least in part to inhaled Ras from the pressurized flow of breathable gas. Preferably the equilibrium passage is defined by a bias pressure passage restricted to dampen operation of the second valve means. More preferably the bias pressure passage includes a bias pressure tube. Preferably the valve body includes a swivel connector at the inlet port, the first valve means connected to the swivel connector. More preferably the first valve means is a non-return valve including a flexible flap connected to the swivel connector. Even more preferably the swivel connector includes a central post to which the flexible flap is mounted. Preferably the respiratory valve apparatus also comprises a user interface connected to the valve body. More preferably the user interface is integral with the valve body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Operation and design of the various aspects and embodiments of the invention are described in the following description and drawings. 
         FIG. 1  is a side view of a system for providing respiratory therapy to a user that uses a respiratory valve apparatus according to a first aspect of the present invention; 
         FIG. 2  is an isometric view of a nasal user interface which is coupled to the respiratory valve apparatus of  FIG. 1 ; 
         FIG. 3  is an exploded isometric view of the system shown in  FIG. 2 ; 
         FIG. 4  is an exploded isometric view of the respiratory valve apparatus shown in  FIGS. 1, 2 and 3 ; 
         FIG. 5  is an isometric and partly cut-away view of the respiratory valve apparatus shown in  FIG. 4  and showing a zone of detail for subsequent views; 
         FIG. 5 a    is an enlarged view of the detail highlighted in  FIG. 5 ; 
         FIG. 6  is a pictorial and partly cut-away view of the respiratory valve apparatus of  FIG. 4 , showing the path of breathable gas during an inhalation phase; 
         FIG. 7  is a pictorial and partly cut-away view of the respiratory valve apparatus of  FIG. 4 , showing the path of breathable gas during an exhalation phase; 
         FIG. 8  is an isometric view of a first embodiment of a rigid valve body of the valve apparatus of  FIG. 4  and showing vertical section plane A-A and near horizontal section plane B-B which passes through the centre line of the inlet port of the aforementioned valve body; 
         FIG. 9  is a sectional side view of the rigid valve body of  FIG. 8  through plane A-A and showing a zone of detail for subsequent views; 
         FIGS. 9 a  to 9 e    are enlarged views of alternative embodiments of the exhaust apertures highlighted in  FIG. 9 : 
         FIG. 10  is a sectional view of the rigid valve body of  FIG. 8  through plane B-B: 
         FIG. 11  is an isometric view of the cap of the respiratory valve apparatus of  FIG. 4 ; 
         FIG. 12  is an isometric view of a second embodiment of the cap for the respiratory valve apparatus; 
         FIG. 13  is an isometric view of a second embodiment of the rigid valve body of the respiratory apparatus and adapted to receive the cap of  FIG. 12 ; 
         FIG. 14  is an isometric and partly cut-away view of a third embodiment of the rigid valve body of the respiratory apparatus and featuring a grating to mitigate undesired deflection modes of the inlet valve; 
         FIG. 15  is an isometric view of the swivel connector of the respiratory valve apparatus of  FIG. 4  and showing a vertical section plane A-A, 
         FIG. 16  is a sectional side view of the swivel connector of  FIG. 15  through the section plane A-A and showing a. zone of detail for subsequent views; 
         FIG. 16 a    is a detail view showing the installation of components to the swivel connector of  FIG. 15 ; 
         FIG. 16 b    is a detail view showing a deformation applied to the swivel connector of  FIG. 15  in order to fasten the components shown in  FIG. 16   a;    
         FIG. 16 c    is a detail view showing a further embodiment for the installation of components to the swivel connector of  FIG. 15 ; 
         FIG. 16 d    is a detail view showing a deformation applied to the swivel-connector of  FIG. 15  in order to fasten the components shown in  FIG. 16   c;    
         FIG. 16 e    is a detail view showing a further embodiment for the installation of components to the swivel connector of  FIG. 15 ; 
         FIG. 16 f    is a detail view showing a deformation applied to the swivel connector of  FIG. 15  in order to fasten the components shown in  FIG. 16   e;    
         FIG. 16 g    is a detail view showing components installed to a swivel connector similar to that of  FIG. 15  and retained by a separate fastener; 
         FIG. 17  is an isometric view of the balanced pressure valve of the respiratory valve apparatus of  FIG. 4  and showing a vertical sectional plane A-A; 
         FIG. 18  is a sectional side view of the balanced pressure valve of  FIG. 17  through section plane A-A; 
         FIG. 19  is an isometric view of a second embodiment of the balanced pressure valve of the respiratory valve apparatus of  FIG. 4  and showing a vertical sectional plane A-A; 
         FIG. 20  is a sectional side view of the balanced pressure valve of  FIG. 19  through section plane A-A; 
         FIG. 21  is an isometric exploded and partly cut-away view of a respiratory valve apparatus according to a second embodiment of the present invention; 
         FIG. 22  is a side view of the respiratory valve apparatus of  FIG. 21 ; 
         FIG. 23  is a pictorial and partly cut-away view of the respiratory valve apparatus of  FIG. 21  in which the end cap is omitted for viewing clarity, and further showing the path of breathable gas during an inhalation phase; 
         FIG. 24  is a similar view to that of  FIG. 23 , and further showing the path of exhaled gas during an exhalation phase; 
         FIG. 25  is a pictorial exploded and partly cut-away view of the respiratory valve apparatus according to a third embodiment of the present invention and showing a zone of detail is for subsequent views; 
         FIG. 25 a    is an enlarged view of a detail highlighted in  FIG. 25 ; 
         FIG. 26  is a sectional side view of the respiratory valve apparatus shown in  FIG. 25 ; 
         FIG. 27  is a similar view to that of  FIG. 26 , and further showing the path of breathable gas during an inhalation phase; 
         FIG. 28  is a similar view to that of  FIG. 26 , and further showing the path of exhaled gas during an exhalation phase; 
         FIG. 29  is an isolated sectional side view of the flexible membrane and flange of the balanced pressure valve used in the respiratory valve apparatus of  FIG. 25 ; 
         FIG. 30  is a sectional side view of another embodiment of the flexible membrane and flange of the balanced pressure valve used in the respiratory valve apparatus of  FIG. 25 : 
         FIG. 31  is a sectional side view of another embodiment of the flexible membrane and flange of the balanced pressure valve used in the respiratory valve apparatus of  FIG. 25 ; 
         FIG. 32  is a pictorial and partly cut-away view of yet another embodiment of the flexible membrane used in the respiratory valve apparatus of  FIG. 25 ; 
         FIG. 33  is a sectional side view of one form of barbed end cap used in the respiratory valve apparatus of  FIG. 25 ; 
         FIG. 34  is a sectional side view of another form of barbed end cap used in the respiratory valve apparatus of  FIG. 25 ; 
         FIG. 35  is a sectional side view of the barbed end cap shown in  FIG. 34  fitted to a flexible membrane of balanced pressure valve used in the respiratory valve apparatus of  FIG. 25 ; 
         FIG. 36  is a pictorial exploded view of the respiratory valve apparatus according to a second aspect of the present invention; 
         FIG. 37  is a sectional side view of a cut-away portion of the respiratory valve apparatus of  FIG. 36 , when assembled, and further showing the path of breathable gas during an inhalation phase; 
         FIG. 38  is a similar view to that of  FIG. 37 , and further showing the path of exhaled gas during an early stage of an exhalation phase; 
         FIG. 39  is a similar view to that of  FIG. 37 , and further showing the path of exhaled gas during a later stage of an exhalation phase; 
         FIG. 40  is an isometric exploded view of a combined user interface and respiratory valve apparatus according to a third aspect of the present invention; 
         FIG. 41  is an isometric view of the combined user interface and respiratory valve apparatus of  FIG. 40  in an assembled state; 
         FIG. 42  is a pictorial and partly cut-away view of the respiratory valve apparatus of  FIG. 40 , and further showing the path of breathable gas during an inhalation phase; 
         FIG. 43  is a pictorial and partly cut-away view of the respiratory valve apparatus of  FIG. 40 , and further showing the path of breathable gas during an exhalation phase. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment of a respiratory valve apparatus according to a first aspect of the present invention is shown in  FIGS. 1 to 7 .  FIG. 1  shows the apparatus  10  in use in a system for providing respiratory therapy. Apparatus  10  is coupled or sealably secured to a mask or user interface  12  covering the nose of a user, and is fed with a pressurised flow of breathable gas from a pressure source comprising gas flow generator  14  and delivery tube  16 , whereby breathable gas is delivered to the airway of the user. 
     It will be appreciated by those skilled in the art that although a nasal user interface is depicted here, alternatives such as an oronasal, oral appliance tracheostomy or endotracheal tube may also be applicable. 
     The user may be an individual undergoing respiratory therapy, and the breathable gas may be enriched with a therapeutic gas, such as oxygen, or include a therapeutic agent, and be in a variety of forms, such as a nebulized mist, powder or gas. 
       FIG. 2  shows the respiratory valve apparatus  10  installed in a nasal user interface or mask  12 .  FIG. 3  shows an exploded view of the assembly of  FIG. 2  including a retaining clip  18  applicable to retain the apparatus  10  to the mask  12 , although it will be appreciated by persons skilled in the art that the outlet port may feature shoulders, circlip retention grooves or alternate structural forms for retaining the valve apparatus  10  to the mask  12 . Alternatively, the valve apparatus may be incorporated into the body of mask ( FIGS. 40 to 43 ). 
     The respiratory valve apparatus  10  delivers a pressurised flow of breathable gas to the airway of the user, and may be used in conjunction with a user interface and ( FIGS. 4 and 5 ) comprises a rigid valve body  20  which includes an inlet port  22  for continuously receiving breathable gas under pressure from the gas flow generator  14  or other ventilator device. There is an outlet port  24  which, via the mask  12 , releases the breathable gas to the user&#39;s airway during an inhalation phase and receives exhaled gas during an exhalation phase of the user&#39;s respiratory cycle. 
     A breathable gas flow passage  34  (as shown by the path of the unbroken arrows in  FIG. 6 ) communicates between the inlet port  22  and the outlet port  24 . Additional elements such as the inlet swivel connector  76  and optional humidification element  61  may extend this passage. There is an exhaust port  28  for releasing the exhaled gas to atmosphere. Exhaust port  28  includes a plurality of circumferentially spaced exhaust apertures  30 . An exhaled gas flow passage  36  (as shown by the path of the unbroken arrows in  FIG. 7 ) communicates between the outlet port  24  and exhaust ports  28 . 
     A first valve  70  is located in the breathable gas flow passage  34  and divides that to passage into an upstream portion  35  and a downstream portion  37 . In this embodiment, the first valve  70  is a non-return or one-way valve. The non-return valve  70  comprises a flexible flap  72  ( FIG. 4 ) which is weakly biased to a closed position under neutral differential pressure (as depicted in  FIG. 5 ) and under greater differential pressure from inlet  78  is deflected into two halves about a generally vertical line ( FIG. 6 ) to an opened position. Non-return valve  70  is retained by central hole  74  to the swivel connector  76  and locates on the shoulder  92  ( FIG. 16 ) defined by central post  82  and mounting post  84  which is subsequently heat staked to form mushroom head  86  ( FIGS. 16 b - f   ). As would be apparent to one skilled in the art, many structural forms of retention are possible including placing the valve directly to the swivel  76  ( FIGS. 16 a  to 16 b   ), heat-staking with added flat washer(s)  128  ( FIGS. 16 c  to 16 d   ), heat-staking with added contoured washer(s)  129  ( FIGS. 16 e  to 16 f   ) or retention by an additional screw or fastening element  87  ( FIG. 16 g   ). Once installed on the swivel  76 , the swivel and valve assembly may then be installed onto the inlet port  22  of the rigid valve body  20 . Retention of the swivel  76  to the valve body  20  may be achieved by structural forms such as the elastic engagement of the rib  88  and groove  90  of the swivel  76  ( FIG. 16 ) to the rib  46  and groove  48  of the valve body  20  ( FIGS. 9, 10 ). Slots  50  may optionally be provided on the inlet port  22  in order to reduce the forces required to install the swivel  76  to the valve body  20 . 
     If the valve  70  were to distort during operation into the inlet port  22 , function of respiratory valve apparatus  10  would be impaired. To mitigate this circumstance, one or more stops  56  may be provided within the inlet port  22 . Stops  56  have a ramp-like form in order to minimise resistance to insertion, but maximise resistance to withdrawal of valve  70  from rigid valve body  20 . 
     Other structural forms of stop are shown in  FIG. 14  attached to rigid valve body  178  comprising a grating  180 , which includes a large aperture  182  through which valve  70  is passed during installation, and a plurality of webs  184  supporting aperture  182 . 
     It will be appreciated by skilled persons in the art that the non-return valve  70  may take alternate structural forms that are all weakly biased to a closed position. 
     The non-return valve  70  will open under pressure of breathable gas received through the inlet swivel connector  76  and then through the inlet port  22  during the inhalation phase ( FIG. 6 ), and so permit flow of breathable gas to the user, and will close under pressure of exhaled gas received through the outlet port  24  during the exhalation phase ( FIG. 7 ), despite the maintenance of a pressurised flow of breathable gas through the inlet port  22  during the exhalation phase. 
     By the closing of the non-return valve  70 , the exhaled gas received through the outlet port  24  is prevented from exiting through the inlet port  22 , but flows through the exhaled gas flow passage  36 . 
     As shown in  FIGS. 4, 5, 6 and 7 , an optional heat and moisture exchange (HME) element  61  may be added to the outlet port  24 . The HME element  61  comprises a housing  63  with an HME insert  69  preferably constructed from an open 
     cell foam treated with a hygroscopic material such as calcium chloride or material with similar hygroscopic properties, or a hydrophobic filter material as described in the prior art. Choice of material will be dependent on efficiency of capture and release of heat and moisture and resistance to gas flow. Insert  69  may optionally be treated with anti-bacterial agents to mitigate colonisation of the insert by microbes. Absorbed heat and condensation would be available for release back into breathable gas flow passage  34  and thence into the user&#39;s airway to reduce drying thereof. Annular housing  63  comprises a cylindrical body  65  and shoulder  67  adapted to locate and sealably retain humidification element  61  to outlet port  24 . It will be appreciated by those skilled in the art that whilst the form of the housing in this case is cylindrical, its form would be tailored to match that of the outlet port be it cylindrical, oval or other suitable cross section. Additionally, both the retention of housing  63  by outlet port  24  and humidification element  61  by housing  63  may be by frictional means as depicted or alternatively by positive means such as screw thread, barbs, bayonet, adhesive or other suitable means apparent to those skilled in the art. 
     A second valve  98  is located in exhaled gas flow passage  36  and, in this embodiment, is a balanced pressure valve. Balanced pressure valve  98  ( FIGS. 17 to 18 ) comprises a flexible membrane  100 , which is weakly biased to an expanded position where it closes the exhaust apertures  30  under ambient or neutral pressure (as shown in  FIG. 5 ) and under pressure of breathable gas received through the inlet port  22  during the inhalation phase ( FIG. 6 ). Balanced pressure valve  98  further comprises a flange portion  110  which is sealably retained between the valve receptacle  32  of the valve body  20  ( FIGS. 4, 5 ) and the compression rim  150  of the cap  138  and optionally with spacing gasket  130  ( FIGS. 4, 5, 11 ). A hole  112  ( FIGS. 4 and 17 ) provides continuity of breathable gas equilibrium passage  38  ( FIG. 5 ) which passes through it and corresponding holes  136  and  134  in gasket  130 . Flexible membrane  100  of balanced pressure valve  98  includes a main body portion  102  of generally cylindrical form and preferably forming an expanding taper towards an expanded annular sealing portion  104  and closed first end  106  at its base, and an open second end  108  at the junction between the flanged portion  110  and main body portion  102 . 
     Cap  1   38  ( FIGS. 4, 11 ) includes a wall  142  which locates within the corresponding peripheral wall  42  of the valve receptacle  32  of the valve body  20 , and is retained thereto by engagement of the fastening rib  40  (which may optionally be either continuous, or broken as shown for lower installation forces) with the groove  140  of the wall  142 . Chambers  146  and  148  linked by channel  144  effect continuity of the breathable gas equilibrium passage  38 . 
     The flexible membrane  100  will, under pressure of exhaled gas flowing through the exhaled gas flow passage  36  during the exhalation phase ( FIG. 7 ), flexibly deform to a semi or fully collapsed position so as to open the exhaust apertures  30 , whereby the exhaled gas is released to atmosphere. 
     The exhaust apertures  30  ( FIG. 5 a   ) are generally tapered longitudinally by angle Θ 2 , which is at least large enough to accommodate tooling draft and through transverse section may comprise a contracting taper Θ 1  to enhance noise reduction by the emitted jet of air. 
     Although any suitable width of the exhaust apertures may be selected it will preferably be in the range 0.2 to 1 mm at their widest part to mitigate noise of exhausting gas, and provide a total area including all apertures open to the exterior preferably in the range 50 to 200 square mm with embodiments manufactured with aperture areas to suit a particular rate of exhaust flow and hence clinical application. 
     The converging end geometry of the exhaust apertures  30  may optionally comprise a variety of forms as shown in  FIGS. 9 a  to 9 e   . Dramatic contractions towards the ends of the apertures as shown in  FIGS. 9 a  and 9 c    and staggered end geometries shown in  FIGS. 9 d  and 9 e    further accentuate the rate of increase in exhaust aperture area revealed in relation to displacements of the flexible membrane  100 . Contraction and stagger produces small airflows at small displacements of the membrane  100  and proportionally increasing airflows as membrane displacement advances to uncover greater aperture area. This further achieves a cushioning effect as the user transitions from exhalation to inhalation, mitigating the tendency for sudden transitional tensioning of the membrane during early phase exhalation and the corresponding pulsing sensation produced thereby which would otherwise be experienced with uniform aperture end geometry such as shown in  FIG. 9 b   . It will be apparent to those skilled in the art that the end geometries shown here may be used singularly or in combination and that these represent a sample of possible options to achieve the performance goals described. 
     In an alternative embodiment ( FIGS. 19 and 20 ), balanced valve  1   14  features a similar flanged portion  124 , although it includes a flexible membrane  1   16  having a generally cylindrical main body portion  118  and preferably forming a converging taper towards a closed first end  120  at its base, and an open second end  122  at the junction between the flanged portion  124  and main body portion  118  which sealably conforms against the inside of the exhaust port  28  of the valve body  20 . 
     Membrane  100  and  1   16  of balanced valves  98  and  1   14  are preferably manufactured from flexible polyethylene film with thickness less than 50 micrometres and preferably, in the range 2 to 10 micrometres and manufactured by vacuum forming, although other manufacturing techniques may be deployed. This combination of material and thickness emphasizes flexibility over elasticity such that any increase in the effort of breathing caused by membrane stiffness is minimized. This is particularly significant in continuous positive pressure treatment where it is preferred to limit pressure increases on exhalation above source pressure wherein balanced valve  98  or  1   14  in combination with area of exhaust port  28  and individual apertures  30  is designed to limit exhalation pressure swing during breathing to less than 2 cm of water. and preferably to less than 0.5 cm of water. 
     In an alternative embodiment ( FIGS. 12, 13 ), the cap  152  now includes an external peripheral wall  166  which envelopes corresponding wall  172  of valve body  168 . Retention is achieved by engagement of the internal fastening rib  154  of the cap  152  within the corresponding groove  174  of the valve body  168  and a chamfer or ramp  176  may be provided to facilitate smooth engagement thereby. 
     Another embodiment of a respiratory valve apparatus according to the present invention is shown in  FIGS. 21 to 24 . Respiratory valve apparatus  282  delivers a pressurised flow Of breathable gas to the airway of a user, and comprises a rigid valve body  288  which includes an inlet port  290  for continuously receiving breathable gas under pressure from a pressure source or gas flow generator. There is an outlet port  292  which, via a mask, releases the breathable gas to the user&#39;s airway during an inhalation phase and receives exhaled gas during an exhalation phase of the user&#39;s respiratory cycle. A breathable gas flow passage  294  (as shown by the path of the unbroken arrows in  FIG. 23 ) communicates between the inlet port  290  and the outlet port  292 . 
     There is an exhaust port  296  for releasing the exhaled gas to atmosphere. The exhaust port  296  includes a plurality of circumferentially spaced apart exhaust apertures  298 . 
     An exhaled gas flow passage  300  (as shown by the path of the unbroken arrows in  FIG. 24 ) communicates between the outlet port  292  and the exhaust port  296 . 
     A first valve  302  is located in the passage  294  and divides that passage into an upstream portion  306  and a downstream portion  308 . The first valve  302  is a nonreturn valve. Rigid valve body  288  has a valve receptacle  310  comprising a peripheral sealing rim  314 , a mounting bar  316  which vertically and symmetrically bridges passage  294  at the junction of the upstream and downstream portions  306 ,  308 . Mounting bar  316  has in a central position, a keyed mounting hole  318  adapted to engage in a fixed orientation the keyed stem  322  and barb  320  of non-return valve  302 . Non-return valve  302  comprises a flexible flap  312  which is weakly biased to a closed position under ambient pressure and which sealably engages peripheral sealing rim  314  during exhalation and, when in an opened position, is pivotally deflected into two halves about a line or lines aligned with mounting bar  316 . Nonreturn valve  302  may optionally have provided on its rear face a groove or grooves  304  which provide a line or lines of reduced stiffness and correspondingly facilitate more pronounced deflection about these lines. 
     The non-return valve  302  will open under pressure of breathable gas received through the inlet port  290  during the inhalation phase ( FIG. 23 ), and so permit, flow of breathable gas to the user, and will close under pressure of exhaled gas received through the outlet port  292  during the exhalation phase ( FIG. 24 ), despite the maintenance of a pressurised flow of breathable gas through the inlet port  290  during the exhalation phase. 
     By the closing of the non-return valve  302 , the exhaled gas received through the outlet port  292  is prevented from exiting through the inlet port  290 , but flows through the exhaled gas flow passage  300  ( FIG. 24 ). A second valve  328  is located in the exhaled gas flow passage  300  and is a balanced pressure valve. Balanced pressure valve  328  is similar in function to balanced pressure valve  98  ( FIG. 17, 18 ) and similarly comprises a flexible membrane  284  which has a sock-like structure defining an internal cavity  286 , and has a generally cylindrical main body portion  324 , the outer surface of which corresponds to the inner wall of the valve body against which it seals circumferentially, a closed first end  326  which is preferably hemispherical and an open second end  330 , whereby operation of the valve controls the opening and closing of the exhaust apertures  298  that are circumferentially spaced apart along a wall of the valve body. The flexible membrane  284  is weakly biased to an expanded position where it closes exhaust apertures  298  under ambient pressure and under pressure of breathable gas received through the inlet port  290  during the inhalation phase ( FIG. 23 ). The flexible membrane  284  will, under pressure of exhaled gas flowing through the exhaled gas flow passage  300  during the exhalation phase, flexibly deform to a collapsed position so as to open the exhaust apertures  298 , whereby the exhaled gas is released to atmosphere ( FIG. 24 ). Balanced pressure valve  328  is preferably manufactured from a moldable elastomer such as liquid silicone rubber with thickness typically less than 100 micrometres, although greater thickness may be used depending on the application. 
     The flexible membrane  284  has a retaining flange  332  by which it is fitted circumferentially against the wall of the exhaust port  296  of the valve body  288 . The retaining flange  332  has an outermost downward lip  334  which engages around uppermost shoulder segments  336  of a collar portion  338  of the exhaust port  296 . The retaining flange  332  also has a lowermost groove  318  which engages around an inner ridge  340  of the collar portion  338 . A retaining cap  342  is engaged around the retaining flange  332 , such that the retaining flange  332  is sandwiched between the upper retaining cap  342  and the lower collar portion  338 . The retaining cap  342  is optionally aligned with the collar portion  338  by notches  344  formed in the side wall  348  of the cap  342  which engage optional protrusions  350  formed on the side wall  352  of the collar portion  338 . The side wall of the retaining flange  332  has one or more bias pressure  10  flow holes  356 , and the side wall  348  of the retaining cap  342  has locking slots  358  for receiving there through respective shoulder segments  336  of the collar portion  338  when the retaining flange  332  is sandwiched between the upper retaining cap  342  and the lower collar portion  338  of the exhaust port  296 . Retaining cap  342  may optionally have a locating boss  346  which when assembled, projects downwards into the open second end  330  of balanced pressure valve  328  and thereby effecting more positive engagement of the valve  328 . 
     The respiratory valve apparatus  282  also includes breathable gas equilibrium passage  360  (as shown by the path of the broken arrows in  FIGS. 23 and 24 ) defined by a bias pressure tube  362 , which is in gas flow communication between the inlet port  290  and the exhaust port  296 . The bias pressure tube  362  is formed integrally with the valve body  288 , being internal of the body at the inlet port  290  but external of the exhaust port  296 . 
     To accommodate the flexible flap  312  of the non-return valve  302 ′ within the partly internally obstructed inlet port  290 , flexible flap  312  has a cut-out portion  364  in the outer shape of the bias pressure tube  362  so as to maintain a generally air-tight barrier between the upstream and downstream portions  306 ,  308  of the passage  294  when the non-return valve is in a closed position. 
     The end opening of the bias pressure tube  362  in the exhaust port  296  opens out into a circular passage  366  between the side wall  352  of the collar portion  338  and the side wall  354  of the retaining flange  332 , which is a sealed annular space except for the bias pressure flow holes  356  in the side wall  354  leading to the internal cavity  286  defined by the sock-like structure of the flexible membrane  284 . 
     During the inhalation phase, when a pressurised flow of breathable gas is delivered into the valve body  288  through the inlet port  290 , a volume of breathable gas is diverted into. and is maintained within, the breathable gas equilibrium passage  360 , and hence within the internal cavity  286  of the flexible membrane  284 , at an equilibrium pressure sufficient to maintain the flexible membrane  284  in an expanded position where it closes the exhaust apertures  298 , despite a larger volume of breathable gas flowing through the breathable gas flow passage  294 . 
     During the exhalation phase, when the non-return valve  302  is forced to close by the greater pressure of the exhaled gas within the downstream portion  308  of the passage  294  than the pressure of the breathable gas entering the inlet port  290 , the pressure of exhaled gas within the exhaled gas flow passage  300  is sufficiently greater than the equilibrium pressure of the breathable gas maintained within the internal cavity  286  of the flexible membrane  284  to cause the flexible membrane  284  to flexibly deform to a collapsed position and thereby open the exhaust apertures  298  so as to permit release of the exhaled gas to atmosphere.  FIG. 22  shows a side view of respiratory valve apparatus  282  indicating that alternate configurations are possible where the angular orientation Θ of the exhaust port  296  to the horizontal may vary, preferably between 90° and 180°. It will be apparent to those skilled in the art that the potential for corresponding angular variations between the inlet port and exhaust port may be applicable to prior and subsequent embodiments in this application. 
     It can be appreciated that while exhaust apertures  298  in the respiratory valve apparatus of  FIGS. 21 to 24  are shown circumferentially around exhaust port  296  they may also be configured as depicted in the first aspect of the invention, namely they may embody longitudinal slots as shown in  FIGS. 9, 9   a - e  and as previously described. 
     Another embodiment of a respiratory valve apparatus according to the present invention is shown in  FIGS. 25 to 28 . 
     The respiratory valve apparatus  368  delivers a pressurised flow of breathable gas to the airway of the user, and comprises a rigid valve body  370  which includes an inlet port  372  for continuously receiving breathable gas under pressure from the gas flow generator  14  or other ventilator device. There is an outlet port  374  which, via the mask  12 , releases the breathable gas to the user&#39;s airway during an inhalation phase and receives exhaled gas during an exhalation phase of the user&#39;s respiratory cycle. 
     A breathable gas flow passage  376  (as shown by the path of the unbroken arrows in  FIG. 27 ) communicates between the inlet port  372  and the outlet port  374 . 
     There is an exhaust port  378  for releasing the exhaled gas to atmosphere. The exhaust port  378  includes a plurality of circumferentially spaced apart exhaust apertures  380 . An exhaled gas flow passage  382  (as shown by the path of the unbroken arrows in  FIG. 28 ) communicates between the outlet port  374  and the exhaust port  378 . 
     A first valve  386  is located in the passage  376  and divides that passage into an upstream portion  388  and a downstream portion  392 . In this embodiment, the first valve  386  is a non-return or one-way valve. The non-return valve  386  comprises a flexible flap  394  which is weakly biased to a closed position under ambient pressure ( FIG. 26 ) and, when in an opened position, is deflected into two halves about a central line defined by a mounting bar  396 . The non-return valve  386  is retained in a receptacle  384  comprising a peripheral sealing rim  390 , a mounting bar  396  which symmetrically bridges the passage  376  at the junction of the upstream and downstream portions  388 ,  392 , and a central mounting hole  400 . Retention of the valve  386  in the receptacle  384  is accomplished by insertion of the barb  398  into the central mounting hole  400 . In a closed position the flexible membrane  394  of valve  386  sealably engages the peripheral sealing rim  390  of the receptacle  384 . 
     It will be appreciated by skilled persons in the art that the non-return valve  386  may take alternate structural forms that are all weakly biased to a closed position. 
     The non-return valve  386  will open under pressure of breathable gas received through the inlet port  372  during the inhalation phase ( FIG. 27 ), and so permit flow of breathable gas to the user, and will close under pressure of exhaled gas received through the outlet port  374  during the exhalation phase ( FIG. 28 ), despite the maintenance of a pressurised flow of breathable gas through the inlet port  372  during the exhalation phase. 
     By the closing of the non-return valve  386 , the exhaled gas received through the outlet port  374  is prevented from exiting through the inlet port  372 , but flows through the exhaled gas flow passage  382 . 
     A second valve  404  is located in the passage  382  and, in this embodiment, is a balanced pressure valve. The balanced pressure valve  404  comprises a flexible membrane  408  which is weakly biased to an expanded position where it closes the exhaust apertures  380  under ambient pressure ( FIG. 26 ) and under pressure of breathable gas received through the inlet port  372  during the inhalation phase ( FIG. 27 ) and a flange portion  430  which is sealably retained between the mounting shoulder  402  of the exhaust port  378  and the flanged portion  424  of the barbed end cap  428 . The flexible membrane  408  will, under pressure of exhaled gas flowing through the exhaled gas flow passage  382  during the exhalation phase ( FIG. 28 ), flexibly deform to a collapsed position so as to open the exhaust apertures  380 , whereby the exhaled gas is released to atmosphere. 
     In this embodiment of the balanced pressure valve, and in the embodiments shown in  FIGS. 29 to 32  and  FIG. 35 , the flexible membrane  408  (as specifically shown in  FIG. 29 ) has a sock-like structure defining an internal cavity  412 , and comprises a main body portion  414 , the outer surface of which corresponds to the preferably generally cylindrical inner wall of the valve body against which it seals circumferentially, a closed first end  41   6  which is preferably hemispherical and an open second end  420 , whereby operation of the valve controls the opening and closing of the exhaust apertures  380  that are circumferentially spaced apart along a wall of the exhaust port  378  of the valve body. It is further apparent that flexible membrane  408  may also be tapered in shape as shown in  FIGS. 29 and 31  with the membrane becoming progressively smaller in circumference from the open end  420  to the closed end  416 . Alternatively the sock-like structure is tapered in shape with the flexible membrane becoming progressively larger in circumference from its open end to its closed end, as suggested at  315  in  FIG. 31 . Flexible membrane  408  of balanced pressure valve  404  is similar in structure and function to flexible membrane  284  of balanced pressure valve  328  ( FIGS. 21, 23 and 24 ). Balanced pressure valve  404  is likewise preferably molded from an elastomer such as silicone. 
     Another embodiment of the flexible membrane used in the balanced pressure valve is shown in  FIG. 30 . Balanced pressure valve  406  is similar in structure and function to balanced pressure valve  404 , and like features which have like structure and function are identified with like numbers. Balanced pressure valve  406  differs from balanced pressure valve  404  in that it has a closed first end  418  which is more planar than hemispherical. 
     Another embodiment of the flexible membrane used in the balanced pressure valve is shown in  FIG. 31 . Balanced pressure valve  434  is similar in structure and function to balanced pressure valve  404 , and like features which have like structure and function are identified with like numbers. Balanced pressure valve  434  differs from balanced pressure valve  404  in that it has an inner lining  432  or layer formed from a viscoelastic material or other material whose mechanical properties will dampen membrane vibration or minimise fluttering instabilities in the motion of the flexible membrane, without interfering with the operation of the flexible membrane in response to varying pressures. The flexible membranes of valves  404  and  434  are each connected to a flange portion  430  which is used for securing each flexible membrane in its exhaust port  378 . 
       FIG. 32  shows a balanced pressure valve  436  and a further embodiment of the flexible membrane  438  including annular ribs  440  located on the internal cavity of the w membrane, which selectively stiffen the membrane against vibratory deflection away from the surface surrounding the exhaust apertures against which the membrane closes. The ribs add hoop stiffness to the membrane whilst maintaining sufficient flexibility to allow progressive or sequential deflection. In a further embodiment, annular ribs  440  could be replaced by lumps similarly located on the internal cavity, of greater thickness than the flexible membrane  438 , and s thereby having greater mass and correspondingly enhancing damping by virtue of increased inertial properties. 
     It will be appreciated by persons skilled in the art that the balanced pressure valve  404  may take many alternate structural forms or any combination of the features previously described. Referring back to  FIGS. 25 to 28 , the respiratory valve apparatus  368  also includes breathable gas equilibrium passage  442  defined by a bias pressure tube  444 , which is in gas flow communication between the upstream portion  388  of passage  376  and the exhaust port  378 . The bias pressure tube  444  is attached to the valve body  370  by tubular barbs  446 ,  448  penetrating the upstream and downstream ends of the bias tube  444 . Barb  446  extends upwardly from the upstream portion  388  of passage  376 , and barb  448  extends centrally from the end cap  428 . 
     During the inhalation phase ( FIG. 27 ), when a pressurised flow of breathable gas is delivered into the valve body  370  through the inlet port  372 , a volume of breathable gas is diverted into, and is maintained within, the breathable gas equilibrium passage  442 , and hence within the internal cavity  412  of the flexible membrane  408 , at an equilibrium pressure sufficient to maintain the flexible membrane  408  in an expanded position where it closes the exhaust apertures  28 , despite a larger volume of breathable gas flowing through the breathable gas flow passage  376 . 
     During the exhalation phase ( FIG. 28 ), when non-return valve  386  is forced to close by the greater pressure of exhaled gas within downstream portion  392  of passage  376  than the pressure of the breathable gas entering inlet port  372 , the pressure of exhaled gas within exhaled gas flow passage  382  is sufficiently greater than the equilibrium pressure of the breathable gas maintained within internal cavity  412  of flexible membrane  408  to cause the flexible membrane  408  to flexibly deform to a collapsed position and thereby open the exhaust apertures  380  so as to permit release of the exhaled gas to atmosphere. 
     Bias pressure tube  444  may also have a constriction  452  as shown in  FIG. 25 a    within its internal passage or have its overall internal diameter selected so as to pneumatically dampen the response of membrane  408  during inhalation and exhalation phases. It can be appreciated that the restriction may be integral to any of the components defining the breathable Ras equilibrium passage, or alternatively an additional element added specifically for this purpose. 
       FIGS. 33 to 35  show alternate embodiments of a barbed end cap that may be used in the respiratory valve apparatus of the present invention. End cap  428  (used in the apparatus of  FIGS. 25 to 28 ) has a circumferential flange  424  in the form of two external steps and end cap  426  has a circumferential flange  422  in the form of one external step. The additional external step of the flange  424  in the end cap  428  prevents inward distortion of the flange  430  of the balanced pressure valve  404 , by engaging. the internal portion of the flange  430  against corresponding sides of the flange  424 . The one external step of the flange  422  in the end cap  426  will, in contrast, require either adhesive bonding or frictional engagement with the flange  430  of the flexible membrane  408  to prevent such inward distortion, because end cap  426  has no features to otherwise impede inwards movement of flange  430 . 
     It can be appreciated that while exhaust apertures  380  in the respiratory valve apparatus of  FIGS. 25 to 28  are shown circumferentially around exhaust port  378  they may also be configured as depicted in earlier embodiments of first aspect of the invention, for example they may embody longitudinal slots as shown in  FIGS. 9, 9   a - e  as previously described. 
     A second aspect of the respiratory valve apparatus according to the present invention is shown in  FIGS. 36 to 39 . The respiratory valve apparatus  454  utilises a sliding piston  456  in place of the flexible membrane  408  (of the apparatus of  FIG. 25 ) in its balanced pressure valve. 
     The apparatus  454  comprises a rigid valve body  458  which includes an inlet port  460  for continuously receiving the breathable gas, and an outlet port  462  for releasing the breathable gas to the user during an inhalation phase, and for receiving exhaled gas during an exhalation phase of the user&#39;s respiratory cycle. 
     A breathable gas flow passage  464  (as shown by the path of the unbroken arrows in  FIG. 37 ) communicates between the inlet port  460  and the outlet port  462 . 
     There is an exhaust port  466  for releasing the exhaled gas to atmosphere. The exhaust port  466  includes at least one circumferential exhaust aperture  468 . An exhaled gas flow passage  470  (as shown by the path of the unbroken arrows in  FIGS. 38 and 39 ) communicates between the outlet port  462  and the exhaust port  466 . 
     A first valve  472  is located in the passage  464  and divides that passage into an upstream portion  476  and a downstream portion  480 . In this embodiment, the first valve  472  is a similar non-return valve to that used in the apparatus of  FIG. 25 . Like features between the valves  386  and  472  are identified by like numbers. The earlier description of the structure and function of the valve  386  also applies to the structure and function of the valve  472 . 
     The non-return valve  472  is retained in a receptacle  474 . In this embodiment, the receptacle  474  is similar to receptacle  384  used in the apparatus of  FIG. 25 . Like features between the receptacles  384  and  474  are identified by like numbers. The earlier description of the structure and function of the receptacle  384  also applies to the structure. and function of the receptacle  474 . Additionally, receptacle  474  is presented in this embodiment as a separate attached unit, rather than the integral receptacle  384  of  FIGS. 25 to 28  although it will be appreciated that similar integral configurations are possible. 
     As described above, the non-return valve  472  will open wider pressure of breathable gas received through the inlet port  460  during the inhalation phase ( FIG. 37 ), and so permit flow of breathable gas to the user, and will close under pressure of exhaled gas received through the outlet port  462  during the exhalation phase ( FIGS. 38 and 39 ), despite the maintenance of a pressurised flow of breathable gas through the inlet port  460  during the exhalation phase. 
     By the closing of the non-return valve  472 , the exhaled gas received through the outlet port  462  is prevented from exiting through the inlet port  460 , but flows through the exhaled gas flow passage  470 . 
     A second valve  456  is located in the passage  470  and, in this embodiment, is a sliding piston and balanced pressure valve. The sliding piston  456  comprises a cylindrical body  494  having a first open end  490  and a proximal annular end lip  496  and a second closed end  492  and proximal annular lip  500 . 
     The sliding piston  456  is weakly biased to a retracted position where it closes the exhaust port  466  or each exhaust. aperture  468  under ambient pressure and under pressure of breathable gas received through the inlet port  460  during the inhalation phase ( FIG. 37 ). During the inhalation phase, the sliding piston  456  is maintained in its retracted position by the pressure applied and by annular end lip  496  which abuts against shoulder  486 . 
     Under the pressure of exhaled gas flowing through the exhaled gas flow passage  470  during the exhalation phase ( FIGS. 38 and 39 ), non-return valve  472  closes sealably against its receptacle  474 , and the sliding piston  456  will move to an extended position so as to open the exhaust port  466  or each exhaust aperture  468 , whereby the exhaled gas is released to atmosphere. In its extended position, the sliding piston  456  is located circumferentially within the inner wall of the exhaust port  466  of the valve body  458  and is retained therein by an end cap  484  having a central aperture  488 . There is a breathable gas equilibrium passage  482  defined by a bias pressure tube  478 , which is in gas flow communication between the upstream portion  476  of passage  464  and the exhaust port  466 . The bias pressure tube  478  is attached at its respective ends to the valve body  458  by engaging through apertures  488  and  498 . During the inhalation phase, when a pressurised flow of breathable gas is delivered into the valve body  458  through the inlet port  460 , a volume of breathable gas is diverted into, and is maintained within, the breathable gas equilibrium passage  482  at an equilibrium pressure sufficient to maintain the sliding piston  456  in a retracted position where it closes the exhaust aperture  468 , despite a larger volume of breathable gas flowing through the breathable gas flow passage  464 . 
     During the exhalation phase, when the non-return valve  472  is forced to close by the greater pressure of the exhaled gas within the downstream portion  480  of the passage  464  than the pressure of the breathable gas entering the inlet port  460 , the pressure of exhaled gas within the exhaled gas flow passage  470  is sufficiently greater than the equilibrium pressure of the breathable gas maintained within the breathable gas equilibrium passage  482  to cause the sliding piston  456  to move to an extended position where it extends away from the outlet port.  462 , and thereby open the exhaust aperture  468  so as to permit release of exhaled gas to atmosphere. 
     It can be appreciated that while exhaust apertures  468  in the current aspect are shown circumferentially around exhaust port  466  they may also be configured as depicted in the first aspect of the invention, namely they may embody longitudinal slots as shown in  FIGS. 9, 9   a - e  and as previously described. 
     A third aspect of the respiratory valve apparatus according to the present invention is shown in  FIGS. 40 to 43 . The respiratory valve apparatus  502  integrates both user interface and respiratory valve functions. 
     The respiratory valve apparatus  502  delivers a pressurised flow of breathable gas to the airway of the user, and comprises rigid mask and valve body left half  510  and right half  512  which are joinable and includes an inlet port  524  for continuously receiving breathable gas under pressure from the gas flow generator  14  or other ventilator device. There is an outlet port  526  which via the nasal cushion and seal interface  508 , releases the breathable gas to the user&#39;s airway during an inhalation phase and receives exhaled gas during an exhalation phase of the user&#39;s respiratory cycle. 
     A breathable gas flow passage  516  (as shown by the path of unbroken arrows in  FIG. 42 ) communicates between the inlet port  524  and the outlet port  526 . Additional elements such as the swivel connector  514  may extend this passage. 
     There is an exhaust port  528  for releasing the exhaled gas to atmosphere. Exhaust port  528  includes a plurality of circumferentially spaced exhaust apertures  522 . 
     An exhaled gas flow passage  518  (as shown by the path of unbroken arrows in  FIG. 43 ) communicates between the outlet port  526  and exhaust port  528 . 
     A first valve  504  is located in the breathable gas flow passage  516  and divides that passage into an upstream portion  530  and a downstream portion  532 . The first valve  504  is in this embodiment a non-return valve of similar operation to valve  386  ( FIG. 25 ). During an inhalation phase ( FIG. 42 ) non-return valve  504  will open under pressure of breathable gas received through the inlet swivel connector  514  and then through inlet port  524  to permit a flow of breathable gas to the user, and will close under pressure of exhaled gas received through the outlet port  526  during the exhalation phase ( FIG. 43 ), despite the maintenance of a pressurized flow of breathable gas through the inlet port  524  during the exhalation phase. 
     By the closing of the non-return valve  504 , the exhaled gas received through outlet port  526  is prevented from exiting through inlet port  524 , but flows through the exhaust gas flow passage  518  ( FIG. 43 ). 
     A second valve  506  is located in the passage  518  and, in this embodiment, is a balanced pressure valve of similar structure and function to valve  404  ( FIG. 25 ). The balanced pressure valve  506  closes the exhaust apertures  522  of exhaust port  528  under ambient pressure ( FIG. 41 ) and under pressure of breathable gas received through inlet port  524  during the inhalation phase ( FIG. 42 ). During an exhalation phase ( FIG. 43 ), the pressure of exhaled gas flowing through the exhaled gas flow passage  518  will exceed that of the breathable gas equilibrium passage  520 , causing the balanced pressure valve  506  to open the exhaust apertures  522 , thereby allowing the release of exhaled gas to atmosphere. 
     Nasal cushion and seal interface  508  is preferably manufactured from either a solid elastomer or a foam with either closed cell structure and alternatively an outer skin, or a foam with open cell structure and an outer skin. Mask and valve body halves  510  and  512  are preferably manufactured by either vacuum forming sheet plastic or injection moulding and bonding there-between preferably achieved by either ultrasonic welding, heat staking, adhesive or the application of fasteners. Fastening of the user interface  508  and valves  504  and  506  to mask body halves  510  and  512  is preferably achieved by adhesive, however alternatives, such as over-moulding, or the use of fasteners may also be applicable. 
     It will be appreciated that alternative user interfaces such as full face, oronasal or nasal prong could be similarly adapted to integrate respiratory valve apparatus. Similarly, it will be appreciated that alternative non-symmetrical structural configurations could also be adopted. 
     It can further be appreciated that while exhaust apertures  522  in the current aspect are shown circumferentially around exhaust port  528  they may also be configured as depicted in the first aspect of the invention, namely they may embody longitudinal slots as shown in  FIGS. 9, 9   a - e  and as previously described. It will be appreciated by persons skilled in the art that one advantage of the present invention in its preferred embodiments is that, in the absence of an unintentional leak from the user interface, it allows only the tidal volume of exhaled gas to be vented to atmosphere preserving administered gases, humidity and pharmacologic agents. This may also facilitate efficiencies in design and construction of accessory devices such as humidifiers and flow generators. 
     A still further advantage of the present invention in its preferred embodiments is that it provides substantial separation of breathable gas and exhaled gas, such that a user will not to any significant extent rebreathe exhaled gas during the full range of breathing rates and tidal volumes thereby improving the safety and efficacy of therapy. It can be appreciated that substantially eliminating accumulation of expired gas within the pressure source, by directing all tidal volume to atmosphere as described by the present invention facilitates more dynamic pressure delivery strategies to a user without increasing risk of rebreathing expired gas. For example, delivered pressure during a breathing cycle may be lower than required in the prior art, regardless of the rate and depth of breathing, to obviate rebreathing. A further example is as described in U.S. Patent Application 2009/0095297, wherein pressure is dropped during user cycled exhalation such that tidal volume is vented to atmosphere under controlled elastic recoil and immediately or soon thereafter before a user triggered inspiratory effort, pressure from the pressure source is returned to the pre-exhalation level, whereupon the cycle is repeated. Such a pressure delivery profile is facilitated by the present invention in its preferred embodiments, whereas the prior art will present significant risk of rebreathing exhaled tidal volume under these circumstances. In the case of the prior art a fraction of expired tidal volume is stored temporarily within the pressure source, in particular the pressure delivery tube. There will be insufficient time for the exhaled tidal volume so stored to be flushed to atmosphere before the pressure is automatically increased to re-inflate a user&#39;s respiratory system. On reintroduction of pressure an unacceptable proportion of expired tidal volume may be reintroduced into a user&#39;s airway and respiratory system. It can be appreciated that under these operating circumstances breathable gas from the pressure source should retain little or no expired tidal volume during lung emptying as occurs with the present invention. 
     It can also be appreciated that the invention in its preferred embodiments is able to maintain any delivered pressure level from the pressure source within a user&#39;s airway and that first and second valve means as described will be fully closed in absence of unintentional leaks when no user breathing effort is present, that is when there is no ingress or egress of tidal volume to or from a user&#39;s airway. For example, when the invention in its preferred embodiments is used with a constant pressure source over a. breathing cycle, that is CPAP, then a single pressure will be effectively maintained Within a user&#39;s airway, subject to any associated pressure fluctuations or swings associated with a user&#39;s inspiratory and expiratory efforts. However, there will be no flow in the inlet or outlet ports when pressures from the source and within a user&#39;s airway are equal. If however the pressure from the source is reduced during a breathing cycle from an upper pressure as may be the case during bi-level therapy, then lung volume will be elastically reduced and the volume of displaced air will be expelled and flow through the outlet port of the invention. Once pressures within a user&#39;s airways and lungs have equilibrated with the pressure source, this new pressure will again be maintained in the user&#39;s airway until another pressure is established by the pressure source. For example, if the pressure were to be then increased by the pressure source, breathable gas would correspondingly flow from the inlet port into the user&#39;s airway thereby reinflating the lungs and establishing and maintaining a new upper pressure level. A further safety advantage of the present invention in its preferred embodiments apparent from the preceding descriptions is that it may also function as a non-rebreathing valve (i.e. in an anti-asphyxia device) if the pressure source fails to generate sufficient flow to provide adequate ventilation to a user. This may occur for example during power, electrical or mechanical failure. Under such circumstances, during exhalation the non-return valve will remain closed and air will be directed to atmosphere through the exhaust apertures as the flexible membrane of the balanced pressure valve is deflected to the open position by exhaled flow. On inspiration, the balanced pressure valve will remain open, since no positive bias pressure is available from the pressure source. In normal operation, negative pressure during inhalation within the user interface will be low enough not to cause the flexible membrane of the balanced pressure valve to close the exhaust apertures and atmospheric air will be inhaled through those apertures. Alternatively, should the flexible membrane reinflate due to sufficiently negative pressure in proximity to the exhaust apertures on strong inhalation, breathable gas can also passage unidirectionally through the non-return valve allowing the user to draw unpressurised breathable gas from the pressure source providing it is of a fan, impellor or other open type. 
     A still further advantage of the present invention in its preferred embodiments is that exhaled gas from a user is vented to atmosphere at a lower volumetric rate of flow relative to the prior art when using continuous venting of source pressurised gas. In lowering the flow rate of exhaled gas, the present invention in its preferred embodiments minimizes the dispersion of infectious particles along with the risk of cross-infection. The invention in its preferred embodiments provides a number of benefits over continuous venting of source pressurised gas as described in the prior art. These benefits include:
         Reduced carbon dioxide rebreathing as source pressure decreases or breathing rate and depth increases providing improved therapy safety and efficacy   Provides more efficient use of breathable gas from a source of pressurised gas   Only tidal volume of exhaled gas is vented to atmosphere preserving administered gases, humidity or pharmacologic agents   May reduce transmission of exhaled infectious particles with the exhaled gas stream   Minimises flow of pressurized gas onto adjacent bed partner when used in the home care setting   Provides improved safety in case of power or general failure of source of pressurized breathable gas without need for additional non rebreathing valves       

     When used with positive pressure sealing interface means such as endotracheal tube or tracheostomy the invention in its preferred embodiments is able to provide exhalation of tidal volume without need for source controlled exhalation valves reducing complexity and reliability of treatment with minimal noise from exhaled gas flow. 
     It will be readily apparent to persons skilled in the art that various modifications may be made in details of design, construction and operation of the respiratory valve apparatus described above without departing from the scope or ambit of the present invention.