Abstract:
A flow regulation vent for use in controlling a vent flow of washout gas in a system for supplying breathable gas pressurized to within a therapeutic pressure range above atmospheric pressure to a patient in the treatment of a sleep disordered breathing condition is disclosed. The flow regulation vent may include a housing having at least one vent orifice and at least one fixed bleed orifice; and a flap configured to regulate the vent flow of washout gas through the at least one vent orifice, wherein the at least one fixed bleed orifice is not covered by the flap in any position and the flow regulation vent provides a minimum vent flow independent of the position of the flap.

Description:
This application is a continuation of U.S. patent application Ser. No. 15/001,521, filed Jan. 20, 2016, pending, which is a continuation of U.S. patent application Ser. No. 13/891,237, filed May 10, 2013, now U.S. Pat. No. 9,278,186, which is a continuation of U.S. patent application Ser. No. 10/433,980, filed Jun. 10, 2003, now U.S. Pat. No. 8,439,035, which is the U.S. national phase of International Application No. PCT/AU01/01658, filed Dec. 21, 2001, and claims priority to U.S. Provisional Application No. 60/257,171, filed Dec. 22, 2000, the entire contents of each of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a vent valve apparatus for use with a system for supplying breathable gas pressurized above atmospheric pressure to a human. 
     The invention has been developed primarily for use in controlling the venting of washout gas in a continuous positive airway pressure (CPAP) gas delivery systems used, for example, in the treatment of obstructive sleep apnea (OSA) and similar sleep disordered breathing conditions. The invention may also be used in conjunction with suitable mask and gas delivery systems for the application of assisted ventilation treatment. 
     The term “mask” is herein intended to include face masks, nose masks, mouth masks, appendages in the vicinity of any of these masks and the like. 
     BACKGROUND OF THE INVENTION 
     Treatment of OSA by CPAP gas delivery systems involves the continuous delivery of air (or breathable gas) pressurized above atmospheric pressure to a patient&#39;s airways via a conduit and a mask. CPAP pressures of 4 cm H 2 0 to 30 cm H 2 0 are typically used for treatment of sleep disordered breathing due to OSA and/or central apnea, depending on patient requirements. 
     Treatment pressures for assisted ventilation can range up to 32 cm H 2 0 and beyond, depending on patient requirements. 
     For either the treatment of OSA or the application of assisted ventilation, the pressure of the gas delivered to patients can be constant level, bi-level (in synchronism with patient inspiration and expiration) or automatically adjusting in level. Throughout this specification the reference to CPAP is intended to incorporate a reference to any one of, or combinations of, these forms of pressure delivery. The prior art method for providing CPAP treatment includes a vent for gas washout of the gas flow. The vent is normally located at or near the mask or in the gas delivery conduit. The flow of gas through the vent is essential for removal of exhaled gases from the breathing circuit. Adequate gas washout is achieved by selecting a vent size and configuration that will allow a minimum safe gas flow at the lowest operating CPAP pressure, which typically can be as low as, around 4 cm H 2 0 for adults and 2 cm H 2 0 in pediatric applications. 
     Existing vent configurations include single or multiple holes, foam or other diffusers, slots and combinations thereof. A reference herein to a vent may be understood to include a reference to one or more holes, foam or other diffusers, slots or any combination of them. 
     It is obviously desirable for a CPAP system to have as wide a pressure range as is feasible in order that a standard configuration may adequately provide the unique treatment require by a variety of users. Increasing CPAP pressure results in more gas passing through the vent which in turn creates more noise. Existing prior art vents can produce excessive noise when CPAP pressures are raised above about 4 cm H 2 0. This noise can adversely affect patient and bed-partner comfort. At higher pressures, existing vents are also inefficient as they allow more gas through the vent than is required for adequate exhaust gas washout and thereby require the flow generator to provide more flow than is necessary in order to maintain the required treatment pressure. Further, where treatment gas is being supplied, such as oxygen, surplus treatment gas is vented and thereby wasted unnecessarily. A similar waste occurs where the supplied gas is humidified. 
     The flow of gas from the gas delivery system through the vent to atmosphere creates noise as the delivered gas, and upon expiration the patient expired gas including C0 2 , passes through the vent to atmosphere. A CPAP system must have a rate of flow through the vent to atmosphere that ensures a clinically undesirable level of expired gas is not retained within the breathing circuit (i.e. within the gas supply conduit and mask chamber). This retention occurs as a result of the exhaled gas not being vented to atmosphere during the exhalation phase of respiration but rather moving down the gas conduit towards the flow generator or accumulating within the mask chamber dead space. An adequate flow of gas to atmosphere may be achieved by selecting the suitable vent size for the clinically desirable pressure treatment range and volume of gas made available by the flow generator to achieve the desired treatment pressure range. Typically this selection involves a compromise being struck between the choice of a vent size that is sufficiently large to achieve an adequate flow rate at the low end of the pressure range and yet cause no greater than an acceptable noise level as the pressure increases through the pressure range. In addition, a large vent which would allow for a generous wash out flow rate at the low end of the pressure range will dictate that the flow generator must have adequate capacity to provide the flow necessary to achieve the desired pressures higher in the pressure range. In short, where the vent size is chosen to deliver a quiet gas wash out flow rate at the higher pressure levels of the pressure range it may be inadequate to allow acceptable wash out flow at the desired lowest end of the pressure range. Also a vent with sufficient size to achieve an adequate wash out flow rate at pressures low in the pressure range tend to generate unacceptable noise at the desired higher end of the pressure range. In addition the choice of a larger vent dictates that the source of gas have capacity to deliver the requisite flow rates for the higher pressure levels and as such the gas source will tend to consume more power and generate louder noise and require additional noise attenuating features so as to keep the total noise within acceptable limits. 
     Because of the constraints on CPAP system design arising from the vent a choice may be made to limit the lower or upper achievable pressure i.e. for a given upper or lower pressure the delta P between that pressure and the other extreme of the range may be inconveniently constrained. The delta P would be chosen so as to achieve the desired aims of adequate wash out of exhaled gas at the lowest end of the pressure range while capping the noise generated and power consumed at the higher end of the pressure range. Such limitations on the choice of upper or lower pressures and the delta P can seriously confine the usefulness of CPAP system as it is desirable for a standard configuration to have the capacity to deliver the widest pressure range so as to be capable of meeting the clinical requirement of as many users as possible. Achievement of this aim is particularly significant where the CPAP treatment involves the operation of a control algorithm that varies the pressure delivered to the user during the period of treatment (for example on a breath-by-breath basis between two or more pressures or in a more complex manner during the period of treatment). Similarly a computer controlled CPAP system that varies the pressure during the period of treatment in accordance with a control algorithm will include operating parameters which reflect the vent characteristic of the breathing circuit. Because of this it can be undesirable to change from a mask specified for the control algorithm for concern that the new mask should introduce a vent characteristic which is not within the operating parameters of the control algorithm. This inability to change masks because of the accompanying introduction of unknown or incompatible vent characteristics can be adverse to patient compliance with CPAP treatment. This is because a patient may only tolerate CPAP treatment where it is delivered through a particular mask and that mask is incompatible with the prescribed CPAP system control algorithm. Accordingly another aim of the present invention is to provide for a method of configuring and making a vent which can change the vent characteristic of a mask so that the mask may better comply with the operating parameters of a CPAP system control algorithm. 
     A further aim of the present invention is a method and apparatus for a system of venting which creates a vent having a flow area which varies with changes in pressure occurring at part of or the whole of a CPAP system pressure operating range. 
     It is known in the art for a CPAP system breathing circuits to include valves that restrict or block venting to atmosphere in given circumstances. 
     U.S. Pat. No. 5,685,296 to Zdrojkowski discloses a Flow Regulating Valve and Method. In the first embodiment, a rigid insert 52 having a central axial opening 54 is connected to a resilient diaphragm 42. As gas supply pressure increases, the diaphragm 42 flexes toward valve body member 38 and opening 54 moves over a body portion 70 of regulating pin 62, thereby decreasing the flow area between opening 54 and regulating pin 62 and maintaining a relatively constant gas flow rate even at the higher gas pressure. In additional embodiments, gas supply pressure is used to move flexible diaphragms 42′ and 42″ toward respective valve body walls, thereby decreasing the gas flow areas between the respective diaphragms and the valve body walls and preventing higher gas flows at higher gas pressures. 
     U.S. Pat. No. 6,006,748 to Hollis discloses a Vent Valve Apparatus which is adapted to progressively restrict a flow area of a washout vent as the pressure of the gas supply increases. In two embodiments disclosed therein, a flexible diaphragm 20 sensitive to the pressure of the gas supply is connected by a rigid wire rod 23 to a conical plug 18 positioned in a conical orifice 15. As the pressure of the gas supply increases, the diaphragm 20 bulges outward. This moves the rod 23 and conical plug 18 such that the conical plug 18 is drawn into the orifice 15, thereby decreasing the flow area of the vent between the plug 18 and orifice 15 and restricting the flow of gasses through the vent. In a third embodiment, an aerodynamic wing 30 replaces the diaphragm 20 and moves the conical plug in relation to gas flow past the aerodynamic surfaces of the wing. 
     While each of these references discloses embodiments that restrict gas flow as the pressure of the gas supply increases, there is a desire to provide a flow regulation vent that is simpler and cheaper to manufacture while providing the opportunity to have the flow through the vent vary as the pressure varies in a manner that is not limited to achieving a constant flow rate. 
     These valves are generally known as non-rebreath or anti-asphyxia valves. An example of a non-rebreath valve is U.S. Pat. No. 5,438,981 to Starr et al. for an Automatic Safety Valve And Diffuser For Nasal And/Or Oral Gas Delivery Mask which includes a valve element 32 that can pivot between a first position and a second position to allow inflow into a mask from either a gas flow generator or the atmosphere. The safety valve does not restrict gas flow as the pressure of the gas supply increases. 
     Other examples of safety valves can be found in U.S. Pat. Nos. 5,896,857, 6,189,532 (Hely/Lithgow assigned to ResMed Limited) and WO 00/38772 (Walker et. al assigned to ResMed Limited). 
     An embodiment of the vent of the present invention could also serve as a non-rebreath or antiasphyxia valve. 
     SUMMARY OF THE INVENTION 
     The present invention is a flow regulation vent for regulating flow from a pressurized gas supply. The vent includes a fixed portion adapted to engage a gas supply conduit and a spring force biased movable portion connected by a hinge to the fixed portion and flowingly connected to the pressurized gas supply. The fixed portion includes a gas flow orifice. The movable portion is pivotally movable between a relaxed position and a fully pressurized position. At a specified minimum operating pressure, the movable portion is pivoted by the spring force away from the fixed portion to the relaxed position to establish a first gas flow area between the movable portion and the gas flow orifice. At a specified greater operating pressure, the pressurized gas offsets the spring force to pivot the movable portion to the fully pressurized position adjacent the fixed portion to establish a minimum gas flow area between the movable portion and the gas flow orifice. In a preferred embodiment, the fixed portion and the movable portion are unitarily formed from a single piece of material, such as a sheet of stainless steel or a sheet of plastic. 
     By tuning the operating characteristics of the flow regulation vent (i.e. the size of the gas flow orifice at a given pressure), the flow rate curve (being the flow through the vent) can be tailored to be relatively constant across a specified operating pressure range or to be a non-constant flow curve over a specified operating pressure range. 
     In a further embodiment, the flow regulation vent can operate as a flow meter by including a strain gauge mounted between the fixed portion and the movable portion for measuring the position of the movable portion and providing an indicator of flow through the vent. The signal generated by the strain gauge transducer will be used with the pressure to determine the flow of gas through the vent. 
     In an alternative embodiment of the present invention the flow regulation vent includes a flexible flap portion having a portion engaging or attached to a fixed housing so that a free portion of the flap can move within a given range with respect to the housing. One side of the flap is exposed to an interior of the mask shell or gas flow conduit that is pressurized when the CPAP system mask is in use and another side is positioned toward an atmosphere side of the vent. 
     The housing includes a vent orifice positioned beneath the free portion of the flap with a portion of the housing surrounding the vent orifice being curved. While flexible, the flap has a level of natural rigidity that will provide a spring resistance against bending of the flap. In a relaxed state, the free portion of the flap will leave the vent orifice uncovered and a gas flow area between the vent orifice and the flap will be at a maximum. When the CPAP system is in use a force will act against the spring resistance of the flap and the free portion of the flap will tend to move toward the vent orifice. As the free portion of the flap moves closer to the vent orifice with increasing mask pressure, it follows the curved surface of the housing, progressively closing the vent orifice and reducing the gas flow area between the vent orifice and the flap. The interaction between the increasing mask pressure and decreasing gas flow area acts to reduce the gas flow rate through the vent as compared to the flow that would be achieved with a vent of constant gas flow area. 
     An alternative embodiment of the flow regulation vent of the present invention opens an auxiliary exhalation vent orifice during exhalation to allow higher exhalation gas flow to atmosphere. An embodiment may also include a non-rebreath valve or anti-asphyxias valve function that reduces or eliminates exhaled gas being retained in the gas circuit after the end of exhalation. These embodiments serve the desired aim of eliminating or at least reducing the occurrence of a user rebreathing exhaled gas. 
     In yet another embodiment the vent of the present invention could be configured so as to facilitate the retention in the mask of a desired level of exhaled breath including CO 2 . The desired level of retention would be directed towards augmenting a prescribed treatment, where some CO 2  retention may serve to counter the patient&#39;s own excessive exhalation of CO 2 . 
     The flow regulation vent of the present invention is simple and inexpensive to manufacture but provides effective, easily tailored flow regulation. The flow regulation vent reduces operating noise of the CPAP system by reducing the volume of gas flow required from the flow generator at high pressures, as well as thus reducing the work output of the flow generator. The vent also reduces rebreathing of CO 2  and other exhaled gas and provides for faster air pressure rise time, increasing the effectiveness of the CPAP system and patient compliance with CPAP treatment. 
     The invention will now be described in detail in conjunction with the following drawings in which like reference numerals designate like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of the flow regulation valve of the present invention; 
         FIG. 2  is a side plan view of the flow regulation valve of  FIG. 1  in a fully pressurized position; 
         FIG. 3  is a side plan view of the flow regulation valve of  FIG. 1  in a relaxed position: 
         FIG. 4  is an exploded perspective view of the flow regulation valve of the  FIG. 1  in combination with base and cover: 
         FIG. 5  is a side plan view of the exploded view of  FIG. 4 : 
         FIG. 6  is a sectional view taken along section line  6 - 6  in  FIG. 5 : 
         FIG. 7  is a side plan view of the flow regulation vent of  FIG. 1  attached to a shell of a breathing mask; 
         FIG. 8  is a side plan view of the flow regulation vent of  FIG. 1  attached to a gas supply tube connecting a shell of a breathing mask to a pressurized gas supply; 
         FIG. 9  is a sectional view of an alternative embodiment of the flow regulation vent of the present invention; 
         FIG. 10  is a graph of flow rate vs. pressure of a mask utilizing the vent of the present invention in comparison to conventional masks: 
         FIG. 11  is a perspective exploded view of an alternative embodiment of the present invention: 
         FIG. 12  is a sectional view of an alternative embodiment of a flow regulation vent of the present invention; 
         FIG. 13  is a chart showing the relationship between a radius of curvature and a deflection angle for a given pressure at which a flap of the embodiment of  FIG. 12  completely closes the vent: 
         FIGS. 14 and 15  are perspective views of an alternative embodiment of a flow regulation vent of the present invention: 
         FIG. 16  is a sectional view of the embodiment of  FIGS. 14 and 15 ; 
         FIG. 17  is a perspective view of an alternative embodiment of a flow regulation vent of the present invention: 
         FIG. 18  is a sectional view of the embodiment of  FIG. 17 ; 
         FIG. 19  is a perspective view of an alternative embodiment of a flow regulation vent of the present invention: 
         FIG. 20  is a sectional view of the embodiment of  FIG. 19 : 
         FIGS. 21-23  are perspective views of alternative embodiments of flow regulation vents of the present invention; 
         FIG. 24  is a perspective view of an alternative embodiment of a flow regulation vent of the present invention connected to a mask shell: 
         FIGS. 25 and 26  are perspective views of an alternative embodiment of flow regulation vents of the present invention; 
         FIG. 27  is a sectional view of the embodiment of  FIGS. 25 and 26 ; 
         FIG. 28  is a sectional view of a cover and mounted flap of a vent of a configuration similar to the embodiment shown in  FIGS. 25-27 , with the flap shown in three different positions based on mask pressure exposed to the flap; 
         FIG. 29  is a perspective view of a flow regulation vent similar to the embodiment of  FIGS. 25-27  connected to a mask shell: 
         FIG. 30  is an exploded view of an alternative embodiment of a flow regulation vent of the present invention: 
         FIG. 31  is a perspective view of the embodiment of  FIG. 30 : 
         FIG. 32  is a sectional view of the embodiment of  FIG. 30 ; 
         FIGS. 33 and 34  show two charts comparing the flow performance of a standard ResMed™ Mirage® mask with a ResMed™ Mirage® mask utilizing a vent according to one of the embodiments of  FIGS. 12-32 ; 
         FIG. 35  is a flow generator side perspective view of an alternative embodiment flow regulation vent of the present invention; 
         FIG. 36  is a mask side perspective view of the vent of  FIG. 35 ; 
         FIG. 37  is a flow generator side perspective view of a housing of the vent of  FIG. 35 ; 
         FIG. 38  is a side perspective view of the housing of  FIG. 37 ; 
         FIG. 39  is a front view of a flap of the vent of  FIG. 35 ; 
         FIGS. 40-42  are partial sectional views of the vent of  FIG. 35  showing gas flow through the vent during different stages of operation; 
         FIG. 43  is an exploded perspective view of an alternative configuration of the vent of  FIG. 35 ; 
         FIG. 44  is an exploded perspective view of the vent of  FIG. 43  in combination with a mask elbow joint; 
         FIG. 45  is a perspective view of the vent of  FIG. 43  positioned in a mask elbow joint; and 
         FIG. 46  is a partial sectional view of a modification of the embodiment of  FIG. 12 : 
         FIG. 47  is a partial sectional view of a modification of the embodiment of  FIG. 12 ; 
         FIG. 48  is a partial top plan view of a modification of the housing of the embodiment of  FIG. 12 ; and 
         FIG. 49  is a partial sectional view of a modification of the embodiment of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A flow regulation vent  10  is shown in  FIGS. 1-3 , and, in this embodiment, is circular. The flow regulation vent  10  is constructed from a unitary sheet of material and includes a movable portion  12  pivotally attached at one end to a fixed portion  14  by unitary hinge  16 . Movable portion  12  has an outer perimeter  18 , which, in the embodiment shown, is substantially circular. Fixed portion  14  includes an orifice  20 , which, in the embodiment shown, is also substantially circular and which is slightly larger in diameter than the diameter of the outer perimeter  18  to provide a gap  22  therebetween when the movable portion is in a fully pressurized position. See  FIG. 2 , which shows a side view of the vent  10  when in the fully pressurized position. Movable portion  12  can optionally include one or more bleed orifices  24  and fixed portion  14  can optionally include one or more bleed orifices  26 . 
       FIG. 4  shows the flow regulation vent  10  in an exploded perspective view in combination with a base portion  30  and a cover  40 . The base portion  30  can be an integral part of a breathing mask shell  32  for covering the mouth and/or nostrils of the patient  50  (see  FIG. 7 ) or can be an integral part of a gas flow tube or conduit  34  that connects the shell  32  to a pressurized gas supply (see  FIG. 8 ). Alternatively, the base portion  30  can be a separate unit attachable to the shell  32  or tube  34 . The base portion  30  includes a support ring  36  that supports an outer periphery of the vent  10  and one or more orifices  38  for connecting the vent  10  to the pressurized gas supply. Alternatively, the bottom of the base portion can be open to the mask shell or gas conduit, but the utilization of a floor with orifices  38  is preferred when the vent  10  is mounted to the mask shell to reduce access to the vent  10  from the interior of the mask shell and prevent accidental damage to the vent  10  from the interior of the mask shell. The cover  40  fits over and is connected to the base portion  30  to fix the vent in place. The cover includes one or more orifices  42  for venting gas to the atmosphere from the vent  10 . The number, size, positioning and shape of the orifices  38  and  42  can be altered as appropriate for the specific application to alter gas flow and noise levels. In the preferred embodiment, cover  40  has 18 orifices of 1.2 mm diameter to provide a level of noise reduction. Alternatively, the cover  40  can be made of a wire mesh or other mesh such as in accordance with co-pending U.S. patent application Ser. No. 09/570,907 filed May 15, 2000 presently unpublished the contents of which are incorporated herein by reference. The cover can be attached to the base portion in any known manner, including snap-fit, screw-on or glued. The snap-fit or screw-on connection is preferred since this provides for ease of cleaning or replacing the vent  10 .  FIGS. 4 and 5  show projections  44  on support ring  36  that can engage an indentation  46  on the interior of the cover  40  to provide a snap-fit. 
     As can be seen in  FIGS. 5, 7 and 8 , in the relaxed position, the movable portion  12  of vent  10  (shown in phantom) is pivoted away from the fixed portion  14  toward the mask shell  32  or gas supply tube  34 , i.e. towards the pressurized gas supply and away from the atmosphere. 
     The operation of the flow regulation vent  10  will now be described. At the minimum safe gas flow at the lowest operating CPAP pressure of, say, 2-4 cm H 2 0, the movable portion  12  is biased by the force from the spring hinge  16  into a relaxed position pivoted away from the fixed portion  14  and toward the pressurized gas supply. See  FIGS. 3, 7 and 8 . This provides a maximum gas flow area between the movable portion  12  and the orifice  20 . Thus, at such a low pressure, the flow area is maximized for allowing gas to easily vent from the mask shell  32  to the atmosphere. This design can also act as an anti-asphyxia valve designed to be open with a large flow area to the atmosphere at low or no pressure. For instance, if the flow generator stops working due to a malfunction, the vent  10  remains open, allowing the patient to continue breathing while reducing the risk of asphyxiation, or even the perception thereof, by the patient. A specific flow area is required for the achievement of an anti-asphyxia effect that is usually larger than would be necessary to achieve the lower end of the pressure range during normal operation. Therefore, an antiasphyxia embodiment can be designed to include an even larger flow area that is fully exposed and provides an adequate anti-asphyxia effect when there is no pressure in the system (as is the case when there is a failure of power or the flow generator and when the anti-asphyxia effect is required). That specific flow area would then close somewhat when the CPAP system is operating as intended at the lowest/lower pressure range and from there the movable portion continues to reduce the flow area as designed with increasing pressure. 
     However, as CPAP pressure increases, a force acts on the surface of the movable portion  12  to counteract the force of the spring hinge  16  and move the movable portion  12  toward the fixed portion  14 . This action causes a continuous reduction in the gas flow area between the movable portion  12  and the orifice  20 . Once the maximum designed operating pressure is reached, the gas flow area between the movable portion  12  and the orifice  20  is at a minimum. A further increase in pressure will not lead to a further reduction of the gas flow area. In the preferred embodiment, the minimum gas flow area is achieved when the movable portion  12  and the fixed portion  14  are essentially coplanar, i.e. lie in the same plane, and the gap  22  between the orifice  20  and the outer periphery  18  of movable portion  12  is minimized. See  FIG. 2 . 
     Thus, by the present invention, a gas flow area for allowing gas to escape from the CPAP system to atmosphere is reduced as the pressure of the gas supply increases. In this way, the total flow rate for gas from the CPAP system is reduced (as compared to a fixed gas flow area vent) even though the pressure of the gas is increasing. Through appropriate tuning of the flow regulation vent  10  within a specified operating pressure range of the CPAP system, a desired flow rate curve can be obtained, including a flow rate curve that is substantially flat across the specified operating pressure range. In alternative embodiments, the flow regulation vent  10  can be tuned to provide an increasing flow rate curve or even a decreasing flow rate curve, if the specific application warrants such, or even different combinations of flat, rising and falling curves at different segments within the specified operating range. 
     The flow regulation vent  10  can be tuned to deliver differing flow rate curves in response to varying CPAP system requirements in a number of ways, used separately or in conjunction with one another. Generally, such tuning can be achieved by altering the ratio between the maximum gas flow area and the minimum gas flow area and/or altering the resistance of the movable portion  12  to movement as a function of the pressure of the gas. Thus, vent  10  can be tuned by 1) altering the pivot angle of the movable portion  12  with respect to the fixed portion  14  in the relaxed position; 2) altering the ratio of the area of the orifice  20  with respect to the outer periphery  18  of the movable portion  12 ; 3) altering the shape or size of the orifice  20  and/or outer periphery  18 ; 4) changing the vent material to provide a different rigidity; 5) altering the thickness of the vent  10  to change rigidity; and/or 6) altering the cross-sectional area and/or configuration of the hinge  16  to alter rigidity. Other methods can also be used to change the tuning of the vent, including, for instance, different heat treatment procedures for vents made of metal, etc. 
     In addition, one or more apertures of various shapes can be provided on the movable portion  12  to alter the rigidity of the movable portion  12  and/or alter a surface pressure gradient on the movable portion  12  when exposed to the pressurized gas. Of course, if a desired minimum bleed flow is desired that is not provided for by the clearance between the orifice  20  and the outer periphery  18 , one or more bleed orifices  24  and/or  26  can be provided in the movable and fixed portions, respectively. Further, it is also contemplated that a multi-stage vent could be provided by utilizing a plurality of movable portions with different operating parameters in conjunction with respective fixed portion orifices or even to provide a second movable portion/orifice combination on the movable portion  12  itself. In any of these alternatives, it may also be desirable to provide positive operating stops on either the fixed or movable portions to positively limit travel of the movable portion in either direction. However, the use of positive stops may be avoided where their addition would increase noise (when the stops engage/disengage) to an extent that would be considered undesirable. 
     In a preferred embodiment, the vent  10  is constructed from a unitary sheet of material such as stainless steel or other metal or plastic, although other materials exhibiting the desired combination of rigidity, flexibility, springiness and resistance to bending fatigue can also be used. In such an embodiment, the vent can be formed by stamping, laser cutting, water jet cutting, and molding or by other known methods. In one preferred embodiment, the vent is cut from a single sheet of 0.1 mm thick polyester film of the type conventionally used for overhead projector transparencies. Such film can be obtained from the Orbit company in Australia, as well as from other suppliers such as 3M and Xerox. In this embodiment, the movable portion has an outer diameter of 13 mm and the fixed portion has an outer diameter of 20 mm (although this is not critical), with a gap between the movable and fixed portions of 0.2 mm. 
     The shape of the vent need not be circular but can be any desired shape. The shape can even be asymmetrical so that it can only be positioned in the base portion in the correct orientation, i.e., with the movable portion  12  pivoted toward the mask/gas supply tube in the relaxed position and not toward the atmosphere. Alternatively, a correct orientation of the vent can be assured by providing an outer edge of the fixed portion with asymmetrically positioned notches or tabs to engage similarly positioned tabs/notches provided in the base portion. In an alternative embodiment, the movable portion and fixed portion can be separate components affixed to one another through use of a hinge on either component or even through use of a separate hinge. In addition, the resistance of the movable portion to movement can be increased by utilization of an auxiliary spring member, which, in a simple form, could merely be an additional piece of rigid material overlaying and attached to the hinge  16 . By providing a readily removable cover  40  over the vent  10 , the flow rate characteristics of the mask can be easily and inexpensively tailored to an individual&#39;s clinical need, merely by exchanging the vent  10  with an alternative vent  10  having different operating parameters. In addition interchangeable flaps and covers with orifices to atmosphere may be substituted so as to change operating parameters. 
     The vent also need not be essentially flat, as in the present embodiment, but can have different profiles as appropriate. For instance, the vent can have a convex or concave profile. Furthermore, the thicknesses of the movable portion and/or the fixed portion can be increased and the edges of the orifice and/or movable portion can be rounded to provide a smoother gas flow through the vent, with potential gains in noise reduction. In one such embodiment, as shown in  FIG. 9  in the fully pressurized position, the rounded outer periphery  118  of the movable portion  112  of vent  110  can even overlap the rounded inner edge of the orifice  120  in the fixed portion  114 , with opposing surfaces of the two portions configured in a complementary manner to smooth airflow through the gap  122  therebetween. In such thicker, rounded embodiments, the movable and fixed portions would preferably be manufactured as separate components and would be pivotally connected together by a separate hinge that can be made of a different material. For instance, the movable and fixed portions  112  and  114  could be made of molded plastic and the hinge  116  made of metal and attached to the other components with adhesive. A small, polymeric bumper can be attached to one of the portions  112 ,  114  in the gap  122  to reduce noise should the two portions contact one another in the fully pressurized position. 
     In a preferred embodiment, the base and cover are made from machine grade polycarbonate, preferably clear. Such material can be obtained from the Dotmar company in Sydney. Australia. An alternative material is Bayer Makrolon 2458 clear polycarbonate by Bayer AG. Other materials from other suppliers can also be used. 
       FIG. 1  shows an exploded perspective view of an alternative embodiment of the flow regulation vent assembly. In this embodiment, a pin  62  that is mounted or molded to the cover  40  contacts an edge of the movable portion  12  to push the movable portion open into the relaxed position. The pin  62  is preferably positioned at an edge of the movable portion  12  approximately 30° around the perimeter of the movable portion  12  from the hinge  16 . A tab  64  on the fixed portion  14  engaging a slot  66  on the cover  40  provides the correct rotational orientation of the movable portion  12  with respect to the pin  62 . The height of the pin  62  is determined to provide the desired lift to the movable portion. The pin  62  holds the movable portion in the open position until the pressure in the mask starts to rise and the movable portion starts to close. Since the movable portion and hinge are relatively flexible the movable portion will bend and move toward the fully pressurized position. In this embodiment, the pin  62  prevents the movable portion from being completely coplanar with the fixed portion in the fully pressurized position. Nonetheless, the effective flow area through the vent is still reduced sufficiently to reduce the flow rate through the vent as pressure increases (as compared to a conventional fixed area vent). An advantage of this embodiment is that use of the pin  62  provides an exacting positioning of the movable portion in the open, relaxed position, which can be important when making the vent from thin plastic. Thus, the movable portion need not be pre-formed to be in the open state but can be pre-formed to be in a closed state, with the pin moving the movable portion to the open state. Another advantage of this embodiment is that the vent can be symmetrical from side to side so that either side can be placed toward the mask. In an alternative embodiment, the pin  62  can be replaced by a curved or sloped ramp. 
     Although the preferred embodiments discussed above utilize a movable portion that is positioned in the interior of the fixed portion, it is contemplated that a reverse configuration can be used where the exterior portion of the vent is movable and the interior portion is fixed to the base portion or cover. It is also contemplated that different vent embodiments can be created utilizing different combinations of alternative structures discussed herein.  FIG. 10  shows a comparison of the performance of the preferred embodiment vent with conventionally vented CPAP masks “A” and “B”. As can be seen, the conventional masks “A” and “B” have sharply increasing flow rate curves while a mask utilizing the vent of the present invention has a less steep flow rate curve. Thus, at 16 cm H 2 0, the mask utilizing the flow regulation vent of the present invention has a flow rate of approximately half of the lower limit flow rate of conventional mask “A” at 16 cm H 2 0 and a flow rate of less than half of the average flow rate of conventional mask “B” at 16 cm H 2 0. 
     Additional testing has shown that with a bilevel CPAP system such as VPAP II by ResMed Limited, a shortened rise time to the target mask pressure from when the patient begins inspiration is achieved using the present invention vent, as compared to a mask using a conventional fixed area vent. If the rise time in pressure is too long, the patient has the feeling of not getting sufficient air upon inhalation. Thus, a shorter rise time is preferred. In one test, the rise time for the present invention vent was approximately 250 ms, as compared to 300 ms in a conventional mask. Achieving the improved rise time performance by incorporating into a CPAP system the vent of the present invention is a less expensive alternative to achieving the same result by increasing flow generator performance. 
     Testing has also shown that in a CPAP mode where pressure in the mask is desired to remain relatively constant, the present invention vent is effective in doing so, as are conventional fixed flow area vents. Accordingly a vent of the present invention is compatible with constant pressure CPAP and may be used to ensure that a CPAP system delivers adequate exhaled gas wash out across the pressure range notwithstanding that the pressure remains fixed for a given patient during a period of treatment. Furthermore in a mode where the flow generator is shut off, testing has shown that the present invention vent acts as an effective anti-asphyxiation valve, providing pressure in the mask that is substantially the same as if the mask was opened to the atmosphere by removing the gas supply tube  34  from the mask. This is especially important in case of flow generator malfunction to reduce the risk of asphyxiation, or even the perception thereof, by the patient. 
     The flow regulation vent of the present invention operates to reduce a flow area of the vent as pressure within the mask increases so as to reduce the flow rate of the vent as compared to a conventional fixed area vent. This is accomplished by progressively moving a movable portion of the vent with respect to increasing pressure to progressively reduce a flow area between the movable portion of the vent and a fixed portion of the vent. The progressive movement of the movable portion can be accomplished by applying a spring force to the movable portion to progressively resist movement of the movable portion accompanying the increasing pressure. 
     In a modification of the present invention, a strain gauge  60  can be optionally attached by known means between the movable portion  12  and the fixed portion  14  to determine a pivot angle between the movable portion  12  and fixed portion  14 . See  FIGS. 1-3 . If the vent  10  is constructed of plastic, the strain gauge can be embedded in the vent  10 . The measurement of the pivot angle, taken in conjunction with the operating parameters of the vent  10  and the pressure of the gas, can then be used to calculate flow though the vent, thus allowing the vent to also function as a flow meter. Signals indicative of the pivot angle can be processed in the vicinity of the vent, say by a processor located on the mask the gas supply conduit or headgear which secures the mask. Alternatively the processor may be located at a distance from the vent, say at the flow generator or in another location. In all instances the transmission of the signals indicative of the pivot angle from the vicinity of the vent to the processor may be achieved by any suitable means such as by conductive wire, optical or wireless transmission. 
     An alternative embodiment of the present invention is shown in  FIG. 12 . In this embodiment, a flow regulation vent  150  includes a flexible flap portion  152  attached at a first end  156  to a fixed housing  154  so that a free end  158  of the flap  152  can move within a given range with respect to the housing  154 . Thus, the flap acts as a cantilever arm with the first end  156  fixed and the free end  158  movable. The housing  154  is connected to a mask shell or gas flow conduit. The vent housing  154  can be a separate component attachable to the mask shell or gas flow conduit, or can be integrated with such components. A side  153  of the flap  152  is exposed to an interior chamber of the mask shell or gas flow conduit that is pressurized to a pressure different than the exterior atmospheric pressure when the mask is in use. A side  155  of the flap is positioned toward an atmosphere side of the vent  150 . The housing  154  includes a vent orifice  160  positioned beneath the free end  158  of the flap  152 . A portion of the housing  154  surrounding the vent orifice  160  is curved to provide a surface  161  having a radius of curvature  162  about a single axis. The flap  152  comes into contact with the housing  154  at the surface  161 . As shown in  FIG. 12 , the flap  152  is in a relaxed state such that the vent orifice  160  is completely uncovered and a gas flow area between the vent orifice  160  and the flap  152  is at a maximum. 
     While flexible, the flap  152  has a level of natural rigidity that will resist bending of the flap  152  and will provide a spring resistance against bending of the flap. When the mask is in use, a force will act against this spring resistance of the flap  152  and cause the free end  158  of the flap  152  to move toward the vent orifice  160 . As the free end  158  of the flap  152  moves closer to the vent orifice  160  with increasing mask pressure, it will follow the radius of curvature  162  of surface  161  of the housing  154 , progressively closing the vent orifice  160  and reducing the gas flow area between the vent orifice  160  and the flap  152 . The amount the flap  152  can move between the relaxed state and a state where the vent orifice  160  is completely covered is shown as a maximum deflection angle  164 , measurable in degrees. As discussed with respect to previous embodiments above, the interaction between the increasing mask pressure and decreasing gas flow area acts to reduce the gas flow rate through the vent  150  as compared to standard fixed flow area vents. 
     The vent  150  can be tuned to provide different relationships between mask pressure and gas flow area. Such tuning can be accomplished by changing the thickness of the flap  152 , the material the flap  152  is made of, or the radius of curvature  162 , where a larger radius will allow the flap  152  to progressively close the vent orifice  160  under lower mask pressures as compared to a smaller radius of curvature  162 . The curved surface  161  is shown as being convex. However, in alternative embodiments, a concave curved surface can also be used.  FIG. 13  shows the relationship between the radius of curvature and the deflection angle for a given pressure at which the flap completely closes the vent. Although other radius of curvature and deflection angles can be used, the chart shows the radius of curvature to be greater than 21 mm and between 26 and 41 mm with the deflection angle between 15 and 25 degrees. The flexing of a flap fixed at one end is governed by the following equation:
 
1/ r=L   2   W /(2 EI )  (Eq. 1)
         Where:
           r=radius of curvature   L=length of flap to deflect from an initial position to closing of the vent orifice   W=uniform load per unit length (air pressure×surface area/length)
               W=Pb where
                   P=mask air pressure   b=flap width   
                   
               E=modulus of elasticity of the flap material   I=section moment of inertia of the flap
               I=bt 3 /12 where
                   t=flap thickness   
                   
               L can be expressed in terms of arc radius and angle
               L=ra where
                   a=deflection angle in radians   
                   
               
               

     By substituting for L in Eq. 1 and solving for angle a, the following equation for the deflection angle of the flap is derived:
 
 a =( Et   3 /6 Pr   3 ) 1/2   (Eq. 2)
 
     The vent  150  of this embodiment is intended for the use with a mask that requires a higher vent flow rate at low pressure. The vent  150  can be designed to work alone or in combination with a fixed bleed such as a fixed flow area bleed orifice. Where the vent  150  operates alone, it is preferably designed so that the flap  152  does not completely cover the vent orifice  160  and fully close the vent  150  under normal operating conditions. 
     It is preferable that the flap  152  be constructed of a lightweight material for fast response to pressure changes in the mask. However, the material must have sufficient stiffness to provide a spring bias against pressure changes in the mask yet the working stress of the flap is preferably designed to be below the endurance limit of the material to prevent fatigue failure from the repetitive alternating stress imposed by opening and closing the vent orifice. The strain is preferably designed to be below 1% at maximum deflection to prevent creep failure. The material thickness, material properties and the radius of curvature of the housing mainly control the stress and strain of the flap  152  and one or more of these parameters can be altered to adjust the stress and strain in the flap. The flap material is preferably made of a thin film and of a grade acceptable for medical application. Tolerances in the material thickness are preferably less than 10% to reduce variability in performance. 
     It is preferred that the curved surface  161  of the housing  154  be of a high finish of 8 micron or better and free of irregularities in order to achieve an airtight seal when the flap  152  is fully closed. The vent orifice  160  can be of any desired shape, including a rectangular window or grouped series of smaller orifices. The use of a symmetrically shaped orifice of constant width and length, such as a rectangle, will make the reduction in cross-sectional area of the vent orifice more uniform as the flap progressively closes the vent orifice. However, the use of an orifice of non-constant width and/or length can be used to specifically tailor the overall flow rate through the vent  150  as mask pressure changes. Fillets of 0.5 mm minimum and draft angles of 3-6 degrees can be used on the vent orifice  160  to reduce air noise. 
     One specific advantage of this embodiment as compared to known vents is the ability and ease with which the flow characteristics of the vent can be altered at specific pressures within the expected operating pressure range. By altering the characteristics of the housing and flap as discussed above, the flow rate through the vent can be altered depending on the pressure level. 
     For instance, in certain situations it may be desirable to quickly reduce flow through the vent as pressure increases above a certain specified pressure level. This can be accomplished by using a housing  154  that has a curved surface  161  with an increasing radius of curvature beyond a point where the flap  152  would be expected to contact the curved surfaced at the specified pressure level. See  FIG. 46 , where the curved surface  161  of the housing  154  has a first radius of curvature from a fixed end  156  of the flap  152  up to change point  165  and a larger second radius of curvature beyond change point  165 . With such embodiments utilizing curved surfaces  161  with an increased radius of curvature beyond a change point, it will take smaller incremental pressure increases above the specified pressure level to bring more of the flap  152  into contact with the curved surface  161  to close more of the orifice  160 . Thus, a gas flow area between the flap  152  and the curved surface  161  will decrease at a faster rate above the specified pressure level. 
     It is even contemplated that beyond the change point, some embodiments could have flat surfaces  161 , i.e., having an infinite radius of curvature. See  FIG. 47 . In such embodiments, the flap would come into complete contact with the curved surface above the specified pressure level, thereby closing the vent orifice  160  above the specified pressure level. In such an embodiment, venting above the specified pressure level would have to be through a fixed area bleed orifice on the flow regulation vent or mask assembly. It is also contemplated that the radius of curvature of the surface  161  could increase in discrete steps beyond a certain change point  165  or continuously increase beyond a certain change point  165 . Under certain circumstances, the radius of curvature can be decreased beyond the change point  165  to provide an opposite effect where the rate of reduction of the gas flow area between the flap  152  and the vent orifice  160  decreases beyond the change point as the pressure increases. 
     A similar result can be achieved by reducing the cross-sectional area of the vent orifice  160  beyond the change point  165 . See  FIG. 48  where the width of the orifice  160  begins to decrease at change point  165  and decrease further at second change point  167 . In this embodiment, the gas flow area between the flap  152  and the vent orifice  160  will decrease at an increasing rate beyond change point  165  (associated with a first specified pressure level) and decrease at an even faster rate beyond second change point  167  (associated with a second specified pressure level). The change in width of the orifice  160  can be at one or more discrete points, can be continuous within specified ranges or can be increasing or decreasing within specified ranges. The change in cross-sectional area of the vent orifice can also be accomplished by positioning an insert of a desired width profile in the vent orifice  160  to effectively alter the width of the vent orifice. As with the example above, the opposite effect can also be accomplished by reducing a width of the vent orifice  160  before the change point  165 . Where the vent orifice  160  comprises a plurality of smaller spaced apart orifices, the effect can be achieved by altering the area of one or more of the orifices with respect to the other orifices as they) are positioned further from the fixed end  156  of the flap  152 . 
     A similar result can be achieved by reducing the thickness (and thus rigidity) of the flap  152  beyond a change point  169  on the flap  152 . See  FIG. 49 . In this embodiment, the less rigid outer portion of the flap  152  will flex more easily toward the curved surface  161  beyond change point  169  (associated with a specified pressure level) and close the vent orifice  160  at a faster rate. The opposite effect can be achieved by increasing the rigidity of the flap  152  as one or more points outboard of the fixed end  156 . The change in thickness can be at one or more discrete points, can be continuous within specified ranges or can be increasing or decreasing within specified ranges. Of course, the rigidity of the flap  152  can be altered along its length in other manners as well, such as by the use of an auxiliary stiffening rib of varying rigidity in conjunction with the flap  152  to achieve the same results. 
     One or more of these tuning mechanisms can be used in conjunction with each other to readily and effectively provide an unlimited ability to precisely tune the gas flow characteristics of the flow regulation vent  150  at any point within an anticipated operating pressure range. 
     The flap can be attached to the housing by riveting, screwing, clamping, use of adhesive or other known methods. The flap can also be attached to the housing by being positioned in a slot in the housing, the slot preferably forming a friction fit between the housing and the flap. 
     In general, the vent of this embodiment will operate under the following conditions. A large deflection angle will cause higher initial airflow through the vent, but will delay closure of the vent. A large radius of curvature will cause the flap to close at lower pressure. A large vent will cause higher initial airflow and the size of the vent orifice is limited by the ability of the flap to seal the vent orifice with no deformation. In masks utilizing a fixed area bleed vent, the end of the flap  152  can extend beyond the end of the vent orifice  160  in order to maintain positive air pressure acting on the flap to keep it closed at high pressure once it is shut. An overlap of 1 mm or greater is considered adequate. A bleed vent can be provided in the flow regulation vent by undercutting a portion of the surface  161  through to the vent orifice  160  such that the undercut potion can still flow gas to the vent orifice  160  even when the flap is in complete contact with the surface  161 . A bleed vent can also be provided by placing an orifice in the flap  152  that allows gas to flow though the flap  152  to the orifice  160  even when the flap  152  is in complete contact with the surface  161 . 
     The flap  152  is preferably made of a material such as polyester film. The film can be slit to size, and then cut to length. Holes can be punched in the film for location purposes. The housing is preferably made of a moldable clear material for ease of cleaning and visibility. In a preferred embodiment, the vent  150  is detachable from the mask or gas flow conduit. This facilitates replacement in case of damage, the ability to fine tune vent operation for specific applications and the ability to upgrade with improved designs. 
       FIGS. 14-16  disclose an alternative embodiment of the vent  150  mounted to a swivel elbow joint  170  for connecting a gas flow conduit/tube to a mask shell.  FIGS. 14 and 15  are perspective views of the vent from different angles and  FIG. 16  is a sectional view of the vent  150 . In this embodiment, the vent  150  is constructed on a cover  172  used to cover a vent chamber housing  174  mounted on the swivel joint  170 . The vent  150  communicates with an interior of the swivel joint  170  and thus, the mask shell, via passage  182 . The cover  172  includes snap arms  178  for engaging slots  180  to hold the cover  172  on the housing  174 , although other known attachment mechanisms can also be used for this purpose. The vent  150  includes a flap  152 , a vent orifice  160  and a curved surface  161  as in the embodiment of  FIG. 12  (see  FIG. 26 ). In this embodiment, the vent orifice  160  is rectangular. However, this embodiment also includes a fixed bleed orifice  176  that remains open to provide a minimum vent flow even when the flap  152  completely covers the orifice  160  and the vent  150  is closed. The vent  150  of this embodiment is detachable from the swivel joint  150  for replacement and/or cleaning. 
       FIGS. 17-26  disclose an alternative embodiment of the vent  150 . In this embodiment, the vent housing  154  is formed as a semi-circular clip that can detachably clip onto the swivel elbow joint  170 . The vent  150  communicates with an interior of the swivel joint  170  and thus, the mask shell, via passage  182 . This embodiment includes two parallel rectangular vent orifices  160  and a plurality of circular fixed bleed orifices  176 . Otherwise, the vent  150  of this embodiment operates similarly to the vent  150  of  FIGS. 14-16 . 
       FIGS. 19-20  disclose an alternative embodiment of the vent  150 . In this embodiment, the vent housing  154  is formed as a clip that can detachably clip onto the swivel elbow joint  170 . The vent  150  communicates with an interior of the swivel joint  170  and thus, the mask shell, via passage  182 . This embodiment includes a single rectangular vent orifice  160  but does not include a fixed bleed orifice. Otherwise, the vent  150  of this embodiment operates similarly to the vent  150  of  FIGS. 14-16 . 
       FIGS. 21-23  disclose alternative embodiments of the vent  150 . In these embodiments, the vent housing  154  is circular for detachable attachment to a circular mount on a mask shell or gas flow conduit. In the embodiments of  FIGS. 21 and 23 , the vent orifice  160  is oval shaped. In the embodiment of  FIG. 22 , the vent orifice  160  is shaped as a series of interconnected channels. The embodiments of  FIGS. 21 and 22  do not include fixed bleed orifices while the embodiment of  FIG. 23  includes a plurality of fixed bleed orifices  176  that extend in parallel along opposite sides of the orifice  160 . In each of these embodiments, the vent orifice is formed on a cover  172  for attachment to the circular housing  154 , similarly to the embodiment of  FIGS. 14-16 . The housing  154  can be provided with an orientation projection  184  for engaging a notch  186  in the cover to rotationally orient the cover  172  with respect to the housing  154 . Otherwise, the vent  150  of these embodiments operates similarly to the vent  150  of  FIGS. 14-16 . 
       FIG. 24  discloses an embodiment of a vent  150  similar to the embodiment of  FIGS. 14-16 , as well as disclosing how the swivel elbow joint  170  is attached to a mask shell  190  of known construction. Mask shell  190  includes a pair of parallel ports  192  that are in fluid communication with the mask interior. 
       FIGS. 25-27  disclose an alternative embodiment of the vent  150  where the vent housing  154  is generally rectangular in shape and includes a pair of mounting bosses  194  adapted to engage the pair of parallel flow ports  192  (see  FIG. 24 ) to allow flow from an interior of the mask shell  190  to the vent  150 . The mounting bosses are sized and configured to be retained on the flow ports  192  by a friction fit, although other known retention mechanisms can also be used. Since the mask shell  190  is of a known design in current production (Ultra MIRAGE® by ResMed Limited), the configuration of this embodiment allows the easy retrofitting of that known mask with the variable vent of the present invention. The housing  154  includes a plurality of internal ribs  196  and seating pads  202  for engaging and positioning a diffuser  198  within the housing  154 . As shown in  FIG. 27 , when the diffuser  198  is properly positioned in the housing  154 , a gas chamber  206  is formed that is in communication with passages  204  in bosses  194 , which are in turn, in communication with the interior of the mask shell via flow ports  192 . The diffuser  198  includes a plurality of orifices  200  through which gas in chamber  206  can pass to flow toward the vent orifice  160 . The plurality of spaced-apart orifices  200  acts to diffuse the gas flow from the two passages  204  to more evenly act on the flap  152 . 
     The diffuser  198  also includes a pair of extending retaining walls  208  for engaging a center portion of the flap  152  to position the flap  152  against the convex curved surface  161  of the cover  172 . In this embodiment, the flap  152  is not attached to the cover  172  at one of its ends, but rather, flexes from its center to, in effect, create two interconnected flaps  152 . The cover  172  includes a centrally located projecting pin  212  to engage a centrally located positioning bore  210  on the flap  152  to position the flap  152  with respect to the cover  172  and prevent lateral movement of the flap  152 . The internal ribs  196  of the housing  154  are positioned alongside the flap  152  to prevent the flap  152  from rotating within the housing  154 . In an alternative embodiment, the bore  210  and pin  212  can have an asymmetrical configuration to prevent rotation of the flap  152 . The flap  152  can also be staked or riveted to the cover  172 . The vent cover can be retained to the housing by a snap fit, friction fit, adhesive or other known retention mechanism. The vent cover  172  includes a vent orifice  160  in the form of a plurality of spaced-apart round orifices. This embodiment does not include a fixed bleed orifice but such a fixed bleed orifice can be provided on the vent  150  or elsewhere on the mask shell or gas flow conduit. Otherwise, the vent  150  of this embodiment operates similarly to the vent  150  of  FIGS. 14-16 , with each outboard side of the flap  152  movable in response to mask pressure to progressively close a respective portion of the vent orifice  160 . 
       FIG. 28  shows a cover  172  and mounted flap  152  of a configuration similar to the configuration shown in  FIGS. 25-27 , with the flap  152  in three different positions based on mask pressure exposed to the flap  152 . In the first position, the flap  152  is entirely open. In the second position, increased mask pressure has moved the outboard ends of the flap  152  toward the convex curved surface  161  of the vent cover  172  to partially obstruct flow through the vent orifices  160 . In the third position, mask pressure has increased to the point that the outboard ends of the flap  152  have moved further toward the curved surface  161  to completely close the vent orifices  160 . All of the embodiments shown in  FIGS. 12-32  operate similarly. 
       FIG. 29  discloses a mask shell of the type shown in  FIG. 24  with a vent  150  similar to the type disclosed in  FIGS. 25-28  attached to the flow ports  192 . In this embodiment, the vent orifice  160  is configured as two oval orifices. 
       FIGS. 30-32  disclose an embodiment similar to the embodiment disclosed in  FIGS. 25-29  but where the curved surface  161  on cover  172  is concave. In this embodiment, the housing  154  includes a plurality of raised walls  220  connected to an internal floor of the housing  154  to both support the flap  152  and to diffuse air/gas flow from passages  204 . The vent cover  172  also includes a plurality of raised posts  222  surrounding the curved surface  161  to position and retain the flap  152  over the curved surface  161 . The walls  220  and posts  222  interact to maintain the flap  152  in the desired position over the curved surface  161  when the vent cover  172  is installed on the housing  154 , as can be best seen in  FIG. 32 . In the embodiments shown in  FIGS. 12-29 , the flap  152  is fixed at its center and the outboard ends of the flap  152  move over the convex curved surface  161  to vary the vent orifice  160 . In this embodiment however, the curved surface is  161  is concave and the flap  152  is not fixed to the vent cover  172  at any point. As opposed to the previous embodiments where the flap bends from one fixed end or from the center, in this embodiment, the flap  152  bends from both outboard ends  224  such that the flap center  226  bows toward the concave curved surface  161  under increasing mask pressure to progressively close the vent orifice  160 . This embodiment also includes a fixed bleed orifice  176 . 
       FIGS. 33 and 34  show two charts comparing the flow performance of a standard ResMed™ Mirage® mask with a ResMed™ Mirage® mask utilizing a vent according to one of the embodiments of  FIGS. 12-32 . In  FIG. 37  the chart shows the flow performance of the mask utilizing a vent  150  (including a fixed bleed orifice  176 ) as compared to the standard mask. The flow rate for the inventive mask is substantially higher at low mask pressures but tapers off at higher mask pressures to be only slightly higher than the standard mask. In effect, the closing of the variable vent  150  is delayed somewhat as shown by the hump in the curve at lower mask pressures. This delayed closure can be achieved by utilizing a curved surface  161  with a smaller radius of curvature or a thicker, stiffer flap  152 . 
       FIG. 34  shows a comparison between a standard ResMed™ Mirage® mask with a ResMed™ Mirage® mask utilizing a vent according to one of the embodiments of  FIGS. 12-32 . The solid curve is for the standard mask. The box curve is for a mask continuing to utilize the fixed bleed orifices of the standard mask but also using a variable vent  150  (having no fixed bleed orifice). This curve shows a higher flow rate at lower mask pressures when the variable vent  150  is open but then overlays the standard curve once the variable vent  150  is closed and flow is only through the fixed bleed orifices of the standard mask. The initial hump in the curve was achieved by using a larger flap deflection angle  164  of 22 degrees and a larger radius of curvature  162  of curved surface  161  of 35 mm. The diamond curve is for a mask utilizing only the variable vent  150 , with no fixed bleed orifice in the vent  150  or the mask. This curves shows flow at lower mask pressures that decreases as mask pressure rises until the vent  150  completely closes and there is no flow at all. 
     The flow regulation vent of the present invention is simple and inexpensive to manufacture, especially when cut made from a flat, unitary disk as described above, but provides effective, easily tailored flow regulation. With such an effective flow regulation vent, the flow generator is delivering higher pressure and need not be sized to have the additional capacity to handle increased flow rates at higher pressures, as with conventional CPAP systems. Noise from the flow generator motor can also be reduced since the motor can operate at lower RPM to deliver the reduced volume of high pressure airflow. The vent also acts as a sound barrier, reducing the level of noise from the interior of the mask, including noise created by the flow generator that escapes to the atmosphere. Further, the reduced flow rate at high pressure results in less noise generation from the airflow itself. The vent also reduces rebreathing of C0 2  and provides for faster air pressure rise time, increasing the effectiveness of the CPAP treatment. Each of these benefits promotes patient compliance with CPAP treatment. 
       FIGS. 35-42  show an alternative embodiment of the present invention. A flow regulation vent  250  includes a generally round flap portion  252  and a generally tubular fixed housing portion  254 . The fixed housing portion  254  includes a user side  256  adapted to be connected to a mask and a flow generator side  258  adapted to be connected to a pressurized supply of gas from a flow generator to position the flow regulation vent  250  between the mask and the flow generator. The fixed housing portion  254  further includes a primary vent orifice  260  positioned near the user side of the housing and a secondary vent orifice  262  positioned near the flow generator side of the housing  254 , each flowingly connected to an exhaust orifice  264  (see  FIG. 38 ) exposed to the atmosphere to allow gas flow between each of the primary vent orifice  260  and secondary vent orifice  262  and the exhaust orifice  264 . In the embodiment shown, the secondary vent orifice  262  is in the form of a plurality of smaller orifices  266  but can also have other configurations, as discussed above. See  FIG. 37 . The secondary vent orifice  262  is positioned on a curved surface  274  of the fixed housing portion  254  and is adapted to engage a movable portion  278  of the flap portion  252 . 
     The fixed housing portion  254  also includes a flap seating flange  268 , against which a fixed portion  276  of the flap portion  252  seats and a projecting orientation pin  270  for engaging an orientation orifice  272  in the flap portion  252  for properly orienting the flap portion  252  with respect to the fixed housing portion  254  when the flow regulation vent  250  is assembled. A hinge portion  280  connects the movable portion  278  of the flap portion  252  to the fixed portion  276 . In the preferred embodiment, a radially outer portion of the curved surface  274  generally smoothly transitions to the flap seating flange  268  to provide a continuous surface against which the movable flap portion  278  can engage as it moves from a relaxed position to a flexed position. 
     The vent  250  of this embodiment operates as follows, with special reference being made to  FIGS. 40-42 .  FIG. 40  shows the vent  250  during inhalation by the user. The air flow from the flow generator (shown as upward pointing arrows in the Figure) has overcome a natural spring force of the flap  252  to move the movable portion  278  of the flap  252  toward the user, increasing a flow area between the movable portion  278  and the fixed portion  276  of the flap  252 . This allows ample air flow to the user during inhalation and prevents any feeling of asphyxiation. The movement of the movable portion  278  has also brought more of the movable portion  278  into contact with more of the curved surface  274  and progressively reduced a flow area between movable portion  278  and the curved surface  274  to reduce flow through the secondary vent orifice  262 . This reduces a total flow area through vent orifices  262  and  260  to reduce flow through the exhaust orifice  264  from air flow from the flow generator or from exhalation. 
     During exhalation, as shown in  FIG. 41 , the spring force of the flap  252  has returned the movable portion  278  of the flap  252  to a relaxed position, minimizing the flow area through the flap  252 . This acts as a non-rebreathing mechanism, minimizing any exhalation into the flow generator conduit and creating CO 2  buildup there that will be rebreathed by the user and similarly acts as a one-way valve to prevent oxygen from going back into the flow generator conduit should the flow generator stop working due to malfunction. This also minimizes any incoming gas flow from the flow generator during exhalation. The movement of the movable portion  278  has also uncovered the secondary vent orifice  262  flow area to add that area to that of the flow area of primary vent orifice  260  and increase a total outflow area of the vent  250  for the exhalation gases. With the increased total outflow area, as well as less flow through the total outflow area due to inflow from the flow generator, the exhalation gases can exit the mask at a greater flow rate. This increases CO 2  outflow from the mask and decreases undesirable CO 2  buildup in the mask. The vent  250  also results in lower mask pressure during exhalation as a result of the increased total outflow area and decreases the pressure rise time in the mask, as compared to conventional masks. 
     As shown in  FIG. 42 , the vent  250  also acts effectively as an anti-asphyxia valve in the event that the flow generator ceases operation. In such a situation, the movable portion  278  of the flap  252  remains in the relaxed, closed position, keeping the secondary vent orifice  262  open and increasing the total flow area (in combination with primary vent orifice  260 ) for allowing outside air into the mask during inhalation by the user. The vent  250  eliminates the need for providing other vents on the mask itself. 
     An alternative configuration of the flow regulation vent  250  is shown in  FIGS. 43-45 . In this configuration, the housing  254  is relatively narrow so that it can be inserted into a slot  284  in a swivel elbow joint  270 . The flap  252  is somewhat T-shaped with the movable portion  278  of the flap  252  being a relatively large proportion of the flap  252  and the fixed portion  276  of the flap  252  being a relatively small proportion of the flap  252 . In this configuration, the flap  252  is held in place with respect to the housing  254  by a flap cover plate  286  that attaches to the housing  254  and sandwiches the fixed portion  276  therebetween. The cover plate can also be configured to contact a flow generator side of the movable portion  278  when in the relaxed position to prevent reverse flow from exhalation into the flow generator conduit. In this embodiment, the secondary vent orifice  262  is generally rectangular and is not positioned on a curved surface of the housing  254 . This is not as important with the flow regulation vent  250  as it is in previous embodiments, since it is not as important to have a progressively increasing or decreasing flow area through the vent orifice  262 . Rather, it is more important that the flow area through the vent orifice  262  be small during inhalation and large during exhalation. This embodiment otherwise operates as does the embodiment of  FIGS. 35-42 . An exhalation flow deflector  288  can be attached to the elbow joint  270  to direct the flow of exhalation gas outside the mask. See  FIG. 45 . The flap cover plate can be attached to the housing  254  by welding, adhesive, snap fit or other known attachment methods. 
     In the preferred embodiment, the flap  252  is constructed from thin polyester sheet with a flap diameter of 21.5 mm (positioned in a housing inside diameter of 23 mm), a flap thickness of 0.004 inch and a flap hinge width of 7 mm. The flow characteristics through the vent  250  can be tailored as desired by altering the flap characteristics, including thickness, movable portion area, material and hinge width. A fixed area orifice can also be provided through the vent  250  between the flow generator and the mask to provide flow from the flow generator should the movable portion  278  of the flap become stuck closed. As with embodiments discussed above, the vent  250  can operate as a flow meter by measuring a pressure drop across the vent  250  or by measuring an electrical signal from a strain gauge attached to the flap  252 . The orifice  260  can also be configured to provide a high resistance to inflow and a low resistance to outflow. 
     It is intended that various aspects of the embodiments discussed above can be used in different combinations to create new embodiments of the present invention. 
     It will be apparent to those skilled in the art that various modifications and variations may be made without departing from the scope of the present invention. Thus, it is intended that the present invention covers the modifications and variations of the invention.