RESPIRATORY MASK HAVING GAS WASHOUT VENT & GAS WASHOUT VENT ASSEMBLY FOR A RESPIRATORY MASK BACKGROUND OF INVENTION 5 Field of the Invention The present invention relates to a respiratory mask and a vent for a respiratory mask. General Background and Related Art The application of Continuous Positive Airway Pressure (CPAP) via a nasal 10 mask is a common ameliorative treatment for sleep disordered breathing (SDB) including obstructive sleep apnea (OSA) as described in the applicant's US Patent No. 4,944,310. In CPAP treatment for OSA, air or other breathable gas is supplied to the entrance of a patient's airways at a pressure elevated above atmospheric pressure, typically in the range 3-20 cm H 2 0 as measured in the patient interface. It is also 15 known for the level of treatment pressure to vary during a period of treatment in accordance with patient need, that form of CPAP being known as automatically adjusting nasal CPAP treatment as described in the applicant's US Patent No. 5,245,995. Non-invasive positive pressure ventilation (NIPPV) is another form of 20 treatment for breathing disorders including sleep disorder breathing. In a basic form NIPPV involves a relatively high pressure of gas being provided in the patient interface during the inspiratory phase of respiration and a relatively low pressure or atmospheric pressure being provided in the patient interface during the expiratory phase of respiration. In other NIPPV modes the pressure can be made to vary in a 25 complex manner throughout the respiratory cycle. For example, the pressure at the patient interface during inspiration or expiration can be varied through the period of treatment as disclosed in the applicant's International PCT Patent Application No WO 98/12965 and International PCT Patent Application No WO 99/61088. In this specification any reference to CPAP treatment is to be understood as 30 embracing all of the above-described forms of ventilatory treatment or assistance. Typically, the patient interface for CPAP treatment consists of a nasal mask. The nasal mask is generally defined by a mask shell which forms an inner cavity defined by its interior surface, mask cushion and the user's face, a gas inlet which may or may not include a separate component such as a swivel elbow. Alternatively,
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a nose-mouth mask or full-face mask or nasal prongs or nasal pillows can be used. In this specification any reference to a mask is to be understood as incorporating a reference to a nasal mask, nose-mouth mask, full face mask, nasal prongs or nasal pillows unless otherwise specifically indicated. The mask incorporates, or has in close 5 proximity, a gas washout vent for venting exhaled gases to atmosphere. The gas washout vent (the vent) is sometimes referred to as a CO 2 washout vent. It is important that the apparatus is quiet and comfortable to encourage patient compliance with therapy. The exhausting to atmosphere of exhaled gas through the vent creates noise. As CPAP and NIPPV treatments are normally administered while 10 the patient is sleeping, minimization of such noise is desirable for both the comfort of the patient and any bed partner. From a clinical perspective it is desirable for a mask and vent combination to maximize both the elimination of exhaled CO 2 through the vent and also the inhalation of the supplied breathable gas. In this way, retention of exhaled CO 2 within 15 the mask, which is "re-breathed' by the wearer, is minimized. Generally by locating the vent in the mask shell CO 2 washout will be superior to locating the same vent between the mask shell and the breathable gas supply conduit. It is desirable to minimise the weight of the vent assembly for greater patient comfort. 20 Systems for the delivery of nasal CPAP treatment often incorporate in-line humidifiers to minimize drying of the nasal mucosa and increase patient comfort. Accordingly, it is also desirable that a vent not block when used with humidified gas. It is also desirable that a vent be easily cleaned or economically disposable. A number of vent configurations are known. One approach to vent 25 configuration is to create within the mask shell one or more openings that allow for the flow of exhaust gas from the inner cavity to atmosphere. The exhaust flow may be directed through the incorporation of an additional pipe extending out from the opening located on the mask shell outer surface. The applicant's nasal mask system known by the name ResMed Modular 30 Mask System incorporates an outlet vent located in the swivel elbow connected to the mask shell. The ports defining the vent have the same cross-sectional thickness and are formed from the same polycarbonate material that is used to form the swivel elbow and mask shell frame. 2 The whisper swivel, manufactured by Respironics, Inc provides three slots on the circumference of a generally cylindrical attachment piece. In use, the attachment piece is to be interposed between the mask shell and the gas conduit. The attachment piece is made of the same material and thickness as is used to make the mask shell. 5 European Patent No. 0 697 225 discloses a vent formed from a porous sintered material. A known vent, manufactured by Gottleib Weinmann Gerate Fur Medizin Und Arbeitsschutz GmbH and Co. comprises a generally cylindrical insert to be interposed in use, between the mask shell and the gas conduit. The insert includes a window 10 which is covered with a porous sintered material of approximately 3-4 mm thickness. Another type of vent intended to be inserted between the mask shell and the breathable gas supply conduit is the E-Vent N by Draeger medizintechnik GmbH (the Draeger vent). The Draeger vent comprises a stack of 21 annular disks, which have slots in their adjacent surfaces for gas to flow therethough. Each slot has a length of 5 15 to 7 mm as measured along the path from the interior of the vent to atmosphere. The applicant produces a respiratory mask known as the MIRAGE® nasal mask system and the MIRAGE® full-face mask (the MIRAGE mask). The MIRAGE® mask has a crescent shaped opening in the mask shell in which is located a complementary shaped crescent elastometric insert with six holes therein which 20 constitutes the vent. The elastomeric inset has a crossectional thickness of 3 to 4mm. The vent of the type used in the MIRAGE® is described in International Patent Application No. WO 98/34665 and Australian Patent No 712236. It is an object of the present invention to provide an alternative form of vent that is suitable for use in a respiratory mask. 25 SUMMARY OF THE INVENTION The present invention provides a vent assembly suitable for use with a mask used in CPAP treatment wherein the vent assembly is a thin air permeable membrane. In one form of the invention, the membrane is thinner than the mask frame. 30 In another form of the invention, the membrane is thinner than 0.5 mm. In another form of the invention the membrane has an approximate thickness of 0.05 mm. 3 In another form of the invention the membrane is constructed from a hydrophobic material such as polytetrafluoroethylene (PTFE). In another form of the invention the membrane is constructed from expanded PTFE. 5 In another form of the invention the expanded PTFE membrane is mounted on a polypropylene scrim. In another form, the pores of the membrane have a reference pore size of 10 to 15 microns. In another form of the invention the membrane is constructed from stainless 10 steel. In another form of the invention the membrane of the vent has a superficial cross-sectional area of approximately 500 mm 2 . In another form of the invention the vent assembly comprises a membrane attached to a vent frame, the vent assembly forming an insert which can be 15 removeably attached to a mask fame. In another form of the invention there is provided a respiratory mask for communicating breathable gas to the entrance of a wearer's airways, the mask including (i) mask shell, (ii) a gas inlet and (iii) an opening into which an insert constructed from a thin air permeable membrane with a corresponding shape may be 20 placed. The opening may be positioned in the mask shell or in the gas inlet. In one form, the mask includes a mask shell with an integrally formed gas inlet and the opening is provided in the mask shell remote the inlet. In another form, the mask includes a mask shell with an integrally formed gas inlet and the opening is provided in the gas inlet. In yet another form, the mask includes a mask shell with a 25 separately formed gas inlet attached thereto and the opening is provided in the mask shell remote the inlet. In still yet another form, the mask includes a mask shell with a separately formed gas inlet attached thereto and the opening is provided in the gas inlet. The present invention also provides a respiratory mask arrangement for 30 communicating breathable gas to the entrance of a wearer's airways, the mask arrangement including a vent assembly comprising an opening with a thin air permeable membrane extending across an opening. The present invention also provides an apparatus for delivering CPAP which apparatus includes a mask arrangement for communicating breathable gas to the 4 entrance of a wearer's airways, the mask arrangement including a gas washout vent assembly comprising an opening with a thin air permeable membrane extending across said opening. 5 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a respiratory mask according to a first embodiment of the invention; Fig. 2 is a perspective view of a respiratory mask according to a second embodiment of the invention; 10 Fig. 3 is a perspective view of a respiratory mask according to a third embodiment of the invention; Fig. 4 is a partial cross-sectional view of a vent assembly according to a fourth embodiment of the invention; Fig. 5 is a partial cross-sectional view of a vent assembly according to a fifth 15 embodiment the invention; Fig. 6 is a perspective view of a respiratory mask according to sixth embodiment of the invention; Fig. 7 is a perspective view of a full-face mask according to a seventh embodiment of the invention. 20 Fig. 8 is an enlarged detailed view of an insert suitable for use with the masks shown in Fig. 6; and Fig 7. Fig. 9 is a perspective view of a vent assembly according to an eighth embodiment of the invention where the thin air permeable membrane is located in a cylindrical position on a tube suitable for attachment to the mask elbow. 25 DETAILED DESCRIPTION Fig. I shows a nasal respiratory mask 10 according to a first embodiment of the invention. The mask 10 includes a rigid plastic mask shell 12, which has a peripheral flange 14 for mounting of a cushion (not shown) to the shell 12. The cushion abuts the 30 wearer's face in use and is well known in the art. The flange 14 includes slots 15 for the connection of mask restraining straps (not shown) that extend around the head of the wearer to maintain the mask 10 adjacent to the wearer's face. The straps are also known in the art. The shell 12 also includes an arm 16, which terminates in a fitting 5 18 that is adapted to connect to a forehead support (not shown), which is also known in the art. The mask shell 12 includes a breathable gas inlet 20 which is rotatably mounted to the shell 12. The inlet 20 has a first end 22 which is adapted for connection with a 5 breathable gas supply conduit (not shown) and a second end 24 which is adapted to connect to, and communicate the supplied gas to the interior of the shell 12 for subsequent communication with the wearer's airways. The mask 10 includes a gas washout vent constituted by an opening 26 in the shell 12 across which extends a thin air permeable membrane 28. 10 In the Fig I embodiment, the thin air permeable membrane 28 is a stainless steel sheet approximately 0.45mm thick having holes with a diameter approximately 0.1 mm in diameter. The total open area is approximately 5% of the total superficial surface area of the sheet. The dimensions of the sheet are approximately 322mm 2 . The holes are laser cut into the stainless steel. The holes are desirably laser cut or flame 15 cut through the stainless steel. Preferably the holes have a diameter of less than 0.2 mm, and preferably provide a total open area of approximately 1% to 25% of the superficial surface area of the steel. The holes may be tapered (in a gradual or stepped manner) through their internal bore. In use, if the smaller end of the vent's openings are located on the 20 atmosphere side the opportunity for blockage occurring by the insertion of particulate matter will be minimized. Alternatively, the larger end of the vent's openings may be located on the atmosphere side which may make the vent quieter. Fig. 2 shows a nasal respiratory mask 40 according to a second embodiment of the invention. Like reference numerals to those used in describing the first 25 embodiment will be used to denote like features in respect of the second embodiment. Accordingly, the mask 40 has a shell 12 with a gas inlet 20. Instead of the slots 15 of the first embodiment the mask shell includes openings 42 which are adapted to snap engage with connection fittings (not shown) provided on the end of mask restraining straps (not shown). Further, instead of the arm 16 and fitting 18, the mask 40 includes 30 an adjustable forehead support mechanism indicated generally by the reference numeral 44. The mask 40 also includes a vent constituted by an opening 26 formed in the gas inlet 20 across which extends a thin air permeable membrane 28. 6 Fig. 3 shows a mask 60 according to a third embodiment of the present invention. Although this particular embodiment is directed to a nasal mask, it should noted that various vent arrangements can be used with various mask arrangements. Once again like reference numerals to those used in describing features of the first 5 embodiment shall be used to denote like features in respect of the third embodiment. The mask 60 includes a mask shell 12 with an integrally formed fixed gas inlet 62. A cushion 64 is attached to the peripheral flange 14 of the shell 12. The shell 12 also includes slotted extensions 66 for connecting headgear (not shown) to the mask. The mask 60 includes an opening 26 across which is extended a thin air permeable 10 membrane 28 of identical construction to the ePFTE membrane discussed below in relation to the mask 40 shown in Fig. 6. Fig. 4 shows a cross-section of vent assembly 110. There is provided a membrane 114 interposed between an outer element 112 and an inner element 116. This arrangement provides for a simple assembly. There is a corresponding opening 15 115 in the outer element 112 and inner element 116 to allow for the passage of air through the membrane. The inner element 116 may form part of the mask frame or of a separate insert to be positioned in an opening in the mask frame. Fig. 5 shows an alternative cross-section of a vent assembly 110. There is provided a stainless steel membrane insert 118 positioned over the inner element 120. 20 There is an opening 119 in the inner element 120 to allow for the passage of air through the membrane. The inner element 119 may form part of the mask frame or of a separate insert to be positioned in an opening in the mask frame. Fig. 6. shows a nasal respiratory mask 80 according to a sixth embodiment of the invention. The mask 80 is similar to the second embodiment of the mask 40 25 shown in Fig. 2 and like reference numerals have been used to indicate like features with respect to the second embodiment. In the mask 40 of Fig. 2, the vent is provided in the gas inlet 20, whereas in the mask 80 the vent is provided in the shell 12. More particularly, the mask 80 includes two cylindrical inserts 82 which have an inner opening 26 across which extends the thin air permeable material 28. The thin air 30 permeable material is made from GORE-TEX* product attached to a polypropylene scrim having an area of 481mm 2 . The membrane is constructed from expanded polytetrafluoroethylene (ePTFE). The inventors have identified GORE-TEX* ePTFE product manufactured by W. L. Gore & Associates, Inc. of Maryland USA (GORE 7 TEX* membrane) as being a suitable material for constructing a membrane. In one preferred form, the GORE-TEX* membrane has the following characteristics: Membrane material 100% expanded polytetrafluoroethylene Reference pore size 10-15 micron Bubble Point typical minimum individual 0.02 bar Airflow 0.37 LPM/cm 2 Thickness 0.05 mm Substrate polypropylene scrim 5 Fig, 7 shows a seventh embodiment of a full-face respiratory mask 100 according to the invention. Once again like reference numerals to those used in denoting like features with previous embodiments have been used to denote like features in respect of this embodiment. The mask 100 is similar to the mask 80 shown in Fig.6 in that the vent is provided in the inserts 82. However the mask 100 uses 10 slotted extensions 66 to attach mask restraining straps (not shown), not openings 42. As best seen in Fig. 8, which is a close-up view of the insert shown in Fig. 6, the insert 82 is comprises a cylindrical portion 86 sized to be a snug fit into a circular orifice 88 provided in the mask shell 12. The insert 82 located against the outer surface of the shell 12 by a peripheral flange 90. The inserts may be glued in position. 15 Fig. 9 shows a further embodiment of the invention in which an in-line vent assembly is provided. Like numerals are used to indicate like features with previous embodiments. In this embodiment, the in-line vent assembly comprises a generally cylindrical shaped vent frame with "windows" or "ports" covered with a membrane as described above. 20 The thin air permeable membrane of the present invention may be attached to the mask by any suitable means. For example the stainless steel vent describe above may be attached to a polycarbonate mask shell by way of hot glue adhesive (for example) or any other suitable adhesive. The durability sought to be achieved will determine the suitable approach for attachment. 25 In a further embodiment there is provided a means to indicate the volume of air that has passed through the vent, or alternatively the time that the vent assembly has been used. When a sufficient volume of air has passed through the vent assembly, 8 or the assembly has been used for a sufficient time and may have become blocked, the indicator will signal that the vent assembly should be replaced. For convenience, the thin air permeable membrane can be provided in an insert which is releasably attachable to the mask shell via a push-fit mechanism, as 5 shown in Fig. 8. Preferably on at least the outer surface of the insert there is provided at least one cross-piece that protects the air permeable membrane from being damaged as it is located into the receiving orifice of the mask shell. This approach will allow for the easy placement, removal and replacement of a vent insert while retaining the other components of the mask. While the insert may be configured to take the form of 10 any requisite shape preferably the insert has a circular circumferential shape defining a cylindrical insert which has a frictional fit within a corresponding circular orifice in the mask shell or gas inlet. Formation of the vent through use of an insert configuration facilitates the selection and fitting of a vent to suit a user's requirements. For example where a low 15 treatment pressure is required the associated flow will also be relatively small compared with flow required to achieve a higher treatment pressure. In such circumstances a relatively large vent area may be adopted to facilitate achievement of the clinically desirable mask CO 2 washout rate. Should a higher treatment pressure be required then the previously selected vent may be exchanged for a vent being more 20 restrictive to flow. The more restrictive vent will allow achievement of the clinically desirable mask CO 2 washout rate while avoiding the intensity of noise and exhaust gas jetting that would occur had the previously selected low pressure vent been used with the higher treatment pressure. Locating the vent in the mask shell results in an improvement in the 25 minimization of C0 2 retention within the mask compared to locating the vent as an inline mask component. Although the invention has been described with reference to specific examples, it is to be understood that these examples are merely illustrative of the application of the principles of the invention. Thus it is to be understood that 30 numerous modifications may be made in the illustrative examples of the invention and other arrangements may be devised without departing from the spirit and scope of the invention. 9