fig1 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 wearer &# 39 ; 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 &# 39 ; s face . the straps are also known in the art . the shell 12 also includes an arm 16 , which terminates in a fitting 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 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 &# 39 ; 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 . in the fig1 embodiment , the thin air permeable membrane 28 is a stainless steel sheet approximately 0 . 45 mm 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 322 mm 2 . the holes are laser cut into the stainless steel . the holes are desirably laser cut or flame 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 &# 39 ; s openings are located on the atmosphere side the opportunity for blockage occurring by the insertion of particulate matter will be minimized . alternatively , the larger end of the vent &# 39 ; s openings may be located on the atmosphere side which may make the vent quieter . fig2 shows a nasal respiratory mask 40 according to a second embodiment of the invention . like reference numerals to those used in describing the first 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 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 . fig3 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 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 membrane 28 of identical construction to the epfte membrane discussed below in relation to the mask 40 shown in fig6 . fig4 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 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 . fig5 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 . 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 . fig6 . 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 shown in fig2 and like reference numerals have been used to indicate like features with respect to the second embodiment . in the mask 40 of fig2 , 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 permeable material is made from gore - tex ® product attached to a polypropylene scrim having an area of 481 mm 2 . the membrane is constructed from expanded polytetrafluoroethylene ( eptfe ). the inventors have identified gore - tex ® eptfe product manufactured by w . l . gore & amp ; associates , inc . of maryland usa ( gore - 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 fig7 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 fig6 in that the vent is provided in the inserts 82 . however the mask 100 uses slotted extensions 66 to attach mask restraining straps ( not shown ), not openings 42 . as best seen in fig8 , which is a close - up view of the insert shown in fig6 , 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 . fig9 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 . 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 . 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 , 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 shown in fig8 . 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 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 &# 39 ; s requirements . for example where a low 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 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 minimization of co 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 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 .
the present invention provides a vent assembly suitable for use with a respiratory mask of the type used in cpap treatment . in one embodiment the vent is made of a thin air permeable membrane . generally , the membrane is thinner than 0 . 5 mm . the membrane can be made of a hydrophobic material such as polytetrafluoroethylene . the membrane can also be fabricated from expanded ptfe . the expanded ptfe membrane is mounted on a polypropylene scrim . the pores of the membrane have a reference pore size of 10 to 15 microns . in an alternative embodiment , the vent assembly includes a vent constructed from stainless steel . in another embodiment the membrane has a superficial cross - sectional area of approximately 500 mm 2 . in another embodiment the vent assembly comprises a membrane attached to a vent frame , the vent assembly forming an insert which can be removeably attached to a mask fame .