# Patent

## Title
Mask and vent assembly therefor

## Abstract

A mask (10) for use with a system for supplying breathable gas pressurized above atmospheric pressure to a human or animal's airways. The mask (10) includes a mask shell (12) which is, in use, in fluid communication with a gas supply conduit and a gas washout vent assembly (20). The gas washout vent assembly (20) includes at least one gas washout orifice (22) extending from a first side of the vent assembly (20) positioned, in use, adjacent the human or animal's face and a second side positioned, in use, adjacent the atmosphere. The cross-sectional area of the orifice (22) at the first side is larger than the cross-sectional area of the orifice (22) at the second side.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 10/298,845, filed Nov. 19, 2002, now allowed, which is a continuation of U.S. application Ser. No. 09/452,558, filed Dec. 1, 1999, now U.S. Pat. No. 6,561,191, which is a continuation-in-part of U.S. application Ser. No. 09/021,541, filed Feb. 10, 1998, now U.S. Pat. No. 6,561,190, each incorporated herein by reference in its entirety.


FIELD OF THE INVENTION
The present invention relates to a mask and a vent assembly therefor.
The mask and vent assembly according to the invention have been developed primarily for the venting of washout gas in the application of continuous positive airway pressure (CPAP) treatment in conjunction with a system for supplying breathable gas pressurised above atmospheric pressure to a human or animal. Such a system is used, for example, in the treatment of obstructive sleep apnea (OSA) and similar sleep disordered breathing conditions. However, the invention is also suitable for other purposes including, for example, the application of assisted ventilation or respiration.
The term “mask” is herein intended to include face masks, nose masks, mouth masks, nasal pillows, appendages in the vicinity of any of these devices and the like.
BACKGROUND OF THE INVENTION
Treatment of OSA by CPAP flow generator systems involves the continuous delivery of air (or other breathable gas) pressurised above atmospheric pressure to a patient's airways via a conduit and a mask.
For either the treatment of OSA or the application of assisted ventilation, the pressure of the gas delivered to a patient can be at a constant level, bi-level (ie. in synchronism with patient inspiration and expiration) or automatically adjusting in level to match therapeutic need. 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 masks used in CPAP treatment generally include a vent for washout of the gas to atmosphere. The vent is normally located in the mask or in the gas delivery conduit adjacent the mask. The washout of gas through the vent is essential for removal of exhaled gases from the breathing circuit to prevent carbon dioxide “re-breathing” or build-up, both of which represent a health risk to the mask wearer. 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 H2O for adults and 2 cm H2O in paediatric applications.
Prior art masks are generally comprised of a rigid plastic shell which covers the wearer's nose and/or mouth. A flexible or resilient rim (or cushion) is attached to the periphery of the shell which abuts and seals against the wearer's face to provide a gas-tight seal around the nose and/or mouth.
A prior art washout vent utilized one or more holes or slits in the rigid shell or in a rigid portion of the delivery conduit to allow the washout gas to vent to atmosphere. In some masks, the holes or slits were formed during the moulding process. In others, they were drilled or cut as a separate step after the shell or conduit had been moulded.
The flow of gas out the holes or slits in the shell or conduit to atmosphere creates noise and turbulence at the hole or slit outlet as the delivered gas, and upon expiration, the patient-expired gas (including CO2) exits. Bi-level and autosetting gas delivery regimes tend to generate more noise than a constant level gas delivery regime. This is thought to be due to the extra turbulence created by the gas accelerating and decelerating as it cycles between relatively low and relatively high pressures. The noise adversely affects patient and bed-partner comfort.
Another prior art vent included hollow rivets or plugs manufactured from stainless steel or other rigid materials attached to openings in the rigid shell. The outer edges of the rivets were rounded to help reduce noise. However, this approach was expensive, required an extra production step and did not prove effective in reducing noise.
Another approach to reduce noise involved the use of sintered filters at the gas outlet of the mask shell. However, the filters were prone to blocking, especially in the presence of moisture. Accordingly, sintered filters were impractical for use in CPAP treatment as they were easily blocked by the moisture from the patient's respiratory system or humidifiers or during the necessary regular cleaning of the mask and associated componentry.
Foam filters wrapped around the air outlets in the shell were also attempted. However, they also suffered from the disadvantages of being prone to blocking, difficult to clean and requiring constant replacement.
Remote outlet tubes have been used to distance the noise source from the patient. However, these tubes are difficult to clean, are prone to entanglement by the patient and/or their bed partner and suffer the further disadvantage that a volume of exhausted gas is retained in the tube adjacent the mask.
It is an object of the present invention to substantially overcome or at least ameliorate the prior art disadvantages and, in particular, to reduce the noise generated by gas washout through a mask.
SUMMARY OF THE INVENTION
Accordingly, the invention, in a first aspect, discloses a mask for use with a system for supplying breathable gas pressurised above atmospheric pressure to a human or animal's airways, the mask includes a mask shell which is, in use, in fluid communication with a gas supply conduit; and a gas washout vent assembly, the gas washout vent assembly includes at least one gas washout orifice extending from a first side of the vent assembly positioned, in use, adjacent the human or animal's face and a second side positioned, in use, adjacent the atmosphere and the cross-sectional area of the orifice at the first side is larger than the cross-sectional area of the orifice at the second side.
In a second aspect, the invention discloses a vent assembly for the washout of gas from a mask or conduit used with a system for supplying breathable gas pressurized above atmospheric pressure to a human or animal, the vent assembly includes at least one gas washout orifice extending from a first side of the vent assembly placed, in use, adjacent the human or animal's face to a second side placed, in use, adjacent the atmosphere, the cross-sectional area of the orifice at the first side is larger than the cross-sectional area of the orifice at the second side.
Preferably, the orifice includes a first substantially cylindrical portion adjacent the first side, a second cylindrical portion adjacent the second side and a tapering portion between the first and second substantially cylindrical portions.
Preferably also, the second substantially cylindrical portion and the tapering portion are of approximately equal thickness in the axial, direction of the orifice and are thicker than the first substantially cylindrical portion.
Desirably, the vent assembly includes a plurality of said orifices therethrough.
Desirably also, each of said orifices is separated from the other(s) of said orifices by at least the diameter of the orifice at the second side.
In an aspect, at least the region of the mask shell or conduit surrounding or adjacent the vent assembly is formed of a relatively flexible elastomeric material. In an embodiment, the entire mask is formed from the elastomeric material. In another embodiment, the vent assembly is in the form of an insert of relatively flexible elastomeric material.



BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by way of examples only, with reference to the accompanying drawings in which:
FIG. 1 is a perspective view of a first embodiment;
FIG. 2 is a perspective view of a second embodiment;
FIG. 3 is a perspective view of a third embodiment;
FIG. 4 is a perspective view of a fourth embodiment;
FIG. 5 is a perspective view of a fifth embodiment;
FIG. 6 is a perspective view of a sixth embodiment;
FIG. 7 is a perspective view of a seventh embodiment;
FIG. 8 is a partial cross-sectional view of the first embodiment along the line 8-8 of FIG. 1;
FIG. 9 is a perspective view of an eighth embodiment;
FIG. 10 is a plan view of the insert of the third embodiment;
FIG. 11 is a cross-sectional view of the third embodiment insert along the line 11-11 of FIG. 10; and
FIG. 12 is a partial cross-sectional view of the third embodiment insert along the line 12-12 of FIG. 10.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring firstly to FIG. 1, there is shown a mask 10 for use with a system (not shown) for supplying breathable gas pressurised above atmospheric pressure to a human or animal's airways. The mask includes a rigid plastics shell 12 having an inlet tube 14 for connection to a supply conduit to communicate breathable gas from a flow generator (not shown) to the nasal passages of the mask wearer. The mask shell 12 also includes a flexible sealing membrane 16 which is used to provide a gas tight seal between the face of the wearer and the interior of the shell 12. The shell 12 also includes lugs 18 for connecting the mask 10 to a head strap (not shown) to retain the mask in place.
The mask includes a SilasticTM insert 20 through which is provided an orifice 22 for gas washout. As best shown in FIG. 8, the insert 20 has a recess or groove 24 around its periphery. A correspondingly sized opening 26 bounded by a rim 28 is provided in the shell 12 to enable the insert 20 to be retained in place in the fashion of a grommet. The opening 26 can be moulded in the shell 12 or drilled or punched as a post-moulding step. The flexibility of the SilasticTM allows the insert 20 to be initially squeezed through the opening 26 before resiliently expanding to the configuration shown in FIG. 8 and engaging the rim 28.
As seen in FIG. 8, orifice 22 has a cross-sectional contour from a face side of the orifice to an atmosphere side of the orifice. In FIG. 8, the contour is shown as being symmetrical between the face side of the orifice and the atmosphere side of the orifice with a central portion of the orifice contour being of constant diameter. After the insert 20 is positioned in opening 26 of mask shell 12, the contour remains substantially constant in size as gas is passed therethrough.
FIGS. 2 to 7 show further embodiments in which corresponding reference numerals are used to indicate like features. In all these embodiments the insert 20 has an external groove or recess 24 which engages the rim 28 of a corresponding shaped opening 26 in the mask shell 12 to retain the insert 20 in place.
In the embodiment shown in FIGS. 2 to 5 and 7 the insert 20 includes more than one orifice 22. In the embodiment shown in FIG. 6, two inserts 20 are provided in the shell 12.
In the embodiment shown in FIG. 9, the insert 20 is provided in a gas supply conduit 30.
FIGS. 10 to 12 show the insert 20 of the third embodiment of FIG. 3. The dimensions 32, 34, 36, 38, 40, 42 and 45 are approximately diameter 1.73 mm, diameter 3.30 mm, 28.80 mm, 19.00 mm, 1.20 mm, 1.20 mm and 3.60 mm respectively.
The side 44 of the insert 20 faces the patient's face in use and the side 46 faces atmosphere.
The mask shell 12 is manufactured from polycarbonate. Other rigid plastics materials can equally be used. The insert 20 can be manufactured from an elastomer sold as SilasticTM (produced by the Dow Corning Corporation) or a thermoplastic elastomer sold as SantopreneTM (produced by Monsanto). Other flexible elastomeric materials can be used also.
The mask 10 produces less noise than an identical mask having a similar sized and shaped orifice(s) formed directly in the mask shell 12 instead of formed in the flexible insert 20. It is thought that the noise reduction occurs due to the flexible insert 20 damping vibrations caused by air passage through the orifice(s) 22 which produce vibrations or similar in the mask shell 12.
A prototype of the embodiment of the invention shown in FIG. 3 has been tested over a range of constant and bi-level CPAP treatment pressures. For comparison purposes, an identical mask to that shown in FIG. 3 but formed entirely from polycarbonate and having six identical arcuately spaced holes 22 drilled directly through the mask shell was also tested. In both masks the six holes had a diameter of 1.7 mm. The results of the test are summarised in the Tables below:







TABLE 1







Constant level gas delivery









Pressure
Noise levels 1 m from mask (dBA)










(cm H2O)
With flexible insert
Without flexible insert












4
26.8
35.2


10
33.4
43.1


18
39.3
49.2
















TABLE 2







Bi-level gas delivery









Pressure
Noise levels 1 m from mask (dBA)










(cm H2O)
With flexible insert
Without flexible insert





 5-10
30.8-38.5
37.2-43.0


10-15
38.6-43.7
42.9-47.9









As the results show, the mask shown in FIG. 3 produced less radiated noise than a similar mask not including the flexible elastomeric insert 20 representing a significant advantage in terms of the comfort of the mask wearer and their bed partner.
In addition to the noise reduction discussed above, the masks 10 possesses other advantages over those of the prior art. Firstly, the insert 20 is very easy to install into the mask shell 12 during either assembly of the mask which, is often supplied in kit form, or before and after cleaning which is regularly required and often carried out in the home environment. Secondly, the mask shell 12 may be produced with a single size of opening 26 and provided with a range of different inserts 20 which allows the outlet size to be “tuned” to give an optimum gas washout rate for a particular patient's treatment pressure level.
Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art, that the invention may be embodied in many other forms.

## Claims

1. A CPAP mask assembly for use with a system for supplying breathable gas pressurized above atmospheric pressure to a human or animal, the mask assembly comprising:
a mask including a rigid plastics shell and an inlet tube adapted to connect to a gas supply conduit, the shell provided with a seal to contact the human or animal in use; and
a vent assembly for gas washout of CO2-laden gas from the mask or the conduit provided to the mask while reducing noise generated by gas washout, the vent assembly including an insert attachable to the mask or conduit, the insert having at least one gas washout orifice extending from an inner side of the insert oriented, in use, towards the human or animal's face to an outer side oriented, in use, towards the atmosphere, the gas washout orifice being open in use, wherein
the inner side of the insert includes an inner flanged end portion that is structured to extend inside the mask or conduit and the outer side of the insert includes an outer flanged end portion that is structured to extend outside the mask or conduit, and
the insert includes a plurality of said orifices,
wherein a cross-sectional area of each orifice at the inner side of the insert is larger than a cross-sectional area of each orifice at the outer side of the insert, and the insert is insertable into an opening in the mask or conduit in only a single orientation which a smaller end of each orifice is positioned at the outer side of the insert.

2. A CPAP mask assembly as claimed in claim 1, wherein said orifices are separated from each other by at least a diameter of the orifice at the outer side.

3. A CPAP mask assembly as claimed in claim 1, wherein the cross-sectional area of each orifice at the inner side is substantially larger than the cross-sectional area of the orifice at the outer side.

4. A CPAP mask assembly for use with a system for supplying breathable gas pressurized above atmospheric pressure to a human or animal, the CPAP mask assembly comprising:
a mask adapted to be in communication with the pressurized gas in use, the mask having an inlet tube that is, in use, adapted to be in fluid communication with a gas supply conduit, said mask including a shell made from polycarbonate or another rigid plastic material, the shell being provided with a sealing membrane to provide a gas tight seal with the user in use; and
a vent assembly to exhaust CO2-laden gas from the mask while reducing noise generated by gas washout, the vent assembly comprising:
at least four gas washout orifices, each said orifice having an inner side that, in use, is oriented towards the user's face, and an outer side that, in use, is oriented towards atmosphere, each said orifice being open in use at least during the inhalation and exhalation phases of the user's breathing cycle, wherein
each said orifice has a substantially round cross section along its length, and
a cross-sectional size or area of each orifice at the inner side is larger than a cross-sectional size or area of the orifice at the outer side, and a transition portion between the inner and outer sides of each said orifice has a cross-sectional size or area that varies along at least a portion of a length thereof,
wherein the vent assembly comprises an insert attachable to the mask, and six orifices extending through the insert, and
wherein the insert is insertable into an opening in the mask in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the insert.

5. A CPAP mask assembly as claimed in claim 4, wherein the length of each said orifice is greater than an inner diameter at the inner side of each said orifice.

6. A CPAP mask assembly as claimed in claim 4, wherein said orifices are separated from each other by at least a diameter of the orifice at the outer side.

7. A CPAP mask assembly as claimed in claim 4, wherein an axis defined within the orifice extends substantially linearly from the inner side to the outer side.

8. A CPAP mask assembly as claimed in claim 4, wherein the shell includes a lug on each side thereof for connecting the mask to a headgear strap to retain the mask in place relative to the user in use.

9. A CPAP mask assembly as claimed in claim 4, wherein the length of each orifice is about 3 times a thickness of the shell.

10. A CPAP mask assembly as claimed in claim 4, wherein each said orifice has an inner diameter at the inner side that is approximately twice an outer diameter at the outer side of the orifice.

11. The CPAP mask assembly of claim 10, wherein the length of each said orifice is about 3.60 mm, and each said orifice has an inner diameter at the inner side of about 3.30 mm, and an outer diameter at the outer side of about 1.73 mm.

12. The CPAP mask assembly of claim 4, wherein the vent assembly includes six orifices uniformly spaced and separated.

13. The CPAP mask assembly of claim 12, wherein the vent assembly is arranged such that orifices provided at each end of the vent assembly are aligned along a horizontal axis.

14. The CPAP mask assembly of claim 12, wherein the vent assembly is arranged such that all the orifices are generally aligned along a horizontal axis.

15. A CPAP mask assembly as claimed in claim 4, wherein the cross-sectional size or area of each orifice at the inner side is substantially larger than the cross-sectional size or area of the orifice at the outer side.

16. A CPAP mask assembly for use with a system for supplying breathable gas pressurized above atmospheric pressure to a human for treatment of obstructive sleep apnea, the CPAP mask assembly comprising:
a mask including a rigid plastic shell and an inlet tube adapted to connect to a gas supply conduit, the shell provided with a flexible seal to seal against the human in use, the mask comprising polycarbonate material; and
a vent assembly provided to continuously exhaust CO2-laden gas from the mask during the entire breathing cycle of the human while reducing noise generated by gas washout, the mask including only a single opening for purposes of CO2 washout, the single opening being delineated by a rim of said polycarbonate material, the vent assembly comprising only a single insert provided to the single opening in the mask and including a plurality of gas washout orifices,
wherein at least one portion of the insert includes an inner portion defining a first flange that engages an inner surface of the rim, and an outer portion defining a second flange that engages an outer surface of the rim, the first and second flanges defining a recess or groove therebetween to receive the rim of the mask,
wherein a cross-sectional area of each orifice at an inner side of the insert is larger than a cross-sectional area of the orifice at an outer side of the insert,
wherein the insert comprises a material that is less rigid compared to the polycarbonate material of the rim,
wherein an outer end of each orifice at the outer side of the insert is positioned outward of the outer surface of the rim,
wherein the insert includes a main body and the first and second flanges are provided at the perimeter of the main body, the plurality of gas washout orifices are provided in the main body and the first and second flanges are radially outward of the plurality of gas washout orifices, and
wherein the insert is insertable into the single opening in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the insert.

17. A CPAP mask assembly as claimed in claim 16, wherein the material of the insert is different from the material of the rim.

18. A CPAP mask assembly as claimed in claim 16, wherein a thickness of the vent assembly is greater than an inner diameter of each orifice.

19. A CPAP mask assembly as claimed in claim 16, wherein each orifice includes a cross-sectional contour that remains substantially constant in size upon application of pressurized gas at a level suitable for CPAP treatment.

20. A CPAP mask assembly as claimed in claim 16, wherein each orifice includes a cross-sectional contour that remains substantially constant in size upon application of pressurized gas less than about 18 cmH2O.

21. A CPAP mask assembly as claimed in claim 16, wherein each orifice includes a cross-sectional contour that remains substantially constant in size upon application of pressurized gas in the range of 4-18 cmH2O.

22. A CPAP mask assembly as claimed in claim 16, wherein each orifice includes a cross-sectional contour that remains substantially constant in size upon application of pressurized gas in the range of 5-15 cmH2O.

23. A CPAP mask assembly as claimed in claim 16, wherein a ratio of a diameter at the inner side of each orifice to a diameter at the outer side of the orifice is about 2:1.

24. A CPAP mask assembly as claimed in claim 16, wherein the mask includes a nose mask.

25. A CPAP mask assembly as claimed in claim 16, wherein the vent assembly includes six orifices.

26. A CPAP mask assembly as claimed in claim 16, wherein a length of each said orifice is greater than an inner diameter at the inner side of each said orifice.

27. A CPAP mask assembly as claimed in claim 16, wherein the shell includes a slotted lug on each side thereof for connecting the mask to a headgear strap to retain the mask in place relative to the user in use.

28. A CPAP mask assembly as claimed in claim 16, wherein each orifice includes a transition portion between the inner and outer sides that has a cross-sectional size or area that varies or tapers along at least a portion of a length thereof.

29. A CPAP mask assembly as claimed in claim 16, wherein each said orifice has a substantially round cross section along its length.

30. A CPAP mask assembly as claimed in claim 16, wherein the insert includes all of the orifices.

31. A CPAP mask assembly as claimed in claim 16, wherein the insert includes three or more gas washout orifices.

32. A CPAP mask assembly as claimed in claim 31, wherein the insert includes four or more gas washout orifices.

33. The CPAP mask assembly as claimed in claim 16, wherein the insert has a thickness that is greater than a surrounding thickness of the mask.

34. A CPAP mask assembly as claimed in claim 16, wherein the first and second flanges, respectfully, extend at substantially right angles relative to a longitudinal axis of the washout orifices.

35. A CPAP mask assembly as claimed in claim 16, wherein a thickness of the recess or groove is approximately equal to a thickness of each of the inner and outer portions.

36. A CPAP mask assembly as claimed in claim 16, wherein the insert is sufficiently flexible to allow the insert to be squeezed through the opening after which it expands to engage the rim.

37. A CPAP mask assembly as claimed in claim 16, wherein the shell is made of polycarbonate.

38. A CPAP mask assembly as claimed in claim 16, wherein the cross-sectional area of each orifice at the inner side of the insert is substantially larger than the cross-sectional area of the orifice at the outer side of the insert.

39. A CPAP mask assembly as claimed in claim 16, wherein the second flange of the insert, as seen in cross-section, includes first and second flange portions on opposing sides of the insert to receive the rim on opposing sides of the single opening, and the first and second flange portions on each side of the insert extend a substantially equal distance from the single opening.

40. A CPAP mask assembly for use with a system for supplying breathable gas for treatment of obstructive sleep apnea, the CPAP mask assembly comprising:
(a) a rigid plastic shell having an inlet tube for connection to a conduit;
(b) a flexible sealing membrane attached to the rigid plastic shell, the flexible sealing membrane and the rigid plastic shell together defining a breathing chamber; and
(c) a vent assembly, the vent assembly comprising:
(i) an opening in the mask assembly for receiving a removable vent insert, the opening bounded by a rim; and
(ii) the vent insert having more than one orifice, the vent insert having, in at least one cross-sectional view, at least one groove at its periphery, the vent insert being removably attached to the mask assembly by the engagement of the at least one groove and the rim,

wherein the vent insert is structured and configured to reduce noise during gas washout, and
wherein a cross-sectional area of each orifice at an inner side of the vent insert is larger than a cross-sectional area of each orifice at an outer side of the vent insert, and the vent insert is insertable into the opening in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the vent insert.

41. The CPAP mask assembly of claim 40, wherein the opening is moulded.

42. The CPAP mask assembly of claim 41, wherein the removable vent insert is easy to install into the mask assembly, the removable vent insert is made of a material more flexible than the rigid plastic shell and inlet tube thereby allowing the vent insert to be initially squeezed through the opening before the vent insert resiliently expands to engage the rim of the opening.

43. The CPAP mask assembly of claim 40, wherein the removable vent insert is easy to install into the mask assembly, the removable vent insert is made of a material more flexible than the rigid plastic shell and inlet tube thereby allowing the vent insert to be initially squeezed through the opening before the vent insert resiliently expands to engage the rim of the opening.

44. The CPAP mask assembly of claim 40, wherein the removable vent insert has at least six orifices.

45. The CPAP mask assembly of claim 40, wherein the cross-sectional area of each orifice in the removable vent insert is substantially larger on the inner side than on the outer side.

46. A CPAP mask for use with a system for supplying breathable gas for treatment of obstructive sleep apnea, the CPAP mask, comprising:
(a) a vent assembly, the vent assembly comprising:
(i) an opening in the CPAP mask for receiving a removable vent insert, the opening bounded by a rim; and
(ii) the vent insert having more than one orifice, the vent insert having, in at least one cross-sectional view, at least one groove at its periphery, the vent insert being removably attached to the CPAP mask by the engagement of the at least one groove and the rim,

wherein the vent insert is structured and configured to reduce noise during gas washout, and
wherein a cross-sectional area of each orifice at an inner side of the vent insert is larger than a cross-sectional area of each orifice at an outer side of the vent insert, and the vent insert is insertable into the opening in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the vent insert.

47. The CPAP mask of claim 46, wherein the CPAP mask is formed of a polycarbonate.

48. The CPAP mask of claim 47, wherein the opening is moulded.

49. The CPAP mask of claim 46, wherein the removable vent insert is easy to install into the CPAP mask, the removable vent insert is made of a material more flexible than the CPAP mask thereby allowing the vent insert to be initially squeezed through the opening before the vent insert resiliently expands to engage the rim of the opening.

50. The CPAP mask of claim 46, wherein the removable vent insert has at least six orifices.

51. The CPAP mask assembly of claim 46, wherein the cross-sectional area of each orifice in the removable vent insert is substantially larger on the inner side than on the outer side.

52. A CPAP mask assembly for use with a system for supplying breathable gas for treatment of obstructive sleep apnea, the CPAP mask assembly comprising:
(a) a rigid plastic shell having an inlet tube for connection to a conduit;
(b) a flexible sealing membrane attached to the rigid plastic shell, the flexible sealing membrane and the rigid plastic shell together defining a breathing chamber; and
(c) a vent assembly, the vent assembly comprising:
a vent insert received in an opening in the CPAP mask assembly, the opening in the CPAP mask assembly bounded by a rim, and
the vent insert having more than one orifice, the vent insert having, in at least one cross-sectional view, at least one groove at its periphery, the at least one groove and the rim engaged when the vent insert is attached,

wherein the vent insert is structured and configured to reduce noise during gas washout, and
wherein a cross-sectional area of each orifice at an inner side of the vent insert is larger than a cross-sectional area of each orifice at an outer side of the vent insert, and the vent insert is insertable into the opening in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the vent insert.

53. The CPAP mask assembly of claim 52, wherein the opening comprises a moulded opening.

54. The CPAP mask assembly of claim 52, wherein the vent insert is constructed and arranged to allow the vent insert to be temporarily flexed to allow insertion of a flange through the opening before the vent insert returns to its unflexed position whereby the flange engages under the rim of the opening.

55. The CPAP mask assembly of claim 54, wherein the vent insert comprises a material that is more flexible than the rigid plastics shell and inlet tube.

56. The CPAP mask assembly of claim 52, wherein the vent insert comprises a material that is more flexible than the rigid plastic shell and inlet tube thereby allowing the vent insert to be initially squeezed through the opening before the vent insert resiliently expands to engage the rim of the opening.

57. The CPAP mask assembly of claim 52, wherein the vent insert has at least four orifices.

58. The CPAP mask assembly of claim 52, wherein the vent insert is provided to the shell.

59. The CPAP mask assembly of claim 52, wherein the vent insert is constructed and arranged to allow the vent insert to be temporarily flexed to allow an edge of the rim to enter the groove, after which the vent insert returns to its unflexed position thereby securing the vent insert relative to the rim.

60. A CPAP mask for use with a system for supplying breathable gas for treatment of obstructive sleep apnea, the CPAP mask, comprising:
a vent assembly, the vent assembly comprising:
a vent insert received in an opening for purposes of CO2 washout in the CPAP mask while reducing noise generated by CO2 washout, the opening in the CPAP mask bounded by a rim, and
the vent insert having more than one orifice, the vent insert having, in at least one cross-sectional view, at least one groove at its periphery, the at least one groove and the rim engaged when the vent insert is inserted into the opening,

wherein the at least one groove of the vent insert and the opening have non-circular, complementary shapes, and
wherein a cross-sectional area of each orifice at an inner side of the vent insert is larger than a cross-sectional area of each orifice at an outer side of the vent insert, and the vent insert is insertable into the opening in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the vent insert.

61. The CPAP mask of claim 60, wherein the CPAP mask is formed of a polycarbonate.

62. The CPAP mask of claim 61, wherein the opening comprises a moulded opening.

63. The CPAP mask of claim 60, wherein the vent insert is constructed and arranged to allow the vent insert to be temporarily flexed to allow insertion of a flange through the opening before the vent insert returns to its unflexed position whereby the flange engages under the rim of the opening.

64. The CPAP mask of claim 63, wherein the vent insert comprises a material that is more flexible than the CPAP mask.

65. The CPAP mask of claim 60, wherein the vent insert has at least four orifices.

66. The CPAP mask of claim 60, wherein the vent insert is constructed and arranged to allow the vent insert to be temporarily flexed to allow an edge of the rim to enter the groove, after which the vent insert returns to its unflexed position thereby securing the vent insert relative to the rim.

67. A CPAP mask for use with a system for supplying breathable gas for treatment of obstructive sleep apnea, the CPAP mask, comprising:
a vent assembly, the vent assembly comprising:
a vent insert received in an opening for purposes of CO2 washout in the CPAP mask, and
the vent insert having more than one orifice,
wherein the vent insert is structured and configured to reduce noise during CO2 washout, and
wherein a cross-sectional area of each orifice at an inner side of the vent insert larger than a cross-sectional area of each orifice at an outer side of the vent insert, and the vent insert is insertable into the opening in only a single orientation in which a smaller end of each orifice is positioned at the outer side of the vent insert.