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 xe2x80x9cmaskxe2x80x9d 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.
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 xe2x80x9cre-breathingxe2x80x9d 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.
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.