Abstract:
A swivel elbow includes a conduit pathway separated from a vent airflow pathway for conducting respective gas flow streams in use. The swivel elbow further includes baffle structure that separates the respective gas flow streams so that they do not interfere with each other within an interior (e.g., breathing chamber) of a mask to which the elbow is connected in use.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. national phase of international application PCT/AU2005/001091 filed 25 Jul. 2005 which designated the U.S. and claims the benefit of U.S. Provisional Application No. 60/590,338 filed 23 Jul. 2004, each incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a swivel elbow for use with patient interfaces used in the treatment of sleep disordered breathing. 
     2. Background of the Invention 
     Since the invention by Colin Sullivan of the use of nasal Continuous Positive Airway Pressure (nasal CPAP) to treat “snoring sickness” (see U.S. Pat. No. 4,944,310) there have a number of advances directed towards improving the noise and comfort of therapy. In nasal CPAP therapy, a supply of air at positive pressure is delivered to the entrance of a patient&#39;s airways via an air delivery conduit and some form of patient interface, such as a mask. The early masks were custom made for each patient and glued on each night. A typical mask comprises:
         (i) a frame which defines a nose-receiving mask cavity;   (ii) a seal-forming face-contacting cushion which in use is positioned between the frame and the patient&#39;s face; and   (iii) a vent to atmosphere which amongst other things allows exhaled CO 2  to vent to atmosphere, thus reducing CO 2  rebreathing.       

     It is generally desirable for the treatment system (including the source of pressurised air and the patient interface) to be as quiet as possible so as not to disturb sleep. 
     The supply of air at positive pressure may be provided by a blower, sometimes referred to as a flow generator. Such devices typically include an electric motor and impeller housed in a volute. Spinning the motor (and thus the impeller) generates a flow of air. When the flow is attached to an air circuit, a pressure is created due to the impedance of the circuit. Spinning the motor faster generates a supply of air at higher pressure, but also more noise. As a fluid such as air flows through a pipe or conduit it loses pressure. Bends and curves in the pipe affect the amount of pressure loss. See Perry&#39;s Chemical Engineers Handbook 6th Edition, McGrawHill, 1984, Section 5, Fluid and Particle mechanics. The greater the pressure drop in each component (i.e. the higher the impedance) of the air circuit (for example along the air delivery conduit) the harder the blower must work in order to provide sufficient pressure in the patient interface. The harder the blower has to work, the greater noise it will generate. Thus generally it is important to design components in the air path to have a low impedance. 
     A further reason for minimising the impedance of components in the air path is to minimise pressure swings as the pressure fluctuates within the mask due to the patient breathing. A higher entry impedance at the mask will lead to a higher pressure difference between inspiration and expiration, which may lead to patient discomfort and additional cyclic noise. 
     The process of air venting from the mask creates noise. Since patients must wear their mask all night while sleeping, there is a need for the vent to be quiet. Some quiet vents are described in U.S. Pat. No. 6,561,190 (Kwok et al.) and U.S. Pat. No. 6,561,191 (Kwok et al.). The contents of these two patents are hereby expressly incorporated by cross-reference. 
     Whilst in some mask designs—such as the ResMed MIRAGE® mask—the air delivery conduit is fixed in position in relation to the frame, other masks—such as the ResMed ULTRA MIRAGE® mask—include a swivel elbow. The swivel elbow enables the air delivery conduit to rotate with respect to the mask. This enables a patient to place the air delivery conduit in a preferred position such as over the head or on the left or right sides. Absent a swivel, inadvertent movement of the air delivery conduit can disrupt the seal and thus therapy. 
     In designing hard parts for patient interfaces, such as a mask frame and elbow constructed from polycarbonate or similar materials, regard must be had to how the part will be moulded. For ease of manufacture, the tool from which a component is manufactured generally has two parts that form the shape of the component. Once the component has been formed, the tool is opened by withdrawing one part along a ‘line of draw’ that is of constant radius (including a straight line). Parts must be designed within the constraints of what is manufacturable. 
     Some swivel elbows, such as the one used in ResMed&#39;s ULTRA MIRAGE® mask, incorporate a vent. See U.S. Pat. No. 6,691,707 (Gunaratnam et al.). Incorporating a vent in a swivel elbow can allow the patient some control over the direction in which air is vented. Thus the vented air may be directed away from the patient or anyone sleeping close by. Incorporation of a vent in an elbow can simplify moulding of the mask frame. 
     Vent flow rate, and hence vent CO 2  flow rate is a function of the pressure differential between the mask interior and ambient pressure. The higher the differential, the higher the flow rate. With a fixed vent, whether adequate CO 2  washout occurs is defined by what happens at the lowest operating mask pressure, typically 4 cmH 2 O. The flow rate is also a function of vent geometry. 
     In some prior art vents incorporated in elbows air entering the elbow from a blower can short-circuit the mask and pass straight out the vent. 
     Another known swivel elbow which includes a vent is described in International Patent Application PCT/AU2003/001162 (published as WO 2004/022147) Drew et al. the contents of which are hereby expressly incorporated by cross-reference. This elbow includes a baffle in the elbow as described in the &#39;1162 PCT application. A commercial version of this elbow is found in ResMed&#39;s ACTIVA mask system. 
     A potential problem with including a baffle in the elbow is that while it may assist with CO 2  washout, it may impede flow from the blower. Increased impedance from a baffle may require a blower to work harder to generate enough pressure and thus result in increased noise. A poorly designed baffle and corresponding vent may be unnecessarily noisy. A possible way of avoiding increased impedance is to make the elbow larger overall, however this is undesirable for other reasons such as aesthetics. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention there is provided a compact low impedance swivel elbow incorporating a quiet vent which provides adequate CO 2  washout. 
     In accordance with another aspect of the invention there is provided a swivel elbow including a baffle which divides an interior of the elbow into a conduit pathway and a vent pathway, the baffle being positioned adjacent an interior wall of the conduit pathway. 
     In accordance with another aspect of the invention, there is provided a swivel elbow including a baffle which defines a part annular vent pathway. 
     In accordance with another aspect of the invention, there is provided a swivel elbow having separate conduit and vent airflow pathways for conducting respective airflow streams in use and further including baffle structure which separates said respective airflow streams so that they do not interfere with each other within an interior of a mask to which the elbow is connected in use. 
     Another aspect of the invention is a swivel elbow which incorporates a pressure port. 
     In accordance with another aspect of the invention there is provided a noise reducing structure for separating two airflow streams that pass close to one another. 
     In accordance with still another aspect of the invention, there is provided a vent elbow comprising a generally L-shaped main body having a frame engaging portion and a base portion, a conduit pathway to deliver an incoming gas flow stream from the base portion towards the frame engaging portion, a vent pathway to allow an outgoing gas flow stream to flow from the frame engaging portion towards a vent formed in the main body, a cylindrical portion extending from the frame engaging portion, and a baffle provided to the main body to separate and at least partly define the conduit pathway and the vent pathway, wherein the baffle is part annular or part cylindrical and has an axis that is concentric with an axis of the cylindrical portion. 
     These and other aspects of the invention are described in and/or apparent from the following description of exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of prior art ACTIVA mask assembly (Reproduced from U.S. Design D486,226); 
         FIG. 2  shows a side view of a patient wearing prior art mask assembly including a cushion, frame and swivel elbow. The mask assembly is shown in cross-section. (Reproduced from PCT/AU2003/001162); 
         FIG. 3  shows a rear view of prior art ACTIVA swivel elbow including a baffle; 
         FIG. 4  shows a side view of prior art ACTIVA swivel elbow; 
         FIG. 5  shows a side view of a swivel elbow in accordance with a first embodiment of the invention; 
         FIG. 6  shows a rear perspective view of the elbow of  FIG. 5 ; 
         FIG. 7  shows a rear view of the elbow of  FIG. 5 ; 
         FIG. 8  shows a front view of an elbow in accordance with a second embodiment of the invention. In this and following figures, thicker lines show edges while thinner lines are tangent lines on surfaces; 
         FIGS. 9   a - 9   e  show cross-sections of different variants taken along line  9   a - 9   a  of the elbow of  FIG. 8 . The circle shown in  FIGS. 9   a - 9   e  is the approximate region a detail of which is shown in  FIG. 10 ; 
         FIG. 10  shows detail from  FIG. 9   a  with and exemplary approximate dimensions shown in millimeters; and 
         FIG. 11  shows a rear view of the elbow of  FIG. 8  and thus shows a view similar to  FIG. 7 . The dimension shown is in millimeter s. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is noted that two embodiments are shown in the figures. The first embodiment has 100-series numbers, the second embodiment 200-series number. Corresponding features have corresponding numbers, hence the baffle is numbered  160  in the first embodiment and  260  in the second embodiment. Where in the following description reference is made to a feature for one embodiment, generally the same description applies to the second embodiment. 
     In a preferred form the invention is applied to a swivel elbow. Preferably the swivel elbow is removably replaceable on a mask frame. 
     A swivel elbow  130  in accordance with an embodiment of the invention has a main body that is generally L-shaped. It comprises a frame-engaging portion  135  and base portion  150 . The base portion  150  includes a generally cylindrical section  155  over which in use an end of an air delivery conduit (not shown) may be friction fit (or otherwise engaged). The frame engaging portion  135  includes a series of slots  137  adapted to engage with a frame (not shown). The frame engaging portion also includes generally cylindrical portion  180 . See  FIG. 5  to  FIG. 9   a - 9   e.    
     Within the swivel elbow  130  two fluid pathways are defined by baffle  160 . A conduit pathway  175  allows flow between the air delivery conduit and the mask cavity. A vent pathway  170  allows flow between the mask cavity and atmosphere. In one form the vent pathway  170  forms part of an annulus. In a preferred form the baffle  160  is part-cylindrical, having an axis  161  (See  FIG. 7 ). It is arranged to be generally concentric with cylindrical portion  180 , that is their respective axes are generally co-linear. The ends of the baffle  160  subtend an angle of up to 180° with respect to its axis. It is noted that extending the baffle so that it subtended an angle greater than 180° might cause it to impinge on the conduit pathway  175 . Hence preferably the angle is 180°. In a preferred form, the baffle  160  is spaced from generally cylindrical portion  180  by four spacers  165  and vent pathway  170 . In other forms, fewer or more spacers may be used. By increasing the angle which the baffle  160  subtends, it is possible to decrease the height of the spacers  165 . In this way the baffle  160  is positioned generally close to cylindrical portion  180  and does not impinge significantly on air flowing along conduit pathway  175 . This arrangement leaves the cross-section of the incoming conduit pathway  175  as close to a circle as possible which provides minimum impedance for a given elbow size. Thus the impedance of the conduit pathway  175  is not significantly increased by the presence of the baffle  160 . 
     As best seen in the cross-sections shown in  FIGS. 9   a - 9   e  and  10  the vent pathway  270  generally tapers from a wider inlet  272  positioned adjacent the mask interior to a narrow outlet  274  positioned adjacent atmosphere. In a preferred form the vent pathway  270  has a first approximately constant cross-section region  273  along approximately ⅔ of its length from the inlet  272 . A second narrower approximately constant cross-section  276  lies adjacent narrow outlet  274 . In between the first and second approximately constant cross-section regions  273 ,  276  there is a tapering region  277 . See  FIG. 10 . 
     In a preferred form a lip portion  162 ,  262  the baffle  160 ,  260  extends more than approximately 1.5 mm, preferably 1.8 mm into the mask cavity. Compare  FIG. 4  and  FIG. 5 . See also  FIG. 9   a  and  FIG. 10 . Extending the baffle reduces noise and improves CO 2  washout. An extension of less than 1.5 mm may not provide significant improvement in noise and CO2 washout. Extending the baffle beyond 2.5 mm may increase the risk that it will interfere with the tip of a patient&#39;s nose in use. Hence an extension of 1.8 mm is preferred. 
     In some prior devices air from the conduit pathway  275  can flow directly across the inlet  272  of the vent pathway  270  causing an audible tone. The extension of the baffle reduces or prevents air from so passing directly over the inlet  272  and thus reduces or eliminates the tone. 
     Extension of the baffle into the mask cavity directs incoming airstreams or gas streams (via conduit pathway  175 ) and outgoing airstreams (via vent pathway  170 ) so that they do not interfere with each other. Prior art baffles tend to give rise to the situation where there are two narrow and relative fast airstreams moving parallel to each other but in opposite directions. The extended baffle causes the airstreams to remain separate until the inlet air has slowed down and the two airstreams have diverged. Hence there will be much less interference between the streams and the resultant turbulence and increase in noise is avoided. 
     There are other ways of separating the incoming and outgoing airstreams. Shaping the baffle within the mask cavity as shown in  FIGS. 9   b - 9   e  may help to reduce interference between the incoming and outgoing airstreams. These variations have been developed to help separate the two airpaths and to minimise the shear and turbulence between the streams. The baffles shown in  FIGS. 9   b  and  9   c  have portions that are shaped to direct the incoming air away from the outgoing airstream and vent. As well as reducing noise, this assists in allowing the fresh inlet air to reach the patient. The baffle shown in  FIG. 9   d  has a portion that is shaped to smoothly guide the exhaled air into the vent pathway.  FIG. 9   e  combines the advantages of  9   c  and  9   d . The aforesaid portions of the baffle may be upwardly-and/or downwardly curved or bent to orient the gas stream(s) in the desired location. 
     A swivel elbow as shown in  FIGS. 5-7  and  8 ,  9   a  and  10  is relatively simple to mould since all features are in the line of draw. The variations shown in  FIG. 9   b - 9   e  may be more complicated to mould. 
     Another aspect of the invention is the swivel elbow port and its respective port cap. The swivel elbow port  205  is illustrated in  FIGS. 8 ,  9   a  and  11 , and the port cap  115  is illustrated in  FIGS. 5-7 . By connecting suitable tubing to this port it is possible to inject medical gas (such as O  2 ) into the air circuit, sample gas or take a pressure reading amongst other things. The cap  115  is used to seal the port when the port is not in use. The swivel elbow is removably replaceable. A swivel elbow in accordance with an embodiment of the invention may include none, one or more such ports. For some treatment applications, e.g. home based treatment, access ports may not be required and so a mask may be fitted with an elbow which has no port. For other treatment applications, such as in a hospital, ports may be desired and thus an elbow with a suitable number of ports may be fitted. A problem with some prior art masks which included ports is that the caps kept falling off in use. While this may be tolerated in treatment situations where ports may occasionally be used, in other treatment situations where ports are not desired, this is a nuisance. This it is possible to overcome this problem by assembling a patient&#39;s mask with the appropriate number of ports, which may include no ports. Another advantage of including the ports on the swivel elbow is that any associated tubing is less likely to tangle with an air delivery conduit if the air delivery conduit is moved. 
     Although the invention has been described with reference to preferred embodiments, it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention. For example whilst the invention is particularly useful for use with a swivel elbow, the principles of the invention are applicable to masks with non-swivelling elbows. Furthermore, the principles of the invention which reduce interference between incoming and outgoing air streams are applicable to masks without elbows at all, but simply having inflowing and out flowing airstreams near one another.