Abstract:
A patient interface assembly adapted to be connected to a gas supply pump to deliver breathable gas to the inlet of a patient&#39;s respiratory system comprises a supply conduit, a patient interface, a branch swivel connector including a Y-piece and an elbow that swivels relative to the Y-piece, the branch swivel connector being adapted to be located to the rear of a patient&#39;s head in use and being connected to the supply conduit. A pair of inlet tubes each have a first end positioned in use near a mouth of a patient and are connected to the nose mask, a middle portion arranged to pass across a cheek of a patient and an end portion being joined to the Y-piece of the branch connector. The inlet tubes have a flat configuration and are provided with a plurality of internal ribs which prevent the tubes being crushed. A strap is secured to the patient interface and adapted to pass around the sides and rear of the head to hold the patient interface in position on the head.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Divisional Application of U.S. application Ser. No. 08/524,148, filed Sep. 6, 1995, now abandoned, which is a Continuation of U.S. application Ser. No. 07/994,153, filed Dec. 21, 1992, now abandoned, the specifications and drawings of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an improved CPAP respiratory apparatus which will increase patient comfort and therefore compliance. 
   The fundamental disclosure of CPAP is made in the specification of PCT/AU82/00063 published under WO 82/03548 which discloses the supply of air to the nose of the patient at an elevated pressure, the air being supplied through a large bore inlet tube. The elevated pressure at which the air is supplied is approximately 10 cm water gauge although pressures in the range of approximately 5–20 cm water gauge are encountered. However, this pressure is measured while the patient is not breathing and as the patient inspires and expires the pressure in the patient&#39;s mask rises and falls typically by approximately 1–2 cm above and below the steady state level. The large bore inlet tube has an unrestricted internal diameter of approximately 20 mm and thus does not introduce unacceptably high pressure drops and swings in the gas delivery system during breathing. All commercially available devices have standardised to this size and arrangement. 
   For the patient, the work of breathing increases in proportion to the size of the pressure swing during the respiration cycle. In particular, the discomfort experienced by the patient also increases in proportion to the increase in mask pressure during breathing out. In order to deliver the patient&#39;s breathing air requirements without significant pressure loss in the supply tube, which would create a relatively large pressure swing during the breathing cycle, the delivery tube and inlet to the nose mask were selected to be substantially unrestricted and to have the large bore of approximately 20 mm. 
   However, this arrangement and tubing size are not particularly convenient as far as the comfort of the patient and control of the treatment are concerned. In practice, patients wearing nose masks or equivalent devices including such tubing can turn only from side to side and the freedom of movement of the patient is impaired by the tubing. If the tubing and nose mask could be made smaller, and more acute changes in the direction of air flow tolerated, a much more comfortable and acceptable air delivery system would result. Also if a humidifier and/or a filter could be placed between the pump and the mask, then patient comfort could be increased. Similarly, if a flow measuring device could be so located, control of treatment could be enhanced. 
   It is the object of the present invention to substantially overcome or ameliorate the above mentioned difficulties by the provision of a CPAP respiratory apparatus which maintains the pressure of air or other breathable gas at the point of immediate access to the patient&#39;s respiratory system substantially constant notwithstanding in-line components which introduce appreciable pressure drops. 
   It is appreciated that increasing the resistance to flow in the supply tube results in an increased pressure drop between the “pump end” and “patient end” of the delivery tube. In order to compensate for this pressure drop between the ends of the delivery tube whilst maintaining flow, the present invention seeks to maintain the air pressure at the “patient end” substantially constant. This is done by sensing the pressure within the nose mask, or equivalent device, itself. 
   Two known commercially available CPAP respiratory devices involve some pressure or air flow control. One of these is the device sold by RESPIRONICS of the USA under the trade name BiPAP in which the supply pressure can be switched between a lower pressure and a higher pressure in accordance with the patient&#39;s respiratory cycle in order to assist the patient&#39;s breathing effort. This switching is achieved by sensing air flow through a sensor in the pump of the air supply system. Another commercially available device sold by HEALTHDYNE also of the USA has a control mechanism which controls the pressure at the outlet of the air pump. 
   Both of these commercially available devices use the standard large bore 20 mm inlet tubing which is substantially unrestricted downstream of the pump outlet and will not operate satisfactorily with pressure drop inducing components such as small bore tubing. This is thought (as will be apparent from the experimental data given hereafter) to be due to the large pressure drop which causes large pressure swings in the nose mask as the patient inspires and expires. In particular, because these prior art devices do not attempt to derive the signal to control the operation at the air pump as near to the patient&#39;s respiratory system as possible, and downstream of all pressure drop inducing components, there is a problem of time lags and phase shifts as regards the supply of air to and from the patient. It has been experimentally determined by the applicant that by sensing the pressure at the patient&#39;s mask and servo-controlling same to be substantially constant, the problems introduced by the pressure drop created in the supply tubing, can be substantially overcome. 
   SUMMARY OF THE INVENTION 
   In accordance with the first aspect of the present invention there is disclosed a CPAP respiratory apparatus comprising a breathable gas delivery device adapted to deliver breathable gas to the inlet of a patient&#39;s respiratory system, a breathable gas supply means having an outlet and arranged to supply breathable gas to said outlet at a pressure above atmospheric pressure, and a flexible conduit having an internal bore and being connected between said outlet and said gas delivery device wherein a pressure transducer is connected to said device to sense the pressure at said respiratory system inlet, and a servo-controller is connected to both said gas supply means and said pressure transducer to adjust the operation of said gas supply means to maintain the pressure at said respiratory system inlet substantially constant. 
   Preferably, at least that portion of said conduit closest to said nose mask has an internal bore which is relatively small compared with the remainder of the conduit. 
   In accordance with a second aspect of the present invention there is disclosed a method of operating a breathable gas supply means of a CPAP respiratory apparatus comprising a breathable gas delivery device adapted to deliver breathable gas to the inlet of a patient&#39;s respiratory system and connected by a flexible conduit to an outlet of said gas supply means to receive breathable gas therefrom at a pressure above atmospheric pressure, said method comprising the steps of sensing the pressure supplied to said respiratory system inlet by said gas delivery device, and using the sensed pressure to servo-control said gas supply means to maintain the pressure at said respiratory system inlet substantially constant. 
   Preferably, at least one pressure drop inducing device is located in the gas supply line between the pump and patient. 

   
     BRIEF DISCUSSION OF THE DRAWINGS 
     Some embodiments of the present invention will now be described with reference to the drawings in which: 
       FIG. 1  is a schematic perspective view of the nose mask and air supply tube of the CPAP respiratory apparatus of a first embodiment; 
       FIG. 2  is a partial perspective view of the mask only with its membrane distended; 
       FIGS. 3 and 4  are cross-sectional views along the lines III—III and IV—IV of  FIG. 1  respectively; 
       FIG. 5  is a perspective view of the nose mask, harness and supply conduit of a second embodiment; 
       FIG. 6  is a view similar to  FIG. 5 , but of a third embodiment; 
       FIG. 7  is a view similar to  FIG. 5 , but of a fourth embodiment; 
       FIG. 8  is a view similar to  FIG. 5 , but of a fifth embodiment; 
       FIG. 9  is an enlarged view of the branched connector of  FIG. 8 ; 
       FIG. 10  Is a front view of the nose mask of  FIG. 8 ; 
       FIG. 11  is a side elevation of an alternative nose mask; 
       FIG. 12  is a plan view of the nose mask of  FIG. 11 ; 
       FIG. 13  is a cross-sectional view taken along the line XIII—XIII of  FIGS. 5 ,  6  and  11 ; 
       FIG. 14  is a cross-sectional view taken along the line XIV—XIV of  FIG. 7 ; 
       FIG. 15  is a cross-sectional view taken along the line XV—XV of  FIGS. 9 and 10 ; and 
       FIG. 16  is a view similar to  FIG. 1  but illustrating a further embodiment having various different types of pressure drop inducing components. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
   As seen in  FIG. 1  a nose mask  1  is of generally known configuration and is substantially as disclosed in Australian Patent Application No. 77110/91. The mask  1  takes the form of a shell  2  of firm plastics which is shaped to fit over at least the nose region of the patient. A distendable membrane  7  is mounted on the shell  2  and forms a face contacting portion for the mask  1 . The shell  2  and membrane  7  together define a chamber which receives the patient&#39;s nose. The chamber communicates with an air or other breathable gas supply aperture to which a short length of supply tube  3  is connected. The aperture is preferably provided with at swivel joint  4  so that the supply tube  3  can rotate relative to the remainder of the face mask  1 . This prevents the supply tube  3  from becoming inadvertently twisted. The nose mask  1  is retained on the patient  5  by means of conventional straps  6 . 
   In the vicinity of the swivel joint  4  are located a series of apertures  8  through which air or other breathable gas exits to atmosphere as indicated by the arrows in the drawing. Pressure is sensed in the interior chamber of mask  1  by a thin flexible pipe  10  which is connected a pressure transducer  11  which provides a continuously sensed electrical output signal carried on cable  18  to a servo-controller  17  for the pump  14 . 
   The supply tube  3  is of a small bore (typically having an effective internal diameter of 9–15 mm) and thus the patient whilst sleeping cannot roll onto an uncomfortable large bore tube. As indicated in  FIG. 3  the small bore supply tube  3  in this embodiment has a substantially triangular cross-sectional shape and is flexible. The term “effective internal diameter” means the diameter of a tube of circular internal cross-sectional shape which has the same internal cross-sectional area. 
   Preferably a swivel joint  12  connects the small bore supply tube  3  to a substantially conventional large bore supply tube  13 . The pipe  10  which typically has a very small bore, or the cable  18  can conveniently be connected alongside the supply tubes  3 ,  13 . This supports the pipe  10  yet enables the pressure transducer  11  to be located either at, or remote from, the nose mask  1 . If desired, the pipe  10  and tube  3  can be combined in a single moulding. Alternatively, if the pressure transducer  11  is located within, or adjacent to, the mask the electrical outputs signal cable  18  of the transducer can be conveyed to the servo-controller  17  via small pipe  10 . 
   The large bore supply tube  13  is connected to a pump  14  which consists essentially of an electric motor  15  and fan  16 . The pump  14  preferably supplies air, however, other breathable gases such as mixtures of air and oxygen can be supplied in known fashion. The term “air” shall be used hereafter for such gases. The electric motor  15  is controlled by a substantially conventional servo-controller  17  which receives as an input, the output from the pressure transducer  11 . If desired, the pipe  10  can be sufficiently long to locate the transducer  11  at the pump  14 . 
   It will be apparent to those skilled in the art that the pressure transducer  11  and servo-controller  17  enable the operation of the electric motor  15  to be controlled so as to maintain the air pressure within the nose mask  1  substantially constant throughout the respiration cycle. As a result, the electric motor  15  accommodates in its operation the fluctuating internal pressure drop created by both the patient&#39;s breathing and the small bore of the supply tube  3 . In particular, the supply conduit interconnecting the mask  1  and air pump  14  can now have a small bore (in the range of from 9 to 15 mm in internal diameter) over at least part of its length. Particularly over that section in the region of the patient&#39;s face and head. This represents a decrease in available cross-sectional area of the supply tube  3  from 43.75% to 79.75% respectively. 
   Because the supply tube  3  has such a reduced bore, the tube is much more flexible and comfortable for the user and can conveniently be fixed to the straps  6  used for holding the nose mask on the patient&#39;s face. In particular, it is not generally possible to lie upon the 20 mm large bore tubing without feeling discomfort, however, with the relatively small bore supply tube  3  this is possible. As a consequence, the patient&#39;s comfort is substantially increased. This increases the patient&#39;s compliance, especially after the more pronounced symptoms of sleep apnea have been initially ameliorated. The increased compliance is of particular importance in the long term treatment of the patient. 
   Comparative Tests 
   The above described apparatus was tested alongside the above mentioned commercially available BiPAP (Respironics) device and TRANQUILITY PLUS device (the trade name of the Healthdyne product). 
   For the experiment, the large bore supply tube  13  took the form of standard 20 mm bore tubing. The length of the small bore supply tube  3  was 17 cm. All three units were tested with the same breathing simulator which delivered a substantially sinusoidal air flow having a 500 ml tidal volume at 12 cycles/minute. The peak flow during both inspiration and expiration was 50–60 litres per minute. 
   For each air pump arrangement (BiPAP, TRANQUILITY PLUS and air pump  14 ) three types of masks were used. The first was a conventional mask with a 20 mm constant diameter supply tube (in the case of BiPAP and TRANQUILITY PLUS the mask and tube were as supplied with the equipment). The second mask was the mask  1  with the supply tube  3  being of circular cross-section and of 15 mm internal diameter. The third mask was the mask  1  but with 9 mm internal diameter for the supply tube  3 . 
   The results for 5 different levels of CPAP pressure (0, 5, 10, 15 and either 17 or 20 cm water gauge) are set out in Table 1. The figures given are air pressures in cm of water gauge with P stat  being the average or static pressure within the mask whilst ΔP tot  is the combined pressure swing during the inspiration/expiration cycle of the breathing simulator. 
   It can be seen that the combined pressure swing ΔP tot  increases significantly with decreasing tubing diameter with the HEALTHDYNE and BiPAP units, while the servo-controlled unit  14  maintains pressure in the mask  1  generally to better than 1 cm total swing for all sizes of tubing. It follows therefore that an improved result allowing the use of the more comfortable small bore tubing, has been achieved. 
   A second embodiment is illustrated in  FIG. 5  where like parts are indicated by a designator increased in magnitude by 20. Thus, the mask  21  of the second embodiment is a face mask and includes a pressure transducer  11  located within the mask  21  as indicated by broken lines in  FIG. 5 . The transducer  11  Is located within the mask  21  and between the patient&#39;s nose and the apertures  8 . A substantially similar arrangement of straps  26  retains the nose mask  21  in position. As indicated in  FIG. 13 , the cross-sectional shape of the small bore inlet tube  23  is circular. Again, the small bore inlet tube  23  is connected to the conventional large bore inlet tube  13  by means of a substantially conventional swivel joint  12 . 
   A third embodiment is illustrated in  FIG. 6  in which the nose mask  21  and small bore inlet tube  23  are substantially as in  FIG. 5 . However, a flow orifice  111  (preferably of the type disclosed in U.S. Pat. No. 4,006,635 [Billette]) only is located in the mask  21  and is connected by two small tubes  210  to a flow transducer  110 . The tubes  210  are located one upstream and one downstream of the flow orifice  111 . As before, the pressure transducer  11  is connected to the mask  21  via the tube  10 . In addition, a cap  29  with straps  36  is provided for the patient in order to secure the small bore inlet tube  23 . 
   A fourth embodiment is illustrated in  FIG. 7  in which like parts have their designator increased in magnitude by 40 relevant to the embodiment of  FIG. 1 . It will be seen that the configuration of the nose mask  41  is changed so as to provide a swivel joint  49  which is sufficiently large to accommodate the pressure transducer  11  which is again located downstream of the apertures  48 . The configuration of the straps  46  is also different and provides an alternative securing arrangement. 
     FIGS. 8–10  illustrate a fifth embodiment in which a nose mask  51  is supplied by means of a split or dual inlet tubes  53  each of which is supplied from a branch swivel connector  54  illustrated in more detail in  FIG. 9 . The connector  54  is located to the rear of the patient&#39;s head and the nose mask  51  is secured in position by means of a forehead strap  56 . 
   As seen in  FIG. 9 , the branch connector  54  includes an elbow  61  which swivels as indicated by the arrow in  FIG. 95  relative to a Y-piece  62 . The inlet tubes  53  are sealed directly to the Y-piece  62 . 
     FIG. 10  illustrates further detail of the nose mask  51  and, in particular, illustrates the cavity  64  which receives the patient&#39;s nose. The flow orifice  111  is located within the inlet to the cavity  64  as are the exit apertures  68 . It will be seen that the inlet tubes  53  extend across each cheek of the patient and alongside the nose mask  51 . As seen in  FIG. 15 , the inlet tubes  53  preferably have a flat configuration and are provided with a plurality of internal ribs  69  which prevent the inlet tube  53  being crushed between the pillow and the patient&#39;s head. 
   Turning now to  FIGS. 11 and 12 , a still further embodiment of the nose mask  71  is illustrated. The nose mask  71  has a substantially rigid outer shell  72  which has an inlet  73  of substantially circular cross-section which includes exit apertures  78  and is sufficiently large to accommodate the pressure transducer  11  as illustrated (or the flow transducer  110 -not illustrated). Sealingly connected to the outer shell  72  is a soft membrane  77  which is shown in  FIGS. 11 and 12  in its distended position and has a nose receiving aperture  79 . Once the nose of the patient is inserted into the aperture  79 , the membrane  77  then conforms itself to the surface of the patient&#39;s skin thereby providing an effective seal. 
   As indicated in  FIG. 14 , if desired the inlet tube  35  and equivalents, can be provided with two internal passageways  80  which can be used either to transmit pressure from the region adjacent the patient&#39;s nose or to locate the electric cable(s) from transducers. 
   With the above described distendable mask, the deformable membrane has hitherto stretched and compressed with changes in the mask pressure. This oscillation is somewhat disturbing to the patient and is substantially eliminated in accordance with the above since the servo-controller  17  maintains the mask pressure substantially constant throughout the respiration cycle. 
   Furthermore, most of the noise escaping from a CPAP device comes either from the air inlet or air outlet. This can be reduced by placing baffles in the air inlet and/or the air outlet, but with the prior art devices this is at the expense of increasing the pressure drop and pressure swings in the mask during inspiration and expiration. 
   In accordance with the above described arrangements, this additional baffling can be added and the pressure swings that would otherwise result can be compensated for by servo-controlling the pressure in the mask. Since mask comfort and noise level are the two most important determinants of patient comfort and compliance, this represents a substantial advantage. 
   Like the small bore tube  3 ,  23  and the connector  54 , such baffles represent pressure drop inducing components. As indicated in  FIG. 16 , such components can take the form of baffles  301 , sharp bends  302 , a filter  303 , a high pressure drop air outlet diffuser  304  having a diverting tube to direct flow away from a sleeping partner, a flow orifice  111  and a humidifier  306  such as a hydroscopic condensing humidifier made by ICOR AB of Sweden. The pressure drop introduced by any or all of these “accessories” can be accommodated so as to maintain the pressure at the patient&#39;s nose substantially constant. 
   If desired, the transducers  11 , 110  can be located at or near the mask as illustrated and connected by cables  18  to the control apparatus  17 . Alternatively, the tubes  10 , 210  can be sufficiently long to enable the transducers  11 ,  110  to be located adjacent the pump  14 . This arrangement has the advantage that no electric cables are located near the patient. 
   In addition, if the positions of the flow orifice  111  and humidifier  306  shown in  FIG. 16  are reversed, then a combined sensing arrangement is possible. In this arrangement the flow orifice  111  is connected to the flow transducer  110  as before via two tubes  210 . The downstream one of the tubes  210  is branched to provide the tube  10  for the pressure transducer  11 . 
   The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the art can be made thereto without departing from the scope of the present invention. 
   For example, although a nose mask is described and illustrated in detail, a full face mask or nasal prongs (not shown) can also be used. 
   
     
       
             
             
             
             
             
             
             
             
             
             
             
           
             
           
             
             
             
             
             
             
             
             
             
             
             
           
             
           
             
             
             
             
             
             
             
             
             
             
             
           
             
           
             
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE I 
             
             
                 
                 
             
             
                 
               Pstat 
               ΔPtot 
               Pstat 
               ΔPtot 
               Pstat 
               ΔPtot 
               Pstat 
               ΔPtot 
               Pstat 
               ΔPtot 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               BIPAP UNIT 
             
           
        
         
             
               Conv. Mask 
                 
                 
               0.5 
               1.00 
               10.0 
               1.20 
               15.0 
               1.40 
               20.00 
               2.40 
             
             
               New Mask 15 
                 
                 
                 
               1.20 
                 
               1.30 
                 
               1.40 
                 
               2.00 
             
             
               New Mask 9 
                 
                 
                 
               2.20 
                 
               2.70 
                 
               3.80 
                 
               5.40 
             
           
        
         
             
               HEALTHDYNE TRANQUILITY PLUS 
             
           
        
         
             
               Conv. Mask 
               0.0 
               0.70 
               5.0 
               0.85 
               10.0 
               1.10 
               15.0 
               1.35 
               17.00 
               1.40 
             
             
               New Mask 15 
                 
               1.20 
                 
               1.40 
                 
               1.80 
                 
               2.10 
                 
               2.20 
             
             
               New Mask 9 
                 
               2.20 
                 
               3.20 
                 
               4.20 
                 
               4.80 
                 
               5.10 
             
           
        
         
             
               SERVO-CONTROLLED UNIT 14 
             
           
        
         
             
               Conv. Mask 
               0.0 
               0.40 
               5.0 
               0.35 
               10.0 
               0.45 
               15.0 
               0.60 
               20.00 
               0.90 
             
             
               New Mask 15 
                 
               0.75 
                 
               0.48 
                 
               0.52 
                 
               0.65 
                 
               0.95 
             
             
               New Mask 9 
                 
               0.90 
                 
               1.05 
                 
               0.65 
                 
               0.75 
                 
               0.90