Abstract:
A device for indicating an internal pressure of an inflatable medical cuff, comprising: a membrane movable in response to a difference between the internal pressure and an external pressure; and movable indication means operably associated with the membrane for indicating, in use of the device, the internal pressure.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and device for pressure indication. In particular, the invention relates to a method and device for indicating an air pressure of a cuff of a laryngeal mask or other airway device. 
   BACKGROUND OF THE INVENTION 
   In 1988, Dr. Archie Brain invented a new airway device, called the laryngeal mask, for delivering anaesthetic gases and oxygen to an unconscious patient during an operation. Prior to this, patients were commonly anaesthetised using an Endotrachael Tube running through the vocal cords of the patient into the trachea. Alternatively, a face mask connected to a gas source was held over the patient&#39;s nose and mouth. 
   In use, the laryngeal mask is inserted into the pharangeal space (throat) deflated, and when correct positioning is observed, an inflatable cuff around the outside of the mask is inflated with air using a syringe. The syringe is inserted into an end of a valve which is connected to the cuff by a small tube, such that air passed through the valve from the syringe is used to inflate the cuff. There are presently eight different sizes of the laryngeal mask available, and while all require different volumes of air to inflate the cuff, the maximum recommended intra cuff pressure is 60 cm H 2 O. 
   The Laryngeal Mask is now the preferred choice of anaesthetists in many countries, and its use continues to grow. The greater prevalence in use of the laryngeal mask has brought certain possible dangers to light. Due to human errors and the possible diffusion of nitrous oxide into the silicone cuff, the cuff may be excessively inflated. Such excessive inflation of the cuff has the potential to damage nerves and tissues around the hypopharynx, and this potential danger has been the subject of scientific papers in anaesthesia journals. 
   In order to prevent over-inflation of the cuff, it is desirable to check the intra cuff pressure when the laryngeal mask is in place. Currently, the means for checking the air pressure of the cuff include a small inflatable bulb in fluid communication with the cuff inflation tube. To get an indication of the cuff pressure, medical personnel squeeze the bulb under slight pressure from their figures to thus subjectively judge the intra cuff pressure. Alternatively, a manometer may be connected to the cuff inflation valve in order to obtain an accurate pressure measurement. Manometers are, however, relatively expensive and somewhat bulky and can be inconvenient to connect to the cuff inflation valve as the valve is usually close to the patient&#39;s mouth. 
   Similar problems can occur in other airway devices having an inflatable cuff, such as endotracheal tubes, for example. 
   The present invention attempts to address or ameliorate one or more of the shortcomings of the prior art, or to at least provide a useful alternative thereto. 
   SUMMARY OF THE INVENTION 
   The present invention provides, in one aspect, a device for indicating an internal pressure of an inflatable medical cuff, comprising:
         a membrane movable in response to a difference between said internal pressure and an external pressure; and   movable indication means operably associated with said membrane for indicating, in use of the device, said internal pressure.       

   Preferably, the internal fluid volume of the membrane is at atmospheric pressure, while the external fluid volume is at the internal cuff pressure. Preferably, the membrane is closed at one end and open to atmospheric pressure at the other end. 
   Preferably, the membrane and movable indicator are housed in a hollow body of the device. Preferably, the hollow body is at least partly transparent. Preferably, the hollow body is autoclavable and formed of one of polysulphone, Lexan (polyphalate carbonate) and Ultem (polyetherlmide). Alternatively, the device may be non-autoclavable and disposable, in which case polyvinylchloride (PVC) can be used for some or all of the device materials. Preferably, the hollow body is cylindrical and has scale markings thereon with which a position of the movable indicator may be compared to indicate the internal cuff pressure. 
   The pressure indication device has distal and proximal ends, the proximal end being defined as the end of the device which is connectable to a cuff inflation tube, while the distal end is defined as being opposite to the proximal end. In one embodiment, the proximal end has a male luer for providing a removable fluid connection to the cuff inflation tube. In another embodiment, the proximal end is directly and non-removably connected to the cuff inflation tube. 
   Preferably, the distal end of the device is adapted to connect to a syringe. In one embodiment, the distal end includes a spring loaded valve and a female luer and is arranged so that the valve is actuated upon insertion of the syringe into the female luer, thereby opening the valve and allowing fluid communication from or into the syringe into or from the device. In an alternative embodiment, a stopper valve is used instead of the spring loaded valve. The stopper valve operates to provide a similar effect. 
   Preferably, the membrane is a bellows. Preferably, the bellows is formed of silicone. Preferably, a wall thickness of the bellows is about 0.3 to 0.5 millimeters. More preferably, the thickness is about 0.4 millimeters. Preferably, pleats of the bellows have an opening angle of about sixty degrees when the bellows is in a relaxed state. Preferably, a flange is provided at the open end of the bellows for engaging a rebate in the proximal end of the device. Preferably, inner and outer chambers are defined by the membrane and the hollow body, where the inner chamber is the internal fluid volume of the membrane and the outer chamber is the external fluid volume bounded by the hollow body on the outside and the membrane on the inside. Preferably, the outer chamber includes a substantially annular chamber between a wall of the hollow body and the membrane and an end chamber between the distal end of the device and the closed end of the membrane, the end chamber and annular chamber being separated by the movable indicator. 
   Preferably, the movable indicator is connected to the membrane at the closed end thereof. Preferably, the movable indicator is formed as a disk having a peripheral flange. Preferably, the flange has one or more channels therein for allowing fluid communication between the end and the annular chambers. Alternatively, the channels may be provided in an inner part of the disk instead of the flange. Preferably, the flange has an indication mark on an outside surface thereof, the mark being visible through said hollow member. Advantageously, the indication mark co-operates with markings on the hollow body to provide an indication of whether the cuff pressure is at a safe pressure. 
   A further aspect of the invention provides a bellows for use in a pressure indication device for inflatable medical cuff, the bellows having:
         an open end and a closed end;   indication means disposed proximate said closed end for providing a pressure indication;   said closed end being movable relative to said open end in response to differential pressure between regions outside and inside of the bellows, such that said indication means is movable to indicate said differential pressure.       

   Preferably, the inside region is at atmospheric pressure, while the outside region is at an internal cuff pressure of a cuff of a laryngeal mask. Preferably, the open end has a vent to atmospheric pressure. Preferably, the open end has a flange connectable to a bellows holding member. Preferably, the indication means includes a movable indicator connected to said closed end by connection means. Preferably, the connection means includes a boss formed on said closed end. Preferably, the bellows is formed of silicone, has a wall thickness of about 0.4 millimeters and has pleats having an opening angle of about sixty degrees in a relaxed state of the bellows. 
   In a further aspect of the invention there is provided a pressure responsive device, including:
         a hollow body;   a membrane disposed within said hollow body and defining inner and outer chambers within said hollow body, the membrane being responsive to differential pressure between said inner and outer chambers; and   a movable indicator disposed toward a closed end of the membrane, the movable indicator being visible through said hollow body for indicating said differential pressure.       

   A still further aspect of the invention provides a method of determining an internal pressure of a cuff of a laryngeal mask or other airway device, including providing a pressure indication device at a cuff inflation tube of the laryngeal mask or other airway device, the pressure indication device having a membrane moveable in response to a difference between side internal pressure and an external pressure and a movable indicator operably associated with said membrane for indicating said internal pressure, and viewing said movable indicator to determine said internal pressure. 
   Preferably, if the determined internal pressure of the cuff is below a minimum level indicated on the pressure indication device, the method further includes connecting a syringe to the pressure indication device and injecting air into the cuff inflation tube from the syringe through the pressure indication device. Alternatively, if the determined internal pressure of the cuff is above a maximum level indicated on the pressure indication device, the method further includes connecting a syringe to an end of the pressure indication device and withdrawing air from the cuff inflation tube through the pressure indication device into the syringe. Preferably, following or during withdrawal or injection of air from or into the cuff inflation tube, the method further includes performing the viewing step again. 
   In another aspect, the invention provides a laryngeal mask having a pressure indication device as described above connected thereto. In another aspect, the invention provides a laryngeal mask having a pressure responsive device as described above connected thereto. 
   In another aspect, the invention provides a method of administering a breathing gas to a patient, including locating a cuff of a laryngeal mask adjacent a hypopharynx of a patient, the laryngeal mask having a pressure indication device as described above connected to a cuff inflation tube of the laryngeal mask, inflating the cuff, determining an internal pressure of the cuff according to the method described above, adjusting the internal pressure of the cuff according to the determined cuff pressure, and administering the breathing gas to the patient through the laryngeal mask. 
   The pressure indication device may advantageously be used with other medical devices having an air-inflatable cuff, including for example, endotracheal tubes, laryngeal tubes, naso-tracheal tubes, univent tubes and combitudes. The pressure indication device is also applicable to airway management devices (AMDs) and Portex laryngeal masks. 
   Advantageously, the pressure indication device of the invention is small and easily connected to, or integrally formed on the end of, the cuff inflation tube of the airway device so as to provide a simple and easily referenced indication of the internal cuff pressure. This means that medical personnel can easily obtain an objective measure of the cuff pressure without having to connect a manometer to the cuff inflation tube. The pressure indication device and methods of use therefor may advantageously reduce the incidence of damage to the hypopharynx area of a patient due to over-inflation of the cuff of the laryngeal mask or damage otherwise resulting from inadequate inflation or over-inflation of an inflatable medical cuff. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side cross-sectional view of a pressure indication device of an embodiment of the invention; 
       FIG. 2  is an end view of an indicator plate for use in the indication device shown in  FIG. 1 ; 
       FIG. 3  is a side view of the indication device of  FIG. 1 , showing example markings on the outside of the device; 
       FIG. 4  illustrates an example of use of a pressure indication device of an embodiment of the invention with a cuff inflation line and a syringe; 
       FIG. 5  is a side cross-sectional view of a bellows according to an embodiment of the invention; 
       FIG. 6  is a side cross-sectional view of a pressure indication device of another embodiment of the invention; 
       FIG. 7  is an illustration of a laryngeal mask having the pressure indication device shown in  FIG. 6 ; 
       FIG. 8  is a side cross-sectional view of a pressure indication device according to a further embodiment of the invention; 
       FIG. 9  is a side cross-sectional view of a bellows according to a further embodiment of the invention; 
       FIG. 10A  is an end side cross-sectional view of a hollow tube for use in the pressure indication device; 
       FIG. 10B  is an isometric view of the hollow body of  FIG. 10A ; 
       FIG. 11A  is an isometric view of a male end of the pressure indication device shown in  FIG. 8 ;  FIG. 11B  is an end view of the male end shown in  FIG. 11A ; 
       FIG. 11C  is a cross-section view taken along lines B-B of the male end shown in  FIG. 11B ; 
       FIG. 11D  is a side view of the male end shown in  FIG. 11A ; 
       FIG. 11E  is a side cross-sectional view of the male end shown in  FIG. 11D , taken along the lines A-A; 
       FIG. 12A  is an isometric view of a valve plate used in the pressure indication device shown in  FIG. 8 ; 
       FIG. 12B  is an end view of the valve plate shown in  FIG. 12A ; 
       FIG. 12C  is a side cross-sectional view of the valve plate shown in  FIG. 12B , taken along the lines A-A; 
       FIG. 13  is an end view of a stopper used in the pressure indication device shown in  FIG. 8 ; and 
       FIG. 14  is a partial cross-sectional side view of a female end of the pressure indication device of  FIG. 8 , illustrating the stopper in a compressed state whereby fluid communication is enabled. 
   

   Embodiments of the invention are hereinafter described, by way of example only with reference to the accompanying drawings. 
   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Embodiments of the invention are described hereinafter generally in relation to their applicability to laryngeal masks. It should be understood, however, that the invention is also applicable to other applications requiring pressure, indications of a similar magnitude and to other airway devices having air-inflatable cuffs, such as some endotrachial tubes. 
   Referring to  FIG. 1 , one embodiment of the invention relates to a pressure indicator device  10  having a cylindrical outer wall  12 , a bellows  14  and female and male ends  16 ,  17  (also referred to as the distal and proximal ends, respectively). The bellows  14  is located inside the outer wall  12  and moves concentrically therewithin in response to differential pressure. One end of the bellows  14  is attached to the male end  17 , while the other end of the bellows, in its relaxed state, extends towards the female end  16 . At the male end  17 , the bellows is open to atmospheric pressure via a vent passage  30 , while the other end of the bellows  14  is closed and has a boss  19  disposed thereon. The boss  19  engages a hole  39  in an indicator  38  connected to the bellows  14  at the closed end thereof. 
   The outer wall  12  is formed of polysulphone or alternatively another substantially clear autoclavable material such as Lexan (polyphalate carbonate) and Ultem (polyetherlmide). The outer diameter of the outer wall  12  is preferably between about 9 millimeters and 15 millimeters and ideally about 12.8 millimeters (which is about 0.5 inches). The clarity of the outer wall  12  allows the position of the indicator  38  to be seen through the outer wall  12 . The length of the device  10  is preferably in the order of about 40 mm total, although this may vary somewhat, depending on requirements. 
   The location of the bellows  14  within the outer wall  12  and the female and male ends  16 ,  17  serve to define an inner chamber  34  within the bellows  14 , which is at atmospheric pressure due to the vent passage  30 , and an outer chamber  32  defined by the roughly annular space between the bellows  14  and the outer wall  12  and between the closed end of the bellows  14  and the female end  16 . This outer chamber  32  is in fluid communication with the cuff inflation line through a male luer outlet passage  31  so that, in use of the indicator device  10 , the outer chamber  32  is at the same pressure as the cuff of the laryngeal mask and the inner chamber  34  is at atmospheric pressure. This differential pressure causes the bellows  14  to contract from its elongated relaxed state, thereby moving the indicator  38 . The bellows  14  is calibrated so as to consistently contract or compress to the same degree in response to the same cuff pressure. 
   The indicator  38  has a runner  18  which has a substantially parallel surface to that of the interior surface of the outer wall  12  so as to keep an indication mark  43  (which runs circumferentially around the outside of the runner  18 ) substantially perpendicular to the direction of travel of the bellows along an axial center of the indicator device  10  and to keep the movement of the bellows  14  substantially axially centered. As shown in  FIG. 2 , the indicator  38  is formed of a circular disk  42  with the runner  18  constituting a peripheral annular flange extending perpendicularly to the plane of the plate in both directions. The plate  42  has the hole  39  in the center thereof for receiving boss  19  in order to attach the indicator  38  to the bellows  14 . A suitable adhesive may be used as well as, or instead of, the mechanical connection of the boss  19  and hole  39 . The runner  18  has a number of channels  41  in its outer surface extending in the axial direction of the indication device  10  for allowing fluid communication from an end chamber  36  adjacent to the female end  16  with the annular part of outer chamber  32  bounded by the outer wall  12  on the outside and the pleats of the bellows  14  on the inside. As an alternative to the channels  41  being formed on the outside of the runner  18 , they may be formed in the plate  42  towards an outer circumference thereof, but not so central as to be blocked by the end of the bellows  14  adjacent to the boss  19 . 
   The runner  18  effectively provides a means of stabilizing the axial movement of the bellows under compression or relaxation. Other stabilizing means may be provided, however, such as a pin attached at the closed end of the bellows  14  in place of boss  19  and extending axially within a fixed guiding sheath anchored toward or at the female end. 
   The male end  17  of the indication device  10  is formed so as to provide the vent passage  30  to the inside of the bellows  14  and an outlet passage  31  for connection to the cuff inflation line. The male end  17  is formed as a male luer fitting  26 , having the outlet passage  31  at the center thereof, an outer male luer wall  28  and a female luer thread  29  on the inside of the luer wall  28  for engaging a male thread of a female luer fitting, if necessary. The male end  17  also has a circular rebate  27  for receiving an end flange  56  (shown in  FIG. 5 ) of the bellows  14 . The end flange  56  is secured in the rebate  27  by a suitable adhesive for providing an air tight seal under pressures in the order of 60 cm H 2 O. 
   The male end  17  is, in the embodiment pictured in  FIG. 1 , formed separately to the outer wall  12 . However, an alternative embodiment of the invention provides that the male end  17  be formed integrally with the outer wall  12 , in which case the axial orientation of the bellows  14  is reversed and the vent passage  30  and rebates  27  are provided in an altered female end  16 . Similarly, the embodiment shown in  FIG. 1  illustrates that the female end  16  is formed separately to the outer wall  12 , but in an alternative embodiment, these are formed integrally, without the need for joining the two portions with adhesives. In a preferred embodiment, the male end  17  is formed separately to the outer wall  12  but has the bellows  14  attached thereto during assembly, while the outer wall  12  and female end  16  are formed integrally and are placed over the bellows  14  and adhered to the male end  17  by suitable adhesives such an alternative embodiment is shown, for example in  FIG. 8 . Regardless of whether the outer wall  12  is formed integrally with the male and/or female end  17 ,  16 , the axial orientation of the bellows  14  may be reversed (e.g. so that the open end of the bellows  14  is connected to the female end  16  on a rebate suitably formed therein and the closed end extends towards the male end  17 ). 
   The female end  16  has a spring-loaded stop valve  20  housed therein which is normally closed to seal off the outer chamber  32  (including end chamber  36 ) at the female end  16 . The valve  20  can be opened in response to depression of a valve actuator  21  against the action of a spring  23 . Once the actuator  21  is released, the spring  23  acts against the actuator  21  to return it to its normal extended position and hence close the valve  20 . The actuator  21  extends within a female luer fitting  22  whereby, when a male luer fitting is inserted into the female luer fitting  22 , the actuator  21  is depressed, thus opening the valve  20 . This would occur if, for example, a male luer fitting on the end of a syringe were inserted into the female luer fitting  22 . With the actuator  21  depressed, the valve  20  is opened and air from the syringe can be injected into the outer chamber  32 . The female luer fitting  22  is provided with female luer bosses  24  for engagement with a male luer thread, such as that shown on syringe  70  in  FIG. 4  by reference numeral  74 . 
   Referring now to  FIG. 3 , the pressure indication device  10  has an indication region  44  around the circumference of the outer wall  12  for providing a scale against which to compare the position of the indicator  38 . The indication region  44  includes an over-pressure band  46  and an at-pressure band  48 . The over-pressure band  46  has an upper indication bound  45  defining the upper bound of the indication region  44  and over-pressure band  46 . A maximum pressure line  47  separates the over-pressure band  46  from the at-pressure band  48 , the maximum pressure line  47  corresponding to an intra cuff pressure of 60 cm H 2 O. The at-pressure band  48  has a lower indication bound  49 , below which the corresponding intra cuff pressure is considered sub-optimal. The at-pressure band  48  represents a range of pressures below the maximum pressure of 60 cm H 2 O which are considered to be within an optimal intra cuff pressure range, such that when the indicator  38  is visible in the at-pressure band  48 , the cuff pressure does not need to be altered. If the indicator  38  is visible above the maximum pressure line  47 , air should be withdrawn from the cuff until the indicator  38  enters the at-pressure band  48 . If the indicator  38  is visible beyond the upper indication bound  45 , this indicates extreme over-pressure of the cuff, and the cuff pressure should be reduced immediately. In a preferred embodiment, the over-pressure band  46  is lightly colored in red, while the at-pressure band  48  is lightly colored in green, with the indicator mark  43  of the indicator  38  being visible through the outer wall  12  as a black line circumferentially running around the outside of the runner  18 . The coloring of the over-pressure band  46  and at-pressure band  48  should not be so dark as to obscure the visibility of the indicator mark  43 . The at-pressure band  48  may, for example represent pressures between 55 and 60 cm H 2 O, while the over-pressure band  46  may represent pressures between, say, 60 and 64 cm H 2 O. Other pressure ranges may be appropriate for airway devices other than the laryngeal mask. 
   Referring now to  FIG. 4 , an illustration of use of the pressure indication device  10  is described. The male luer fitting  26  is insertable into the standard female luer fitting of a cuff inflation line  60 . The cuff inflation line  60  has an inflation valve  61  housing the female luer fitting, a cuff pressure indicator balloon  62  connected to the inflation valve  61  on the cuff side thereof and a cuff inflation tube  64  connecting the cuff (not shown) with the cuff pressure indicator balloon  62 . 
   When the pressure indication device  10  is fitted onto the cuff inflation line  60  by inserting the male luer fitting  26  into the female luer fitting of the inflation valve  61 , the inflation valve  61  is actuated (opened). This then provides fluid communication from the cuff inflation tube  64  through to the outer chamber  32  of the pressure indication device  10 , whereupon, if the cuff pressure is greater than atmospheric, the bellows  14  will be compressed due to the pressure difference, resulting in visible movement of the indicator  38  within or relative to the indication region  44 . Relevant medical personnel can then read the relative pressure within the cuff from the position of the indicator  38  within or relative to the indication region  44 . 
   If the position of the indicator  38  relative to the indication region  44  indicates that the cuff pressure is too great or too small, the medical personnel can then withdraw or inject air from or into the cuff by using a syringe  70 . This is done by inserting a male luer  72  of the syringe  70  into the female luer fitting  22  of the pressure indication device  10  and twisting the syringe  70  so as to engage the male luer thread  74  with the female luer bosses  24 . The insertion of the syringe male luer  72  into the female luer fitting  22  actuates the valve  20  and thereby allows air to be withdrawn or injected from or into the outer chamber  32  (via end chamber  36 ) and thence from or into the cuff inflation line  60 . The injection or withdrawal of air is performed by movement of a plunger  76  within the syringe  70 . This plunger  76  seals an end chamber  78  in the end of the syringe  70  so as to maintain the pressure in the cuff inflation line  60  during the cuff pressure adjustment. 
   While adjusting the air pressure in the cuff, the medical personnel monitor the position of the indicator  38  relative to the indication region  44  so as to ensure optimal cuff pressure is obtained (i.e. by locating the indicator  38  within the at-pressure band  48 ). Thus, the pressure indication device  10  advantageously allows an initial check of the internal cuff pressure, subsequent adjustment of the cuff pressure using a syringe and continuous cuff pressure monitoring during and after use of the syringe  70 . This is a better and more objective means of determining the cuff pressure than by manually finger-squeezing the cuff pressure indicator balloon or by periodically or sequentially taking pressure measurements with a manometer, which involves detaching the manometer and attaching a syringe each time the cuff pressure needs to be adjusted. 
   Shown in  FIG. 5  is an embodiment of the bellows  14  in a relaxed state. The bellows  14  is formed of silicone and has a wall thickness, w, of about 0.3 to 0.52 millimeters, but preferably closer to 0.3 mm. The thickness of the bellows is greater however, at a closed end  54  where the boss  19  is located and may be slightly increased at the flange  56  at its open end  52 . In its relaxed state, the length of the bellows  14  is about 28 millimeters and pleats of the bellows  14  open at an angle, φ, of about sixty degrees. The inner diameter, x, of the bellows  14  is preferred to be about 5 millimeters, and is more preferably about 4.9 millimeters. An outer diameter, y, of the bellows  14  is preferred to be about 9.6 millimeters, and more preferably 9.62 millimeters. The precise dimensions, thickness, length, opening angle, number of pleats and configuration of the bellows may be subject to some variation while still being capable of performing the invention. The silicone bellows  14  is preferably formed of silicone rubber having shore hardness A 40 to 60. Depending on the shore hardness of the bellows  14 , the location of the indication region  44  on the outer wall  12  will vary in proximity to the proximal and distal ends of the indication device  10 . The precise location of the indication region  44  and maximum pressure line  47  is determined by calibration using a manometer. 
     FIG. 6  illustrates an alternative embodiment of the pressure indication device, represented by reference numeral  110 . The pressure indication device  110  is substantially the same as the embodiment shown in  FIG. 1  and designated by reference numeral  10 , except that it is intended for permanent connection to the cuff inflation tube  64  in place of the inflation valve  61  and cuff pressure indicator balloon  62 . Like reference numerals in  FIG. 6  represent like features and the preceding description of those features applies to this embodiment. 
   In pressure indication device  110 , a male end  117  is provided without a male luer fitting but instead having an outlet  128  to which the cuff inflation tube  64  is connected (for example, by suitable adhesives), and the outer chamber  32  is then placed in fluid communication with the cuff inflation tube  64  through outlet passage  131 . Instead of having a protruding tubular outlet  128  such as that illustrated in  FIG. 6 , the cuff inflation tube  64  may otherwise be fixed to the male end  117  so as to be in fluid communication with the outer chamber  32 . 
   Advantageously, the pressure indication device  110  is assembled as part of a laryngeal mask assembly  125  (see  FIG. 7 ) so as to replace the cuff pressure indicator balloon and inflation valve  62 ,  61  and obviate the need to connect and disconnect the pressure indication device  10  to the cuff inflation line  60  of the laryngeal mask. The pressure indication device  110  is intended to be fitted to a reusable laryngeal mask or other airway device and must therefore be autoclavable. Advantageously, the materials selected for the device  110  (and  10 ) are all autoclavable. 
   An example of a laryngeal mask having the pressure indication device  110  assembled as part thereof is shown in  FIG. 7 , designated by reference numeral  125 .  FIG. 7  shows the pressure indication device  110  connected at the end of cuff inflation tube  64 , which is in-turn connected to the cuff  120 . 
   Referring now to  FIGS. 8 to 14 , a further embodiment of the pressure indication device is shown, designated by reference numeral  210 . This embodiment is similar to the previously described embodiments in many respects, including, for example, its functional interaction with the cuff inflation tube and syringe and in having markings on its cylindrical body for facilitating the pressure indication function. This embodiment is different, however, in that a different embodiment of the bellows is employed, together with modified female and male ends. For purposes of clarity and ease of understanding, the following description of this further embodiment will focus on functional differences between this embodiment and the previously described embodiments. Where different features are not mentioned, this implies that the relevant functional features are the same or similar between the embodiments. 
   Referring to  FIG. 8  specifically, pressure indicator device  210  has an outer wall  212  for housing a bellows  214  and has a male end  217  and oppositely disposed female end  216 . The bellows  214  is affixed by a suitable adhesive to a rebate portion  227  of the male end  217  and the male end  217  is fixed similarly to the outer wall  212 . Materials used in this embodiment are generally the same as those used in the embodiments previously described and thus the adhesives chosen to affix the bellows  214  to the male end  217  and the male end  217  in turn to the outer wall  212  should therefore be suitable for adhering (for example) silicone to Lexan and Lexan to Lexan, respectively. In this embodiment, the outer wall of female end  216  is integrally formed with outer wall  212 . From outer wall  212 , female end  216  tapers frustoconically toward an end opening  226 . Apart from being fixed to the male end  217 , the bellows  214  is moveable longitudinally within the body of the device  210  in response to a pressure difference between inner chamber  234  and outer chamber  232 . The inner chamber  234  is bounded by the inside walls of the bellows  214  and the male end  217  but is vented to atmosphere through atmospheric vent passages  230  in the male end  217 . 
   The outer chamber  232  is bounded by the outer wall  212  and the outer surface of the bellows  214 . The outer wall  212  has a number of internal wall ribs  213  (better illustrated in  FIGS. 10A and 10B ) which protrude from the internal wall surface of outer wall  212  by about 0.25 mm. These internal wall ribs  213  serve to act as a guiding means for stabilizing the bellows  214  as it moves within the outer wall  212 . Between the internal wall ribs  213  there is an interrupted annular gap through which fluid may communicate past the largest diameter pleats (described further below) of the bellows  214 . 
   An outlet passage  231  in the male end  217  has an outlet fluid communication channel  240  (shown in  FIGS. 11A ,  11 D and  11 E) which communicates with the outer chamber  232  so as to place the outer chamber  232  at the internal cuff pressure when the male end of the device  210  is fitted to the cuff inflation tube. At the other end of the device  210 , the outer chamber  232  includes a female end chamber  236  which communicates through the female end  216  to a syringe when valve  220  is actuated. 
   Valve  220  includes a valve support plate  219  seated against the end of the internal wall ribs  213  and supporting a valve stopper  221 . The valve stopper  221  is generally hemispherical and hollowed and somewhat cup-shaped, with a concave face facing toward the bellows  214 . The stopper  221  has a head portion  223  disposed on its outer convex face for fitting into and loosely engaging the female end opening  226 . In its relaxed state, the stopper  221  sits against the inside wall of the female end  216  adjacent the female end opening  226 . This interior wall portion contacted by the stopper is generally frustoconical such that, when the outer convex surface of the stopper  221  sits against that portion of the interior wall of the female end  216 , female end opening  226  is occluded. Even in its relaxed state, the stopper  221  is still slightly compressed between the frustoconical interior wall of female end  216  and the valve support  219  so as to seal the valve  220 . The stopper  221  rests, at its bottom or outer annular edge, against a stopper support flange  228  (shown in  FIGS. 12A ,  12 B and  12 C). 
   The stopper  221  has a channel or notch  224  formed in its head portion  223 . This channel  224  allows passage of fluid from the syringe around the head portion  223  and into the female end  216  when the stopper  221  is depressed by the syringe nozzle (in its compressed state), as shown in  FIG. 14 . With reference also to  FIG. 14 , when the stopper  221  is depressed by the insertion of the syringe nozzle  72  into the female end opening  226 , the outer convex surface of the stopper  221  adjacent head portion  223  is moved away from the interior frustoconical wall of female end  216 , leaving a gap therebetween. Also, as head portion  223  does not itself occlude the female end opening  226 , but rather leaves a small gap, this allows fluid to flow from the syringe nozzle  72  through the head channel  224 , around the head portion  223  and through the gap created between the outer convex wall of the stopper  221  and the interior frustoconical wall of the female end  216 . The fluid is then free to flow through female end fluid channels  222  in the valve support plate  219 . The female end fluid channels  222  are formed as a number, for example 4, of holes formed toward a radially outer peripheral area of the valve support plate  219  (as shown in  FIGS. 12A ,  12 B and  12 C). The direction of fluid flow into female end  216  from syringe head  72  is depicted in  FIG. 14  by small arrows. Fluid may flow similarly in the other direction in response to suction from the syringe when the stopper is compressed. 
   The valve support plate  219  has a center hole  225  in the center thereof in communication with the concave underside of stopper  221 . Center hole  225  serves to prevent a vacuum forming inside the stopper  221  after it is compressed. Such a vacuum would hinder the stopper  221  from returning its relaxed state and would thus impair the function of valve  220 . As can be appreciated from the described arrangement, the natural state of valve  220  is closed, due to the tendency of the flexible stopper  221  to return to its relaxed state, by which it occludes female end opening  226 . To accomplish this, the stopper  221  is preferably formed of silicone rubber and has a minimum shore hardness A 60 . 
     FIG. 9  shows a cross-sectional representation of bellows  214  (in its relaxed state) of a roughly cylindrical form, for use in the pressure indication device  210 . The bellows  214  may alternatively be employed in other embodiments of the pressure indication device, providing those embodiments have internal wall ribs  213  for guiding and stabilizing the movement of the bellows as it contracts or expands within the device. In this embodiment of the bellows, the closed end portion  254  is modified somewhat relative to the previously described embodiment. The closed end  254  does not have a boss formed on its end. The indication mark is not provided on a flange connected to the closed end, but is instead formed on an outer annular edge of the last pleat toward the closed end  254 . The indication marks are shown in  FIGS. 8 and 9  and designated by reference numeral  238 . These indication marks may be formed as a paint or dye applied to the bellows  214  after it is molded. This paint or dye must contrast with the color of the material used for the bellows. For example, if the bellows is formed of a white or generally translucent material, the indicator mark  238  should be of a contrasting dark color. In this embodiment, as there is no disk and flange to guide movement of the bellows within the outer wall  212 , this guiding and stabilizing function is performed by larger diameter pleats  260  (including the end pleat on which indicator marks  238  are formed), in cooperation with the internal wall ribs  213 . In the embodiment of the bellows  214  shown in  FIG. 9 , the last and third-from-last pleats in the bellows are enlarged so as to contact the internal wall ribs  213  as they move longitudinally within the device  210 . This stabilization is important so that the orientation of the indication mark relative to the scale markings on the outside of the outer wall  212  is not skewed during compression of the bellows  214 , which may lead to an inaccurate pressure reading. Additionally, thickened portions  239  are provided between the last three pleats toward the closed end  254  to stiffen the end part of the bellows  214 . This stiffening assists in stabilizing the travel of the bellows and thus the indication mark. The thickened portions  239  also serve to minimize movement of the last pleat on which the indication mark  238  is formed relative to other pleats during compression of the bellows  214 , thus assisting in calibration of the device for serving its pressure indication function. These thickened portions  239  may be formed around the circumference of the bellows  214  in the relevant pleat position or may be formed as circumferentially spaced ribs connecting the pleats at a number of points for stiffening thereof. 
   As shown in  FIG. 9 , the outer diameter of smaller pleats  258 , indicated by a, is less than the outer diameter of the larger diameter pleats  260 , represented by b. The difference between b and a is preferably about 0.2 mm. For example, b may be 9.8 mm and a may be 9.6 mm. 
   The bellows  214  has a flange  256  at its open end  252  for fitting into and engaging with the rebate  227 , as shown in  FIG. 8 . 
   The function of male end  217  is generally illustrated with respect to  FIG. 8 . Referring now to  FIGS. 11A to 11E , it can be seen that the representation of male end  217  in  FIG. 8  corresponds to that of  FIG. 11C . What is not shown in  FIG. 8 , however, is how outlet fluid communication channel  240  communicates with outer chamber  232 . This is more apparent from  FIGS. 11A ,  11 D and  11 E, in which it can be seen that the male end  217  has flattened side portions  241  where the outlet fluid communication channel  240  opens out to the sides of male end  217 . These flattened side portions  241  allow a small gap between the outside of male end  217  and outer wall  212  for providing fluid communication between outlet fluid communication channel  240  (and thus outlet passage  231 ) and outer chamber  232 . 
   Referring now to  FIGS. 12A ,  12 B and  12 C, it can be seen that the valve support plate  219  is generally disk shaped, but with a center hole  225  and additional holes formed toward a periphery of the disk shape so as to provide female end fluid channels  222 . A raised portion of the disk acts as the stopper support flange  228  for supporting the outer annular base of the stopper  221 , while the center hole  225  communicates with the concave interior of the stopper  221 . The valve support plate  219  may be held in place within the outer wall  212  by use of adhesives or may be chemically, ultrasonically or laser welded in place. For additional support, valve support plate  219  may sit against the ends of internal wall ribs  213 . This additional support may assist in resisting the force required to be applied to the stopper  221  to open the valve  220 . The valve support plate  219  and male end  217  are preferably formed of Lexan or Ultem for the autoclavable embodiment or PVC for the non-autoclavable embodiment. 
     FIG. 13  is an end view of the stopper  221 , as if looking toward it through the female end opening  226 . The stopper head portion  223  and channel  224  are located centrally on an apex of the stopper  221 . The depth of channel  224  need only be in the order of 0.5 mm or less, while the thickness of the head portion  223  may be in the order of 2 mm. The diameter of head portion  223  is formed so as to provide only a loose fit within female end opening  226  and is not intended to, in itself, occlude the opening. 
   While certain embodiments of the invention have been described above, it is specifically envisaged that different parts of the different embodiments may be used interchangeably to arrive at further embodiments. For example, the bellows  14 , guiding arrangement and indicating mark arrangement shown and described in  FIGS. 1 ,  2  and  5  may be substituted with that described in relation to  FIGS. 8 and 9 . Also, the male and female ends shown and described in relation to  FIGS. 8 ,  11 A to  11 E,  12 A to  12 C,  13  and  14 , may be used in place of male and female ends  17  and  16  described with reference to  FIG. 1 . Additionally, autoclavable and non autoclavable versions of each embodiment are envisaged, the difference residing in the choice of materials for each component. Further, a version of the embodiment shown and described in relation to  FIG. 8  is envisaged for use in a manner similar to that shown and described in relation to  FIG. 6 , such that it is non removably connected to a cuff inflation tube  64  of a laryngeal mask  125 , such as is shown in  FIG. 7 .