Patent Publication Number: US-2023149652-A1

Title: Nasal respiratory mask

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part of U.S. Non-Provisional patent application Ser. No. 17/505,332, filed Oct. 19, 2021, which is a Continuation-in-Part of International Patent Application No. PCT/GB2021/051911, filed Jul. 23, 2021. Both applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a nasal respiratory mask, a nasal respiratory mask for a high flow oxygen therapy apparatus, a nasal respiratory mask system, and a high flow oxygen therapy apparatus. 
     BACKGROUND OF THE INVENTION 
     High Flow Oxygen Therapy (HFOT) delivers an air/oxygen gas mix to the patient at flow rates that exceed the patient&#39;s peak inspiratory flow rates at various minute volumes. The air flow is heated, humidified, and oxygen enriched. The respiratory support offered by HFOT is non-invasive. 
     HFOT is typically delivered to the patient by nasal cannula, which offers many advantages over traditional face masks in that the nasal cannulas allow a patient to eat and are tolerated by patients for longer periods. However, extended wear of a nasal cannula can still lead to patient discomfort, for example, pressure sores on the nasal septum. Nasal respiratory masks offer an alternative to nasal cannulas. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a method of administering high flow oxygen therapy to a patient using a high flow oxygen therapy apparatus, the apparatus comprising: a nasal respiratory mask comprising a mask frame; a mask cushion on the mask frame, the mask frame and mask cushion defining a nasal breathing cavity; a hose attachment portion; and a mask aperture having an adjustable flow restrictor for restricting the flow of gas from the nasal breathing cavity directly to ambient; and an oxygen enriched air supply having a variable flow rate output for supplying heated, humidified oxygen enriched air to the patient, the method comprising: providing the nasal respiratory mask to the patient&#39;s face so that the nasal breathing cavity surrounds the patient&#39;s nose and not the mouth, and the mask cushion contacts and substantially seals against the face of the patient; connecting a hose between the hose attachment portion of the mask and the oxygen enriched air supply; adjusting the variable flow rate output of the oxygen enriched air supply to provide a flow rate of the heated, humidified oxygen enriched air to the mask which just exceeds the patient&#39;s peak nasal inspiratory flow rate; adjusting the adjustable flow restrictor of the mask to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A further aspect of the invention provides a method of administering high flow oxygen therapy to a patient using a high flow oxygen therapy system, the system comprising: a plurality of nasal respiratory masks, each mask comprising: a mask frame; a mask cushion on the mask frame, the mask frame and mask cushion defining a nasal breathing cavity; a hose attachment portion; and a mask aperture for restricting the flow of gas from the nasal breathing cavity directly to ambient, each of the plurality of masks having a differently sized mask aperture for restricting the flow of gas from the nasal breathing cavity directly to ambient by a different amount; and an oxygen enriched air supply having a variable flow rate output for supplying heated, humidified oxygen enriched air to the patient, the method comprising: selecting one of the plurality of nasal respiratory masks; providing the selected nasal respiratory mask to the patient&#39;s face so that the nasal breathing cavity surrounds the patient&#39;s nose and not the mouth, and the mask cushion contacts and substantially seals against the face of the patient; connecting a hose between the hose attachment portion of the mask and the oxygen enriched air supply; adjusting the variable flow rate output of the oxygen enriched air supply to provide a flow rate of the heated, humidified oxygen enriched air to the mask which just exceeds the patient&#39;s peak nasal inspiratory flow rate; wherein the selected nasal respiratory mask is selected to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A further aspect of the invention provides a method of administering high flow oxygen therapy to a patient using a high flow oxygen therapy system, the system comprising: a plurality of nasal respiratory masks, each mask comprising: a mask frame; a mask cushion on the mask frame, the mask frame and mask cushion defining a nasal breathing cavity; a hose attachment portion; and a mask aperture, wherein the mask aperture has a passive one-way valve configured to move from a closed position in which air is restricted from flowing through the one-way valve, to an open position in which air can flow from the nasal breathing cavity through the one-way valve directly to ambient, each of the plurality of masks having a different one-way valve with a different pre-set valve opening pressure of less than 1 kPa; and an oxygen enriched air supply having a variable flow rate output for supplying heated, humidified oxygen enriched air to the patient, the method comprising: selecting one of the plurality of nasal respiratory masks; providing the selected nasal respiratory mask to the patient&#39;s face so that the nasal breathing cavity surrounds the patient&#39;s nose and not the mouth, and the mask cushion contacts and substantially seals against the face of the patient; connecting a hose between the hose attachment portion of the mask and the oxygen enriched air supply; adjusting the variable flow rate output of the oxygen enriched air supply to provide a flow rate of the heated, humidified oxygen enriched air to the mask; wherein the selected nasal respiratory mask is selected to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A further aspect of the invention provides a method of administering high flow oxygen therapy to a patient using a high flow oxygen therapy system, the system comprising: a nasal respiratory mask comprising: a mask frame; a mask cushion on the mask frame, the mask frame and mask cushion defining a nasal breathing cavity; a hose attachment portion; and a mask aperture, wherein the mask aperture has a passive one-way valve configured to move from a closed position in which air is restricted from flowing through the one-way valve, to an open position in which air can flow from the nasal breathing cavity through the one-way valve directly to ambient, wherein the one-way valve has a variable valve opening pressure; and an oxygen enriched air supply having a variable flow rate output for supplying heated, humidified oxygen enriched air to the patient, the method comprising: selecting one of the plurality of nasal respiratory masks; providing the selected nasal respiratory mask to the patient&#39;s face so that the nasal breathing cavity surrounds the patient&#39;s nose and not the mouth, and the mask cushion contacts and substantially seals against the face of the patient; connecting a hose between the hose attachment portion of the mask and the oxygen enriched air supply; adjusting the variable flow rate output of the oxygen enriched air supply to provide a flow rate of the heated, humidified oxygen enriched air to the mask; adjusting the valve opening pressure of the mask to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A further aspect of the invention provides a nasal respiratory mask for a high flow oxygen therapy apparatus, comprising: a mask frame; a mask cushion on the mask frame for contacting and substantially sealing against a face of a patient, the mask frame and mask cushion defining a nasal breathing cavity; the mask frame having: a hose attachment portion for attaching a hose for delivering a supply of oxygen enriched air to the patient; and a mask aperture having an adjustable flow restrictor for restricting the flow of gas from the nasal breathing cavity directly to ambient, wherein the adjustable flow restrictor is adjustable to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A further aspect of the invention provides a nasal respiratory mask for a high flow oxygen therapy apparatus, comprising: a mask frame; a mask cushion on the mask frame for contacting and substantially sealing against a face of a patient, the mask frame and mask cushion defining a nasal breathing cavity; the mask frame having: a hose attachment portion for attaching a hose for delivering a supply of oxygen enriched air to the patient; and a mask aperture having a passive one-way valve configured to move from a closed position in which air is restricted from flowing through the one-way valve, to an open position in which air can flow from the nasal breathing cavity through the one-way valve directly to ambient outside the mask, wherein the one-way valve has a valve opening pressure for maintaining a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A further aspect of the invention provides a nasal respiratory mask for a high flow oxygen therapy apparatus, comprising: a mask frame; a mask cushion on the mask frame for contacting and substantially sealing against a face of a patient, the mask frame and mask cushion defining a nasal breathing cavity; the mask frame having: a hose attachment portion for attaching a hose for delivering a supply of oxygen enriched air to the patient; and a mask aperture for restricting the flow of gas from the nasal breathing cavity directly to ambient, wherein the mask aperture is dimensioned for maintaining a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     A nasal respiratory mask is a mask that covers the nasal passages of a patient but not the mouth passage, in contrast to a face mask. The nasal respiratory mask may be arranged to extend from a position above the nasal passages to a position above the mouth passage. The nasal respiratory mask may be arranged to extend from the bridge of a patient&#39;s nose to their upper lip, or extend between a position below the bridge of a patient&#39;s nose to a position above the patient&#39;s upper lip. 
     The adjustable flow restrictor may be adjusted to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of approximately 0.5 kPa during the administering of high flow oxygen therapy to the patient, preferably 0.3 to 0.7 kPa, preferably 0.4 to 0.6 kPa. The flow restrictor may be manually adjusted. 
     The mask aperture may further comprise either a vent or a passive one-way valve. The vent or valve may be located adjacent the flow restrictor and fluidically coupled in the same flow passage as the flow restrictor, preferably immediately upstream of the flow restrictor in the flow direction of exhaled gas from the nasal breathing cavity towards ambient. 
     The vent defines an opening that remains open during use. In particular, the vent remains open during both inhalation and exhalation of the patient. The vent may be sized to restrict the flow of gas from the nasal breathing cavity to ambient, so as to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of at least 0.2 kPa during the administering of high flow oxygen therapy to the patient, preferably approximately 0.5 kPa, preferably 0.3 to 0.7 kPa, preferably 0.4 to 0.6 kPa. 
     The passive one-way valve may have a valve member configured to move from a closed position in which air is restricted from flowing through the one-way valve, to an open position in which air can flow from the nasal breathing cavity through the one-way valve towards the flow restrictor and to outside the mask. The passive one-way valve may be fully passive and operable to open and close only in direct dependence on the pressure of air in the mask aperture flow passage between the nasal breathing cavity and ambient. 
     The one-way valve may have a movable valve member that is unbiased and free floating. Alternatively, the valve member may be biased to provide a valve opening pressure of less than 1 kPa and/or a valve opening pressure which is greater than 0.2 kPa. The valve opening pressure may be pre-set or may be variable, e.g. by manual adjustment. The valve opening pressure may not be adjustable by a pressure feedback mechanism. 
     The valve opening pressure may be less than 0.8 kPa. 
     The vent may have a flow passage cross-sectional area that is larger than the flow passage cross sectional area of the flow restrictor (where provided) when in the fully open position. This ensures that the vent does not impinge the flow when the flow restrictor is fully open. 
     A flow rate through the one-way valve in the open position and/or a flow rate through the vent and/or through the flow restrictor may be configured to be at least 2 litres per minute, preferably at least 5 litres per minute. 
     The one-way valve may be a flapper valve or a lift-check valve. 
     The mask frame may have a generally domed shape. 
     The mask cushion may comprise a thermoplastic elastomer and/or silicone. 
     The mask cushion and at least a perimeter of the mask frame may be integrally formed of the same material. 
     At least a portion of the mask frame may comprise a substantially less flexible material than the material of the mask cushion. 
     The mask cushion may be inflatable and deflatable. 
     The hose attachment portion may be substantially centrally located on a vertical centre line of the mask frame. 
     The hose attachment portion may be located towards a lower end of the mask frame. The hose attachment portion may be located towards a lower end of the mask frame and arranged so as to be adjacent a middle of a user&#39;s mouth when worn. 
     The nasal respiratory mask may comprise two of the mask apertures spaced substantially symmetrically about a vertical centre line of the mask frame. 
     The two mask apertures may be located towards a lower end of the mask frame so as to be adjacent either side of a user&#39;s mouth when worn. 
     The mask frame may be at least partially formed from a water permeable material. 
     At least 50% of the mask frame may be formed from the water permeable material. 
     The water permeable material may be permeable to liquid water and/or water vapour. 
     The hose attachment portion may comprise a swivel connector configured to provide relative rotation between the mask frame and the hose. 
     The nasal respiratory mask may comprise a pair of opposing straps and/or harness extending from the mask frame. 
     The nasal respiratory mask may comprise a carbon dioxide monitoring line connector on the mask frame for attaching a carbon dioxide monitoring line and/or a carbon dioxide sensor. 
     The nasal respiratory mask may comprise a carbon dioxide sensor on the mask frame. 
     The nasal respiratory mask may comprise a carbon dioxide monitoring line attached to the carbon dioxide monitoring line connector and a carbon dioxide sensor attached to the carbon dioxide monitoring line. 
     The carbon dioxide monitoring line may comprise a water permeable material. 
     The nasal respiratory mask may further comprise a filter membrane arranged to cover the mask aperture. 
     The filter membrane may be arranged to cover at least half of the mask frame. 
     The filter membrane may be arranged to cover a patient&#39;s mouth 
     A further aspect of the invention provides a nasal respiratory mask system comprising the nasal respiratory mask of the invention and a hose for attaching to the hose attachment portion of the nasal respiratory mask for delivering a supply of oxygen enriched air to the user. 
     The hose may comprise a water permeable material. 
     The water permeable material may be permeable to liquid water and/or water vapour. 
     The hose may be malleable and/or comprise a malleable member, such that the hose is configured to be deformable and retain a given shape when the hose is manipulated. 
     A further aspect of the invention provides a high flow oxygen therapy apparatus comprising: the nasal respiratory mask system; and an oxygen enriched air supply coupled via the hose to the respiratory mask and configured to supply oxygen enriched air to a user. 
     The oxygen enriched air supply may be configured to deliver a flow rate of at least 5 litres per minute to the user, and preferably a flow rate of between 30 and 60 litres per minute. 
     The oxygen enriched air supply may be configured to deliver a flow rate of less than 70 litres per minute to the user. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG.  1    shows a perspective view of a nasal respiratory mask system according to a first example; 
         FIG.  2    shows a front view of the nasal respiratory mask system; 
         FIG.  3    shows a rear view of the nasal respiratory mask system attached to a head band; 
         FIG.  4    shows a bottom view of the nasal respiratory mask system; 
         FIG.  5    shows a side view of the nasal respiratory mask system with a carbon dioxide sensor connected to the nasal mask via a carbon dioxide monitoring line; 
         FIG.  6    shows the nasal respiratory mask system comprising a malleable member; 
         FIG.  7    shows a schematic of a high flow oxygen therapy apparatus comprising the nasal respiratory mask system; 
         FIGS.  8 A and  8 B  show a one-way umbrella valve; 
         FIGS.  9 A and  9 B  show a flapper valve; 
         FIGS.  10 A and  10 B  show a lift-check valve; 
         FIGS.  11 A and  11 B  show a spring-loaded inline one-way valve; 
         FIGS.  12 A and  12 B  show the spring-loaded inline one-way valve comprising an adjustable flow restrictor; 
         FIG.  13    shows a nasal mask having unbiased one-way valves; 
         FIGS.  14 A and  14 B  show an unbiased flapper valve; 
         FIGS.  15 A and  15 B  show an unbiased inline one-way valve; 
         FIGS.  16 A to  16 G  show a second example of an adjustable flow restrictor; 
         FIG.  17    shows a front view of a nasal respiratory mask system according to a second example; 
         FIG.  18    shows a perspective view of the nasal respiratory mask system; 
         FIG.  19    shows a perspective view of a neo-natal nasal respiratory mask system according to a third example; 
         FIG.  20    shows a front view of a neo-natal nasal respiratory mask system according to a fourth example; 
         FIG.  21    shows a front view of a nasal respiratory mask system according to a fifth example; 
         FIG.  22    shows a front view of a neo-natal nasal respiratory mask system according to a sixth example; 
         FIG.  23 A  shows a filter membrane covering the opening/aperture in one of the valves; 
         FIGS.  23 B and  23 C  show a filter membrane arranged to cover the mask frame; 
         FIG.  23 D  shows a filter membrane arranged to cover the mask frame and a patient&#39;s mouth; 
         FIG.  24    shows a perspective view of a nasal respiratory mask system without a valve. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     A high flow oxygen therapy (HFOT) apparatus delivers heated, humidified, and oxygen enriched air at flow rates many times higher than those used with standard face masks and nasal cannulas. For instance, the flow rates to a standard mask or cannula may be approximately 5 Standard litres per minute (LPM). In contrast, a HFOT apparatus may deliver flow rates of between 30 LPM and 70 LPM, so that a greater area of the patent&#39;s lungs is recruited for gas exchange, further improving blood oxygenation. The flow rate of the supply is controllable and may be set manually. The setting of the HFOT supply flow rate is determined based on the patient&#39;s inspiratory flow rate, for example the HFOT supply flow rate may be set to substantially match or just exceed the patient&#39;s peak inspiratory flow rates and so flow rates outside this range (higher and lower) may be applicable in some situations, for example pre-natal patients may receive a flow rate of 5 LPM or greater, and flow rates in some applications may meet and exceed 100 LPM. It will be understood that higher flow rates (e.g. those exceeding 70 LPM, or exceeding 100 LPM) may be arranged for use in short-term and temporary applications, such as pre-operative oxygenation or when a patient is in respiratory distress. 
     The patient&#39;s inspiratory flow rate is taken to be the inspiratory flow rate during an inhalation of the patient. The inspiratory flow rate may refer to the peak inspiratory flow rate during an inhalation of the patient. 
       FIG.  1    shows an example of a nasal respiratory mask system  2  that may form part of a high flow oxygen therapy apparatus  1 . 
     The nasal respiratory mask system  2  includes a nasal respiratory mask  3 . The nasal respiratory mask  3  includes a mask frame  5 , and a mask cushion  6  on the mask frame  5 . The mask frame  5  and mask cushion  6  define a nasal breathing cavity between an internal side of the mask  3  and the patient. The mask  3  is intended to extend over the face of the patient, thereby covering the nasal passages. The mask  3  is arranged so as to cover the nasal passages but not the mouth passage. The nasal respiratory mask  3  may be arranged to extend from a position above the nasal passages to a position above the mouth passage. The nasal respiratory mask  3  may be arranged to extend from the bridge of a patient&#39;s nose to their upper lip, or extend between a position below the bridge of a patient&#39;s nose to a position above the patient&#39;s upper lip. 
     The mask frame  5  may comprise a material that is less flexible than the material forming the mask cushion  6 . The mask frame  5  may be formed, entirely or in part, of a substantially rigid material in comparison to the structure and/or material of the mask cushion  6 . A substantially rigid material is self-supporting and able to provide the necessary structure that forms and maintains the size of the nasal breathing cavity when the mask frame  5  is in use. The mask frame  5  may form a generally triangular dome shape. In other words, the mask frame  5  may have a shape that is triangular dome shaped in overall appearance but still have deviations from a perfectly triangular dome shape. However, it will be understood that the mask frame  5  may have any suitable shape, such as a circular or oval dome shape. The mask frame  5  may be formed from any suitable material, such as silicone or a thermoplastic elastomer (TPE). 
     The mask cushion  6  is arranged to contact and substantially seal against the face of a user. In this context, substantially seal is intended to refer to the mask cushion  6  as forming a sufficient seal to prevent excessive leaks, for example leaks that may detrimentally lower the flow rate and/or direct flow towards a patent&#39;s eye, thereby causing discomfort or poor performance. The mask cushion  6  may be inflatable and deflatable, thereby assisting in conforming to a patient&#39;s facial anatomy, for example the mask cushion may comprise an air valve that is attachable to an air pump (not shown). 
     The mask cushion  6  is arranged to prevent excess contact pressure being applied to the patient. The mask cushion  6  may be formed from any suitable material, for example silicone, thermoplastic elastomer gel or foam. Preferably the mask cushion  6  is formed of silicone. The mask cushion  6  may be attached to, or integral with, the mask frame  5 . 
     In some examples, the mask frame  5  and mask cushion  6  may be integrally formed from the same material, such that the mask frame  5  and mask cushion  6  form a single part. In alternative examples, the mask frame  5  and mask cushion  6  may be separately formed. 
     The mask frame  5  and mask cushion  6  may be formed of the same material, or the mask frame  5  and mask cushion  6  may be formed of different materials. In one example, the mask frame  5  is formed of a thermoplastic elastomer and the mask cushion  6  is formed of silicone. 
     In other examples, the mask cushion  6  and at least a perimeter of the mask frame  5  may be integrally formed of the same material. In one example, the mask frame  5  may be formed of two materials (e.g. silicone and a thermoplastic elastomer), with the silicone being formed around the perimeter of the mask frame  5  that contacts the mask cushion  6 . The mask cushion  6  may be formed of silicone, such that the silicone of the mask frame  5  extends integrally to form the mask cushion  6 . 
     The mask frame  5  may be at least partially formed from a water permeable material. For example, the material may be permeable to liquid water and/or water vapour. The mask frame  5  may be entirely formed from the permeable material or partially formed from the permeable material. For example, at least 30%, at least 50%, or at least 70% of the mask frame  5  may be formed from the permeable material. A portion of the mask frame  5  may be formed from a permeable material whilst the remainder of the mask frame  5  is formed of a material that is relatively non-permeable (i.e. less permeable than the permeable material). In one example, the perimeter of the mask frame  5  is formed of a non-permeable material whilst the remaining material of the mask frame  5  is formed of the permeable material. 
     The permeable material is a material configured to allow water (e.g. liquid water and/or water vapour) to flow therethrough. The permeable material may be configured to reduce, restrict or prevent flow of gases therethrough. A mask frame  5  formed at least partially of a permeable material reduces the build-up of water in the nasal respiratory mask  3 . Reducing, restricting or preventing the flow of gases through the material prevents a loss of air pressure in the nasal respiratory mask system  2 . 
     The water permeable material of the mask frame  5  may be formed from an amphiphilic material. The water permeable material may be a hydrophobic and hydrophilic poly(ethylene oxide) based block co-polymer. Alternative water permeable materials include: water permeable polytetrafluoroethylene (PTFE); Nafion®; Sympatex®; Arnitel®; Diaplex®; water permeable Hytrel®; and Goretex®, although it will be appreciated that any suitable permeable material may be used. 
     The nasal respiratory mask  3  includes a hose attachment portion  10 . The hose attachment portion may be attached to a hose  11 . The hose  11  may be coupled to a supply of oxygen enriched air  60 . The hose  11  may deliver a supply of oxygen enriched air to the patient from a source of oxygen enriched air  60 . 
     The hose attachment portion  10  may comprise a connector  12  that attaches to the hose  11 . The connector  12  may be a swivel connector  12  that allows the hose  11  to rotate relative to the mask frame  5 . This allows a distal end of the hose  11 , relative to the connector  12 , to be rotated into a convenient position relative to the patient when in use. The swivel connector  12  may provide full 360 degree (or more) rotation of the hose  11  relative to the mask  3 , or the relative rotation of the hose  11  relative to the mask  3  may be restricted to a set angular range. Alternatively, the connector  12  may be fixed in position relative to the mask  3 . 
     As shown in  FIGS.  1  to  4   , the connector  12  may be an elbow connector  12  (in addition or alternatively to being a swivel connector). The elbow connector  12  redirects the air flow through the connector  12  such that the gas flow through the hose attachment portion  10  is angled with respect to the gas flow through the hose  11  adjacent to the elbow connector  12 . The angle of the elbow connector  8  may be between 25 degrees and 90 degrees. In the example shown, the elbow connector  12  has an angle of 90 degrees. The elbow connector  12  provides a sharp right angle where two respective perpendicular sections of the elbow connector  12  meet, although in alternative examples the elbow connector  12  may be swept and/or curved such that the angular change is more gradual and there is no sudden angle change. 
     The connector  12  may be configured to vary its angle. For example, the elbow  12  connector may comprise a hinge mechanism. The hinge mechanism may be configured to allow the connector  12  to move from a first position, configured to redirect gas flow at an angle of 30 degrees, to a second position, configured to redirect gas flow at an angle of 90 degrees. The hinge mechanism may be fixable at a plurality of angular positions. 
     In some examples, the connector  12  may not include an elbow. In this case, the hose  11  may be coupled at one end to a straight connector pivotally or fixedly connected to the hose attachment portion  10 . Due to the flexibility of the hose  11 , the hose  11  may form an elbow or at least provide some flexibility that can increase patient comfort and convenience whilst wearing the nasal cannula. 
     The connection arrangement between the hose attachment portion  10  and the connector  12  may be non air-tight, such that some air/gas is able to escape or enter through the connection. The air/gas leakage through the connection arrangement may be maintained below a target value. The air/gas leakage through the connection may be below 10% at flows of up to 50 Standard litres per minute. In alternative examples, the air/gas leakage may be below 5% at 50 Standard litres per minute. 
     The connection arrangement may be a snap-fit connection, which allows the parts  10 ,  12  to be interlocked by pushing the parts together. A snap-fit connection allows the parts  10 ,  12  to be assembled quickly. The snap-fit connection may be a one-way snap-fit connection that does not permit ready disassembly. 
     As shown best in  FIG.  2   , the hose attachment portion  10  may be substantially centrally located on a vertical centre line of the mask frame  5 . The hose attachment portion  10  may therefore be positioned equidistant from the distal sides of the mask frame  5  and/or mask cushion  6  for locating the hose attachment portion  10  centrally on a patient&#39;s head. The hose attachment portion  10  may be located towards a lower end of the mask frame  5 , as shown in the example of  FIGS.  1  to  4   . This may allow the hose attachment portion  10 , through which the supply of oxygen enriched air enters the nasal breathing cavity, to be adjacent the middle of a user&#39;s mouth when worn. 
     The hose  11  may be flexible or rigid. The hose  11  may be sufficiently resilient to retain a substantially constant cross section of air flow, yet able to bend so that the hose  11  can be comfortably positioned and manoeuvred relative to a patient&#39;s face. The hose  11  may be a corrugated tube, e.g. with a corrugated outer surface. The inner surface of the hose  11  may also be corrugated, although in alternative examples the inner and/or outer surface may be smooth. 
     The hose  11  may be at least partially, and in some cases entirely, formed from a water permeable material, and function similarly to the permeable material of the mask frame  5  described above in that it allows liquid water and/or water vapour therethrough. The permeable material may be configured to restrict or prevent flow of gases therethrough. A hose  11  formed, at least partially, of a permeable material reduces the build-up of water and/or water vapour inside the hose  11 . Reducing, restricting or preventing the flow of gases through the material prevents a loss of air pressure in the nasal respiratory mask system  2 . Similarly, the connector  12  may be at least partially, and in some cases entirely, formed from the water permeable material. 
     The nasal respiratory mask system  2  may include or be connected to a heating arrangement for heating up the oxygen enriched air prior to inhalation by the patient. For example, the hose  11  may comprise a heating wire breathing circuit that heats the air flowing through the hose  11 . The heating wire breathing circuit may be provided in the form of a wire heating element  18  (See  FIG.  4   ). The wire heating element  18  may extend around the hose  11 , e.g. spiral around the hose  11 . The wire heating element  18  may be positioned in any suitable position for heating up the air passing through the hose  11 , for example the wire heating element  18  may be embedded within the wall of the hose  11 , or wrapped around an outer surface of the hose  11 , or within the hose  11  adjacent an inner surface of the hose  11 . 
     Alternatively, or in addition, the nasal respiratory mask system  2  may be connected to a heating unit as described in relation to  FIG.  6   . 
     The nasal respiratory mask  3  may comprise one or more clips  14   a,    14   b  as shown, for example, in  FIG.  4   . A first clip  14   a  may be located between the ends of the hose  11 . A second clip  14   b  may be located on a distal end of the hose  11 , relative to the hose attachment portion  10 . The one or more clips  14   a,    14   b  may be configured for securing the hose  11  to an object. When fastened to an object, either directly or indirectly, the clips  14   a,    14   b  may help to reduce strain on the hose  11  and help to position the hose  11  in use. 
     In some examples, clips  14   a,    14   b  may include one or more holes  14   c  through which a fastening pin  14   d  extends (See clip  14   a  in  FIG.  5   ) so as to fasten the clip  14   a.  The clip  14   a  may comprise a plurality of holes  14   c  so that the size and tightness of the clip  14   a  can be adjusted. Alternatively, the clip  14   a,    14   b  may include a crocodile clip end  14   e,  or similar, that latches onto an object. 
     In some examples, either or both of the clips  14   a,    14   b  may be garment clips for attaching to an item of clothing of the patient, or attachable to a garment clip, or configured for securing the hose  11  to a lanyard (not shown), a strap  15   i,    15   j,  harness or head band  19 . 
     An example of the straps  15   i,    15   j  is shown best in  FIGS.  1  to  4   , which show a first strap  15   i  extending from a first side of the mask  3  and a second strap  15   j  extending from a second side of the mask  3 . The straps  15   i,    15   j  may extend in opposite directions, such that they are opposing straps  15   i,    15   j  extending from the mask frame  5 . However, it will be understood that there may be any number of straps  15   i,    15   j  extending in any suitable direction. The straps  15   i,    15   j  may be flexible. The straps  4  may comprise an elastomer, for example the straps  4  may be made of silicone. 
     The straps  15   i,    15   j  may extend from the mask frame  5 . The straps  15   i,    15   j  may be arranged to extend around part of the patients face. The straps  15   i,    15   j  may be configured to extend around a patient&#39;s head entirely, such that the straps  15   i,    15   j  directly attach to each other. The straps  15   i,    15   j  may be joined to each other so that they form a unitary strap extending from opposing ends of the mask  3 . 
     In alternative examples, the straps  15   i,    15   j  may attach to a harness or patient head band  19 , as shown in  FIG.  3   . This allows different sizes of harness or head band  19  to be selected for a given patient, or for an adjustable harness or head band  19  to be selected. The harness or head band  19  may have attachment portions that attach to corresponding attachment portions on the straps  15   i,    15   j.  As shown in  FIG.  3   , the straps  15   i,    15   j  may include one or more slots  16   i,    16   j  (alternatively referred to as apertures) that provide attachment portions for attaching a patient head band, or clip for attaching a patient head band, although it will be clear that other attachment portions may be provided on the straps  15   i,    15   j  or directly on the mask  3 . 
     The nasal respiratory mask  3  may include means to monitor a carbon dioxide level of the mask  3 , and in particular monitor the nasal breathing cavity. The nasal respiratory mask  3  may comprise a carbon dioxide monitoring line connector  31  for attaching a carbon dioxide sensor  30 , as shown in  FIGS.  1  to  5   . 
       FIG.  5    shows an example of a nasal respiratory mask  3  comprising a carbon dioxide monitoring line  32  extending between the carbon dioxide monitoring line connector  31  and the carbon dioxide sensor  30 , although it will be appreciated the carbon dioxide sensor  30  may connect to the carbon dioxide monitoring line connector  31  directly. In the example shown in  FIG.  5    the carbon dioxide monitoring line connector  31  is located on a lower end of the mask frame  5 , although it will be appreciated that the carbon dioxide monitoring line connector  31  may be located at any suitable position on the mask  3 . The carbon dioxide monitoring line connector  31  may include a cap for sealing a port of the carbon dioxide monitoring line connector  31  when not is use, and/or the carbon dioxide monitoring line connector  31  may be self-sealing so as to substantially prevent through-flow across the port when the carbon dioxide monitoring line  32  is disconnected from the carbon dioxide monitoring line connector  31 . The carbon dioxide monitoring line  32  may include an elbow connector, and/or may form a swivel connection with the carbon dioxide monitoring line connector  31 , as described in relation to hose attachment portion  10  and connector  12 . 
     The carbon dioxide monitoring line  32  may be at least partially, and in some cases entirely, formed from a water permeable material, and function similarly to the permeable material of the mask frame  5  and hose  11  described above in that it allows liquid water and/or water vapour therethrough. The permeable material may be configured to reduce, restrict or prevent flow of gases therethrough. A carbon dioxide monitoring line  32  formed, at least partially, of a permeable material reduces the build-up of water and/or water vapour inside the carbon dioxide monitoring line  32 . Similarly, the carbon dioxide monitoring line connector  31  may be at least partially, and in some cases entirely, formed from the water permeable material. 
     The hose  11  may be malleable, and/or comprise a malleable member  17  (as shown in  FIG.  6   ), such that the hose  11  is able to retain a given shape or position when manipulated (e.g. bent or twisted) into that shape or position. This allows the hose  11  to be positioned so as to improve patient comfort and/or clinician access. The entire length of the hose  11  may be malleable, or comprise a malleable member  17 , or only a portion of the hose  11  may be malleable, or comprise a malleable member  17 . 
     The malleable properties of the malleable member  17  mean that it retains a given shape or position when manipulated (e.g. bent or twisted), such that the hose  11  is able to retain a given shape or position when manipulated into that shape or position. This allows the hose  11  to be positioned so as to improve patient comfort and/or clinician access. The malleable member  17  may extend along the entire length of the hose  11 , or the malleable member  17  may extend along only a portion of the length of the hose  11  (e.g. 50% of the length). In some examples, the malleable member  17  may extend from the hose  11 , i.e. from a location between or at the ends of the hose  11 , and attach to or press against an external object (e.g. part of the patient) or part of the nasal respiratory system  2  (e.g. one of the straps  15   i,    15   j,  or the harness/head band  19 ) so as to support the hose  11  via the object. 
       FIG.  7    shows an example of a high flow oxygen therapy apparatus  1  including a nasal respiratory mask system  2 . 
     The high flow oxygen therapy apparatus  1  may include a carbon dioxide sensor  30  connected to the nasal respiratory mask  3 , as described above. 
     The hose  11  may extend from the nasal respiratory mask  3  to a heating chamber and/or humidification chamber  40  that selectively and controllably heats and humidifies the oxygen enriched air supplied to the patient through the nasal respiratory mask. 
     The heating chamber and/or humidification chamber  40  may be arranged between the mask  3  and a ventilator  50 . The ventilator  50  may be arranged to produce the high flow of oxygen enriched air. The oxygen enriched air may be supplied by an oxygen enriched air supply  60  connected to the ventilator  50 . In some examples, the carbon dioxide sensor  30  may be integrated into the ventilator  50 . 
     The nasal respiratory mask  3  includes at least one mask aperture providing a gas flow path through the mask between the nasal breathing cavity and ambient air outside the mask. The mask aperture has an adjustable flow restrictor for restricting the flow of gas from the nasal breathing cavity directly to ambient. Adjustment of the adjustable flow restrictor is performed to maintain a positive end-expiratory pressure (PEEP) in the nasal breathing cavity of between 0.2 kPa and 1 kPa during the administering of high flow oxygen therapy to the patient. 
     The nasal respiratory mask  3  may include at least one mask aperture  20   i,    20   j.  The example shown in  FIGS.  1  to  4    is shown to include two mask apertures  20   i,    20   j  although the nasal respiratory mask  3  may comprise any number of mask apertures  20   i,    20   j,  for example one, two, three or four. 
     In one arrangement, the mask aperture(s) each include a passive one way valve  20   i,    20   j.  The one or more one-way valves  20   i,    20   j  are configured to move from a closed position in which air is restricted from flowing through the one-way valve  20   i,    20   j  into the nasal breathing cavity, to an open position in which air can flow from the nasal breathing cavity through the one-way valve  20   i,    20   j  to outside the mask. 
     The one-way valves  20   i,    20   j  allow the pressure in the nasal breathing cavity of the mask  3  to drop when it reaches the opening pressure of the valve  20   i,    20   j,  or at least prevents the pressure in the nasal breathing cavity increasing above a given value, e.g. 1 kPa. Above 1 kPa (approximately 10 cmH2O), expiration by the patient into this nasal breathing cavity pressure may become uncomfortable. 
     When a patient breathes in, the pressure in the nasal breathing cavity remains below the valve opening pressure such that the one-way valve  20   i,    20   j  is arranged in the closed position. When the patient breathes out, the pressure in the nasal breathing cavity will increase and may increase above the valve opening pressure such that the one-way valve  20   i,    20   j  moves from the closed position to the open position. This allows air to escape from the nasal breathing cavity and, in particular, allows expired gases from the patient to be expired when breathing out, whilst preventing ambient air being inhaled by the patient through the one-way valve  20   i,    20   j  when breathing in. 
     The one-way valve(s)  20   i,    20   j  may have a valve opening pressure of approximately 0.5 kPa (approximately 5 cm H2O), which is expected to be sufficient to wash out gases expired by the patient. The valve opening pressure may be less than 1 kPa (approximately 10 cm H2O). Although it will be appreciated that the valve opening pressure may be any suitable value. The valve opening pressure may be less than 0.8 kPa (approximately 8 cm H2O) or less than 0.6 kPa (approximately 6 cm H2O). The valve opening pressure may be greater than 0.2 kPa (approximately 2 cm H2O), greater than 0.3 kPa (approximately 3 cm H2O) or greater than 0.4 kPa (approximately 4 cm H2O). 
     Adjustment of the valve opening pressure may enable adjustment of the flow restriction provided by the one-way valve in the mask aperture. The valve opening pressure adjustment may be performed manually, i.e. without and closed loop control of the valve so that the one-way valve remains fully passive in its operation to open and close. 
     The mask cushion  6  may be arranged to form a seal that is maintained up to pressures that at least match the opening pressure of the valve(s)  20   i,    20   j,  so as to prevent excessive leakages of air below the opening pressure. 
     The desirable flow rate through the one-way valve  20   i,    20   j  is determined based on ensuring the pressure in the nasal breathing cavity is maintained at a suitable level during expiration of the patient. The flow rate through the one-way valve  20   i,    20   j  in the open position may be at least 2 litres per minute, or preferably at least 5 litres per minute. To prevent excessive pressure loss through the one-way valve  20   i,    20   j,  the flow rate through the one-way valve  20   i,    20   j  in the open position may be less than 20 litres per minute in use. 
     The one-way valve(s)  20   i,    20   j  defines an opening  21  that extends through the mask frame  5  in the open position, and through which the air can flow from the nasal breathing cavity to outside the mask  3 . The opening may have a cross-sectional area of between 0.5 cm 2  and 15 cm 2 , and preferably between 1 cm 2  and 8 cm 2 . 
     The one-way valve(s)  20   i,    20   j  may be any suitable one-way valve. 
       FIGS.  8 A and  8 B  show an example in which the one-way valve  20   i,    20   j  is a one-way umbrella valve  20   a.  As shown in  FIG.  8 A , the umbrella valve  20   a  has a generally umbrella shaped valve member  22   a  comprising a dome-shaped diaphragm  23   a  that sits over the opening  21  and prevents air passing through the opening  21 , from outside the mask  3  to inside the nasal breathing cavity of the mask  3 , when the valve  20   a  is in the closed position. When the flow of air is reversed, such that when air pressure is applied to the bottom of the diaphragm  23   a,  the diaphragm is forced upwards at the valve opening pressure so that the diaphragm  23   a  assumes an upwardly curved shape that allows air to pass through the opening from the nasal breathing cavity to outside the mask  3 , as shown in  FIG.  8 B . 
       FIGS.  9 A and  9 B  show an example in which the one-way valve  20   i,    20   j  is a flapper valve  20   b.  The flapper valve  20   b  comprises a hinge or pivot mechanism  25   b  that provides limited rotation of the valve member  22   b,  so that air travelling through the opening  21  from outside the mask  3  to inside the nasal breathing cavity of the mask  3  moves or holds the valve member  22   b  in a closed position. When the air flow is reversed, the valve member  22   b  is moveable through an angle range permitted by the hinge  25   b,  so as to move the valve member  22   b  from the closed position ( FIG.  9 A ) to the open position ( FIG.  9 B ). The movement of the hinge  25   b  from the closed position to the open position is restricted, so that the air pressure must reach a minimum (valve opening) pressure. For example, the valve member  22   b  may have a minimum weight that counters any movement, or the hinge  25   b  may be spring-loaded by a spring  24   b.  The hinge  25   b  may be adjustable so as to adjust the valve opening pressure and/or adjust the angular range of the hinge and thereby adjust the size of the opening through the one-way valve. 
       FIGS.  10 A and  10 B  show an example in which the one-way valve  20   i,    20   j  is a lift-check valve  20   c.  The lift-check valve  20   c  comprises a valve member  22   c  biased by a spring  24   c  against a wall of the mask frame  5 , so as to block the opening  21 . When air flows from outside the mask  3  through the opening  21  (See  FIG.  10 A ), the valve member  22   c  remains in a closed position of the valve  20   c  so as to prevent airflow through the opening  21  into the nasal breathing cavity. When the air flow reverses, the air pressure pushes against the valve member  22   c  and against the spring  24   c  so as to move the valve member  22   c  from the closed position ( FIG.  10 A ) to the open position ( FIG.  10 B ). Air is thereby able to flow from inside the nasal breathing cavity, through the opening  21 , and outside the mask  3 . 
       FIGS.  11 A and  11 B  show an example in which the one-way valve  20   i,    20   j  is a spring-loaded inline one-way valve  20   d.    FIG.  11 A  shows the valve  20   d  extending through an opening  21  of the mask frame  5 . The valve  20   d  is in a closed position, such that the valve member  22   d  prevents air travelling through the opening  21  from outside the mask  3  to inside the nasal breathing cavity of the mask  3 . When the air flow is reversed, the air pressure pushes against the valve member  22   d  so as to move the valve member  22   d  from the closed position ( FIG.  11 A ) to the open position ( FIG.  11 B ). A spring  24   d  biases the valve member  22   d  into the closed position, such that the air pressure must reach a minimum pressure value (i.e. valve opening pressure) to move the valve member  22   d  against the spring  24   d,  so as move to the open position and allow air to move through the opening  21 . 
     The spring  24   d  may be selected to provide a particular valve opening pressure, and/or be adjustable so as to vary the valve opening pressure. Where the valve opening pressure is not adjustable, a separate adjustable flow restrictor may be provided in line with the one-way valve. An adjustable flow restrictor  26  configured to adjust a minimum size of the opening  21  on one side of the one-way valve  20   d.  For example,  FIGS.  12 A and  12 B  show an adjustable flow restrictor  26  comprising a wedge-shaped member  27  that engages a neck of the opening  21 . Rotation of the head  28  of the adjustable flow restrictor  26  varies an air gap between the wedge-shaped member  27  and the neck of the opening  21  so as to control the minimum size of the opening  21 .  FIG.  12 A  shows the adjustable flow restrictor  26  in a first position, defining a first minimum size of the opening  21 , and  FIG.  12 B  shows the adjustable flow restrictor  26  in a second position, defining a second minimum size of the opening  21  that is greater than the minimum size of the opening  21  shown in  FIG.  12 A . This allows the flow rate through the one-way valve  20   d  to be variable. It will be understood that any of the above-mentioned one-way valves  20   i,    20   j,    20   a,    20   b,    20   c,    20   d  may comprise an adjustable flow restrictor  26  configured to adjust a minimum size of the opening  21  so as to adjust the flow rate through the one-way valve  20   i,    20   j,    20   a,    20   b,    20   c,    20   d.    
     It will be understood that in some examples, the valve  20   i,    20   j,    20   a,    20   b,    20   c,    20   d  opening and closing pressures may differ slightly due to static friction and other effects causing some hysteresis. 
     In some examples, as shown in  FIG.  13   , the nasal mask  3  may include one or more mask apertures with one-way valves  120   i,    120   j  whose movement from the closed position to the open position is entirely dependent on the pressure in the nasal breathing cavity of the mask  3 , such that the one-way valves do not have a minimum opening pressure. In other words, the valve member may be substantially unbiased, with movement from the closed position to the open position dependent on the pressure in the nasal breathing cavity. It will be appreciated that the one-way valve may have some relative bias due to friction or the weight of the valve member, but that this bias is minimised for all practical purposes. Examples of such unbiased valves will now be described. 
       FIGS.  14 A and  14 B  show an example in which the one-way valve is an unbiased flapper valve  120   b  similar to that shown in  FIGS.  9 A and  9 B  except it does not have a spring  24   b.  The flapper valve  120   b  comprises a hinge or pivot mechanism  25   b  that provides limited rotation of the valve member  22   b,  so that air travelling through the opening  21  from outside the mask  3  to inside the nasal breathing cavity of the mask  3  moves the valve member  22   b  into a closed position. When the air flow is reversed, the valve member  22   b  is moveable through a permitted angle range, so as to move the valve member  22   b  from the closed position ( FIG.  14 A ) to the open position ( FIG.  14 B ). 
       FIGS.  15 A and  15 B  show an example in which the one-way valve is an unbiased inline one-way valve  120   d.    FIG.  15 A  shows the valve  20   d  extending through an opening  21  of the mask frame  5 . The valve  120   d  is in a closed position, such that the valve member  22   d  prevents air travelling through the opening  21  from outside the mask  3  to inside the nasal breathing cavity of the mask  3 . When the air flow is reversed, the air pressure pushes against the valve member  22   d  so as to move the valve member  22   d  from the closed position ( FIG.  15 A ) to the open position ( FIG.  15 B ), allowing air to flow around the valve member and through apertures  22   e.    
     The one-way valves of  FIGS.  14 A , B and  15 A, B are provided in line with an adjustable flow restrictor  26  configured to adjust a minimum size of the opening  21  on one side of the one-way valve, such as shown in  FIGS.  12 A and  12 B . The adjustable flow restrictor  26  may be compatible with any suitable one-way valve, such as the one-way valves  120   b,    120   d  described above. The adjustment of the adjustable flow restrictor  26  may be performed manually. 
       FIGS.  16 A to  16 G  show a further example of an adjustable flow restrictor  126  that can be placed in or over the opening  21  to be used in line with either an open vent or a passive one-way valve, as described above. The adjustable flow restrictor  126  comprising a moveable element  128  that rotates about an axis relative to a fixed element  127  of the adjustable flow restrictor  127 . Rotation of the moveable element  128  (e.g. via a handle  129 ) adjusts an opening through the adjustable flow restrictor  126  through which air can flow. In this way, the effective size of the opening  21  is variable. 
       FIGS.  16 B and  16 C  show the adjustable flow restrictor  126  in a closed position, in which the moveable element  128  has been rotated to entirely cover the opening and prevent air flow therethrough. 
       FIGS.  16 D and  16 E  show the adjustable flow restrictor  126  in an open position, in which the moveable element  128  has been rotated to entirely uncover the opening and allow air flow therethrough. In this particular example, the moveable element  128  is rotated by approximately 180 degrees with respect to its position in the closed position of  FIGS.  16 B and  16 C , however it will be appreciated that the relative degrees of rotation to cover and uncover the opening may vary. 
       FIGS.  16 F and  16 G  show the adjustable flow restrictor  126  in a partially open position, in which the moveable element  128  has been rotated to partially cover the opening and allow some relative air flow therethrough. 
     As will be apparent, the position of the moveable element  127  can be varied to control the effective size (cross sectional area) of the opening  21 . In the example of  FIGS.  16 A to  16 G , the moveable element  128  is rotated between 0 and 180 degrees to vary the degree of the opening that is uncovered, although any suitable range may be provided. 
     The adjustable flow restrictor  26 ,  126  can be tuned so as to adjust the size of the opening on one side of the one-way valve. In this way the nasal breathing cavity can be maintained at a selectable positive pressure value, for instance a positive pressure of between 0.2 kPa and 1 kPa can be maintained. This allows the mask  3  to be used at a range of different HFOT supply flow rates to meet or slightly exceed the peak inspiratory flow rate of the patient, whilst maintaining the positive pressure within the nasal breathing cavity to within an effective range. The effective range ensures the positive pressure is not too high so as to be uncomfortable for the patient and not too low so as to be ineffective at maintain the nasal airways open for gas recruitment to the lungs. A positive pressure above 1 kPa is considered uncomfortable for a patient, whilst a positive pressure below 0.2 kPa reduces the effectiveness of the treatment. The positive pressure may be maintained at a level between 0.4 kPa and 0.8 kPa, or at approximately 0.5 kPa. 
     The mask apertures may be positioned in an optimal position for allowing expiration from the nasal respiratory mask  3 , whilst minimising loss of the oxygen enriched air fed through the hose  11 . Nasal respiratory masks  3  comprising two or more mask apertures may arrange the apertures  20   i,    20   j  to be in a spaced arrangement across the mask frame  5 . As shown best in  FIG.  2   , the mask apertures may be spaced substantially symmetrically about a vertical centre line of the mask frame  5 . Substantially in this context refers to allowing for manufacturing tolerances. This allows the mask apertures to be arranged with respect to the generally symmetrical face of a patient. For example, the mask apertures may be located towards a lower end of the mask frame  5  so as to be adjacent either side of a user&#39;s mouth when worn. The mask apertures may be located on a vector of the nostril, so that air from the nasal passages is directed to the mask apertures. 
     In some examples, the nasal respiratory mask  3 , and particularly the mask frame  5 , may have no gas flow apertures other than the hose attachment portion  10 , the carbon dioxide monitoring line connector  31 , and the mask apertures. This helps to ensure the gas flow into and out of the nasal respiratory mask  3  is controlled. 
       FIGS.  17  and  18    show a nasal respiratory mask system  2  according to a second example. The second example is substantially the same as the first example of a nasal respiratory mask system  2  shown in  FIGS.  1  to  6   , except that the valves  20   i,    20   j  are replaced by vents  29   i,    29   j  and the carbon dioxide monitoring line connector  31  is located substantially in the middle of the mask frame  5 . It will be appreciated that the carbon dioxide monitoring line connector  31  and hose attachment portion  10  may be located at any suitable position on the nasal respiratory mask  3 , for example,  FIGS.  17  and  18    show the carbon dioxide monitoring line connector  31  above the hose attachment portion  10  (above in this context referring to the orientation of the mask  3  when on the patient&#39;s face). 
     In some examples, the carbon dioxide monitoring line connector  31  and hose attachment portion  10  may be one component on the mask  3 . For example, a single connector located on the mask  3  may comprise, or function as, the carbon dioxide monitoring line connector  31  and hose attachment portion  10 . 
     The vents  29   i,    29   j  operate substantially the same as the valves  20   i,    20   j  in the open position of the previous examples, in that they each define an opening/aperture through the mask  3 . In order to ensure an increased pressure is maintained within the mask  3  and delivered to the patient, the vents  29   i,    29   j  each define an opening having a total cross-sectional area (i.e. the opening of each vent  29   i,    29   j  combined) less than a cross-sectional area of the opening through the hose attachment portion  10 . The total cross-sectional area may be less than 80% of the cross-sectional area of the opening through the hose attachment portion  10 , and in some examples less than 50%. 
     In contrast to the one-way valves  20   i,    20   j,    120   i,    120   j  of the previous examples, the vents  29   i,    29   j  remain in an open position so that air can flow from the nasal breathing cavity through each vent  29   i,    29   j  to outside the mask  3  at all times, e.g. during both inhalation and exhalation of the patient. However, the extent to which air flow can travel through the vent  29   i,    29   j  may be variable by using an adjustable flow restrictor  26 ,  126 , such as described above. 
     In particular, the adjustable flow restrictor  26 ,  126  can be tuned so as to adjust the size of the opening through the vent  29   i,    29   j.  In this way the nasal breathing cavity can be maintained at a selectable positive pressure value, for instance a positive pressure of between 0.2 kPa and 1 kPa can be maintained. This allows the mask  3  to be used at a range of different flow rates to meet or slightly exceed the peak inspiratory flow rate of the patient, whilst maintaining the positive pressure within the nasal breathing cavity to within an effective range. The effective range ensures the positive pressure is not too high so as to be uncomfortable for the patient and not too low so as to be ineffective at maintain the nasal airways open for gas recruitment to the lungs. A positive pressure above 1 kPa is considered uncomfortable for a patient, whilst a positive pressure below 0.2 kPa reduces the effectiveness of the treatment and may result in entrainment of ambient air through the vent  29   i,    29   j.  The positive pressure may be maintained at a level between 0.4 kPa and 0.8 kPa, or at approximately 0.5 kPa. 
       FIG.  19    shows a neo-natal nasal respiratory mask system  2  according to a third example. The third example is substantially the same as the first example of a nasal respiratory mask system  2  shown in  FIGS.  1  to  7   , except that the nasal respiratory mask  3  is for neo-natal patients. The size of the nasal respiratory mask  3  of the third example is therefore tailored to dimensions suitable for neo-natal patients. 
     In particular, the nasal respiratory mask  3  may be smaller so as to extend over the face of the neo-natal patient, thereby covering the nasal passages, without extending over the eyes or entirely over the face of the patient. This ensures an adequate seal of the mask  3  to the face of the patient is formed, whilst also maintaining patient comfort. 
     Other features of the neo-natal nasal respiratory mask system  2  may also be smaller with respect to those features of the adult nasal respiratory mask system  2  of the first and second examples. For example, the size of the straps  15   i,    15   j.  The length and/or diameter of the hose  11  may be smaller than the length and/or diameter of the hose  11  of an adult nasal respiratory mask system  2 . 
     Alternatively, many of the features may be the same size as for the adult nasal respiratory mask system  2 . For example, the carbon dioxide monitoring line connector  31  and hose attachment portion  10  may be a standard size, thereby allowing a single carbon dioxide monitoring line  32  or hose  11  to connect to any of the nasal respiratory masks  3 . 
     In the example shown in  FIG.  19   , the hose attachment portion  10  is positioned towards a middle of the mask  3 . However, it will be understood the hose attachment portion  10  may be positioned in any suitable position on the mask  3 . 
     In some examples of a neo-natal nasal respiratory mask system  2 , the flow rates supplied through the hose  11  and to the patient may be less than that required for an adult patient. For example, in order to approximately match or exceed the neo-natal patient&#39;s peak inspiratory flow rates, the flow rate delivered to the patient may be lower than 20 LPM, however it is generally at least 5 LPM. 
     Accordingly, the valve opening pressure may be similarly adjusted and/or adjustable to suit the needs of a neo-natal patient. The one-way valve(s)  20   i,    20   j  may have a valve opening pressure of approximately 0.5 kPa (approximately 5 cm H2O), or a lower value of approximately 0.3 kPa (approximately 3 cm H2O). In other examples, the one-way valve  120   i,    120   j  may be unbiased, such as described in relation to  FIGS.  13  to  15   . 
     It will be understood that the mask  3  may include one-way valves  20   i,    20   j,    120   i,    120   j  and/or vents  29   i,    29   j,  as required. The flow rate through the one-way valves  20   i,    20   j  or vents  29   i,    29   j  may be at least 1 litres per minute, and is preferably at least 3 litres per minute or 5 litres per minute. The flow rate may be approximately linked to the patient&#39;s weight, e.g. 2 litres per minute per kg. 
     In some examples, the mask frame  5  and mask cushion  6  may be integrally formed of a single material, thereby allowing the neo-natal nasal respiratory mask  3  to be lighter for the patient. For example, the neo-natal nasal respiratory mask  3  may be formed of silicone. Alternatively, the construction of the neo-natal nasal respiratory mask  3  may be the same as for the adult nasal respiratory mask  3  described in relation to the first and second examples. 
       FIG.  20    shows a neo-natal nasal respiratory mask system  2  according to a fourth example. The fourth example is substantially the same as the third example of a nasal respiratory mask system  2  shown in  FIG.  19   , except that the carbon dioxide monitoring line connector  31  is located substantially in the middle of the mask  3  above the hose attachment portion  10 , such as is shown and described in relation to the second example of  FIGS.  17  and  18   . 
     In some examples, the mask  3  and/or mask frame  5  may not include a carbon dioxide monitoring line connector  31 .  FIG.  21    shows an example of an adult nasal respiratory mask  3  that does not include a carbon dioxide monitoring line connector  31  and  FIG.  22    shows an example of a neo-natal nasal respiratory mask  3  that does not include a carbon dioxide monitoring line connector  31 . 
     In some examples, a carbon dioxide monitoring line connector  31  may be attached to the hose  11  or other component of the mask  3 , e.g. connector  12 . The carbon dioxide monitoring line connector  31  may be arranged to substantially prevent air flow therethrough when a carbon dioxide monitoring line  32  is not coupled to the carbon dioxide monitoring line connector  31 . For example, the carbon dioxide monitoring line connector  31  may be a pneumatic quick connect coupling or similar, or a connector cap (not shown) may be selectively placed over the connector  31 . 
     In some examples, the one-way valves  20   i,    20   j  and/or vents  29   i,    29   j  may include a filter membrane  39  covering the opening/aperture of the one-way valves  20   i,    20   j  and/or vents  29   i,    29   j,  for example as shown in  FIG.  23 A . The filter membrane  39  may be located against an inner surface of the mask frame  5 , an outer surface of the mask frame  5 , or in the opening between the inner and outer surfaces. The filter membrane  39  may be arranged to reduce the spread of contaminants from the patient&#39;s exhaled breath. In some examples the filter membrane  39  may extend over a significant portion (i.e. over half) of the mask frame  5  and/or extend to cover the patient&#39;s mouth. 
       FIGS.  23 B and  23 C  show an example in which the filter membrane  39  is configured to cover the whole of the mask frame  5 . In some examples, the filter membrane  39  may include straps  38  to assist in attaching the filter membrane  39  to the patient or mask  3 . One or more straps  38  may extend from the filter membrane  39 . As shown in  FIG.  23 B , the straps  38  may be elongate strips arranged to be tied together or to an external object, such as part of the nasal respiratory mask system  2 . Alternatively, the straps  38  may comprise a fastener on an end of the strap  38  for fastening to an adjacent strap  38  or external object. The straps  38  may be arranged as ear straps, which wrap around an ear, or head straps, which wrap around the patient&#39;s head. The straps  38  may be formed of the same material as the filter membrane  39 , or an alternative material. The straps  38  may be elastic or substantially rigid. Alternatively or in addition, the filter membrane  39  may include an elastic strip (not shown) extending circumferentially around the filter membrane  39  to fit over the patient&#39;s head. 
     In some examples, the filter membrane  39  may be arranged to also cover the patient&#39;s mouth.  FIG.  23 D  shows an example substantially the same as the example shown in  FIGS.  23 B and  23 C , except the filter membrane  39  extends over the patient&#39;s mouth in addition to extending over the mask frame  5 . 
     A filter membrane  39  covering at least part the mask frame  5 , as well as optionally the patient&#39;s mouth, can help to reduce the spread of particles from the patient&#39;s exhaled breath to the environment, increasing safety for clinicians and other patients. 
     In some examples, the nasal respiratory mask system  2  may not include a one-way valve or vent.  FIG.  24    shows an example of an adult nasal respiratory mask  3 , arranged to cover the nasal passages but not the mouth, which does not include a one-way valve. The mask cushion  6  may be arranged to form a sufficient seal to prevent excessive leaks, but will not be fully sealed to the patient&#39;s face, for example leaks that may detrimentally lower the flow rate, thereby causing poor performance, but will typically leak more than the nasal respiratory mask systems  2  of  FIGS.  1  to  23    during patient outflow, due to the absence of a one-way valve to evacuate excess air pressure. The leakages from the mask  3  and mask cushion  6  may be designed so as to substantially direct flow away from certain parts of the patient, such as the patient&#39;s eye, thereby improving patient comfort. For example, the mask cushion  6  may be contoured to form a tighter seal at an upper half of the mask cushion  6 , adjacent the patient&#39;s eyes, than at a lower half of the mask cushion  6 , adjacent the patient&#39;s mouth. 
     The masks  3  described above may be configured to engage the patient&#39;s nose without the use of nasal prongs that enter the patient&#39;s nasal passages. This may improve patient comfort, whilst still allowing the delivery of high flow rates of air to the patient. 
     Where the word ‘or’ appears this is to be construed to mean ‘and/or’ such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination. 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.