Therapeutic gas delivery system coupling

Abstract

A therapeutic gas delivery system coupling 1 comprising an inlet 10 coupleable to a therapeutic gas supply; a delivery outlet 20 configured for delivery of therapeutic gas to a subject and a waste outlet coupleable to a gas extraction system. The coupling comprising a body portion 40 having a main cavity 40a located between the inlet and the delivery outlet and an ancillary cavity 40b, located between the main cavity and the waste outlet. The main cavity and ancillary cavity are linked via an exhalation valve 50 moveable between a closed position if a pressure differential between the ancillary cavity and the main cavity is greater than or substantially equal to zero, and an open position if a pressure differential between the ancillary cavity and the main cavity is less than zero. The ancillary cavity comprises a waste valve 60 moveable between an open position if a pressure differential between the ancillary cavity and ambient pressure is less than zero, and a closed position if a pressure differential between the ancillary cavity and ambient pressure is greater than or equal to zero. A therapeutic gas delivery system and exhalation valve assembly is also provided.

Description

TECHNOLOGICAL FIELD Various example embodiments relate to a therapeutic gas delivery system coupling, a therapeutic gas delivery system including such a coupling, and a method of coupling components of a therapeutic gas delivery system using such a coupling. BACKGROUND Therapeutic gases are administered to subjects in support of various clinical, therapeutic and / or other procedures. One such therapeutic gas is nitrous oxide. Nitrous oxide can be administered to a subject as an anaesthetic or for pain relief, for example, in medical, dental, and veterinary contexts. Nitrous oxide is typically administered to a subject as a mixture with oxygen in a ratio 50:50 by volume, though other ratios are possible and are used in dependence upon administration scenarios. Nitrous oxide used as an anaesthetic or for pain relief is not metabolized by a subject nor consumed by a subject. As a result, the nitrous oxide administered to a subject during a procedure is exhaled or otherwise excreted in an amount equal to that originally delivered. Typically a subject to whom nitrous oxide has been administered will exhale, and nitrous oxide will form part of the exhalation gases. The exhalation of a subject may, in some scenarios, be directed away from the subject and other personnel. In some administration scenarios nitrous oxide, both from the supply and in exhaled gases, may escape into the area where it is being administered. A sustained high level of nitrous oxide in an area could lead to potential long-term risk to personnel. Nitrous oxide used in the manner described above is eventually released to the atmosphere. Nitrous oxide is a greenhouse gas, having a lifetime in the atmosphere of greater than 100 years and a global warming potential of about 265 [3] times that of carbon dioxide. New methods and devices for addressing nitrous oxide released as part of therapeutic gas delivery to a subject may be beneficial. BRIEF SUMMARY The scope of protection sought for various example embodiments of the invention is set out by the independent claims. The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention. According to various, but not necessarily all, embodiments there is provided a therapeutic gas delivery system coupling comprising: an inlet coupleable to a therapeutic gas supply; a delivery outlet configured for delivery of therapeutic gas to a subject; a waste outlet coupleable to a gas extraction system; the coupling comprising a body portion having: a main cavity located between the inlet and the delivery outlet; and an ancillary cavity, located between the main cavity and the waste outlet; wherein: the main cavity and ancillary cavity are linked via an exhalation valve, the exhalation valve being moveable between: a closed position if a pressure differential between the ancillary cavity and the main cavity is greater than or substantially equal to zero, and an open position if a pressure differential between the ancillary cavity and the main cavity is less than zero; and wherein the ancillary cavity further comprises a waste valve, the waste valve being moveable between: an open position if a pressure differential between the ancillary cavity and ambient pressure is less than zero, and a closed position if a pressure differential between the ancillary cavity and ambient pressure is greater than or equal to zero. In some embodiments, the therapeutic gas comprises an analgesic gas. In some embodiments, the therapeutic gas comprises nitrous oxide. In some embodiments, at least one of the exhalation valve and / or waste valve comprise: a membrane valve. In some embodiments, the exhalation valve comprises a membrane valve. In some embodiments, the waste valve comprises a membrane valve. In some embodiments, the membrane valve is arranged, when in a closed position, to obstruct gas flow therethrough. In some embodiments, the membrane valve allows unimpeded gas flow when in an open position; is gas tight in a closed position; or in between the two; in dependence upon pressure difference across the valve. In some embodiments, the body portion comprises an assembly of components. In some embodiments, the assembly comprises: an exhalation valve assembly incorporating: the inlet, the delivery outlet and the exhalation valve. In some embodiments, the body portion assembly comprises a diverter portion incorporating: the waste outlet and the waste valve. In some embodiments, the diverter portion and the exhalation valve assembly couple together to form the body portion. In some embodiments, the exhalation valve assembly is removably replaceable from the coupling. In some embodiments, the therapeutic gas delivery coupling comprises an administration device coupled to the outlet for delivery of therapeutic gas to a subject. In some embodiments, the administration device comprises: a mouthpiece or a mask. In some embodiments, the administration device comprises a double skinned mask, comprising an inner mask and an outer mask, wherein the inner mask is shaped to seal to a subject’s face, and wherein the waste valve is in communication with a cavity formed between the inner mask and the outer mask. According to some, but not necessarily all, embodiments, there is provided an exhalation valve assembly as described above for insertion into a therapeutic gas delivery system coupling as described above. According to some, but not necessarily all, embodiments, there is provided a therapeutic gas delivery system incorporating a therapeutic gas delivery system coupling as described above. In some embodiments, the therapeutic gas delivery system comprises: a gas supply, wherein the coupling is provided between the gas supply and administration apparatus for administration of gas to a subject. In some embodiments, the therapeutic gas delivery system is such that the coupling is coupled to the administration apparatus. In some embodiments, the therapeutic gas delivery system comprises: a gas extraction system, and the waste outlet of the coupling is coupled to an inlet of the gas extraction system. In some embodiments, the gas extraction system is configured to apply a negative, or suction, gas flow to the waste outlet of the coupling. In some embodiments, the therapeutic gas delivery system comprises: a flow indicator located downstream of the waste outlet and upstream of gas extraction apparatus. In some embodiments, the therapeutic gas delivery system comprises: a variable volume exhaust gas reservoir, for receiving gas exhaled by a subject. According to some, but not necessarily all, embodiments there is provided a method of providing a therapeutic gas delivery system coupling comprising: an inlet coupleable to a therapeutic gas supply; a delivery outlet configured for delivery of therapeutic gas to a subject; a waste outlet coupleable to a gas extraction system; the method comprising: providing a body portion having a main cavity and an ancillary cavity by: locating the main cavity between the inlet and the delivery outlet; and locating the ancillary cavity, between the main cavity and the waste outlet; linking the main cavity and ancillary cavity via an exhalation valve, the exhalation valve being moveable between: a closed position if a pressure differential between the ancillary cavity and the main cavity is greater than or substantially equal to zero, and an open position if a pressure differential between the ancillary cavity and the main cavity is less than zero; and providing the ancillary cavity with a waste valve, the waste valve being moveable between: an open position if a pressure differential between the ancillary cavity and ambient pressure is less than zero, and a closed position if a pressure differential between the ancillary cavity and ambient pressure is greater than or equal to zero. In some embodiments, the therapeutic gas comprises an analgesic gas. In some embodiments, the therapeutic gas comprises nitrous oxide. In some embodiments, at least one of the exhalation valve and / or waste valve comprise: a membrane valve. In some embodiments, the exhalation valve comprises a membrane valve. In some embodiments, the waste valve comprises a membrane valve. In some embodiments, the membrane valve is arranged, when in a closed position, to obstruct gas flow therethrough. In some embodiments, the membrane valve allows unimpeded gas flow when in an open position; is gas tight in a closed position; or in between the two; in dependence upon pressure difference across the valve. In some embodiments, the body portion comprises an assembly of components. In some embodiments, the assembly comprises: an exhalation valve assembly incorporating: the inlet, the delivery outlet and the exhalation valve. In some embodiments, the body portion assembly comprises a diverter portion incorporating: the waste outlet and the waste valve. In some embodiments, the diverter portion and the exhalation valve assembly couple together to form the body portion. In some embodiments, the exhalation valve assembly is removably replaceable from the coupling. In some embodiments, the therapeutic gas delivery coupling comprises an administration device coupled to the outlet for delivery of therapeutic gas to a subject. In some embodiments, the administration device comprises: a mouthpiece or a mask. In some embodiments, the administration device comprises a double skinned mask, comprising an inner mask and an outer mask, wherein the inner mask is shaped to seal to a subject’s face, and wherein the waste valve is in communication with a cavity formed between the inner mask and the outer mask. According to some, but not necessarily all, embodiments, there is provided a method of providing a therapeutic gas delivery system incorporating a therapeutic gas delivery system coupling as described above. In some embodiments, the method comprises: providing the coupling between a gas supply and administration apparatus for administration of gas to a subject. In some embodiments, the method comprises coupling to the administration apparatus. In some embodiments, the method comprises: providing a gas extraction system, and coupling the waste outlet of the coupling to an inlet of the gas extraction system. In some embodiments, the method comprises configuring the gas extraction system to apply a negative, or suction, gas flow to the waste outlet of the coupling. In some embodiments, the method comprises: locating a flow indicator downstream of the waste outlet and upstream of gas extraction apparatus. In some embodiments, the method comprises: providing a variable volume exhaust gas reservoir, for receiving gas exhaled by a subject. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION Some example embodiments will now be described with reference to the accompanying drawings in which: FIG. 1 illustrates schematically some main components of one arrangement in accordance with described approaches. FIG. 2 is a heavy breathing trace showing flow rate induced in a cavity by a subject without interference from any competing gas flows into, or out of the cavity. FIG. 3 illustrates graphically a calculation of excess exhalation volume which may require capture by a receiving system. FIG. 4 illustrates, as a side view and cutaway sectional view, a flow indicator assembly configured to couple to an exhalation reservoir for use in a therapeutic gas delivery system incorporating a coupling such as that shown in FIG. 1. FIG. 5 illustrates schematically main components of an example therapeutic gas delivery system incorporating a coupling in accordance with the coupling shown schematically in FIG. 1. FIG. 6 is a photograph of a coupling, delivery apparatus and tubing according to an arrangement; and FIG. 7 is a view of a coupling including delivery apparatus according to an arrangement. DETAILED DESCRIPTION Before discussing the example embodiments in any more detail, first an overview of the landscape surrounding various arrangements will be provided. As described above, therapeutic gases are administered to subjects in support of various clinical, therapeutic and / or other procedures. One such therapeutic gas is nitrous oxide. Nitrous oxide can be administered to a subject as an anaesthetic or for pain relief, for example, in medical, dental, and veterinary contexts. Therapeutic gases can be administered to subjects in various ways. One method of administration comprises use of breathing apparatus. Medical or therapeutic breathing apparatus can include a regulator or regulator valve for supplying gas to a patient. Breathing apparatus incorporating regulators can be used in a variety of contexts, for example, for delivery of nitrous oxide mixtures in support of pain relief during procedures such as, for example, childbirth; wound dressing; limb resetting; and other similar cases where pain relief may be beneficial. Such breathing apparatus is not specific to administration of nitrous oxide, and may be used, for example, for administration of oxygen as used in resuscitation, treatment for smoke inhalation and similar. Such breathing apparatus can also be used with other gases. Therapeutic gas breathing apparatus including regulators may be arranged such that the regulator is connected to a gas supply. The gas supply may be configured to provide therapeutic gas at a medium pressure, for example, 3-6 bar. The gas supply may comprise, for example, a gas cylinder or a gas pipeline. An outlet of the regulator can allow gas flow to a subject's mouth or nose via, for example, a mask or a mouthpiece. As a subject draws gas from the apparatus, for example, by inhaling, the regulator is configured to regulate gas flow from the supply to match the demand, maintaining a roughly constant pressure which corresponds to a level which can readily be drawn by a patient at the regulator outlet. That level may, for example, be in the region of 250 Pa-1000 Pa below atmospheric. Such a regulator arrangement is sometimes called a "demand valve". Typical demand valves of the type utilised in therapeutic gas delivery to a subject are described in EP-A-0249322, EP-A-0582419 and EP-A- 610054. Nitrous Oxide Nitrous oxide is used to provide pain relief in many areas of clinical practice. It can be supplied as an equimolar gas mixture with oxygen in pressurised gas cylinders. This analgesic gas mixture, O2 / N2O 50% / 50% v / v, is commonly self-administered during painful procedures, procedural sedation and during labour. The pain relieving mechanisms of nitrous oxide are not completely understood but it is known that the gas is adsorbed into the body through the lungs and the rate of adsorption is dependent on the alveolar-arterial gradient. Nitrous oxide is only temporarily adsorbed by the body and is not metabolised. Almost all nitrous oxide leaves the body in the exhaled breath resulting in elevated concentrations of nitrous oxide local to the site of administration. Occupational exposure to nitrous oxide has been linked to various adverse health effects. 8-hour time weighted exposure limits in different countries typically range from 25 ppm to 100 ppm. As a result, in enclosed indoor environments where nitrous oxide is to be delivered to a subject, active measures can be implemented to reduce the concentration of the gas in the ambient gaseous environment. Various methods have been employed to reduce the concentration of nitrous oxide in an environment, including, for example, passive ventilation; centralised forced ventilation; local exhaust ventilation; and anaesthetic gas scavenging systems (AGSS). Anaesthetic Gas Scavenging Systems (AGSS) Anaesthetic gas scavenging systems are employed in clinical settings to reduce occupational exposure of personnel to volatile anaesthetic vapours and nitrous oxide. Anaesthetic gas scavenging systems were originally designed for use during general anaesthesia to capture waste gases from a breathing circuit and transport them outside the operating theatre. Anaesthetic gas scavenging systems comprise various sub systems: a breathing system; a receiving system; a transfer system and a disposal system. Anaesthetic gas scavenging systems have since been utilised by some healthcare facilities for use with patients receiving other therapeutic gases. Some notable differences between an anaesthetic gas application and a therapeutic medical gas application include: during therapeutic gas application subjects may typically be spontaneously breathing; the subjects may be relatively mobile; the subjects may selfadminister a gas; and the subjects may not be continuously monitored. The capturing efficiency of AGSS system in such circumstances may be less that of a conventional anaesthetic gas capturing application. Nitrous Oxide Destruction Systems can be employed to reduce nitrous oxide in a waste gas stream. Such systems may be configured to transform nitrous oxide into its constituent elements, for example, by exposing the nitrous oxide containing gas to a moderate temperature in a catalytic reactor. Destruction systems can be centralised and multiple areas of an acute hospital may be connected to the centralised destructor unit via an appropriate pipeline system. Alternatively, individual destructor units may be located close to a point of delivery of nitrous oxide. Mobile nitrous oxide decomposition systems for single patient application are described in EP3612289, EP2165756 and US2020282162 and typically induce a fixed flow rate of gas at their inlet. Environmental Spillage of Exhaled Gases In pulmonary medicine, functional residual capacity is defined as the volume of gas remaining in lungs after exhaling normally. Functional residual lung capacity typically falls in a range from 1.7 to 3.5 litres, depending mainly on the size of an individual. After a period of self-administration of O2 / N2O gas, the functional residual volume of gas within lungs of a subject will contain a significant concentration of nitrous oxide, with more nitrous oxide present within subject tissue and subject blood. The nitrous oxide will be rapidly excreted from the body of the subject in exhaled breath. However, such excreted nitrous oxide may not be captured directly by an anaesthetic gas scavenging system during exhalation, since the subject may not exhale into a mouthpiece or mask. Spillage of nitrous oxide into the ambient environment around a subject may therefore occur. Spillage of nitrous oxide into the environment can arise in other ways too including, for example, exhalation through the nose when using a mouthpiece; a poorly fitting mask; the presence of facial hair which can prevent or hinder creation of a seal; and / or a subject not exhaling completely through an exhalation collection device, as a result of speaking, coughing or similar. A major application of O2 / N2O in many countries is pain relief during childbirth. In such a scenario, the gas mix is normally administered at the onset of a contraction and continues until the pain is substantially alleviated. For example, Einnarsson et al [1] reported a typical delivery pattern during labour of ~1 minute of 50% nitrous oxide administration followed by ~2 minutes without administration, repeated with each contraction. It will be appreciated that such an administration regime results in many periods of nitrous oxide being flushed from the body in exhaled air whilst the delivery device is not being used. Einnarsson et al also reported an average end tidal N2O concentration of 36.3%, which rapidly diminished at 30 seconds but persisted at low level for at least 2 minutes. A study by Pinder et al [2] found that use of a mobile destruction unit in a simulated childbirth application reduced ambient nitrous oxide levels by 71% to 81%, depending on the mask type employed, suggesting that 19% to 29% of ambient nitrous oxide was not captured by such an arrangement. Having described the landscape surrounding the invention, approaches adopted by the innovation are now described in overview, before various detailed possible implementations are provided. Arrangements in accordance with described approaches may improve the capture efficiency of exhaled gas, for example, exhaled gas containing nitrous oxide, during and between periods of active administration of therapeutic gases. FIG. 1 illustrates schematically some main components of one arrangement in accordance with described approaches. An aspect of described approaches comprises a therapeutic gas delivery system coupling 1 as shown in FIG. 1. In general terms, an aspect of described approaches is a therapeutic gas delivery system coupling comprising: an inlet 10 coupleable to a therapeutic gas supply; a delivery outlet 20 configured for delivery of therapeutic gas to a subject; and a waste outlet 30 coupleable to a gas extraction system. The coupling comprises a body portion 40 having: a main cavity 40a located between the inlet 10 and the delivery outlet 20; and an ancillary cavity 40b, located between the main cavity 40a and the waste outlet 30. The main cavity 40a and ancillary cavity 40b are linked via an exhalation valve 50. The exhalation valve 50 is moveable between: a closed position if a pressure differential (P2 - Pi) between the ancillary cavity 40b and the main cavity 40a is greater than or substantially equal to zero, and an open position if a pressure differential (P2- Pi) between the ancillary cavity 40b and the main cavity 40a is less than zero. The ancillary cavity 40b further comprises a waste valve 60, the waste valve being moveable between: an open position if a pressure differential (P2- P3) between the ancillary cavity 40b and ambient pressure P3 surrounding the body portion 40 is less than zero, and a closed position if a pressure differential (P2- P3) between the ancillary cavity 40b and ambient pressure P3 surrounding the body portion 40 is greater than or equal to zero. Accordingly, such a coupling may allow for breathing apparatus to operate such that therapeutic gas can be administered to a subject via outlet 20 (in FIG. 1, the outlet is connected to a delivery mask). Whilst therapeutic gas is being delivered, the waste valve may be in the open position, allowing for collection of gas immediately adjacent a subject to a waste system 100, provided slight suction is applied to waste outlet 30. During subject exhalation, the exhalation valve 50 opens to allow a flow of exhaled gas from a subject to the waste system. Waste valve 60 is closed during subject exhalation, therefore preventing escape of exhaled gases to an environment surrounding the subject. Use of a coupling such as that shown in FIG. 1 may therefore allow for collection of exhaled gases in the immediate vicinity of a subject. Such collection may be beneficial if the therapeutic gas delivered via the therapeutic gas delivery system includes nitrous oxide, or any other undesirable gas which may accumulate around the subject. In some arrangements, the therapeutic gas may comprise an analgesic gas. In some arrangements, the therapeutic gas may comprise nitrous oxide. In some arrangements, the exhalation valve may comprise a diaphragm or membrane valve. In some arrangements, the waste valve may comprise a diaphragm or membrane valve. The membrane or diaphragm may be arranged to control, obstruct or isolate a fluid or gas flow. The exhalation and / or waste valve may allow unimpeded gas flow when in an open position; or be gas tight in a closed position; or be between the two; in dependence upon pressure difference between the main and ancillary cavity 11 in the case of the exhalation valve, or pressure difference between the ancillary cavity and ambient air pressure surrounding the ancillary cavity in the case of the waste valve. In some arrangements, the body portion comprises an assembly of components. In some arrangements, the assembly comprises: an exhalation valve assembly 700 incorporating: the inlet 10 configured to be coupleable with a therapeutic gas supply, the delivery outlet 20 and the exhalation valve 50. In some arrangements, the body portion 40 assembly comprises a diverter portion 800 incorporating: the waste outlet 30 coupleable to a gas extraction system and the waste valve 60. In some arrangements, the diverter portion 800 and the exhalation valve assembly 700 couple together to form the body portion 40. In some arrangements, that coupling between 700 and 800 comprises a friction fit arrangement. In some arrangements, that coupling comprises a gas tight coupling. The coupling may, for example, be gas tight between 0.5 to 2.0 bar. In some arrangements the inlet portion 10 may comprise a threaded portion connectable with a component from which a therapeutic gas supply may be obtained. In some arrangements, the inlet portion is configurable to form a gas-tight friction fit with a component from which a therapeutic gas supply may be obtained. In some arrangements, the inlet portion may comprise one or more protrusion or annular rib engageable with a component from which a therapeutic gas supply may be obtained. In some arrangements, the delivery outlet 20 may be coupleable with an administration device for delivery of therapeutic gas to a subject. The administration device may comprise a mouthpiece, mask (as shown in FIG. 1) or similar. In some arrangements, the waste outlet 30 may be configured to couple with tubing or piping as shown in FIG. 1. In some arrangements, an outer surface of the waste outlet may comprise one or more annular ribs or protrusions configured to be engageable with an inner surface of resilient or flexible tubing or piping. Some arrangements recognise that it may be beneficial to make components of the coupling as reusable as possible. Nonetheless, some parts may not support economically feasible reprocessing after use. In the list below, all reusable parts are ones which only come into contact with exhaled gas, rather than ones which are in contact with gas that is then inhaled by a subject. Diverter: Reusable; Exhalation valve assembly: Single patient use; Mouthpiece (optional): Single patient use; Outer mask (optional): Reusable; Inner mask (optional): Single patient use. The coupling 1 shown schematically in FIG. 1 may be used in conjunction with a demand valve 80. The demand valve 80 shown in FIG. 1 is located between the coupling 1 and the therapeutic gas supply. In other words, the inlet of the coupling 1 is coupled to the therapeutic gas supply via the demand valve 80. In order to understand how operation of demand valve 80 may integrate with the operation of a coupling such as that shown in FIG. 1, it is helpful to understand how a subject may inhale and exhale. FIG. 2 is a heavy breathing trace showing flow rate induced in a cavity by a subject without interference from any competing gas flows into, or out of the cavity. In relation to operation of demand valve 80, the main cavity 40a formed may be used to regulate flow from the therapeutic gas supply to the outlet 20. During subject inhalation, the cavity associated with operation of the demand valve 80 requires a minimum level of sub-atmospheric pressure to be reached before gas flow is initiated. The suction level applied, for example, to cavity 40a, by a subject inhaling at outlet 20 increases to a peak and then falls as an inhalation sub-cycle progresses. A typical waveform is shown in FIG. 2. Exhalation by a subject, applied to cavity 40a, causes demand valve 80 to close. Arrangements described recognise that it is necessary to prevent suction, such as that provided by an AGSS 100, being applied directly to an inhalation sensing cavity 40a associated with a demand valve 80 because demand valves typically open at pressures around 100 to 200 Pascals below atmosphere. If too high a suction level is applied to the cavity 40a by an AGSS suction system, the demand valve 80 would open and exhaust fresh gas directly from the therapeutic gas supply to a waste outlet 30. Similarly, if a suction level applied by an AGSS is too high, then during subject inhalation, the scavenging system would compete with the subject for fresh therapeutic gas. A system in which a coupling such as that shown in FIG. 1 may operate may typically operate such that the suction level at the downstream side of the exhalation valve 50 is maintained below the level at which the exhalation valve opens. It is possible to mitigate against issues which may occur during subject exhalation by using 13 a downstream receiving system located downstream of the waste outlet 30. A receiving system typically comprises an indicator to provide visual conformation of adequate gas flow towards the AGSS, a reservoir that prevents spillage of excess exhaled gas and a filter. A receiving system suitable for use in a therapeutic gas 5 delivery system incorporating a coupling such as that shown in FIG. 1 is described in more detail below. Breathing Cycle By way of example, stages of a breathing cycle which may occur if a subject uses a 10 therapeutic gas delivery system which includes a coupling such as that shown in FIG. 1 are now described in detail with reference to TABLE 1 and FIG. 1: PRESSURES MEMBRANE VALVE STATUS STAGE PATIENT PORT GAUGE PRESSURE (Pi) DIVERTER CAVITY GAUGE PRESSURE (Pa) EXHALATION VALVE / DIVERTER CAVITY DIFFERENTIAL PRESSURE EXHALATION MEMBRANE VALVE DIVERTER MEMBRANE VALVE 1. Idle, AGS on 0 kPa -0.01 kPa - 0.01 kPa Partially open Wide open 2. Onset of inhalation -0.1 kPa -0.01 kPa 0.09 kPa Loosely closed Wide open 3. Peak of inhalation -1.5 kPa -0.01 kPa 1.49 kPa Tightly closed Wide open 4. End of inhalation -0.1 kPa -0.01 kPa 0.09 kPa Loosely closed Wide open 5. Onset of exhalation 0.1 kPa OkPa -0.1 kPa Partially open Loosely closed 6. Peak of exhalation 0.6 kPa 0.3 kPa -0.3 kPa Wide open Tightly closed 7. End of exhalation 0.1 kPa OkPa -0.1 kPa Partially open Loosely closed TABLE 1: COUPLING PRESSURE DIFFERENTIAL AND VALVE STATUS ANALYSIS 15 Stage 1: Subject Idle, AGSS (scavenging system) on In a stage of operation in which a gas scavenging flow (suction) is applied to waste outlet 30, and in which no therapeutic gas is being delivered to a subject located at the delivery outlet 20, there is a continuous flow of gas towards the AGSS 100, that flow of gas is pulled through from: the diverter membrane valve 60 from ambient atmosphere surrounding the delivery apparatus, and through the exhalation membrane valve 50 from atmosphere via the patient port (delivery outlet 20). Typical system pressures at this and all subsequent stages in the breathing cycle are shown in TABLE 1 above. Stage 2 - Onset of Subject Inhalation When a subject applies inhalation suction to delivery outlet 20, for example, by placing a mouthpiece into their lips or a mask over their face, and starting to inhale, a sub-atmospheric pressure develops within the main cavity 40a. Initially the sub-atmospheric pressure is too low to open the demand valve 80 and gas continues to be evacuated from the cavity 40a by the subject. When sufficient gas has been evacuated, the sub-atmospheric pressure exceeds demand valve cracking pressure and the demand valve opens, allowing therapeutic gas to pass from the therapeutic gas supply to the subject through demand valve 80. The vacuum developed in cavity 40a increases the closure force acting to seal exhalation membrane valve 50. Stage 3 - Peak inhalation As a subject inhales deeply at delivery outlet 20, the vacuum level within the main cavity 40a increases until the subject reaches a peak inhalation flow, developing the highest closure force on the exhalation valve membrane 50 and forming a gas tight seal between main cavity 40a and ancillary cavity 40b. Stage 4 - End of inhalation Following inhalation peak flow, as the lungs of a subject tend towards the end of an inward tidal volume, the flow from delivery outlet 20 and sub-atmospheric pressure in cavity 40a reduce until the demand valve 80 closes. A brief pause between inhalation and exhalation of a subject often follows or, in some instances a subject may move directly into an exhalation sub-cycle. Stage 5 - Onset of exhalation As a subject begins to breathe out into delivery outlet 20, the pressure inside main cavity 40a rapidly equalises with atmosphere and then becomes slightly positive. The exhalation membrane valve 50 begins to open and gas passes through the exhalation membrane valve 50 into the ancillary cavity 40b. Stage 6 - Peak exhalation A slightly positive pressure is respect to atmosphere (P3) develops within the ancillary cavity 40b when / if a subject’s exhalation flow exceeds the gas scavenging flow applied to the waste outlet 30. During the exhalation sub-cycle, the exhalation flow rate rapidly increases, quickly reaching a peak and then reduces almost linearly with time to the end of the exhalation sub-cycle. Stage 7 - End of exhalation As the lungs tend towards the end of the outward tidal volume, the exhalation membrane valve 50 tends towards the closed position but remains partially open. Scavenged gas is once again extracted from atmosphere via both the waste membrane valve 60 and the patient port 20 via a partially open exhalation membrane valve 50. Application of Coupling to Demand Valves Used with an AGSS As previously described, an Anaesthetic Gas Scavenging System (AGSS) is often employed in combination with the delivery of O2 / N2O analgesic gas mixture to a subject. In order to prevent a vacuum developed within the AGSS the being applied to the patient port 20 of the coupling and thereby competing with a subject for the analgesic gas from a supply, a receiving system can be placed between an AGSS system 100 gas inlet and the main cavity 40a. The receiving system acts as an airbreak, only taking gas from the upstream transfer system when the pressure is slightly positive relative to atmosphere; the slight positive pressure being developed through a subject’s exhalation effort. To benefit from a local-to-subject gas extraction system facilitated by a coupling such as the one described in relation to FIG. 1, functions provided by a typical AGS receiving system can be replicated with one significant modification. Conventional receiving systems entrain room air into the system in a location remote from the patient when the patient is not exhaling into the AGSS. A system incorporating a coupling such as that described in FIG. 1 facilitates an entraining of ambient air close to a site of administration of therapeutic gas, thereby maximising or at least increasing capture efficiency of, for example, nitrous oxide to minimise associated negative consequences of the release of nitrous oxide to the environment. A typical exhalation reservoir is normally an open-ended cylindrical chamber with a vertically oriented axis that provides a temporary housing for large peaks of exhaled gas. The exhalation reservoir prevents escape of exhaled gas if a subject exhales at a flow rate in excess of the gas scavenging flow rate of the AGSS. Without an exhalation reservoir, if a patient’s exhalation flow rate exceeds a flow rate extracted by an AGSS, the exhaled gas would either flow to atmosphere (in an open system) or the pressure increases (in a closed system). The latter may result in negative outcomes: a subject may exhale to atmosphere or attempt to continue exhaling by pressuring a volume within the waste system, both of which are unsatisfactory performance deficiencies. FIG. 3 illustrates graphically a calculation of excess exhalation volume which may require capture by a receiving system. With reference to FIG. 3, it can be calculated that if a scavenging flow rate is 50 litres per minute and a peak exhalation flow rate of a subject is around 90 litres per minute, integrating the flow rate above 50 litres per minute over the time period in which the exhalation flow rate exceeds 50 litres per minute, gives the volume of gas that must be accommodated within an exhalation reservoir to prevent negative outcomes previously described. In the example in FIG. 3 this equates to around 0.4 litres of excess gas that cannot be evacuated during the exhalation phase by the AGSS. FIG. 4 illustrates, as a side view and cutaway sectional view, a flow indicator assembly configured to couple to an exhalation reservoir for use in a therapeutic gas delivery system incorporating a coupling such as that shown in FIG. 1. The flow indicator assembly 400 shown in FIG. 4 comprises an inlet 410, configured to couple to tubing or pipework associated with a subject interface of a therapeutic gas delivery system and receive exhalation gases from that subject interface. The inlet 410 is provided with a mesh filter 415, to collect any debris in the gas flow. The flow indicator assembly 400 further comprises an outlet, 420 coupleable to an inlet of an AGSS system, a conduit 460 linking inlet and outlet, and a reservoir connector 430 in fluid communication with the conduit 460, the reservoir connector being coupleable to an exhalation reservoir bag (not shown in FIG. 4). The flow indicator assembly 400 further includes a flow indicator float 440, moveable within the conduit 460 of the flow indicator assembly against a resilient spring 450 arranged to urge the float 440 against a ledge or shoulder 470 of a wall of the conduit 460. Flow of gas from the inlet 410 to the outlet 420 moves the float 440 in opposition to the resilient force of the spring 450. The flow indicator assembly housing 480 comprises an indicator window 490. When there is no flow between inlet 410 and outlet 420 through the conduit 460, the float 440 is urged against shoulder 470 and is not visible through indicator window 490. When there is gas flow between inlet 410 and outlet 420 through the conduit 460, the float 440 is moved away from shoulder 470, the gas flow moving the float against the resilient force of the spring 450 and the float becomes visible through indicator window 490, allowing a user to see that a flow of gas is being transferred towards the AGSS. Provision of a reservoir bag at connector 430 allows any excess exhalation gas, as described above, to be captured during a subject’s exhalation sub-cycle. Once the exhalation sub-cycle is completed, the reservoir bag is evacuated by the continued suction applied by the AGSS, during the subject’s inhalation sub-cycle. Typically, the inhalation sub-cycle occurs over a shorter time than the exhalation sub-cycle, so it follows that, in practice, the scavenging flow rate of the AGSS should be sufficient to ensure complete evacuation of the reservoir bag during the period in which the exhalation flow rate is below the scavenging flow rate. FIG. 5 illustrates schematically main components of an example therapeutic gas delivery system incorporating a coupling in accordance with the coupling shown schematically in FIG. 1. Some arrangements may provide a therapeutic gas delivery system incorporating a therapeutic gas delivery system coupling as described above. Some arrangements may provide an analgesic gas delivery system. Some arrangements may provide a nitrous oxide gas delivery system. In some arrangements, the system comprises a gas supply or source and the coupling is provided between the gas supply or source and a subject to whom the gas is to be delivered. In some arrangements, the gas delivery system comprises administration apparatus for administration of gas to a subject. In some arrangements, the coupling coupled to such administration apparatus. In some arrangements, the system comprises a gas extraction system. In some arrangements, the waste outlet of the coupling is coupled to an inlet of the gas extraction system. In some arrangements, the gas extraction system is configured to apply a negative or suction gas flow to the waste outlet of the coupling, either via direct connection, or connection to the waste outlet via tubing or piping. In some arrangements, the system comprises a flow indicator. The flow indicator may be located downstream of the waste outlet and upstream of a gas extraction apparatus or system. In some arrangements, the system may comprise an exhaust or exhaled gas reservoir, configured to couple to the waste outlet of the coupling and receive gas exhaled by a subject. The gas delivery system 500 shown in FIG. 5 comprises: a source or supply of therapeutic gas 510. The therapeutic gas may, for example, comprise a therapeutic gas mixture incorporating nitrous oxide. The therapeutic gas is connected to breathing apparatus for a subject 550 via appropriate piping or tubing 520. The breathing apparatus of FIG. 5 comprises: a demand valve 530, a diverter valve 540 such as that shown in FIG. 1, and delivery apparatus 560. Exhaust gases in the form of exhaled gases and gas located in the region of subject 550 are captured by breathing apparatus 540, 560 and passed along waste piping or tubing 570 towards an exhaust gas scavenging system 600. In gas flow line 570, there is located a flow indicator 580 such as that shown schematically in FIG. 4, which includes a reservoir bag 590. It will be appreciated that a therapeutic gas delivery system such as that shown in FIG. 6 may be such that the source or supply of therapeutic gas 510 may, in the case of O2 / N2O gas mixtures, comprise a blended source or a premixed gas in a high pressure gas cylinder. Blenders enable separate sources of nitrous oxide and oxygen gas to be blended at the point of use. Blenders are commonly used where the premixed gas does not have a marketing authorisation, such as in the United States, or where a higher than 50% concentration of nitrous oxide is desired for greater sedative effect. Blenders often have a selectable N2O concentration in the range 30% to 70% by volume. The delivery apparatus 560 may, for example, comprise a mouthpiece, a nasal mask, a face mask or a delivery chamber. In each case, the delivery apparatus is configured to allow delivery of therapeutic gas to the subject 550. The exhaust gas scavenging system 600 may be configured to apply a suction to connector tubing or piping 570. In a conventional system, with no diverter coupling 540 the waste or scavenging tubing 570 between a subject and an air break or nitrous oxide decomposition system, is exposed to an intermittent flow of exhaled air into that tubing. The incoming exhaled gas has a very high water concentration and is initially warm at ~37 °C. It cools in the tubing and water vapour condenses on the tubing inner surface. With continued use, the tubing can have a substantial pool of liquid water develop inside it If the tubing is raised, the liquid water can run either towards the patient or into the AGS supply system, AGS receiver system or nitrous oxide decomposition system and cause equipment malfunction. In a system incorporating a diverter coupling 540 such as that shown in FIG. 1, it is possible to have a continuous flow of gas in the scavenging tubing 570, during inhalation and during pauses in delivery of the gas, as ambient gas surrounding the subject is pulled through the waste valve of the coupling. The majority of the gas passing through the tubing 570 will not be saturated with water vapour. This can help to ensure any water condensing during an exhalation will evaporate into the continuous gas stream, preventing liquid water from collecting and protecting the subject and other equipment from liquid water. FIG. 6 is a photograph of a coupling, delivery apparatus, and tubing according to an arrangement. Like features have been given like numbers to arrangements shown in earlier Figures. FIG. 6 shows a coupling 1, as it can be deployed in a therapeutic gas delivery system. The coupling 1 is connected to a diverter valve 540, which is connected to a gas source via tubing 520. The coupling 1 has a mouthpiece 610 connected to its delivery 20 outlet (not seen in the photograph) which facilitates delivery of therapeutic gas to a subject. The waste outlet 30 of coupling 1 is connected to tubing 570 which leads to an AGSS (not shown). Waste gas valve 60 in the form of a membrane can also be seen in FIG. 6. FIG. 7 is a view of a coupling including delivery apparatus according to an arrangement. Like features have been given like numbers to arrangements shown in earlier Figures. FIG. 7 shows a coupling 1, like that shown schematically in FIG. 1, in which the delivery outlet 20 is connected or coupled to a double-skinned delivery mask 620. An outer mask 650 forms a cavity 670 around an inner mask 630. The cavity 670 is in fluid communication with gas inlet ports, located near the waste valve 60 of the diverter coupling 1. The inner mask 630 is connected to the delivery outlet 20, which is operably connected to the main cavity (not shown) of the coupling and which can be configured to cause operation of a demand valve (not shown in FIG. 7). The double mask arrangement of FIG. 7 brings collection of gas entering or being entrained into an AGSS system as close as possible to a subject’s face and may capture any exhaust or exhale leakage if the inner mask 640 is not fitted well to a face. To ameliorate leakage from the inner mask, a seal 640 may be provided around an edge of the inner mask. The seal 640 may be formed of a soft pliable material to provide a good seal against a subject’s face. It will be appreciated that some arrangements may provide a delivery system for nitrous oxide gas. That delivery system may comprise a gas supply device. The delivery system may comprise an exhalation valve in fluid communication with a gas scavenging conduit. The delivery system may comprise a cavity downstream of the exhalation valve. The cavity downstream of the exhalation valve may be provided with a means to isolate fluid communication of the cavity with a patient gas delivery port during a patient inhalation phase of gas delivery. The delivery system may be configured such that when or if atmospheric pressure occurs at the patient gas delivery port, a gas scavenging flow is substantially entirely supported by a flow of gas from an ambient environment via a diverter membrane valve open to the ambient environment. In some arrangements, if / when atmospheric pressure occurs at the patient gas delivery 21 port, a gas scavenging flow is provided through both a diverter membrane valve and the from the patient gas delivery port via an exhalation valve. In some arrangements, the delivery system may comprise a receiving system. That receiving system may be located downstream of a diverter membrane valve and a transfer system. The receiving system may be configured to entrain atmospheric and exhaled gas interchangeably during a breathing cycle, the atmospheric gas being provided through the diverter membrane valve situation at an upstream end of a transfer system. In some arrangements, a delivery system for nitrous oxide gas comprises a receiving system downstream of a diverter membrane valve and transfer system, and the receiving system is configured to entrain atmospheric and exhaled gas interchangeably during a patient breathing cycle, the atmospheric gas being provided through the diverter membrane valve situation at the upstream end of the transfer system. In some arrangements, the receiving system incorporates a flow indicator. A delivery system for nitrous oxide gas may, according to some arrangements, comprise: a receiving system downstream of a diverter membrane valve and transfer system. The receiving system may be configured to entrain atmospheric and exhaled gas interchangeably during a patient breathing cycle. The atmospheric gas may be provided through a diverter membrane valve situated at an upstream end of a transfer system. In some arrangements, the receiving system may incorporate a filter. In some arrangements, a delivery system for nitrous oxide gas may comprise a receiving system located downstream of a diverter membrane valve and transfer system. The receiving system may be configured to entrain atmospheric and exhaled gas interchangeably during a patient breathing cycle. The atmospheric gas may be provided through a diverter membrane valve situated at an upstream end of a transfer system. The receiving system may comprise and a variable volume reservoir for containment of exhalation gas during exhalation peaks in excess of a waste gas extraction scavenging flow rate. Although example embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon. REFERENCES [1] Einarsson, S., Stenqvist, 0., Bengtsson, A., Noren, H. and Bengtson, J.P. (1996), Gas kinetics during nitrous oxide analgesia for labour. Anaesthesia, 51: 449-452. https: / / doi.Org / 10.1111 / j.1365-2044.1996.tb07790.x [2] Pinder, A., Fang, L., Fieldhouse, A., Goddard, A., Lovett, R., Khan-Perez, J., ... &Shelton, C. (2022). Implementing nitrous oxide cracking technology in the labour ward to reduce occupational exposure and environmental emissions: a quality improvement study*. Anaesthesia, 77(11), 1228-1236. https: / / doi.Org / 10.1111 / anae. 15838 [3] IPCC Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Climate Change 2013 - The Physical Science Basis - Section 8, Table 8.A.1

Claims

1. A therapeutic gas delivery system coupling comprising:an inlet coupleable to a therapeutic gas supply;a delivery outlet configured for delivery of therapeutic gas to a subject;a waste outlet coupleable to a gas extraction system;the coupling comprising a body portion having:a main cavity located between the inlet and the delivery outlet; andan ancillary cavity, located between the main cavity and the waste outlet; wherein the main cavity and ancillary cavity are linked via an exhalation valve, the exhalation valve being moveable between: a closed position if a pressure differential between the ancillary cavity and the main cavity is greater than or substantially equal to zero, and an open position if a pressure differential between the ancillary cavity and the main cavity is less than zero;and wherein the ancillary cavity further comprises a waste valve, the waste valve being moveable between: an open position if a pressure differential between the ancillary cavity and ambient pressure is less than zero, and a closed position if a pressure differential between the ancillary cavity and ambient pressure is greater than or equal to zero.

2. A therapeutic gas delivery system coupling according to claim 1, wherein the therapeutic gas comprises an analgesic gas, and optionally wherein the therapeutic gas comprises nitrous oxide.

3. A therapeutic gas delivery system coupling according to claim 1 or claim 2, wherein at least one of the exhalation valve and waste valve comprise a membrane valve.

4. A therapeutic gas delivery coupling according to claim 3, wherein the membrane valve is arranged to obstruct gas flow therethrough.

5. A therapeutic gas delivery coupling according to claim 3 or claim 4, wherein the membrane valve allows unimpeded gas flow when in an open position; is gas tight in a closed position; or in between unimpeded and gas tight; in dependence upon pressure difference across the valve.

6. A therapeutic gas delivery coupling according to any preceding claim, wherein the body portion comprises an assembly of components.

7. A therapeutic gas delivery coupling according to claim 6, wherein the assembly comprises: an exhalation valve assembly incorporating: the inlet, the delivery outlet and the exhalation valve.

8. A therapeutic gas delivery coupling according to claim 6 or claim 7, wherein the body portion assembly comprises a diverter portion incorporating: the waste outlet and the waste valve.

9. A therapeutic gas delivery coupling according to claim 8, wherein the diverter portion and the exhalation valve assembly couple together to form the body portion.

10. A therapeutic gas delivery coupling according to claim 9, wherein the exhalation valve assembly is removably replaceable from the coupling.

11. A therapeutic gas delivery coupling according to any preceding claim comprising an administration device coupled to the outlet for delivery of therapeutic gas to a subject.

12. A therapeutic gas delivery coupling according to claim 11 wherein the administration device comprises: a mouthpiece or a mask.

13. A therapeutic gas delivery coupling according to claim 12, wherein the administration device comprises a double skinned mask, comprising an inner mask and an outer mask, wherein the inner mask is shaped to seal to a subject’s face, and wherein the waste valve is in communication with a cavity formed between the inner mask and the outer mask.

14. An exhalation valve assembly in accordance with claim 7, for insertion into a coupling according to claim 1.

15. A therapeutic gas delivery system incorporating a therapeutic gas delivery system coupling according to any one of claims 1 to 13.

16. A therapeutic gas delivery system according to claim 15, comprising: a gas supply, wherein the coupling is provided between the gas supply and administration apparatus for administration of gas to a subject.

17. A therapeutic gas delivery system according to claim 16, wherein the coupling coupled to the administration apparatus.

18. A therapeutic gas delivery system according to any one of claims 15 to 17 comprising a gas extraction system, wherein the waste outlet of the coupling is coupled to an inlet of the gas extraction system.

19. A therapeutic gas delivery system according to claim 18, wherein the gas extraction system is configured to apply a negative or suction gas flow to the waste outlet of the coupling.

20. A therapeutic gas delivery system according to any one of claims 15 to 19, wherein the system comprises a flow indicator located downstream of the waste outlet and upstream of gas extraction apparatus.

21. A therapeutic gas delivery system according to any one of claims 15 to 20 comprising a variable volume exhaust gas reservoir, for receiving gas exhaled by a subject.

22. A method of providing a therapeutic gas delivery system coupling comprising: an inlet coupleable to a therapeutic gas supplya delivery outlet configured for delivery of therapeutic gas to a subject;a waste outlet coupleable to a gas extraction system;the method comprising providing a body portion having a main cavity and an ancillary cavity by:locating the main cavity between the inlet and the delivery outlet; andlocating the ancillary cavity, between the main cavity and the waste outlet;linking the main cavity and ancillary cavity via an exhalation valve, the exhalation valve being moveable between: a closed position if a pressure differential between the ancillary cavity and the main cavity is greater than or substantially equal to zero, and an open position if a pressure differential between the ancillary cavity and the main cavity is less than zero;and providing the ancillary cavity with a waste valve, the waste valve being moveable between: an open position if a pressure differential between the ancillary cavity and ambient pressure is less than zero, and a closed position if a pressure differential between the ancillary cavity and ambient pressure is greater than or equal5 to zero.28

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