Abstract:
A pneumatic single-lumen medical gas conserver combines the advantages of typical single-lumen and dual-lumen conservers. In particular, a pneumatic single-lumen conserver can provide a rapid response to patient inhalations without the need for a more expensive dual-lumen cannula hose. In addition, after delivering oxygen the conserver has a specific pneumatically-implemented delay period before being able to detect the next inhalation to inhibit “double pulse” deliveries. In addition to a conserving or pulse flow mode, the conserver can provide a user-selectable gas flow at a continuous or constant flow mode.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/823,456, filed on Aug. 24, 2006 (Attorney Docket No. 2173.3000US01). The entire teachings of the above application are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Gas-conserving regulators include oxygen regulators, which are used to supply a patient with a regulated flow of oxygen. The oxygen is supplied by a source of highly-compressed oxygen, such as from a supply tank, which has its pressure reduced to a low pressure (e.g., 22 PSI) for delivery to the patient. Typical oxygen regulators employ a back-pressure piston to supply a continuous flow of that low pressure oxygen to the patient. Much of that oxygen is wasted because it is not inhaled by the patient. 
         [0003]    To reduce the amount of wasted oxygen, oxygen-conserving regulators have been developed. These conservers tend to limit the oxygen flow to periods of inhalation. The oxygen flow is typically controlled electronically or pneumatically. Of the pneumatic types, there are two common types of systems: single-lumen and dual-lumen. 
         [0004]    In a typical electronic conserver, a solenoid valve controls the flow of oxygen to the patient. The solenoid valve can accurately open to provide the flow of oxygen to the patient when the patient inhales, and close between breaths. Typically, the solenoid valve has high energy requirements and is battery powered. 
         [0005]    In typical dual lumen pneumatic conserving regulators, a reservoir coupled to the oxygen source holds a supply of oxygen for delivery to the patient. Delivery of the oxygen is controlled by a slave diaphragm that separates the reservoir from a control gas chamber. The slave diaphragm seals the opening to a delivery nozzle when the patient is not inhaling and releases the seal from the nozzle opening when the patient inhales. The slave diaphragm is made from a flexible material and is generally pressurized toward the closed position. Operation of the slave diaphragm is controlled by a pilot diaphragm, which is coupled to the patient. When the patient inhales, the pilot diaphragm lifts off an orifice pneumatically connected to the control gas chamber. The oxygen in the control gas chamber is then expelled, creating a pressure drop sufficient to allow the slave diaphragm to move away from the slave nozzle, thus allowing flow to the patient. 
         [0006]    Dual-lumen devices use a cannula with two separate hoses for connecting to the conserver. Depending on the design of the cannula, each hose either serves one or both nostrils of the patient. The conserver likewise has two cannula hose ports. A sensing or pilot port is used exclusively for sensing the vacuum caused by patient inhalation. A slave or delivery port is used exclusively for delivery of oxygen to the patient. 
         [0007]    When the patient inhales, oxygen is delivered by the delivery port through a delivery hose until inhalation ends. Because the two hoses of the cannula do not intermingle, the conserver is able to deliver oxygen the entire time the patient is inhaling. Therefore, dual-lumen conservers are commonly called “demand” conservers. 
         [0008]    In a typical dual-lumen conserver (i.e., demand conserver), when the patient stops inhaling—causing the pilot diaphragm to close—the control chamber builds back to operating pressure (e.g., 22 PSI) almost immediately. Consequently, when the pilot diaphragm shuts against the pilot nozzle, flow to the patient stops. This is usually done by having a preset control flow between 100 cc and 350 cc per minute, depending on the design of the device. The need to stop flow as soon as the pilot diaphragm closes is because, in a demand conserver, the pilot diaphragm stays open as long as the patient inhales. The dual-lumen design of such conservers allows the unit to be sensitive enough to sense the vacuum caused by inhalation. 
         [0009]    In comparison, a typical single-lumen conserver does not have that sensitivity. Single-lumen conservers use only a single cannula hose that serves both nostrils, which is coupled to a single port on the conserver. When no oxygen is flowing through the hose, the conserver can detect when the patient inhales, and oxygen delivery begins. However, once oxygen begins to flow through the hose, the flow of oxygen to the patient overwhelms the device&#39;s ability to sense the vacuum caused by the patient during inhalation and the device will no longer be able to sense when inhalation ends. Therefore, the device is constructed to stop the flow of oxygen after a predetermined amount of time, regardless of the patient&#39;s breathing pattern. There are some pneumatic devices that work this way, and all electronic devices work this way. These conservers are called “pulse” conservers, as they typically give a large pulse of oxygen and then shut themselves off and wait for the next breath. 
         [0010]    Typically, dual-lumen conservers have the advantage of much better performance under all breathing conditions, meaning they deliver the correct amount of oxygen for the patient and work well with the widest variety of breathing patterns. Also, dual-lumen devices can have continuous or constant flow at all settings if required, whereas single-lumen devices typically have only a single continuous flow setting, such as a constant 2 liters per minute (LPM). 
         [0011]    In comparison, single-lumen conservers have the advantages of a simpler (and less expensive) cannula hose, and because they only deliver a pulse of oxygen, these conservers can have a higher conservation ratio (many respiratory professionals believe that oxygen delivered at the end of inhalation is wasted because it does not get to the lungs before being exhaled). However, by controlling the rate of flow after the initial burst of oxygen, a dual-lumen device can be manufactured to conserve as much as a single lumen device. 
         [0012]    One disadvantage of single-lumen pneumatic conservers is that they may be too quick to detect a breath after delivering oxygen. This problem is especially acute when the patient has a long breathing pattern (i.e. few breaths per minute). Because such a patient may still be inhaling on the same breath after oxygen is delivered, the patient may receive a “double pulse” or “multiple pulses” of oxygen for each breath. Electronic conservers generally avoid that problem by not registering a new inhalation until a specified period of time has elapsed since the last detection. 
         [0013]    Furthermore, in typical prior art oxygen-conserving regulators, the inhaling patient receives an initial burst of oxygen from a bolus reservoir, often followed by a steady flow of oxygen at the regulator&#39;s flow rate while inhalation continues or until delivery is stopped. The initial burst volume of gas delivered to the patient at inspiration is equal to the volume of the reservoir multiplied by the pressure of the gas in the reservoir. 
         [0014]    Some examples of oxygen-conserving regulators are described in U.S. Pat. Nos. 6,116,242 to Frye et al., 6,364,161 to Pryor, and 6,752,152 to Gale et al. Other embodiments are described in U.S. application Ser. No. 10/666,115 entitled “Differential Pressure Valve Employing Near-Balanced Pressure” by LeNoir E. Zaiser, which was filed on Sep. 19, 2003 (U.S. Publication No. 20040194829); U.S. application Ser. No. 10/706,872 entitled “Gas Conserving Regulator” by LeNoir E. Zaiser, et al., which was filed on Nov. 12, 2003 (U.S. Publication No. 20040154693); and U.S. application Ser. No. 10/772,220 entitled “Hybrid Electro-Pneumatic Conserver for Oxygen Conserving Regulator” by LeNoir E. Zaiser, et al., which was filed on Feb. 4, 2004 (U.S. Publication No. 20050039752). The teachings of those patents and applications are incorporated herein by reference in their entirety. 
       SUMMARY 
       [0015]    In accordance with particular embodiments of the invention, a pneumatic single-lumen medical gas conserver combines the advantages of typical single- and dual-lumen conservers. In particular, a pneumatic single-lumen conserver can provide a rapid response to patient inhalations without the need for a more expensive dual-lumen cannula hose. In addition, after delivering oxygen the conserver has a specific pneumatically-implemented delay period before being able to detect the next inhalation to inhibit “double pulse” deliveries. 
         [0016]    In accordance with a particular embodiment, a pneumatic medical gas conserver for providing a volume of medical gas to a patient can include a supply port, a patient port, a sensing valve, and a check valve. The supply port receives a regulated flow of a medical gas and the patient port provides a flow of medical gas to a patient through a single-lumen cannula. 
         [0017]    The sensing valve is in gaseous communication with the patient port and can detect an inhalation by the patient so as to trigger delivery of the medical gas to the patient port. 
         [0018]    The check valve can be gaseously disposed between the sensing valve and the patient port. The check valve can decouple the sensing valve from the patient port in response to detection of the inhalation by the sensing valve. More particularly, the check valve can decouple the sensing valve from the patient port before the medical gas is delivered to the patient port, in which case the decoupled sensing valve can be gaseously isolated from the delivered medical gas. 
         [0019]    The conserver can further include a gas regulator in gas communication with the supply port. 
         [0020]    In accordance with another particular embodiment, a pneumatic medical gas conserver for providing a volume of medical gas to a patient can include a patient port, a supply port, a delivery valve, a control valve, a sensing valve, and a check valve. 
         [0021]    The patient port can be coupled to a single-lumen cannula and the supply port can receive a regulated flow of a medical gas; 
         [0022]    The delivery valve can be gaseously disposed between the supply port and the patient port for controlling the flow of medical gas to the patient port. The conserver can further include a user-operable flow valve for selecting between a pulse delivery mode and a constant flow mode. In particular, the constant flow mode can cause the flow of medical gas to bypass the delivery valve. 
         [0023]    The control valve can be in mechanical communication with the delivery valve and in gas communication with the supply port. The control valve can actuate the delivery valve. In addition, the control valve can be tuned to provide a fixed delivery cycle using, for example, an adjustable needle to vent gas and/or a timing orifice disposed in the gas flow path from the supply port to the control valve. 
         [0024]    The sensing valve can be in gas communication with the control valve and the patient port. The sensing valve can detect an inhalation by the patient so as to trigger the control valve to begin a delivery cycle. In particular, the sensing valve can include a flexible membrane. 
         [0025]    The check valve can be in mechanical communication with the control valve and gaseously disposed between the sensing valve and the patient port. The check valve can decouple the sensing valve from the patient port in response to the detection of the inhalation by the sensing valve. 
         [0026]    The conserver can further include a gas regulator that can provide the regulated flow of medical gas from a pressurized supply of the medical gas. The gas regulator can be in gas communication with the supply port. The gas regulator can further provide a user selectable flow of medical gas from a plurality of flow settings. 
         [0027]    In accordance with another particular embodiment, a pneumatic medical gas conserver for providing a volume of medical gas to a patient can include a housing, a gas flow regulator, a patient port, a user-operable mode selector, and a pneumatic delivery circuit. 
         [0028]    The housing can be connectable to a supply of pressurized medical gas. The gas flow regulator can be within the housing and can provide a regulated flow of medical gas from the supply of pressurized medical gas. That regulated flow can include a user-selectable flow rate from a plurality of selectable flow rates. 
         [0029]    The patient port can be within the housing and provides gas communication to a patient through a single-lumen cannula. The user-operable mode selector can be within the housing for providing the regulated flow of medical gas to the patient port at the selected flow rate in either a pulse flow mode or a constant flow mode. 
         [0030]    The pneumatic delivery circuit can be within the housing and can receive the regulated flow of medical gas and deliver the received flow to the patient port in accordance with the mode selector. The delivery circuit can, in particular, include a delivery valve. The regulated flow of medical gas can bypasses the delivery valve when the mode selector is in the constant flow mode setting or the regulated flow of medical gas can travel through the delivery valve when the mode selector is in the constant flow mode setting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0031]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0032]      FIG. 1  is a schematic block diagram of a particular embodiment of a medical gas conserving device. 
           [0033]      FIG. 2  is a schematic diagram of one embodiment of the conserving device of  FIG. 1  at a steady-state condition. 
           [0034]      FIG. 3  is a schematic diagram of the conserving device of  FIG. 2  illustrating gas flow in response to an inhalation by the patient  1 . 
           [0035]      FIG. 4  is a schematic diagram of the conserving device of  FIG. 2  illustrating the conserver&#39;s response to the inhalation of  FIG. 3 . 
           [0036]      FIG. 5  is a schematic diagram of the conserving device of  FIG. 2  illustrating the release of control gas. 
           [0037]      FIG. 6  is a schematic diagram of the conserving device of  FIG. 2  illustrating the control piston at the top of its stroke. 
           [0038]      FIG. 7  is a schematic diagram of the conserving device of  FIG. 2  illustrating the control piston during its down stroke. 
           [0039]      FIG. 8  is an example of another suitable delivery valve that can be used in the conserving device of  FIGS. 2-7 . 
       
    
    
     DETAILED DESCRIPTION  
       [0040]      FIG. 1  is a schematic block diagram of a particular embodiment of a medical gas conserving device. A human or animal patient  1  receives a regulated volume of oxygen from a pressurized supply vessel  3  through a regulator  5  and conserver  10 . Although not required, the regulator  5  and conserver  10  are typically part of a single physical unit that couples to the oxygen supply vessel  3 . As shown, the patient  1  is in gas communication with the conserver  10  through a single-lumen cannula  7  coupled to a cannula port  11 . 
         [0041]    As shown, the conserver  10  includes four valves, a delivery valve  20 , a sensing valve  40 , a control valve  60 , and a check valve  80 . Gas passageways interconnect the valves and interface the conserver  10  with the regulator  5  and the cannula  7 . A regulated flow of gas flows into a main supply passageway  15  from the regulator  5 . The main supply passageway  15  supplies gas to two branch passageways, a delivery passageway  12  interconnected to the delivery valve  20  and a timing passageway  13 , which in turn interconnects to a control passageway  16  coupling the control valve  60  with the sensing valve  40 . The patient is coupled to the check valve  80  via a detection passageway  18  and to the delivery valve  20  via a flow passageway  19 . The sensing valve and the check valve are interconnected by a sensing passageway  17 . 
         [0042]    Although the delivery passageway  12  and the timing passageway  13  are shown as being supplied by a common main supply passageway  15 , that structure is not required. Instead, the timing passageway  13  can be tapped off from the delivery valve passageway  12  or separately supplied from the regulator  5 . Additional valves and flow orifices may be present in physical embodiments, although not shown in the figures. Any suitable supply network can be employed and is contemplated by the following description. 
         [0043]    Briefly, the conserver  10  delivers a volume of oxygen to the patient  1  in response to an inhalation. When the conserver detects an inhalation, using the sensing valve  40 , oxygen begins flowing for a period of time controlled by the delivery valve  20 . The specific volume is determined by the flow rate of the oxygen from the regulator  5  and the amount of time the delivery valve  20  is open. Once inhalation is detected, the check valve  80  decouples the sensing valve  40  from the patient  1  during the delivery cycle so that subsequent breathing actions do not affect the sensing valve  40 —until the conserver  10  is ready to begin a new cycle. 
         [0044]    Further details will be described below with respect to  FIGS. 2-8 . It should be understood that for clarity not all components are shown in the drawings, but the use and placement of such components is within the ability of those of ordinary skill in the art. Furthermore, it is understood that O-rings would commonly be used for sealing purposes, and that more or less O-rings, or different arrangements of O-rings, may be required in a physical product. 
         [0045]      FIG. 2  is a detailed schematic diagram of one embodiment of the conserving device  10  of  FIG. 1  at a steady-state condition. In this state, the patient is not inhaling through the cannula and gas is not flowing to the cannula. The conserver  10  is ready and waiting to deliver oxygen. 
         [0046]    The delivery valve  20  includes a delivery valve body  22 , a delivery valve plate  24 , a delivery actuating rod  26 , and one or more delivery valve springs  28  operating in a delivery valve cavity  25 . Oxygen enters the delivery valve  20  from the delivery passageway  12  through a delivery inlet  21  and exits to the flow passageway  19  through a delivery outlet  29 . Note that unlike some prior pneumatic conservers, there is no gas reservoir to store oxygen for delivery to the patient. 
         [0047]    The delivery valve  20  operates between the delivery inlet  21  and the delivery outlet  29  to open and close. As shown, the delivery valve  20  is closed, with the delivery valve plate  24  face sealed to the delivery valve body  22  through compression of the delivery valve plate  24  against an O-ring by the delivery valve springs  28 . Because the delivery valve  20  is closed, no oxygen is being supplied to the patient. Note that the delivery actuating rod  26  is biased toward and extends into the control valve  60  by a known distance. The delivery valve plate  24  and actuating rod  26  are shown as being separate component parts, which can be fastened together, but the two features can be manufactured as a single valve element. 
         [0048]    The control valve  60  includes a control piston  62  and one or more control springs  66 . The control piston reciprocates within a piston cavity  65 . An O-ring riding with the control piston  62  moveably divides the piston cavity into two chambers, a spring chamber  67  and a control gas chamber  68 . The control gas chamber  68  is also bounded by the delivery valve body  22 , which can be fixed within the piston cavity  65 , with O-rings isolating the control chamber  68  from the delivery valve  20 . 
         [0049]    Oxygen from the timing passageway  13  is directed to a control passageway  16 . As shown, the timing passageway includes a timing orifice  14  to reduce the timing gas flow rate. Oxygen supplied from the control passageway  16  enters the control chamber  68  through a control valve inlet  61  to pressurize the control chamber. In a particular embodiment the timing orifice has a diameter of 0.003 inches. 
         [0050]    The spring chamber  67  is maintained at atmospheric pressure by a vent  69 . As shown, the control chamber  68  is fully pressurized and the control piston  62  is face sealed against an O-ring. 
         [0051]    The sensing valve  40  includes a nozzle  42  separated from a sensing chamber  44  by a flexible membrane  46 , and a bias spring  45 . The bias spring  46  biases the membrane  46  against the nozzle  42  to assist in closing the sensing valve  40 . The nozzle is coupled to the control passageway  16 , which in a particular embodiment interfaces with the sensing valve membrane  46  through a nozzle  42  opening having a diameter of 0.009 inches. Also shown is an atmospheric vent  49  and a sensing port  47  coupled to the sensing passageway  17 . Further details of the sensing valve  40  are described in the above-incorporated U.S. application Ser. No. 10/666,115. As shown, the membrane  46  is sealing the nozzle  42  (and closing the sensing valve  40 ) so that oxygen cannot flow through the nozzle. 
         [0052]    The check valve  80  includes a check plate  82 , at least one spring  84 , and an actuating rod  86 . The check valve interfaces with the sensing passageway  17  through a sensing inlet  87  and with the detection passageway  18  through an outlet  88 . The check valve spring  84  is biased to face seal the check plate  82  against an O-ring so as to disconnect the sensing inlet  87  from the outlet  88 . As shown, the check valve  80  open, with the check plate  82  being unsealed. This occurs because the fully compressed control piston  62  has moved the actuating rod  86  to push the check valve plate  82  away from the sealing O-rings. The delivery valve plate  24  and actuating rod  26  are shown as being separate component parts, which can be fastened together, but the two features can be manufactured as a single valve element. 
         [0053]    In the steady-state condition of  FIG. 2 , the conserver  10  is ready to supply oxygen to the patient. All that is needed to trigger a response is an inhalation from the patient, which would cause a vacuum in the detection passageway  18 . 
         [0054]      FIG. 3  is a schematic diagram illustrating gas flow in response to an inhalation by the patient  1 . As shown, inhalation draws atmospheric gas from the check valve  80 , the sensing passageway  17 , and the sensing chamber  44  through the detection passageway  18 . That action results in a slight vacuum forming in the sensing chamber  44 . The inhalation also tends to draw gas from the delivery valve  20 . 
         [0055]      FIG. 4  is a schematic diagram illustrating the conserver&#39;s response to the inhalation of  FIG. 3 . The vacuum created in the sensing chamber  44  overcomes the force of the bias spring  45 , which causes the membrane  46  to release from the nozzle  42 . This immediately causes the low-pressure gas to flow from the control cavity  68  to atmosphere through the control passageway  16 , the nozzle  42 , and the vent  49 . Note that in the delivery valve  20  the vacuum force is resisted by the delivery springs  28  to maintain the face seal on the delivery plate  24 . 
         [0056]      FIG. 5  is a schematic diagram illustrating the release of control gas. As shown the low-pressure gas from the control gas chamber  68  continues to flow to atmosphere through the vent  49 . As the control gas chamber  68  is evacuated, the control springs  66  urge the control piston  62  to lift off from the face seal and begin an upstroke. Once the face seal is released, the sensing chamber becomes coupled to the control spring chamber  67 , which is maintained at atmospheric pressure by the control vent  69 . 
         [0057]    The movement of the control piston  62  also releases the check valve actuating rod  86 , which allows the check valve spring  84  to compress the check valve plate  82  against its O-ring. The sensing chamber  44  is now decoupled from the patient, even though no oxygen is yet being delivered to the patient (i.e., the delivery valve  20  is closed). That decoupling protects the pilot diaphragm  46  from pressure shocks. Note that the check valve actuating rod  86  is biased toward and extends into the control spring chamber  67 . 
         [0058]    The upstroke of the control piston  62  aids in evacuating the control gas chamber  68  by maintaining pressure in the control gas chamber  68  above atmospheric pressure. The additional pressure caused by the control springs  66  helps to keep the sensing valve  40  open because the gas pressure in the nozzle  42  can continue to overcome the membrane  42  bias. 
         [0059]    During the upstroke, the upward motion of the control piston  62  tends to create a vacuum in the control spring chamber  67  behind the piston. Because the sensing chamber  44  is coupled to the control spring chamber  67 , such a vacuum would tend to hold the sensing valve  40  open. The control vent  69  to atmosphere tends to modulate this vacuum in the control spring chamber  67  and the sensing chamber  44  to avoid damage to the diaphragm  46 . Note that no back-pressure is applied to the sensing chamber  44 . 
         [0060]      FIG. 6  is a schematic diagram illustrating the control piston  62  at the top of its stroke. Before reaching the top of the stroke, however, the control piston  62  engages the delivery actuating rod  26 , which overcomes the force of the delivery springs  28  to unseal the delivery plate  24 . As soon as the delivery plate  24  is unsealed, the delivery valve  20  is open and low-pressure supply oxygen begins flowing from the delivery inlet  21  through the delivery outlet  29  and on to the patient. The oxygen flow continues until the delivery valve  20  is closed. 
         [0061]    Note that to reach the patient, the oxygen enters the flow passageway  19 . Because the detection passageway  18  is connected with the flow passageway  19 , delivery oxygen also flows into the detection passageway  18  and the check valve cavity  85 . Before the delivery valve  20  is opened, however, the check valve  80  has been closed, as described above. Because the check valve  80  is closed, the sensing chamber  44  is isolated from the check valve cavity  85  and, thus, the delivery oxygen flow. The gas flow into the detection passageway  18  will pressurize the check valve cavity  85  and apply further force to seal the check valve plate  82 , but will not harm the check valve  80 . 
         [0062]    At the top of the control piston&#39;s stroke, the control gas chamber  68  is essentially empty or at least depressurized enough so that the bias spring  45  can seal the nozzle  42  with the sensing valve membrane  46 . Once the nozzle is sealed, gas from the control passageway  16  can no longer escape to atmosphere, causing pressure to build. The control gas chamber  68  will now begin pressurizing via timing gas passing through the control passageway  16 . 
         [0063]      FIG. 7  is a schematic diagram illustrating the control piston  62  during its down stroke. As shown, the control gas chamber  68  is being pressurized. As pressure builds toward the supply pressure, the force of the control springs  66  is overcome, allowing the control piston  62  to compress the control springs  66 . At a fixed point during the down stroke, the control piston  62  disengages from the delivery actuating rod  26 , thus allowing the delivery springs  28  to compress the delivery plate  24  against the face seals to close the delivery valve. At the point shown, oxygen is not flowing to the patient and the conserver  10  is not ready to detect an inhalation because the check valve  80  is still closed. 
         [0064]    The length of the delivery actuating rod  26  is chosen to maintain contact with the control piston  26  for a fixed period of time, such as 400 milliseconds. That provides the patient with a pulse of oxygen for that fixed period of time. Because there is no reservoir to provide an initial burst of oxygen, the conserver provides a fixed flow rate during delivery. Furthermore, the timing orifice  14  is used to reduce the flow rate of timing gas into the control gas chamber  68  and thus inhibit the chance of the device resetting too quickly and delivering a double pulse of gas. 
         [0065]    Once the control gas chamber is pressurized, the control piston  62  will engage the check valve actuating rod  86 . The actuating rod  86  will lift the check valve plate  82  away from the O-ring and re-couple the patient to the sensing chamber  44 , as shown in  FIG. 2 . At that stage, the conserver  10  is ready to begin a new delivery cycle. 
         [0066]    One feature of the conserver  10  is its ability to detect an inhalation and, in response, rapidly deliver oxygen to the patient. This is accomplished by maintaining a small control gas cavity so that the control piston  62  has a short stroke. In addition, a low-friction liner can be placed on the wall of the control gas cavity  68  so that the control piston&#39;s O-ring encounters less friction, thus allowing faster piston strokes and providing longer life to the O-ring. Suitable low-friction materials include TEFLON-impregnated nickel, and plastics. The cavities for the delivery valve  20  and the check valve  80  should also be small. 
         [0067]    While the conserver  10  should respond rapidly to an inhalation, it is also important that the conserver does not reset too quickly. The time needed for the control piston  62  to complete its down stroke and engage the check valve actuating rod  86  is timed to reduce the likelihood of a double pulse. That timing is determined by the flow rate of oxygen into the control chamber  68 , which can be set by selecting the diameter of the timing orifice  14  in the timing passageway  13 . During manufacture, the conserver  10  can be set for a one of a plurality of breathing patterns. More generally, an adjustable restrictor could be employed, such as an orifice plate in the regulator  5  ( FIG. 1 ). 
         [0068]    In a particular embodiment, the control piston  62  has a stroke period to accommodate patients with a breathing rate in a range of about 16-50 breaths per minute. More particularly, the control piston  62  has a stoke period of about 1.2 seconds, essentially all of which is consumed by the down stroke as the control chamber  68  pressurizes. The delivery actuating rod  26  is dimensioned so that the delivery valve  20  is open for a duty cycle of about 400 milliseconds during the down stroke of the control piston  62 . Different timings can be used, however, such as a stroke period of about 1.0 seconds and a delivery duty cycle of between 400-600 milliseconds. Because the timing depends on the volume of the control chamber  62 , the use of an adjustable control chamber, employing the techniques described in the above-incorporated U.S. application Ser. No. 10/706,872, can allow the timing to be adjusted in the field. 
         [0069]    It is also noted that the control valve vent  69  has an effect on the length of the pulse of gas to the patient as well as the timing after the pulse before the conserver is reset to its steady-state condition ( FIG. 2 ). In a particular embodiment, an adjustable needle valve is disposed within the vent  69 , which allows the vent orifice  69  to be controlled during assembly so as to tune the conserver to a specific flow profile. In a particular embodiment, the conserver has an adjustable delivery cycle of between 400 milliseconds and 1200 milliseconds, including a 200 millisecond delay from inhalation until the gas delivery begins. 
         [0070]    In addition, the regulator can provide a variable flow rate of oxygen based on a flow rate selected by a user. In this way, the conserver  10  can deliver a variable rate of oxygen for fixed periods of time. The variable flow rate can be provided by the orifice plate in the regulator  5  ( FIG. 1 ). 
         [0071]    It should also be understood that the conserver can include a continuous-flow mode, in addition to the conserving mode described above. In particular, the conserver can include a user-selectable conserving or continuous knob or switch, which can be coupled to a valve that allows supply gas flow through the timing passageway  13  when in conserving mode and blocks supply gas flow through the timing passageway  13  when in continuous mode. In such an embodiment, the regulator orifice plate could employ two sets of flow orifices to provide the appropriate gas flows at both settings. When in conserving mode, a selected pair of orifices would provide oxygen to the delivery passageway  12 , while in conserving mode, only one orifice of the pair would provide oxygen to the delivery passageway  12 , as controlled by the valve. Note that even in the continuous flow mode, the constant flow rate can be selected by the user from a plurality of flow rates, as provided by the orifice plate. A suitable orifice plate for the regulator  5  is described in the above-incorporated U.S. application Ser. No. 10/706,872. 
         [0072]    In another embodiment, the delivery valve  20  is shunted when the conserver is in continuous mode. This embodiment offers certain advantages. One advantage is that possible leaking of timing gas into the control gas chamber  68  would not stop continuous flow because the delivery valve  20  is bypassed. Another advantage is that an initial inhalation is not needed to initiate the continuous flow. 
         [0073]    Although the invention has been described as not requiring a bolus reservoir, one of ordinary skill in the art can add a reservoir. In one embodiment, a reservoir can be formed between the delivery inlet  21  and the delivery valve plate  24  that could be filled between delivery cycles. Another approach is to use the control gas chamber  68  as a reservoir by replacing the sensing valve vent  49  with a passageway to the delivery inlet  21  and adjusting the bias of the sensing membrane accordingly. A reservoir can also be added by reorienting the valves. 
         [0074]    Although a particular valve type is shown for the delivery valve  20  and the check valve  80 , other suitable valves can be substituted. For example, the orientation of the passageways can be reversed with respect to the illustrated valves. In the delivery valve, in particular, the delivery passageway  12  could terminate at an inlet in the delivery valve cavity  25  and the delivery outlet could be through the delivery valve body  22  on the opposite side of the delivery plate  24 . That arrangement would permit the delivery valve cavity  25  to become pressurized and used as a bolus reservoir for supplying an initial burst of oxygen to the patient. A similar valve structure could be used for the check valve  80 . Other valve types can be used and are a design choice. 
         [0075]      FIG. 8  is an example of another suitable delivery valve  20 ′ that can be used in the conserving device of  FIGS. 2-7 . As illustrated, delivery valve body  22 ′ includes an inlet  21 ′ and an outlet  29 ′ that are sealed against a common face of a valve plate  24 ′ by O-rings when the valve is closed and unsealed from the O-rings when the valve is open. Note that in this particular structure, the O-rings are redundant. Also shown are a delivery actuating rod  26 ′ and delivery valve springs  28 ′. 
         [0076]    The conserver components are made from typical materials known in the art to be oxygen compatible, including aluminum, brass, titanium, and nickel. The particular materials used for each component part is an engineering choice. In one particular embodiment, the delivery valve body  22  and the moveable components are fabricated from brass. 
         [0077]    Although the delivery of oxygen is most common use of a medical gas conserver, the delivery of other therapeutic gases is contemplated, for example, nitrous oxide. In addition, the present invention can be employed for non-therapeutic uses, such as for the delivery of lethal gases, or the delivery of other gases for industrial uses. 
         [0078]    While this invention has been particularly shown and described with references to particular embodiments, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention encompassed by the appended claims. For example, various features of the embodiments described and shown can be omitted or combined with each other.