Abstract:
A method and apparatus for conserving oxygen provided for oxygen therapy. The oxygen conserving delivery system includes a sensor for detecting inspiration efforts, and a microcontroller which is triggered by the sensor to open a valve to allow the flow of oxygen to the patient. The system includes a cross-field excitation/detection inspiration sensor using an adaptive full-wave signal detector/tracer. The system also includes a method for conserving energy by a unique combination of start and sustain drive lines coupled to the valve control. The system is able to compensate for variations in battery condition, inlet gas pressures, variations in valve tolerances, and the absence of gas pressure in a fault condition. Finally, a self-adaptive inspiration depth tracking ability ensures constant oxygen delivery volume even when there are variations in inspiration depths rather than changes in respiration rates. This ability enables the system to provide oxygen at a depth and a rate which is a best fit for a patient&#39;s oxygen needs when at rest or at work.

Description:
BACKGROUND 
     1. The Field of the Invention 
     This invention relates generally to the administration of a supplemental, intermittent oxygen supply to a patient. More specifically, the present invention provides a method and apparatus for providing oxygen therapy to a patient, where a system includes the ability to conserve oxygen by only delivering an amount that is usable by the patient during an inspiration cycle. 
     2. The State of the Art 
     The state of the art in oxygen therapy is replete with systems that attempt to conserve oxygen being supplied to a patient. The need to conserve oxygen is a result of the understanding that continuous, long term oxygen therapy is expensive because of the large quantities of oxygen that needed to be provided to a patient. 
     Less sophisticated oxygen therapy systems provide oxygen at a continuous rate without interruption. The result is that all oxygen supplied to the patient during exhalation is wasted. This waste can be substantial considering that approximately two thirds of the respiratory cycle is spent in exhalation. Furthermore, these systems do not adjust for rates or depth of inspiration by the patient. These factors, combined with the fact that the oxygen therapy is generally being conducted using a mobile oxygen container, demonstrate the need for more prudent oxygen conservation. Clearly what is needed is a way to reduce oxygen consumption while maintaining adequate alveolar gas exchange of the patient. 
     To meet this need for oxygen conservation during oxygen therapy, systems have been developed which are designed to deliver oxygen only during patient inspiration. However, the methods and apparatus for accomplishing this goal are many, and are met with varying degrees of success. 
     One method for accomplishing conservation during oxygen therapy is to provide pulsed oxygen delivery to the patient according to some control logic based upon a physician prescribed gas flow rate and the breathing characteristics of the patient. For example, studies support the observation that the first portion of inspiration is the most effective time for oxygen delivery, with the last portion of the patient&#39;s inspiration not actually providing oxygen to the secondary respiratory system (i.e. the blood stream). 
     For example, in U.S. Pat. No. 5,165,397 issued to Arp, the patent apparently teaches a method and apparatus for providing oxygen on demand, where the system is responsive to a patient&#39;s breathing. 
     Similar systems are taught in U.S. Pat. No. 5,099,837 issued to Russel Sr. et al., and in U.S. Pat. No. 5,697,364 issued to Chua et al. All of these systems have numerous drawbacks which prevent them from serving all the functions of an acceptable oxygen conservation device. These drawbacks include, but are not limited to, a failure to adequately adapt to breathing depth (force) of the person using the system. They also fail to take into account changes that occur due to variations in altitude of the user. They also fail to compensate for changes in battery strength, and fail to adequately conserve battery power. 
     It would be advantageous over the state of the art in oxygen therapy to provide a conserving device which is responsive to the breathing rate of the patient. The system should adapt the volume of oxygen delivered depending upon depth of inspiration, and the rate of inspiration. 
     It would also be advantageous to provide a conserving device which did not require training of the patient, so that the system was responsive to the patient, and not the other way around. 
     The system should be capable of making some adjustments to patient inspiration, regardless of whether the patient&#39;s inspiration rate or depth changes rapidly. 
     It would also be an improvement over the state of the art to provide a system which does not require a plurality of oxygen delivery tubes between the conserving device and the patient. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus for providing oxygen therapy which results in a reduction or elimination of variations in oxygen flow. 
     It is another object to provide a method and apparatus for providing a dynamically adaptive delivery of oxygen flow to the patient to thereby better respond to a patient&#39;s changing needs. 
     It is another object to provide a mobile conserving device for use in oxygen therapy which reduces power consumption to thereby extend the life of a battery providing power. 
     The presently preferred embodiment of the present invention is a method and apparatus for conserving oxygen provided for oxygen therapy. The oxygen conserving delivery system includes a sensor for detecting inspiration efforts, and a microcontroller which is triggered by the sensor to open a valve to allow the flow of oxygen to the patient. The system includes a cross-field excitation/detection inspiration sensor using an adaptive full-wave signal detector. The system also includes a method for conserving energy by a unique combination of start and servo sustain drive lines coupled to the valve control. This method of conserving energy also includes the ability to compensate for variations in battery condition, inlet gas pressures, variations in valve tolerances, and the absence of gas pressure in a fault condition. Finally, a self-adaptive inspiration depth tracking ability ensures constant oxygen delivery volume even when there are variations in inspiration depths in addition to changes in respiration rates. 
     In a first aspect of the invention, a unique start and sustain servo is used for the valve which enables the microcontroller to use a minimum amount of energy from the power source to open the valve for each pulse of oxygen that flows therethrough. 
     In another aspect of the invention, this start and sustain method of valve operation automatically controls energy consumption by compensating for a diminishing power factor curve that is associated with batteries. This method also allows for automatic compensation of performance variations within the valves themselves. 
     In another aspect of the invention, electro motive energy which is reflected form the valve is shunted back to the battery to further conserve energy. 
     In another aspect of the invention, the sensor responds to the entire phase of the patient&#39;s respiration efforts and the admission cycle of the oxygen. This is accomplished by calculating a real-time respiration depth-time factor. A system for calculating the depth-time factor is stored as a schedule for providing a constant percentage ratio between inspiration depth time and actual valve on-times, or a sliding bolus. Using this depth-time factor coupled with monitoring the respiration rate, the system is able to provide proper oxygen delivery while compensating for changing respiration activities which change both rate and depth of patient inspiration. 
    
    
     These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the presently preferred embodiment which is made in accordance with the principles of the present invention. The components of the conserving device are shown with respect to a patient and an oxygen source. 
     FIG. 2 is a schematic diagram illustrating a portion of circuitry included within the valve assembly shown in FIG.  1 . This circuitry shows the interaction between the valve, the start and the sustain drive lines. 
     FIG. 3 is a simplified timing diagram which illustrates the cycles of operation of the oxygen conserving device of FIG. 1 including the inspiration sensor input, the start drive line, the sustain drive line, and the power being delivered to the valve. 
     FIG. 4A is a block diagram of the inspiration sensor and its associated sensor circuitry which comprise the inspiration sensor assembly shown in FIG.  1 . 
     FIG. 4B is a schematic diagram of the inspiration sensor and its associated sensor circuitry which comprise the inspiration sensor assembly shown in FIG.  4 A. 
     FIG. 5 is an event timing diagram showing operation of the oxygen conserving device of FIG. 1 under normal operating conditions. 
     FIG. 6 is an event timing diagram showing operation of the oxygen conserving device of FIG. 1 when an apnea condition is detected. 
     FIG. 7 is an event timing diagram showing operation of the oxygen conserving device of FIG. 1 when a flow fault condition is detected. 
     FIG. 8 is a profile view of an external packaging of the OCDS as described in the preferred embodiment of FIG.  1 . 
     FIG. 9 is an elevational profile view of an alternative embodiment, including an oxygen conserving delivery system which is coupled to a primary H.P. reducing regulator, a bypass valve and a cylinder gauge. 
     FIG. 10 is a block diagram of another alternative embodiment of the oxygen conserving delivery system which includes a barometer to enable adjustments in oxygen delivery based on changes in altitude. 
     FIG. 11 is a block diagram of another alternative embodiment of the oxygen conserving delivery system which includes sensors for measuring inspiration effort depths and positive pressures caused by the opening of the valve. 
     FIG. 12 is an event timing diagram of the waveform from inspiration efforts, and a sensor output line (S/D). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. 
     Before describing the invention, it is useful to remember from the outset that this technology has application in both the medical and aviation fields. This is because the same adaptive method taught by the present invention for changing the oxygen delivery rate for a patient who is changing between an ambulatory state and an at-rest state can also be used to adapt an oxygen flow rate to a pilot or passenger based on changes in altitude. 
     The preferred embodiment of the present invention is shown in the block diagram of FIG.  1 . The oxygen conservation device  100  is shown to be comprised of several components. These components will be described by examining how the components interact during operation of the device. 
     An inspiration sensor assembly  102  includes an inspiration sensor  164  (not shown until FIGS. 4A and 4B) which detects inspiration efforts of the patient when the patient inhales. It is assumed that the patient has inserted in the nasal passages a cannula (not shown). The cannula is coupled to the inspiration sensor  164  via a single tube  104 . The inspiration sensor  164  generates a signal to a microprocessor  106  (or any other appropriate microcontroller) when the patient begins a respiration cycle that is detected by the inspiration sensor  164 . In response, the microprocessor  106  generates a signal which opens a valve  162  (FIG. 2) in a valve assembly  108 . The opened valve  162  enables oxygen to flow for a short period of time from an oxygen source  110 , through the valve  162 , and on to the cannula through the tube  106 . At the end of some predetermined or calculated duration, the valve  162  is closed. This cycle then repeats itself at the beginning of every inspiration effort of the patient. It is noted that the inspiration rate may be limited to, for example, 20, 25, 30 and 40 bpm. This is also known as hold-off. 
     In conjunction with the overview of the oxygen delivery process described above, the oxygen conservation device  100  has other useful features which enhance usability of the device. For example, a manually operable on/off switch  112  enables the user to turn off the flow of oxygen to the patient. This same switch  112  is also capable of operating as an effective rate of flow control switch. The amount of oxygen which is delivered to the patient can be set by the user so that an overall physician prescribed flow rate can be maintained under normal breathing conditions. The flow rate switch  112  turns off the device by setting the respiration rate to zero or to an “OFF” switch setting. 
     Status indicators also provide feedback to the user to indicate a condition of the oxygen conserving device  100  that requires attention. The status indicators include both visual indicator means  116 ,  118  and audio indicator means  120 . As will be explained, these conditions include flow fault, apnea, low battery, device error, and positive flow. While the microprocessor  106  determines a condition of flow fault, apnea, and positive flow, a separate low voltage battery detector circuit  122  and a run-time device-error detector circuit  126  (watch dog circuit) are typically used for detection of these conditions. The system also includes a voltage integrity monitor  121 , which should not be confused with the low voltage battery detector circuit  122 . 
     In the presently preferred embodiment, the oxygen conserving device  100  is a mobile device which operates using a battery  124  as the power source. 
     It should be realized that the audio alarm means  120  can alert the user of a serious condition of the device  100  which requires immediate attention. However, for the visually impaired, the audio indicator means  120  can also function to provide the same feedback to the user as is provided by the visual indicator means  116 ,  118 . 
     In the presently preferred embodiment, the visual indicator means  116  is a warning indicator and is thus selected to be a red light. In contrast, the visual indicator means  118  is used to provide feedback to the user about the flow of oxygen from the oxygen source  110 , and is therefore selected to be a green light to indicate that there is a condition of positive flow. 
     It is noted that instead of the red and green indicator lights, the visual indicator means could also be displayed on a liquid crystal display (LCD). This alternative embodiment has the advantage of very low power consumption, and yet providing a very informative and detailed visual display. 
     In the presently preferred embodiment, the visual indicator means  116  and  118  are selected to be lights of any appropriate variety which can be driven by signals from the microprocessor  106 . A diode symbol is used in FIG. 1, but this can be substituted. Those skilled in the art understand the appropriate voltages that can be supplied and used to drive the type of selected visual indicator. 
     The microprocessor selected for use in the presently preferred embodiment has a part number of PIC 16C71. However, it should be understood that any microcontrolling device that can be programmed in firmware can be substituted for this particular integrated circuit. 
     It should also be understood that the instructions for the microprocessor utilized in this presently preferred embodiment are stored in a memory device which is programmable. For example, an EEPROM (electrically erasable and programmable read-only memory) is associated with the microprocessor so that when the oxygen conserving delivery system is activated, the microprocessor prepares itself for use. Preparation includes an internal self-test of the OCDS, and then a one-time flow-fault check. 
     While some of the processes and indicator means described above are well known in the art, there are several aspects of these processes which are unique to the present invention. These novel aspects of the invention will become apparent while describing the components and operational detail of the oxygen conserving device  100 . 
     Beginning with the operation of the valve  162 , it is operated via two drive lines from the microprocessor  106 . FIG. 1 shows that these drive lines are labeled a start drive line  128  and a sustain drive line  130 . 
     FIG. 2 is a schematic diagram illustrating the start drive line  128  from the microprocessor  106 , the sustain drive line  130 , and the valve assembly  108 . The start drive line  128  is a high-current start pulse to meet the initial so-called “kick-in” higher power requirement that is generally associated with electromechanical devices. In contrast, the sustain drive line  130  is a lower (constant current) drive line that keeps the valve  162  energized in a lower power sustain mode once it has been opened by the higher current start drive line  128 . The sustain drive line  130  advantageously provides a constant current to the valve  162  regardless of the condition of the battery&#39;s useful power factor. 
     The start and sustain drive line method operates as follows. The microprocessor  106  detects a signal from the inspiration sensor  164  indicating inhalation by the patient. The microprocessor  106  responds by activating the start drive line  128 . The start drive line  128  initially energizes the valve  162  with the full voltage available from the battery supply  124 . The valve  162  does not place an instantaneous full power load on the battery  124 , but the power builds gradually until a reactance of the coil “exhibits” the electro motive energy required to open the valve  162 . 
     Once the valve  162  has opened, the inspiration sensor  164  (FIG. 1) detects oxygen pressure. The start drive line  128  is then no longer needed, and it is de-energized by the microprocessor  106  so that the sustain drive line can take over during the duration of the on-time required by the valve  162 . A typical ratio of “ON” time for the start drive line versus the sustain drive line is 8:1, where the sustain drive line is on the longest. 
     Reflected electromotive energy from the valve  162  is reverse shunted by the clamping diodes D 1  and D 2  which direct energy back into the circuit which includes the battery  124  and the damping capacitor C(b). The diodes D 1  and D 2  thus also serve as quenching arrestors of any potential electrical spark that may be generated by any back EMF events from the valve  162  while the transistors Q 1  and Q 2  release power being supplied to the valve. This ensures maximum safety of the oxygen conserving device  100  because of its potential use in oxygen rich environments. 
     This dual-line start and sustain drive line method of operating the valve  162  enables the microprocessor  106  to use only the amount of battery energy needed to open the valve and then maintain it in an open position. This is possible because the start drive line  128  is turned off by the microprocessor  106  once the valve  162  has been detected to be open. This method also enables the oxygen conserving device  100  to automatically compensate for a diminishing power factor curve that is associated with batteries, and to also compensate for variations of performance of the valve  162  itself. 
     The valve  162  of the presently preferred embodiment is a low-power electromechanical poppet type of valve. The valve  162  is normally closed when in a non-energized state. In the event of failure of the valve  162  or the oxygen conserving device  102 , the valve will restrict any flow of oxygen to the patient. In this situation, the user must bypass the oxygen conserving device  100  by disconnecting the cannula and the oxygen source from the oxygen conserving device  100  and coupled the them together directly using a coupler barb, allowing oxygen to flow in a preset constant-flow configuration. In an open state, the valve  162  has a minimal amount of restriction on the net flow of oxygen, i.e. about 3%. 
     FIG. 3 is provided as a simplified illustration of an event timing diagram which demonstrates the cycles of operation. It is important to keep in mind during this explanation that the inspiration sensor  164  continues to respond to all of the phases of a respiration cycle, as well as the pressure of oxygen. 
     At time mark (a), the inspiration sensor  164  detects the inspiration effort of the patient by going from high to low. The microprocessor drives both the start drive line  128  and the sustain drive line  130  high. Between time marks (a) and (b), the power consumed by the valve  162  is shown as increasing until reaching a peak at time mark (b) when the valve opens. A sensor output line (S/D)  136  returns to a high output when the oxygen pressure is detected from the oxygen source  110  (FIG.  1 ), and the start drive line  128  goes low as it is no longer needed. It should be realized that this return will dwell longer in proportion to reduced battery voltage. This dwell time  127  is shown as an unknown but typically short duration (on the order to approximately 5 to 10 ms) of time after the start drive line  128  is de-energized. 
     The time between time marks (b) and (c) is indicative of the on-time of the valve  162 , while the time between time marks (c) and (d) is indicative of the remaining portion of the respiration cycle during which no oxygen is being supplied to the patient because the valve has closed at time mark (c). 
     The microprocessor  106  measures the time duration from time mark (a) through time mark (d) to thereby achieve a real-time respiration depth time factor. This time factor is used to help compensate for deep respiration efforts. 
     FIG. 3 indicates that the total “on” time of the valve  162  is shown as time  129  from time mark (a) to time mark (c). The time between inspiration events is shown as the time  131  from time mark (a) to time mark (d). What is not shown because of the lack of scale is that the time  131  is typically ten to twenty times longer than the time  129  during which time the valve is being actuated. 
     It is noted that the reflected electromotive energy is shunted back to the power supplying circuitry at time points  132  and  134  of FIG.  3 . 
     FIG. 4A is a block diagram of the inspiration sensor  164  and its associated sensor circuitry. This diagram is helpful in understanding the overall purpose of the inspiration sensor without having to see the detail provided in FIG.  4 B. 
     Without going into the detailed explanation, the inspiration sensor is comprised of the following components. First, an excitation circuit  140  provides a signal to the sensor  142 . The sensor generates output signals which are indicative of pressure on the sensor. An output signal from the sensor  142  is then rectified and amplified (buffered) at  144 . The signals are then sent through a low pass filter and a high pass filter at  146 . The positive and negative difference signals are then sent to a detector  148 . 
     FIG. 4B is a more detailed schematic diagram of the inspiration sensor  164  and its associated sensor circuitry of the presently preferred embodiment. Some observations about this circuit are that a first comparator circuit  150  (which includes an operational amplifier) is used to generate a square-wave signal of 15 Khz. This signal is generated using passive components R 1 , R 2 , R 3 , R 4  and C 1 . The suggested values (but not the only values possible) for these components are as follows: R 1 =1 mega ohm, R 2 =1 mega ohm, R 3 =1 mega ohm, R 4 =27 kilo ohm, and C 1 =0.001 micro farads. It is noted that a suitable part number for the operational amplifier used in the first comparator circuit is TLC3702. 
     The square wave signal from the first comparator circuit  150  is used to excite the inspiration sensor  164 . In this preferred embodiment, the inspiration sensor  164  is a dual-plate stationary capacitor where a canter plate (moving membrane) is used to capacitively couple the two stationary plates. Changes in pressure move the membrane closer for negative pressures, and further apart for positive pressures because the capacitor plates are caused to couple or uncouple, respectively. The change in pressure results in more or less capacitively coupled current to flow from a driven stationary plate  154  to a receiving stationary plate  156  within the inspiration sensor  164 . 
     The receiving stationary plate  156  is coupled to a full-wave rectifier/detector and current amplifier circuit that is comprised of passive components R 5 , R 6 , R 7 , R 8 , C 2  and C 3 , and of active components Q 1  and Q 2 . The suggested values for these components in this preferred embodiment are as follows: R 5 =100 kilo ohm, R 6 =100 kilo ohm, R 7 =1 mega ohm, R 8 =1 mega ohm, C 2 =0.1 micro farads, and C 3 =0.1 micro farads. Suitable transistors Q 1  and Q 2  can be selected with part numbers 2N3904, 2N3906 or their equivalent. Transistors Q 1  and Q 2  may be bipolar NPN type transistors. 
     The operational amplifier used in the second comparator circuit  152 , which can be identical to the first comparator circuit  150 , is used as a voltage comparator for detecting a voltage that is equal to or greater than a reference voltage. Dual reference voltages are generated by the resistor network R 9 , R 10  and R 11  for the second comparator resistors R 11  and R 12 . Resistors R 11  and R 12  enable a drive impedance from the voltage divider network R 9 , R 10  and R 11  of about 2 mega ohms. 
     The output of the second comparator circuit  152  is held high while it is quiescent. Capacitors C 6  and C 7  are used as coupling capacitors that enable a differentiating counter-augment current to the high impedance dual reference voltage. In other words, complimentary signals having a given rate of change produced through the inspiration sensor  164  are then rectified and buffered to thereby augment the reference voltage so as to be detectable by the second comparator (detector) circuit  152 . The resistor R 13  in association with capacitor C 8  give the second comparator  152  positive feedback so that a controlled amount of dynamic (mono stable) hysteresis ensures that the second comparator  152  generates a clean and jitter-free transition from a high-to-low and from low-to-high state for each valid threshold detection event. The voltage reference follows the amplitude of the square wave generator on a 1:1 basis so that variations in a supply voltage does not effect the relative and dynamic sensitivity of the entire excitation, sensor and rectifier/buffer system from 1.5 to 15 volts. 
     The rectifier/buffer circuitry generates complementary DC voltage outputs in an analog form which is representative of the absolute position of the membrane with respect to the stationary plates  154  and  156  and the amplitude of the square wave signal being generated by the first comparator circuit  150 . 
     Regarding the operational parameters of the present invention, in this description of the preferred embodiment, it is noted that the oxygen conserving device  100  is powered via a 9 or a 3 volt battery. Alternatively, the oxygen conserving device can be operated from an AC power source using a transformer to generate DC voltage. In the event of AC power failure, unplugging the AC adapter will enable the battery to provide energy to the device. 
     The maximum operating inlet pressure for the oxygen conserving device  100  is approximately 50 psig. The maximum, non-operating, inlet pressure the device can tolerate without damage is approximately 115 psig. The oxygen conserving device  100  can also respond to inspiration efforts as measured at the sensor as low as 1 mm of H 2 O. 
     The oxygen conserving device  100  has included therein a table for each flow setting that is selected. The table is based upon a formula for valve-on pulse outputs as follows:        VKt   =         flr        (   te   )       Ir       Vkf   60                              
     Where: 
     Vkt=valve open dwell time in ms 
     Vkf=valve open flow coefficient in liters/min 
     flr=the desired prescribed flow rate in liters/min 
     Ir=the effective inspiration rule (typically 500 ms) during which a person will benefit from all the oxygen that is available 
     te=the standard temperature factor of 1.0 for 25 degrees C. 
     FIG. 5 is provided because it is useful to examine normal operation of the oxygen conserving device  100  in terms of what should be considered an event timing diagram describing normal operation. The event timing diagram shows the start drive line (Sta)  128  and the sustain drive line (Sus)  130  that drive the valve  102 , and an internal envelope signal (Env), all of which are generated in response to inspiration events (Ie) that are derived from the inspiration sensor output (S/D)  136 . The Inspiration events are shown as diamonds, secondary time mark events are shown as triangles, and arbitrary elapsed times are indicated by circles. Event time mark (Evtm) points are alphabetically labeled, and Elapsed time mark points are numerically labeled. 
     The first inspiration event Ie(l) is indicated by the S/D sensor input line going low which starts the valve  162  energizing. After the valve  162  is energized, and opens to allow oxygen to flow, the S/D input line goes back high as a result of oxygen pressure resetting the active inspiration sensor  164 . At this time Evtm(a), a positive-flow status is present and the Sta drive line is released, while the Sus drive line is designed to continue driving open the valve  162  using less energy than the Sta drive line. The Sus drive line continues driving the valve  162  for a predetermined amount of elapsed time Eltm( 1 ). Once the Sus drive line has been active for its predetermined amount of time, the valve  162  is de-energized to return to a normally closed position to stop the flow of oxygen at Evtm(b). The S/D drive line will go back low if a valid inspiration effort is still occurring at Eltm( 2 ). In other words, the oxygen flow will be terminated while the patient is still breathing in. 
     The microprocessor  106  will measure the time from Ie( 1 ) to Ie( 1 )′ to thereby determine the inspiration depth factor for each valid inspiration effort. Time mark Ie( 1 )′ is the time at which the inspiration effort ends, and the patient is about to pause or begin breathing out. Once the valve  162  has been opened and shut, (a valve cycle), the valve will not reopen due to an inspiration event by the patient until the envelope time (Env) has elapsed. Therefore, inspiration efforts will be ignored until after Env goes low at Evtm(d). The envelope time is set according to allowable and preset inspiration rates. For example, an Envelope time of three seconds is used when a maximum allowable respiration rate is set for  20  inspiration events per minute. 
     After EVtm(d), the next detected and allowed inspiration effort on the S/D line occurs at Ie( 3 ), even though an inspiration event was detected at Ie( 2 ). Ie( 2 ) did not trigger the Sta drive line and the Sus drive line because the inspiration event occurred during the envelope when no new inspiration events will trigger a response from the system. 
     Along with normal operation, the present invention is also designed to detect abnormal conditions. One of the abnormal conditions which the present invention is designed to detect is an Apnea condition, or the suspension of breathing by the patient. In the event that inspiration events should fail to be detected within a predetermined period of time, regardless of the presence of oxygen pressure, the oxygen conserving device  100  will respond with an apnea condition. 
     FIG. 6 shows an event timing diagram of what occurs in the oxygen conserving device  100  when an apnea condition is determined to exist. In the presently preferred embodiment, the apnea condition is set at about 30 seconds, although this can be modified. After the envelope time has expired at ELtm(d), the timing diagram indicates that no further inspiration events are detected throughout EVtm( 4 ). The microprocessor  106  then triggers an apnea condition alarm by sending the Apn line high at ELtm(e). The alarm may consist of an audio alarm, a visual alarm, or a combination of the two. 
     As mentioned previously, the oxygen conserving device  100  (FIG. 1) includes a flow fault alarm to inform the patient that the oxygen supply  110  has run out, or that the tubing between the oxygen supply  110  and the cannula has become pinched or disconnected. 
     FIG. 7 is an event timing diagram of what occurs in the oxygen conserving device  100  when a flow fault condition is detected. In the diagram, a flow fault has been detected by the microprocessor  106  between Ie( 1 ) and Ie( 1 )′ as indicated by F—F going high at ELtm(a). If oxygen pressure is not present to reset the S/D sensor output line once a period of time has elapsed to where the valve opens, the S/D sensor output line remains low during the entire inspiration effort. This condition constitutes a flow fault condition when there is insufficient oxygen supply pressure. The system is designed such that the sustain drive line will go low and release the valve to move to its normally closed position on any flow fault alarm detection. Once the oxygen pressure is restored, operation of the system will return to normal as indicated beginning at Ie( 2 ). 
     It should be apparent from the description of the oxygen conserving delivery system (OCDS) of FIG. 1 that the system can be mobile. FIG. 8 is provided as an external profile view of the OCDS in a mobile configuration. All that is externally visible on the device  170  are an inlet connection and tube  172  from the oxygen supply source, an outlet connection and tube  174  to the cannula, a flow fault and apnea condition bulb  176 , a low battery bulb  178 , a positive flow indicator bulb  180 , and a speaker outlet  182  for an audible alarm. Shown in outline is a battery compartment  184  which is visible on a backside of the device  170 . On a top side  186  of the device  170  are two buttons  188  and  190 . These buttons are for increasing (button  188 ) and for decreasing (button  190 ) the effective flow rate setting of the device  170 . 
     It is observed that what is accomplished by the presently preferred embodiment of the present invention is to provide an oxygen conserving device  100  which is capable of portable use. Alternatively, the OCDS of FIG. 1 can be modified, in an alternative embodiment, with a regulator and a bypass valve to create a single package. 
     This single combined system is shown in FIG.  9 . The most obvious difference as compared to FIG. 8 is that the inlet connections of the OCDS of FIG. 8 are moved so that the OCDS  170  of FIG. 9 is now coupled to an H.P. reducing regulator and bypass valve  172 . The OCDS  170  can be coupled to a variety of different types of regulators. This type of combined OCDS unit can be coupled to different types of oxygen cylinders. The H.P. reducing regulator low pressure outlet body parts swivel, enabling a gauge  174  on the regulator to be aligned as desired with respect to a cylinder (not shown). In addition, the OCDS  170  can be rotated and oriented as desired. 
     FIG. 10 shows that in another alternative embodiment of the present invention, it is desirable to be able to compensate for changes in the conditions under which the OCDS  100  (FIG. 1) must operate. Accordingly, another feature that can be added is a barometer  114  which can generate an input signal to the microprocessor  106  to enable the OCDS  100  to compensate for changes in altitude (and thus availability of oxygen). The barometer  114  is used to augment the flow of oxygen to the patient and thereby compensate for moderate barometric changes such as an excursion to a mountain resort or on an airplane. The function of the barometer should not be confused with the ability of the device to adapt to changes in respiration depth and rate. 
     The barometer  114  is used to augment the flow of oxygen to the patient in accordance with a selectable formula. In the presently preferred embodiment, the barometer  114  provides a signal to the microprocessor indicative of changes in altitude. The flow rate of oxygen is modified by a minimum increase of 1.0 liters per minute for every increase of 10,000 ft. in altitude. This change in flow rate can be augmented by as much as 2.0 liters per minute for each increase of 10,000 ft. in altitude. 
     It was previously explained that the present invention responds to the entire phase of the patient&#39;s inspiration efforts and the admission cycle of the oxygen by calculating a real-time respiration depth-time factor. The OCDS contains a table in firmware that describes a delivery schedule that is indexed in two dimensions. The data in the tale is derived from known requirements of human respiratory physiology. The first index (Y) is derived from the average inspiration rate. The second index (X) is derived from the average inspiration dwell time. Using the measured inspiration rate and dwell time of the system, the OCDS compensates for changes in oxygen delivery needs based on real-time measurements. 
     In an alternative embodiment for providing a real-time respiration depth-time factor, the barometer is used to measure the actual inspiration depth/force, in addition to altitude. This alternative embodiment requires a data table having a third index, for the negative pressure depth of the inspiration efforts. 
     FIG. 11 is a block diagram of this alternative embodiment. This embodiment utilizes an Absolute Pressure Transducer (APT) barometer device  200  where the output is coupled to two signal processing modules (or processor amplifiers). A first barometer module  202  scales a signal from the APT for altitude measuring. A second inspiration module  204  performs a track, hold and zoom function in order to measure small negative pressures produced from inspiration efforts. The physical input of the APT is also coupled to the oxygen tube that goes to the patient. 
     The APT enables the present invention to measure inspiration effort depths (the minute negative pressures) and positive pressures caused by the opening of the valve  206 . 
     The microprocessor selects between the processing modules  202  and  204  at times when making the measurements does not interfere with each other. For example, the first barometer module  202  takes measurements synchronously between inspiration efforts where the outlet of the OCDS is in a static or ambient state. This means that the valve  206  is closed and the person using the device is in between an inspiration and an expiration effort, or in between inspiration efforts. 
     FIG. 12 is provided as an event timing diagram which illustrates operation of the APT  200 . At event time (a), an inspiration event is beginning. Shortly thereafter (about 5 ms), the valve  206  opens at event time (b) at which time the ATP responds to the increase in pressure from the supply of oxygen from the valve  206 . During the time between events (b) and (c), the microprocessor will use the last measured absolute ambient pressure reading from the first barometer module  202  as a reference, and switch to read a signal from the second inspiration module  204  where it will hold this reference and magnify and measure small variations in absolute pressures that result from inspiration efforts. The magnification of the signals is accomplished by increasing gain by a factor, in this embodiment, of  250 . A profile and peak negative signal will be an analog to actual inspiration depth efforts. FIG. 12 shows the waveform  210  from inspiration efforts magnified about 250 times to show a “track and zoom” mode of operation. Event (a′) illustrates an inspiration effort that occurs outside of the envelope of time during which it is measured without the interruption of the valve  206  admitting oxygen. 
     At event (c), the microprocessor determines (from real-time measurements of inspiration rate and dwell times) the most stable (quiet) time at which to switch to and read the signal from the first barometer module  202  to thereby make altitude measurements. The first barometer module  202  preferably has a gain factor of only 1.5. The APT  200  has a transfer function that roughly equates to a voltage range between 0.5 to 4.75 volts with an absolute pressure range of 1.15 bar to 0.015 bar (15 mb). 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.