Apparatus for conservative supplemental oxygen therapy
BACKGROUND OF THE INVENTION 
     This invention relates to systems for supplying supplemental oxygen to a patient intermittently with the supply on for a predetermined number of ON respiratory cycles and the supply off for a predetermined number of OFF respiratory cycles. Oxygen saving is further enhanced by shutting the oxygen flow off during the exhalation phase of each ON respiratory cycle. 
     The incidence of chronic obstructive lung disease (C.O.L.D.) is increasing. These patients have inadequate pulmonary gas exchange resulting in inadequate oxygenation of blood passing through the lungs. Inadequate blood oxygenation and consequent body tissue hypoxia account for the disabling effects of the disease. It has been demonstrated that continuous administration of supplemental oxygen by nasal cannula overcomes the functional disability and greatly improves the quality of life as well as prolonging life. It is now common practice for ambulatory patients to carry an oxygen supply weighing nine pounds and lasting 3-4 hours. Prior art devices conserve oxygen by shutting off the gas flow during the exhalation phase of each respiratory cycle, which may double the effective supply. 
     BRIEF SUMMARY OF THE INVENTION 
     Restricted mobility, activity and costs of C.O.L.D. therapy may be effectively reduced by intermittent oxygen therapy. This intermittent oxygen therapy can replace continuous oxygen therapy because of the unusual physiological nature of C.O.L.D.. To conserve oxygen, a number of oxygen supply systems turn off the oxygen flow during exhalation and turn on the oxygen flow only during inhalation. It is an object of this invention to further extend and conserve the oxygen supply by turning the oxygen flow off for a predetermined number of OFF respiratory cycles, and then turning the flow on only during inspirations for a predetermined number of ON cycles. When physical exertion increases oxygen requirements, the respiration rate will also increase, thereby shortening the ON/OFF time intervals correspondingly. Alternatively performance may be further enhanced by programmatically changing cycle numbers, flow rates, and ON/OFF intervals in response to respiratory rate. 
     C.O.L.D. destroys lung tissue and impairs the lung&#39;s blood supply. As a consequence, the gas exchange behavior of the lungs is impaired, resulting in diminished oxygenation of the arterial blood. When such patients breathe air enriched with supplemental oxygen, the partial pressure of oxygen within the lungs is increased. This improves the transfer of oxygen from the lung to the blood and increases the oxygen content of the blood. 
     When oxygen administration is discontinued and the patient is returned to breathing room air, the arterial oxygen content remains elevated for a varying period. The oxygen content only very slowly returns to that value obtained while breathing room air. Depending upon the degree of pulmonary abnormalities, this may require 20-25 minutes. By contrast, patients with previously normal lungs requiring oxygen therapy (example: patients with heart failure), will promptly return to the value obtained while breathing room air (3-5 minutes). 
     Since many C.O.L.D. patients maintain an adequate level of arterial blood oxygenation for a prolonged period following cessation of oxygen administration, advantage is taken of this response by discontinuing oxygen therapy while the oxygen level still remains at an appropriate value. Discontinuation during this time conserves oxygen. When the arterial oxygen saturation has decayed to a predetermined lower value, oxygen administration is restarted, and continues until appropriate oxygenation of the blood is reestablished. At this time administration is again discontinued. These cycles repeat on a continuous basis administering O 2  intermittently to maintain adequate arterial oxygenation. 
     C.O.L.D. patients have many abnormally enlarged spaces within their lungs that are slowly ventilated and have a very poor blood supply. These areas act as &#34;internal reservoirs&#34; which store air enriched with oxygen when such is being breathed. When regular air breathing is resumed, these areas transfer the oxygen enriched air to adjacent areas of the lungs with a better blood supply and act as a source of continued oxygenation of the blood for some time after supplemental oxygen has been discontinued. 
     Recently developed sophisticated instrumentation (Hewett-Packard Ear Oximeter, Model 47201A) allows for the continuous determination of arterial oxygen saturation--an index of oxygen content. This is a painless, non-invasive method. Determination of the time to obtain appropriate oxygenation at a selected flow of oxygen (usually 2L/Min) establishes the &#34;on&#34; time of oxygen administration. After this value is determined, a recording is made of the rate of decline in the oxygen saturation after oxygen has been discontinued. This defines the &#34;off&#34; time. 
     The respiratory rate in cycles per minute is determined. The &#34;on&#34; time and the &#34;off&#34; time in minutes are multiplied by respiratory rate in cycles per minute to get ON cycle number and OFF cycle number. These are the number of respiratory cycles that the gas flow can be ON and OFF in sequence while maintaining the arterial blood oxygen concentration within the two desired levels at the selected flow rate. These three values, ON number, OFF number and flow rate are entered into the oxygen delivery system of the invention. The unit is connected between the oxygen supply source and the patient. Oxygen is generally conveyed from the apparatus to the patient by plastic tubing terminating in the nasal prongs. Thusly, the system continuously and automatically delivers oxygen intermittently to the subject as required. By shutting off gas flow completely during the OFF cycles the system conserves at least 50% of the oxygen when compared to prior art continuous systems. And by shutting off during the expiratory phase of the ON cycles, the system further conserves at least 50% of the remaining supply so that the overall savings of gas may be more than 75% when compared to prior art continuous systems. 
    
    
     It is an object of the invention to provide a method for intermittent oxygen therapy for a C.O.L.D. patient which is supplemental to the patient&#39;s normal room air intake. This is generally delivered via a loosely fitting nasal cannula without altering the pressure relationships within the respiratory tract. When the patient inhales while gas is supplied to the cannula, the volume of gas inhaled is not changed, only the concentration of oxygen is increased. This is in contrast to respirators and ventilators which form a closed circuit with the respiratory tract and provide alternate pressure and suction to move all gas into and out of the lungs at different phases of each respiratory cycle. It is an object of this invention to provide a system which periodically administers supplemental oxygen during an ON interval measured over a predetermined number of respiratory cycles followed by an OFF period extending over a predetermined number of respiratory cycles, wherein the ON and OFF cycle numbers are independently adjustable and are predetermined from prior measurements of the patient&#39;s requirements for such therapy. These and other features, objects and advantages will become more apparant from the following description and drawing of preferred embodiments of the invention wherein: 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a dispensing system for the intermittent administration of oxygen through a cannula to a patient which features the oxygen conserving functions of the invention for shutting off the flow of oxygen during those periods when it is not useful. 
     FIG. 2 is a schematic view of another embodiment of the invention including a more versatile responsiveness to increased respiratory rate. 
    
    
     The specific apparatus illustrated utilizes the operative principles or features of the invention set forth and defined in the appended claims. It is considered reasonably self-evident that these principles or features can be utilized within a wide variety of differently constructed and/or differently appearing oxygen dispensing apparatuses through the use of exercise of routine skill in the art without departing from the essential characteristics of the invention. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, the patient 1 receives a flow of oxygen through a nasal cannula 2 connected to a portable or stationary supply of oxygen 7 which may be a compressed gas cylinder, a liquified gas container, an oxygen concentrator, a piped-in wall outlet or the like. The supply will generally be controlled by a pressure regulator 6, a flow controller 5, a flow meter 4 and an on/off valve 3. Exhale sensor 9 is connected to the patient by sensor connection 8. As illustrated, this may be a gas tube either directly connected to the nasal cannula or to the tubing connected to the nasal cannula. The pressure in sensor connection 8 will change with the respiratory cycle, being lower during the inhale phase and higher during the exhale phase. Exhale sensor 9 converts the gas pressure into an electrical signal, sending out a positive electrical signal to a first command element 18 and also to pulse shaper 10 whenever the gas pressure in connection 8 is not high, i.e. a positive signal output occurs whenever subject is not exhaling. Pulse shaper 10 puts out a single output pulse every time it receives a positive going output signal from exhale sensor 9. The pulse from pulse shaper 10 passes through two-position switch 11 to either ON counter 12 or OFF counter 15. The position of switch 11 is controlled by bistable element 16 through switch control line 20. In a first condition of bistable element 16, it sends a positive output through control line 20 to maintain switch 11 in a first position, wherein pulses from pulse shaper 10 are fed to ON counter 12 which adds up the incoming pulses. At the same time, the positive output of bistable element 16 is also sent to the second ON command element 17. Logic AND element 19 responds to positive signals on both command inputs 17 and 18 by turning on the on/off valve 3. Command element 17 will be sending out a positive signal continuously throughout all the ON respiratory cycles and command element 18 will only send out a signal when the patient is not exhaling. This results in the ON/OFF valve 3 being off during exhale and on during inhale while the ON counter 12 is counting. Preset element 13 has been set to the desired number of ON cycles as determined by prior blood gas measurements. Preset element 13 controls ON counter 12, stopping the count when the preset number of cycles has been reached, and signalling through first input 26 to bistable element 16 to switch it from its first condition to its second condition. In the second condition of bistable element 16 there is a negative signal to ON command 17 and to switch 11 causing switch 11 to assume a second position in which pulses from pulse shaper 10 are now fed to OFF counter 15. And ON/OFF valve 3 remains OFF. Preset element 14 has been set to the desired number of OFF cycles as determined by prior blood gas measurements. Preset element 14 controls OFF counter 15, stopping the count when the preset number of cycles has been reached, and signalling through second input 27 to bistable element 16 to switch it from its second condition to its first condition. The ON counting cycle now begins again and the cycles continuously repeat according to the preset program. 
     The ON/OFF requirements are generally determined by blood gas measurements made when the patient is at rest. When the patient is active, his oxygen requirements will increase. The rate of respiration will rise and the rate of rise and fall of blood oxygen with and without supplemental oxygen can also be expected to increase. Since the ON/OFF periods are based on number of respiratory cycles and not time, the increased respiratory rate will automatically cause the ON/OFF periods to be shorter, which should properly accomodate to the new requirements. 
     The embodiment of FIG. 2 is similar to the first described embodiment with the addition of elements providing a more adaptive response to increased activity to better provide the increased oxygen thereby required. During the preliminary blood gas measurement process, in addition to determining respiratory rate, gas flow rate and ON/OFF cycle numbers at rest, the same information is acquired at increasing levels of physical activity. These data are stored in data memory 28. Ratemeter 21 receives time information from clock 22 and a pulse from pulse shaper 10 for each respiratory cycle. Ratemeter 21 converts these data into respiratory rate information which is transmitted to microcomputer 23. Microcomputer 23 under guidance of programs stored in program memory 24 finds in data memory 28 the particular settings for ON cycle number and OFF cycle number and gas flow rate corresponding to that particular respiratory rate. This may optionally include interpolation processes. Output drivers 25 use this information output from microcomputer 23 to put a new ON cycle number and OFF cycle number in preset elements 13 and 14 respectively and to readjust flow regulator 5 to provide the optimum gas flow rate. 
     Many of the individual functional elements illustrated may be incorporated into a microcomputer. Alternatively, a single common counter may be used with alternative ON preset and OFF preset function. Any of a variety of mechanical, pneumatic, fluidic or electronic sensor elements well known in the prior art may be employed to sense one or more phases of the respiratory cycle including a sensing belt around thorax or abdomen. The individual functional elements illustrated may be embodied in fluidic, mechanical or electrical form or in combinations thereof well known in the prior art without departing from the scope of the invention. 
     The above disclosed invention has a number of particular features which should preferably be employed in combination, although each is useful separately without departure from the scope of the invention. Inasmuch as the invention is subject to many variations, modifications, and changes in detail, it is intended that all matter described above be interpreted as illustrative and not in a limiting sense.