Title:
PULMONARY INFORMATION TRANSMISSION SYSTEM
United States Patent 3726270


Abstract:
A system by which signals pertaining to the pulmonary condition of a patient are transmitted to a location which is remote from the location of the patient. The signals are transmitted as the patient is being tested. pulmonary information in the form of spirometry data and total lung capacity volume are transmitted by means of telephone lines or the like to a location remote from the patient, and at the remote location the transmitted information may be studied by a person who is well acquainted in the art of analysis of such information. Patients at various locations provide information to a central location at which studies of the information are conducted. Thus, a specialist in the art of such studies is in a position to receive information from various patients who may be located at numerous widespread places and who may be many miles from the specialist.



Inventors:
Griffis, Roy A. (Dayton, OH)
Rauterkus, Thomas L. (Dayton, OH)
Torzala, Terence (Dayton, OH)
Application Number:
05/182053
Publication Date:
04/10/1973
Filing Date:
09/20/1971
Assignee:
SYST RES LABOR INC,US
TRAN MED SYST INC,US
Primary Class:
Other Classes:
73/23.3, 128/904, 422/84
International Classes:
A61B5/087; (IPC1-7): A61B5/08
Field of Search:
128/2
View Patent Images:



Other References:

Med. & Biol. Engineering, Vol. 9, pp. 247-254, Pergamon Press, 1971. .
Journ. of Assoc. for Advancement of Med. Instrumentation, Vol. 5, No. 4 July-Aug., 1971, pp. 220-223..
Primary Examiner:
Howell, Kyle L.
Claims:
The invention having thus been described, the following is claimed

1. A pulmonary testing system for providing an electrical signal indicating the peak percent nitrogen expired by a person in a breath and the total nitrogen volume expired by said person during repetitious breathing comprising:

2. The invention according to claim 1 wherein said integrating means includes multiplying means for multiplying said first and second electrical signals prior to integrating the product thereof.

3. The invention according to claim 1 wherein said pulse generator means includes memory means for storing a value equal to the peak percent of nitrogen during each exhalation and delay means for providing said pulse generator means signal during the next inhalation.

4. The invention according to claim 3 wherein said next exhalation is detected by means determining that said second signal is below a certain value.

5. The invention according to claim 3 wherein said second signal is an analog signal having an instantaneous magnitude related to the instantaneous percent of nitrogen during each exhalation and wherein said memory means includes a capacitor which is charged to a voltage related to the maximum magnitude of said second signal.

6. The invention according to claim 5 wherein said pulse generating means include means for discharging said capacitor by determining that the instantaneous magnitude of said second signal is below a certain value.

7. The invention according to claim 1 wherein said system further includes minimum flow detector means for determining when said first signal is below a predetermined value and minimum percent nitrogen detector means for determining when said second signal is below a predetermined value, said integrating means being inhibited from providing a signal whenever said first or said second signals are less than said predetermined values therefor, and said pulse generating means being inhibited from providing a signal whenever said second signal is below said predetermined value therefor.

8. The invention according to claim 7 wherein said integrating means includes multiplying means for multiplying said first and second signals prior to integrating the product thereof.

9. The invention according to claim 8 wherein said integrating means further includes inhibiting means for inhibiting said integrating means from integrating the product of said first and second signals whenever either of said first or second signals is less than said predetermined value therefor.

10. A pulmonary testing system for providing an electrical signal capable of being transmitted over a single pair of telephone lines to a recording device for causing, in response to said transmitted signal, a graphic representation of the peak percent of nitrogen expired by a person during each one of a plurality of successive breaths, and the total volume of nitrogen expired by said person during said plurality of breaths, said person, during said plurality of breaths, inhaling a gaseous mixture containing negligible nitrogen and exhaling a gaseous mixture containing a decreasing percent of nitrogen, said system comprising:

11. The invention according to claim 10 wherein said output means further includes transmitting means which, in response to said added signals, provides a modulated signal as said system electrical signal.

12. The invention according to claim 10 wherein said system further includes minimum detector means for determining whether said first signal is above a first minimum magnitude and whether said second signal is above a second minimum magnitude, said minimum detector means including means causing said multiplying means to be inhibited from providing a signal whenever either of said first or second signals has a magnitude below said respective first and second minimum magnitudes.

13. The invention according to claim 12 wherein said minimum detector means includes means for inhibiting said pulse generating means from providing a signal whenever said second signal has a magnitude below said second minimum magnitude.

14. The invention according to claim 10 wherein said system further includes means for providing a second system electrical signal which represents the spirometry data of a single breath of said person.

15. The invention according to claim 14 wherein said system further includes switching means to select between said first and second system electrical signals,

16. The invention according to claim 10 wherein said pulse generating means includes a storage capacitor for storing electrical energy having a voltage proportional to the highest magnitude of said second signal during each breath, and logic means responsive to said second signal being below a certain magnitude to cause said pulse to be provided during said immediately following inhalation.

17. The invention according to claim 16 wherein said pulse generating means further includes switching means responsive to said logic means for causing said capacitor to discharge after said pulse is provided and before the immediately following exhalation.

18. Circuitry for transmission of pulmonary information from a test location to a remote location by means of a telephone circuit comprising:

19. The system of claim 10 in which the analyzer means comprises: a high output impedance circuit which includes

20. The invention according to claim 19 wherein each of said first and second unidirectional current conducting means is the base-emitter junction of a transistor.

21. The invention according to claim 19 wherein said second operational amplifying means further has an inverting input coupled to the output of said second operational amplifying means.

22. The invention according to claim 19 wherein the output of said circuit is the junction of said second operational amplifying means and said second unidirectional current conducting means.

Description:
BACKGROUND OF THE INVENTION

This invention relates to pulmonary testing and more particularly to a pulmonary testing system for transmitting pulmonary data to a central location.

In the past, tests related to the pulmonary condition of a patient have been observed by a doctor or other specialist at the location of the patient. The system of this invention provides means by which a doctor or other specialist can receive by telephone lines pulmonary test data from various locations, any of which may be remote. For example, patients tested by the specialist may be in cities remote from the location of the doctor or other specialist.

So far as is known, systems now in use which transmit more than one type of pulmonary test data employ more than one pair of telephone lines.

SUMMARY OF THE INVENTION

In accordance with one preferred embodiment of the invention, there is provided a pulmonary testing system for providing an electrical signal indicating the peak percent of nitrogen expired by a person in a breath and the total nitrogen mass or volume expired by the person during repetitious breathing comprising means responsive to the person's repetitious breathing for providing a first electrical signal proportional to the gas flow in each exhaled breath, analyzer means responsive to the repetitious exhaling of the person for providing a second electrical signal indicating the percent of nitrogen during each exhalation, integrating means responsive to the first and second electrical signals for providing a third electrical signal indicating the total amount of nitrogen exhaled by the person, pulse generator means responsive to the second electrical signal for providing a series of pulse signals, each pulse signal indicating the peak percent of nitrogen during a single exhalation, and means for providing a system signal by superimposing the series of pulse signals upon the third electrical signal such that the series of pulse signals occur during the time an inhalation occurs.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram showing a system of this invention for transmission of pulmonary information.

FIG. 2 is a portion of a chart showing the results of a typical spirometry test of a patient tested by the circuitry of this invention.

FIG. 3 is a portion of a chart showing peak nitrogen and functional residual capacity data of a patient tested.

FIGS. 4 - 11 are schematic wiring diagrams of portions of the electrical circuitry of this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides means by which a single pair of telephone lines is employed to transmit spirometry, the percentage of nitrogen present in each expired breath of a patient, and to transmit a signal which is proportional to nitrogen volume or mass, for the determination of functional residual lung capacity (FRC) of the patient.

As shown in FIG. 1, a test unit 8 includes a flowmeter 10, preferably a mass flowmeter, which is shown as having connected thereto a fluid conduit member 12 which is adapted to cover the nose and/or mouth of a patient, for communication with the patient's respiratory system.

The flowmeter 10 provides an electrical output signal which is proportional to a derivative of the volume or mass of the expired breath. A suitable flowmeter, for example, is a device sold by Datametrics Corporation and referred to as In-line Flow Transducer, Series 1000. A switch arm 18 is connected to the input of a transmitter 22 and when connected to a contact point 19, the flowmeter 10 is electrically connected to the transmitter 22. Preferably, the transmitter 22 is a frequency modulation transmitter, preferably having an output frequency in the general range of 1300 hertz. However, a transmitter of another frequency may also be satisfactory. The connection of the switching arm 18 to a contact 15 joins a precision oscillator 17 to the transmitter 22. The transmitter 22 is joined to a start-stop switch 24, which is connected to a telephone circuit 30. The telephone circuit 30 may be a full duplex system and includes a pair of wires or lines. The precision oscillator 17 is used to test the telephone circuit 30. Also joined to the telephone circuit 30 is a signal filter 32 to which is connected an alarm 34.

The telephone circuit 30 extends to a central receiver unit 40 within which is a demodulator 42 connected to the telephone circuit 30. The central receiver unit 40 may be very remote from the test unit 8.

Also within the central unit 40 and joined to the telephone circuit 30 is a signal switch 44. Also within the central receiver unit 40 is an integrator and special purpose computer 45, and a chart recorder instrument 48 which is connected to the integrator and special purpose computer 45. The chart recorder 48 is also connected to the demodulator 42.

Also within the test unit 8 and joined to the fluid conduit 12 is a fluid conduit 60, which, through a valve 62, is connected to a source of oxygen 64. Also connected to the fluid conduit 12 is a fluid conduit 68 which through a valve 70, is connected to a nitrogen percent analyzer 72. A suitable analyzer may for example, be such a device sold by Med-Science Company and referred to as Model 505 Nitralyzer.

The output of the flowmeter 10 is joined by a conductor 75 to one input of an analog multiplier 76, shown schematically in FIG. 4, and to the input of a minimum flow detector 78, shown schematically in FIG. 5. The nitrogen percent analyzer 72 is connected by a conductor 77 to a second input of the multiplier 76, by a conductor 79 to the signal input of a peak percent nitrogen follower 80, shown schematically in FIG. 6, and by a conductor 81 to the input of a minimum percent nitrogen detector 82, shown schematically in FIG. 7. The multiplier 76 has an output connected by a conductor 83 to a multiplier output gate 84, shown schematically in FIG. 8. The multiplier 76 may be any suitable instrument which is capable of multiplication of two analog quantities. The multiplier 76 employed herein is an instrument produced by Hybrid Systems and identified as "107C Transconductance Multiplier" and is shown in FIG. 4 as having a gain adjustment 89 and a balance adjustment 91 connected thereto. The multiplier 76 also has an X terminal, a Y terminal, and a Z terminal.

The peak percent nitrogen follower 80 has an output connected by a conductor 85 to a percent nitrogen sample pulse generator 88, shown schematically in FIG. 9. The percent nitrogen sample pulse generator 88 includes time delay circuitry. The minimum flow detector 78 is connected by a conductor 73 to a first inhibit input of the multiplier output gate 84. The minimum percent nitrogen detector 82 is connected by a conductor 71 to a second inhibit input of the multiplier output gate 84 and to an inhibit input of the percent nitrogen sample pulse generator 88. The percent nitrogen sample pulse generator 88 has output conductors 67 and 69 connected to clear and read inputs of the peak percent nitrogen follower 80.

An integrator 94 shown schematically in FIG. 11, is connected by a conductor 95 to one input of a summer device 92 and receives a signal through a conductor 97 from the multiplier output gate 84. A pair of conductors 103 connects the switch 24 to control inputs of the integrator 94. The summer device 92 is connected by a conductor 99 to a switch contact 96 which is joined through switching arm 18 to the transmitter 22. A conductor 101 connects the percent nitrogen sample pulse generator 88 to a second input of the summer device 92.

A breath switch 100 is connected to the input to the transmitter 22 and to a breath light 102. The breath switch 100 is any suitable switch which, when receiving a signal directly or indirectly from the flowmeter 10, causes the breath light 102 to become energized.

OPERATION

The circuitry of this invention includes means for transmission of spirometry test data as the patient exhales into the fluid conduit 12.

Prior to testing a patient, the switching arm 18 is placed in contact with the switch contact 15, and the precision oscillator 17 applies a test signal to the transmitter 22, which in response thereto applies a frequency modulated signal through the switch 24 to the flowmeter telephone circuit 30, and to the demodulator 42 of the central receiver unit 40 for calibration of these elements.

During the spirometry test, the valves 62 and 70 are closed so that all of the expired breath from the patient flows into the flowmeter 10, and the switching arm 18 is placed in contact with the switch contact 19.

The flowmeter 10 provides an electrical signal which is transmitted through the switch contact 19 and the switching arm 18 to the transmitter 22. The breath switch 100 senses that an expired breath of the patient has caused a signal to be applied to the transmitter 22, and the breath switch 100 causes the breath light 102 to become energized to indicate to the operator that the patient's expiration is causing a signal to be applied to the transmitter 22. The start stop switch 24, when closed, for example by manual operation, permits a signal from the transmitter 22 to flow through the telephone circuit 30 to the demodulator 42 in the central receiver unit 40. The analog signal which is received from the demodulator 42 by the integrator and special purpose computer 45 is integrated with respect to time and the output of the integrator and special purpose computer 45 flows to the chart recorder 48. A signal from the demodulator 42 simultaneously flows directly to the chart recorder 48.

The upper portion of FIG. 2 is a portion of a chart illustrating forced expiratory flow of the patient, and the lower portion of FIG. 2 is a portion of a chart illustrating the integration of the flow to provide volume of the patient's forced exhalation.

When an operator at the central receiver unit 40 wishes to contact the operator at the test unit 8, for example, to indicate that the test is completed, the operator uses the signal switch 44 to apply a signal of a different frequency to the telephone circuit 30, and this signal is detected by the signal back filter 32 which activates the signal back alarm 34. The operators at both units can then connect telephones to the telephone circuit 30 for telephone communication between the test unit location of the patient and the central receiver unit 40.

The circuitry of this invention is also employed to transmit to the central receiver unit 40 information regarding the percent nitrogen in each expired breath of a patient and nitrogen volume, to determine total lung volume and functional residual capacity of the patient. These tests are referred to as "nitrogen washout" tests. These tests are based upon the fact that air normally contains approximately eighty percent nitrogen, and it is assumed that the patient's lungs normally contain air having this percentage of nitrogen.

For these tests, the valves 62 and 70 are opened and the switching arm 18 is placed in contact with switch contact 96. The patient and expires and expires through the fluid conduit member 12. The valve 62 permits flow of oxygen from the source 64 only when the patient is inhaling. Thus, the patient inhales pure oxygen from the source of oxygen 64. The patient exhales into the fluid conduit 12 and the exhaled breath is sensed by the flowmeter 10. A very small portion of the exhaled breath flows through the fluid conduit 68 to the nitrogen percent analyzer 72. An electrical output signal having an instantaneous magnitude proportional to expiratory flow travels from the flowmeter 10 through the conductor 75 to the multiplier 76, and an electrical signal having a magnitude, proportional to the percentage of nitrogen in the expired breath travels through the conductor 77 from the nitrogen percent analyzer 72 to the multiplier 76 and through the the conductor 79 to the peak percent nitrogen follower 80.

During this test, each inspiration by the patient contains pure oxygen and each expiration contains a percentage of nitrogen. As inspiration and expiration continue, the percentage of nitrogen in each expired breath should decrease.

The multiplier 76 thus receives a signal proportional to the expiratory flow and multiplies therewith a signal proportional to the percent of nitrogen in the expired breath, and a resultant signal output of the multiplier 76 (nitrogen flow) travels to the multiplier output gate 84.

The minimum flow detector 78, which is also joined to the multiplier output gate 84, provides a signal to the multiplier output gate 84 only when a signal above minimum flow is received from the flowmeter 10. The minimum percent nitrogen detector 82 provides a signal to the multiplier output gate 84 only when the percentage of nitrogen in an expired breath is above a given minimum value. The multiplier output gate 84 is disabled and thus does not transmit a signal from the multiplier 76 to the integrator 94 unless there is a signal received from the minimum flow detector 78 that an expired breath is occurring and a signal from the minimum percent nitrogen detector 82 that the percentage of nitrogen is above a given minimum.

The integrator 94 thus provides a signal proportional to nitrogen volume in the expired breath. The output signal of the integrator 94 is transmitted through the summer device 92 to the transmitter 22 and then transmitted over the telephone circuit 30 to the central receiver unit 40. The integration appears on the chart recorder 48 in a manner similar to that represented by a reference numeral 105 in FIG. 3.

The peak percent nitrogen follower 80 detects the peak percentage of nitrogen in each expired breath. This peak signal is transmitted to the percent nitrogen pulse generator 88, which includes time delay circuitry. After the percent nitrogen, as detected by the percent nitrogen detector 82, falls below a predetermined minimum during the next inspiration of pure oxygen, and after a given time delay, for example, one hundred and fifty milliseconds or the like, the time delay circuitry transmits this signal to the summer device 92, and the peak signal travels through the summer device 92 to the transmitter 22 and travels over the telephone circuit 30 to the central receiver unit 40. The peak signals are superimposed upon the integration signals at the chart recorder 48 in the manner illustrated by a reference numeral 107 in FIG. 3.

Thus, it is understood that the peak signals 107 are transmitted to the central receiver unit 40 during the time that the patient is inhaling and the integration of the exhalation is transmitted to the central receiver unit 40 during the exhalation period. Thus, the signals corresponding to peak percentage of nitrogen and signals pertaining to functional residual capacity of the patient are transmitted to the central receiver unit 40 over a single pair of telephone lines, and the information received at the central receiver unit 40 appears on the chart in a manner such as that illustrated in FIG. 3.

Details of portions of the system will now be discussed.

A first sample exhalation is actually taken before the first inhalation of pure oxygen, and should indicate that 80 percent nitrogen was present in the air of the conduit 12 immediately before the beginning of the test. When the first inhalation of pure oxygen is made, a sample nitrogen pulse indicating 80 percent nitrogen should be generated. This may serve as a check on the adjustment of the nitrogen analyzer 72 and may serve as a check to indicate if oxygen was present in the breathing tube before the test began.

Conductors 103 extending from the start-stop switch 24 to the integrator 94 are momentarily shorted to discharge or reset a capacitor 221 of the integrator 94, shown in FIG. 11.

As stated above, the multiplier output gate 84 is disabled when the minimum flow detector 78 detects a flow signal below a minimum value or when the minimum percent nitrogen detector 82 detects a nitrogen level less than a minimum value. The reason for this is that the multiplier 76, shown in FIG. 4, is not ideal. With a voltage applied to one of the X or Y terminals and zero volts applied to the other of the X or Y terminals, the output of the multiplier 76 is not zero but is a positive voltage slightly above zero. In addition, the zero adjustment of the output as set by the balance potentiometer 91 has a slight drift. To limit the error due to this non-ideal behavior, the multiplier output gate 84 is employed, into which the output signal from the multiplier 76 is connected. The multiplier output gate 84, shown in FIG. 8, includes a coil 194 which operates a normally closed relay contact 196, through which the multiplier output signal is applied to the integrator 94. When either the flow or percent nitrogen of the patient's expired breath is below a predetermined value, the coil 194 is energized and the relay contact 196 is open. This occurs between expired breaths and after the nitrogen is completely "washed out" of the patient's lungs.

The multiplier output gate 84 includes a transistor 192 having a grounded emitter, and a collector which is connected through a coil 194 to a source of positive voltage V. The conductors 71 and 73 are coupled together through respective forward biased diodes 188 and 190 to the base of transistor 192. Whenever a positive voltage appears on either of the conductors 71 or 73, resulting from detection of the respective minimum percent nitrogen or minimum flow, the transistor 192 becomes conductive. This allows current to flow through the coil 194, opening relay contact 196. A diode 198 is coupled in parallel with the coil 194 and allows the coil 194 to discharge after the transistor 192 again becomes non-conductive.

The minimum flow detector 78, as shown in FIG. 5, includes an operational amplifier 200 operated in the inverting mode with the base-emitter junction of a transistor 202 in the feedback loop thereof. The inverting input to the operational amplifier 200 is coupled to the conductor 75 through a resistor 204, and to a source of negative voltage -V, through resistors 205 and 206. The junction of resistors 205 and 206 is coupled to ground through a resistor 207. The resistors 204, 205, 206 and 207 are selected so that whenever the voltage on the conductor 75 is below a minimum voltage, a negative voltage is applied to the inverting input of the operational amplifier 200, and otherwise a positive voltage is applied thereto.

A positive voltage reverse-biases the emitter-base transistor junction 202, causing the feedback impedance to become extremely high, and the operational amplifier 200 attempts to saturate positively. However, the emitter-base transistor junction 202 breaks down at a voltage before saturation occurs. The negative current from a negative voltage is cancelled when the positive voltage applied on the conductor 75 reaches a sufficiently high value. When the total input voltage increases to a value greater than the threshold, the output of the operational amplifier 200 becomes negative as a result of the net positive current at the negative input. This negative output forward-biases the emitter-base transistor junction 202, and the output of the operational amplifier 200 becomes slightly negative. Thus, as long as the voltage on the conductor 75 is above a minimum value, the output voltage of the minimum flow detector 78 which appears on the conductor 73 is near zero volts, and when the voltage appearing on the conductor 75 falls below the minimum voltage, the output voltage on the conductor 73 becomes substantially positive.

The minimum percent nitrogen detector 82, illustrated in FIG. 7 functions in substantially the same manner as the minimum flow detector 78. The nitrogen threshold is a small percentage of full nitrogen volume. The minimum percent nitrogen detector 82 includes an operational amplifier 208, having a base-emitter transistor junction 209 in the feedback loop thereof. The operational amplifier 208 has a filter circuit 210 in the output thereof, which makes possible smooth output voltage shifts as changes in nitrogen levels occur above and below the trigger threshold of the detector circuit.

The output of the multiplier 76 is connected through the conductor 83, the multiplier output gate 84, and the conductor 97 to the input of the integrator 94. The integrator 94, as shown in FIG. 11, includes an integrating operational amplifier 220 having a capacitor 221 in the feedback loop thereof. The integrator 94 also includes a unity gain inverter 224 which corrects for polarity change which occurs within the operational amplifier 220. The gain of the integrator 94 may, for example, be such that at the output conductor 95 thereof 1 volt equals 1 liter. The input signal on line 97 to the integrator 94 is a voltage proportional to nitrogen flow in liters per minute, and the output of the integrator 94 is nitrogen volume in liters. The gain of the integrator 94 is adjusted by adjusting a potentiometer 228 at the input of the operational amplifier 220.

The output conductor 95 of the integrator 94 is connected to the summer device 92, shown in FIG. 10, which inverts the output signal of the integrator 94 and superimposes upon the output signal of the integrator 94 the signal received through the conductor 101 from the percent nitrogen pulse generator 88. The output of the summer device 92 then travels through the conductor 99, the switch contact 96, and switching arm 18 to the transmitter 22, as stated above.

The peak percent of nitrogen in each expired breath is determined by the peak percent nitrogen follower 80, shown in FIG. 6. This includes an operational amplifier 230, having the base-emitter junction of a transistor 240 and an operational amplifier 232 in the feedback loop thereof. The peak percent nitrogen follower 80 also includes an operational amplifier 234. The output voltage of the operational amplifier 230 is applied through the transistor 240 and through a closed sample relay contact 242 to a storage capacitor 236. When the input voltage appearing on the conductor 79 of the operational amplifier 230 begins to decrease, the output voltage thereof does not decrease because the base-emitter junction of the transistor 240 in the negative feedback circuit of the operational amplifier 230 is reverse-biased by the value of the previous higher voltage stored in the storage capacitor 236. In order for the storage capacitor 236 to maintain the charge, the impedance looking back into the operational amplifier 230 must be extremely high. The operational amplifier 232, having a very high input impedance, is employed for this purpose and, as shown and discussed above, is in the feedback loop of the operational amplifier 230. Use of the transistor 240 as a diode serves to reduce to a negligible value any leakage in this portion of the circuitry.

The operational amplifier 232 serves as a voltage follower buffer with an input impedance on the order of 1011 ohms in the negative feedback loop of the operational amplifier 230.

The negative feedback loop is the output loop, because the input to the operational amplifier 230, which is an inverter, is a positive voltage, for example, 10 volts full scale with 100 percent nitrogen. In addition, the base-emitter junction of a transistor 238 and the transistor 240 are used as diodes, with the base being coupled to the collector. Since the base-emitter transistor junctions begin leakage at a voltage somewhat below actual breakdown, the gain of the operational amplifier 230 is reduced to about one-fifth in order to keep this leakage value negligible. This attenuation is compensated for by a gain created in an output buffer stage which includes the operational amplifier 234, operating in a known manner, to provide the peak percent nitrogen signal to the conductor 85.

When the nitrogen level value exceeds the threshold of the minimum percent nitrogen detector 82 during an exhalation, the logic circuitry in the percent nitrogen sample pulse generator 88 causes a positive voltage to appear on the conductor 69, thereby rendering a transistor 262 conductive and energizing a relay coil 241, shown in FIG. 6, to cause the normally open relay contact 242, which is located between the operational amplifier 230 and the storage capacitor 236, to close. This permits the capacitor 236 to charge up to a peak voltage.

When the nitrogen level goes below the minimum threshold during inhalation of pure oxygen by the patient, the voltage ceases to be applied on the conductor 69, the transistor 262 becomes non-conductive and the coil 241 becomes deenergized and permits the relay contact 242 to open, and a voltage proportional to the peak nitrogen value is stored by the storage capacitor 236.

The storage capacitor 236 is connected to the input of the operational amplifier 234 which has a high impedance input to minimize capacitor discharge.

After the voltage manifesting the nitrogen level at the input conductor 81 falls below the threshold of the minimum percent nitrogen detector 82, there is a slight delay for a reason discussed below, and then the percent nitrogen sample pulse is generated on conductor 101 by percent nitrogen pulse generator 88. Immediately after this, a normally-closed relay contact 252, in the peak percent nitrogen follower 80, which had been open to allow peak nitrogen sampling and storage, is closed by the logic in the percent nitrogen sample pulse generator 88, causing zero voltage to appear on the conductor 67, thereby rendering a transistor 260 non-conductive and deenergizing a coil 250 for closure of the contact 252. The contact 252 remains in the closed position for about 75 milliseconds to allow discharge of capacitor 236. The capacitor 236 is then in a condition to be recharged to a new peak value during the next exhalation.

As previously stated, the percent nitrogen pulse generator 88 functions in close conjunction with the peak percent nitrogen follower 80. The primary control input signal to the pulse generator 88 is the minimum percent nitrogen detector 82 signal appearing on the conductor 71. When the output of the minimum percent nitrogen detector 82 is a low voltage, indicating that the nitrogen level is above the minimum threshold, a NAND gate 258 within a NAND gate unit 259 of the percent nitrogen pulse generator 88, shown in FIG. 9, provides a positive voltage to the conductor 69 to energize the coil 241 of the peak percent nitrogen follower 80 shown in FIG. 6, and closes the contact 242, thereby connecting the output of the operational amplifier 230 to the storage capacitor 236. During this time, the contact 252 is maintained in an open condition, because the coil 250, associated therewith, is energized through the operation of another NAND gate 261 in the NAND gate unit 259.

In order for the contact 252 to be held open, a series of monostable multivibrators 270, 272, and 274 of the percent nitrogen sample generator 88 shown in FIG. 9, are in the zero state, and a function switch 280 is in the FRC (Functional Residual Capacity) position. As the patient exhales the nitrogen from his lungs, a voltage proportional to the peak nitrogen is stored by capacitor 236 in the peak percent nitrogen follower 80. When the patient stops exhaling and again begins inhalation of pure oxygen, the minimum percent nitrogen detector 82 provides a positive voltage on the conductor 71. However, because the output voltage of the filter 210 in FIG. 7 rises slowly, the output voltage provided by a buffer circuit 288 rises slowly. This permits the NAND gate 258 to provide a near zero voltage to the conductor 69, thus deenergizing the relay coil 241 and permitting the contact 242 to open. The zero relay contact 252, however, is still open, because the monostable multivibrators 272 and 274 are still in the low state. However, the leading edge of the output pulse on the conductor 71 from the minimum percent nitrogen detector 82 triggers the monostable multivibrator 270 through the buffer 288. The time constant of this monostable multivibrator 270 is selected so that the output thereof provides a positive voltage for about 150 milliseconds. In addition, the monostable multivibrator 270 is not actually triggered until about 350 milliseconds after the output signal on the conductor 71 from the minimum percent nitrogen detector 82 becomes positive. This is because of the slow rise of the output voltage of the minimum percent nitrogen detector 82 and the buffer 288. The trailing edge of the output pulse of the monostable multivibrator 270 triggers the monostable multivibrator 272. Therefore, from the time the nitrogen level goes below the threshold until the monostable multivibrator 272 fires, about one-half second has elapsed. It is this monostable multivibrator 272 which renders a transistor 294, shown in FIG. 9, conductive, thereby allowing current to flow through a coil 296 and moves a contact 298 connected to the conductor 101 from a ground connection to connection with the conductor 85 from the output of the peak percent nitrogen follower 80. This generates a peak percent nitrogen sample pulse about 75 milliseconds in duration in the percent nitrogen sample pulse generator 88, which is connected through the conductor 101 to the second input of the summer device 92. The pulse is then superimposed on the functional residual capacity represented by reference numeral 105 and is shown by the reference numeral 107 in FIG. 3.

The trailing edge of the monostable multivibrator 272 triggers the monostable multivibrator 274 which also has a time constant selected to generate a pulse 75 milliseconds in duration. The output of the monostable multivibrator 274 is transmitted to the NAND gate 261, which provides a positive voltage to the conductor 67, to the relay coil 250, through a transistor 260, as shown in FIG. 6. When the monostable multivibrator 274 is in its high state and the function switch 280 is in the FRC position, the signal provided to the conductor 67 is near zero volts, so the relay coil 250 is deenergized, and the relay contact 252 closes. This discharges the peak nitrogen storage capacitor 236 and prepares the peak percent nitrogen follower 80 for the next exhalation.

As soon as the minimum percent nitrogen detector 82 again provides a low voltage to the conductor 71, indicating that the nitrogen level is above the minimum, the signal provided by the NAND gate 258 to the conductor 69 becomes positive and the sample relay contact 242 closes again and the cycle repeats.

When the subject is completely "washed out" of nitrogen, the minimum percent nitrogen detector 82 no longer detects nitrogen above the minimum threshold during exhalation, and therefore the voltage on conductor 71 remains positive.

At this point, the sample relay contact 242 remains open and no additional percent nitrogen sample pulses are generated. In addition, the multiplier output gate relay contact 196 remains open and no additional functional residual capacity integration will occur.

Thus, the operator in the central receiver unit 40 knows that functional residual capacity computation is complete.

When the functional residual capacity computation is complete, a function switch 282, shown in FIG. 1, is put in its grounded position. This grounds the nitrogen inputs to the multiplier 76, minimum percent nitrogen detector 82, and peak percent nitrogen follower 80. This causes the minimum percent nitrogen detector 82 to apply a positive voltage to the conductor 71 which holds the relay contact 196 open, thus preventing an output signal from the multiplier 76. This also holds the sample relay contact 242 open which, together with the closed relay contact 252, maintains the peak percent nitrogen follower 80 ready for the first sample of the next test. The relay contact 252 is closed, because the coil 250 is deenergized as a result of the low voltage provided by the NAND gate 261 as controlled by the function switch 280.

Although the preferred embodiment of the system has been described, it will be understood that within the purview of this invention, various changes may be made in the form, details, circuitry components, the combination thereof and mode of operation, which, generally stated, provide a system capable of performance in the manner disclosed and defined in the appended claims.