Heart rate measurement system

• US 4,258,719 A
• Filed: 12/04/1978
• Issued: 03/31/1981
• Est. Priority Date: 12/04/1978
• Status: Expired due to Term

• Current Assignee: Hughes Aircraft Company (Rtx Corporation)
• Original Assignee: Hughes Aircraft Company (Rtx Corporation)
• Inventors: Lewyn, Lanny L.
• Primary Examiner(s): Michell, Robert W.
• Assistant Examiner(s): Jaworski, Francis J.
• Application Number:
  US05/965,816
• Time in Patent Office: 848 Days
• Field of Search: 128/687-690, 328/165, 328/2, 328/167, 307/231, 307/311, 250/214 A, 250/214 AL, 250/214 B, 250/214 C, 250/215
• US Class Current: 600/503
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives
• CPC Class Codes: A61B 5/7242 using integration

Abstract

A pulsed IR reflectance plethysmograph for heart rate measurement and display in a digital watch or as a medical instrument using, for example, a pulsed LED light emitting diode for transmitting light pulses and a photodiode for receiving light pulses reflected from a finger. The LED and photodiode are mounted in a small sensor on the watch or medical instrument. The photodiode is connected directly to a signal conditioning circuit which rapidly removes unwanted asynchronous ambient background light signals such as from sunlight, by means of a switched ambient light subtractor circuit which performs the subtraction without requiring amplification or conversion of the detector signal photocurrent to another electrical parameter. The signal current after cancellation of the ambient background signal is sampled in an integrate and hold circuit to provide a heart systolic pressure wave. The synchronous steady state IR pulse carrier envelope signal is cancelled by a perfect second-order feedback loop via an integrating amplifier and a switched transconductance element that operates with fast response, even under conditions of large overloads that occur at the sensor. Pulses of the heart systolic pressure wave are then detected to determine the pulse rate which is displayed. When only the presence of an unmodulated IR light carrier is to be detected, as in an IR light beam detector, an ambient light subtractor in accordance with the invention is provided in an unswitched configuration that removes ambient light with a minimum number of components over an extremely wide dynamic range.

Claims

1. A pulse rate measuring system for receiving during sample pulse intervals, a pulsed photocurrent signal including an ambient light photocurrent signal and a pulsed photocurrent carrier signal which is amplitude modulated by a heart blood pressure signal, and for providing said heart blood pressure signal to an output terminal, said rate measuring system comprising

a sensor including a pulsed light source and a photodiode having an anode and a cathode with the cathode coupled to a first source of reference potential, the anode providing said pulsed photocurrent signal,

ambient light cancellation means directly coupled to the anode of said photodiode for cancelling the ambient light photocurrent signal during pulse intervals,

first integrator means coupled to said ambient light cancellation means for receiving at least a portion of the pulsed photocurrent signal from said ambient light cancellation means during sample pulse intervals and for integrating and holding said heart blood pressure signal, and first integrator means providing heart blood pressure and error voltage signals to an output terminal, and

feedback means coupled between the output terminal of said first integrator means and the anode of said photodiode and including a second integrator means and a switched transconductance element, said second integrator means including an operational amplifier having an output terminal coupled to said switched transconductance element and having first and second input terminals with said first input terminal coupled to respond to said error voltage signals to control said switched transconductance element to provide a cancelling current during sample pulse intervals, and a second source of reference potential coupled to said second input terminal of said operational amplifier for maintaining the potential substantially constant at said first input terminal of said operational amplifier.

2. The combination of claim 1 in which said ambient light cancellation means includes first and second field-effect transistors having split-drain electrodes and with a source electrode of said first transistor being coupled to the anode of said photodiode, the first drain electrodes of said first and second transistors being coupled together and to said first integrator means and the second drain electrodes of said first and second transistors being coupled together at a node, capacitive means coupled between a source electrode and a gate electrode of said second transistor, and gating means coupled to said node, to a third source of reference potential and to said capacitive means for connecting said capacitive means to said node during periods between said sample pulse intervals and for disconnecting said capacitive means from said node and applying said third reference potential to said node during said sample pulse intervals.

3. The combination of claim 1 in which said switched transconductance element includes switched comparison means for comparing a signal from said second integrator means with a third reference potential and providing cancelling current during said sample pulse intervals.

4. The combination of claim 1 in which said first integrator means includes switching means coupled to said ambient light cancellation means, an integrating capacitor coupled between said switching means and a third source of reference potential and a unity gain operational amplifier having a first input terminal coupled to said integrating capacitor and an output terminal connected to said rate measuring system output terminal.

5. The combination of claim 4 in which said switching means includes a first switch to conduct from said ambient light cancellation means to said integrating capacitor during said sample pulse intervals and a second switch coupled from said ambient light cancellation means to a second input terminal of said unity gain operational amplifier and to said rate measuring system output terminal for conducting between said sample pulse intervals.

6. The combination of claim 1 in which said operational amplifier of said second integrator means further includes a first resistor coupled between the output terminal of said first integrator means and the first input terminal of said operational amplifier, a capacitor and a resistor series coupled between said first input terminal and said output terminal of said operational amplifier and a third source of reference potential coupled to said second input terminal of said operational amplifier.

7. The combination of claim 1 further including discriminator means coupled to said rate measuring system output terminal, rate computation means coupled to said discriminator means and display means coupled to said rate computation means.

8. A system for receiving pulsed photocurrent during sample pulse intervals, said photocurrent including ambient light photocurrent, a steady state pulsed carrier photocurrent and a heart blood pressure modulation photocurrent, and applying a heart blood pressure modulation wave to an output terminal, said system comprising

a photodiode for providing said pulsed photocurrent,

asynchronous cancellation means having an input terminal directly coupled to said photodiode for cancellation of said ambient light photocurrent,

integrate and hold means coupled from said asynchronous cancellation means to said output terminal for providing an error voltage signal proportional to the uncancelled portion of the steady state pulsed carrier photocurrent and for providing a heart pressure modulation wave, and

feedback means coupled between said output terminal and said input terminal of said asynchronous cancellation means, said feedback means including means for integrating and storing said error voltage signal and holding the steady state value of said error voltage on said output terminal independent of subsequent changes of said steady state pulsed carrier photocurrent value.

9. The combination of claim 8 in which said feedback means is a second-order feedback loop.

10. The system as defined in claim 9 in which said second-order feedback loop includes control means for maintaining a feedback voltage at a level V_f proportional to the integral of the difference between said error voltage signal out of said integrate and hold means and a reference voltage VR₄, and means for producing a feedback current to the direct coupled input terminal of said asynchronous cancellation means that is proportional to the difference VR₄ -V_f.

11. The system as defined in claim 10 wherein said means for producing a feedback current is a switched capacitor transconductance means comprised of a capacitor, a first transmission gate connecting a source of said feedback voltage V_f to said capacitor, a second transmission gate connecting a source of said reference voltage VR₄ to said capacitor, means for turning on said second transmission gate while said first transmission gate is turned off between said pulse intervals, and means for turning on said first transmission gate while said second transmission gate is turned off during said pulse intervals.

12. The system as defined in claim 11 further including detecting means coupled to said system output terminal for detecting peaks of voltage in said heart pressure modulation wave developed by said integrate and hold means, second means couled to said first means for computing the rate of said peaks, and third means coupled to said second means for displaying said rate.

13. The system as defined in claim 12 wherein said system is a reflectance plethysmograph having a light emitting diode mounted at a selected distance from said photodiode, and means for periodically triggering said light emitting diode to provide light pulses, wherein said photodiode is mounted to receive at least a portion of the energy of said light pulses emitted from said light emitting diode.

14. A system comprising a photodiode to detect pulsed light during light pulse periods in the presence of asynchronous ambient light, said photodiode responding to said pulsed light to provide an amplitude modulated pulsed carrier photocurrent, signal conditioning means for cancellation of asynchronous ambient light photocurrent for the photocurrent produced by said photodiode, said signal conditioning means connected directly to said photodiode to receive photocurrent, integrate and hold means coupled to said signal conditioning means for integrating and holding a voltage proportional to said pulsed carrier photocurrent over a succession of light pulse periods, a second-order feedback loop coupled from an output terminal of said integrate and hold means to an input terminal of said signal conditioning means, said feedback loop including switched transconductance means for switching a charge of current proportional to a difference voltage between a reference voltage and an output voltage of said integrate and hold means and providing a feedback signal to the input terminals of said signal conditioning means during light pulse periods.

15. The improvement of claim 14 wherein said switched transconductance means is comprised of a series connected capacitor and two gates that are alternately switched into conduction, one gate being switched into conduction for connecting said capacitor to said reference voltage, and one gate being switched into conduction for connecting said capacitor to said output voltage of said integrate and hold means.

16. A system including a photodiode for detecting pulsed light during light pulse periods and for detecting the presence of asynchronous ambient light between said pulse periods and signal conditioning means coupled to said photodiode for cancellation of said asynchronous ambient light photocurrent from photocurrent produced by said photodiode during said pulse periods, said signal conditioning means being comprised of a split-drain field-effect transistor having a source element coupled to said photodiode and storage current mirror means coupled between the split-drain elements of said field-effect transistor, a first one of said split-drain elements serving to provide to said current mirror means between said pulse periods, an output signal proportional to asynchronous ambient light photocurrent received at said source element of said transistor, and a second one of said split-drain elements providing to said current mirror means during said pulse periods, a photocurrent proportional to said asynchronous ambient light photocurrent sampled between pulse periods and providing to an output terminal of said signal conditioning means, a photocurrent signal with the asynchronous ambient light photocurrent cancelled.

17. A pulsed plethysmograph instrument for heart pule rate measurement and display comprising

a light emitting diode;

means for periodically pulsing said light emitting diode to emit pulse of IR radiation during sample pulse intervals;

a photodiode for detecting at least a portion of said radiation and producing a photodiode current proportional to the radiation received, said photodiode current including an asynchronous ambient light current and a pulsed carrier current including a steady state pulsed carrier component and an amplitude modulated heart blood pressure component,

signal conditioning means having an input terminal connected to receive said current produced by said photodiode without amplification for producing at an output terminal a voltage signal proportional to said amplitude modulated heart blood pressure component, said signal conditioning means including,

means for cancellation of current due to asynchronous ambient light detected by said photodiode, and

means for cancellation of the steady state pulsed carrier component of the current produced by said photodiode during said sample pulse intervals;

pulse discriminator means for detecting systolic pulses in said voltage signal produced at the output terminal of said signal conditioning means;

means for computing the pulse rate of the detected systolic pulse; and

means for displaying the pulse rate.

18. A pulsed plethysmograph as defined in claim 17 wherein said means for cancellation of current due to asynchronous ambient light is comprised of

a storage capacitor,

first and second gates,

means for dividing said current received from said photodiode in two current paths, a first path for producing an output current to the means for cancellation of the steady state pulsed carrier component and a second path for producing a current to said storage capacitor through said first gate to store in said capacitor a voltage proportional to said asynchronous ambient light photocurrent amplitude,

means for controlling said second gate to conduct said current in said second path between sample pulse intervals,

means for switching current conduction in said second path through said first gate during said sample pulse intervals, and

means responsive to the voltage stored in said capacitor for shunting from said output current in said first path a current equal to the current that produced the stored voltage on said capacitor, whereby ambient light present is cancelled from said output current during said sample pulse intervals.

19. A pulsed plethysmograph as defined in claim 18 whereiln said means for cancellation of the steady state pulsed carrier component of the current produced by said photodiode during detection of reflected IR radiation pulses is comprised of

first integrating means coupled to said means for cancellation of current due to asynchronous ambient light for integrating said output current during said sample pulse intervals and for storing integrated current during periods between integrations to provide a voltage signal at said output terminal of said signal conditioning means, and

a second-order feedback loop coupled between said output terminal and said input terminal of said signal conditioning means, said loop including second integrating means and switched means for controlling feedback current to be proportional to the difference between a reference voltage and the output voltage of said second integrating means during said sample pulse periods, whereby the pulsed steady state carrier component of the reflected IR radiation is cancelled.

20. In an apparatus having a photodiode for detecting pulsed light energy in the presence of ambient light, signal conditioning means connected to receive at an input terminal a photocurrent from said photodiode without amplification for producing at an output terminal a current signal proportional to said pulsed light energy with cancellation of photocurrent due to said ambient light, said signal conditioning means comprising

a storage capacitor,

means for dividing said photocurrent received from said photodiode into two equal current paths, a first path for producing a current to said output terminal, and a second path for producing a voltage on said storage capacitor proportional to the low frequency components of current from said photodiode in said second path, and

means responsive to said voltage stored on said capacitor for shunting from said output terminal a current equal to said low frequency components of current from said photodiode in said second path due to ambient light.

21. In an apparatus having a photodiode for detecting pulsed light provided by a source of light pulses in the presence of ambient light, signal conditioning means connected to receive at an input terminal a photocurrent from said photodiode for producing at an output terminal a current signal proportional to said pulsed light with cancellation of photocurrent due to said ambient light, comprising

a storage capacitor,

a first gate,

means for equally dividing said photocurrent received from said photodiode into two current paths, a first path for providing a current to said output terminal, and a second path for providing a current to said storage capacitor through said first gate to store on said capacitor a voltage proportional to the current amplitude through said second path,

a second gate coupled to said second path,

means coupled to said first gate for switching off current conduction in said second path through said first gate for a period substantially equal to the period of said light pulses, each time said source provides a light pulse,

means coupled to said second gate for switching on said second gate to conduct said current in said second path for periods while current conduction through said first gate is switched off, and

means responsive to the voltage stored on said capacitor for shunting from the current in said first path a current equal to the current that produced the stored voltage on said capacitor, whereby photocurrent due to ambient light is cancelled from said output terminal during the period when said photodiode is receiving said pulsed light while said first gate is not conducting and said second gate is conducting current.

22. A signal cancelling circuit responsive to a pulsed input signal, said input signal including an asynchronous signal and a modulated pulsed carrier signal, said circuit cancelling said asynchronous signal and applying to an output terminal a signal proportional to said modulated pulsed carrier signal, said circuit comprising

a storage capacitor,

first transistor means responsive to said pulsed input signal for equally dividing said input current into first and second paths, said first path being coupled to said output terminal,

a first gate coupling the second path and said storage capacitor,

second transistor means having first and second paths respectively coupled to the first and second paths of said first transistor means, the conduction through said second path being controlled during pulses,

a source of reference potential,

a second gate coupled between said source of reference potential and said second path of said first and second transistor means, and

timing means coupled to said first and second gates for biasing said first and second gates respectively conducting and nonconducting between the pulses and respectively nonconducting and conducting during the pulses.

23. The combination of claim 22 in which said first and second transistor means are respective first and second split gate field-effect transistors with the source of said first field-effect transistor being responsive to said pulsed input signal and the first and second paths of said first and second transistor means respectively being first and second drains of said first and said second field-effect transistors.

24. The combination of claim 23 in which said first and second field-effect transistors are of opposite conductivity types.

25. Signal conditioning means responsive to an input current signal including an asynchronous current signal and a pulsed current carrier signal for providing at an output terminal a signal proportional to said pulsed current carrier signal with cancellation of said asynchronous current signal comprising

first and second split-drain field-effect transistors each having first and second drain electrodes respectively coupled together and with the second drain electrodes coupled to said output terminal, each transistor having a source electrode and a gate electrode, the source of said first field-effect transistor being responsive to said input current signal,

a source of a first reference potential and a source of a second reference potential respectively coupled to the gate electrode of the first field-effect transistor and the source electrode of said second field-effect transistor,

a resistor coupled between the second drain electrodes of said transistor and the gate electrode of said second field-effect transistor, and

a capacitor coupled between the gate electrode of said second field-effect transistor and said second source of reference potential.