Method and apparatus for measuring arterial blood constituents

• US 5,193,543 A
• Filed: 04/06/1992
• Issued: 03/16/1993
• Est. Priority Date: 12/12/1986
• Status: Expired due to Fees

• Current Assignee: Critikon, Inc.
• Original Assignee: Critikon, Inc.
• Inventors: Yelderman, Mark
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): NASSER, ROBERT L
• Application Number:
  US07/865,774
• Time in Patent Office: 344 Days
• Field of Search: 128/633, 128/665-666, 356/41
• US Class Current: 600/310
• CPC Class Codes: A61B 5/14551 for measuring blood gases

Abstract

A method and apparatus for non-invasively measuring blood constituent concentration and particularly the oxygen saturation of arterial blood. The apparatus includes at least one light source directed to a tissue area of a patient, such as a finger or earlobe. A photodetector receives emitted light passing through the sample, and a receiver circuit analyzes the data and produces an output which is proportional to the oxygen content of blood. The light source is modulated onto a preselected carrier frequency and the receiver circuit is tuned to the carrier frequency so that undesired signals such as ambient light can be filtered out leaving only the signals created by the emitted light passing through the tissue at the light source frequency. The oxygen content is then calculated using the patient'"'"'s known red cell count, thereby correcting the final saturation value for patient anemia. Moreover, the calculation for oxygen saturation incorporates red cell scattering parameters for increased accuracy in clinical environments.

Claims

1. A device for non-invasive measurement of arterial blood constitutes for use in an environment generally illuminated by artificial light, comprising:

driver means for generating a driving signal at a driving signal frequency not substantially present in said artificial light;

light source means responsive to said driving signal for transmitting light, having respective predetermined wavelengths, at said driving signal frequency through a body tissue of a patient so as to detect said arterial blood constituents;

selecting means for selectively activating said light source means for predetermined time periods at each of said respective predetermined wavelengths, said predetermined time periods being defined by at least two successive cycles of said driving signal;

means for applying said driving signal to said light source means during said predetermined time periods so as to cause said light source means to transmit light having said respective predetermined wavelengths at said driving signal frequency through said body tissue of said patient for said at least two successive cycles of said driving signal;

detecting means for detecting at least said light transmitted through said body tissue of said patient at said driving signal frequency during said predetermined time periods and outputting a detection signal;

demodulating means tuned to a narrow band receiving frequency range including said driving signal frequency for demodulating said detection signal from said driving signal so as to get a demodulated output representing the concentration of said blood constituents detected by light at said respective predetermined wavelengths, said narrow band receiving frequency range excluding frequencies substantially present in said artificial light; and

processing means for producing an output proportional to the concentration of said arterial blood constituents from said demodulated output.

2. A device as in claim 1, wherein said processing means further includes means for producing an output proportional to the concentration of saturated hemoglobin oxygen from said demodulated output.

3. A device as in claim 1, wherein said driving signal frequency is modulated by a patient'"'"'s pulse and said demodulating means further includes means for demodulating said patient'"'"'s pulse from said driving signal.

4. A device as in claim 1, wherein said light source means comprises first and second light emitting diodes.

5. A device as in claim 4, wherein said driver means generates a square wave which is applied to said first and second light emitting diodes as said driving signal and said selecting means biases said square wave to cause said first light emitting diode to be activated when said square wave is at a first biased level and said second light emitting diode to be activated when said square wave is at a second biased level.

6. A device as in claim 5, wherein said first light emitting diode emits red light and said second light emitting diode emits infrared light.

7. A device as in claim 6, wherein said first and second light emitting diodes are connected to said driver means in relatively reversed polarities such that said driver means applies said driving signal to only one of said light emitting diodes at any given instant in time.

8. A device as in claim 7, wherein said driver means comprises intensity adjustment means for driving one of said light emitting diodes to produce a higher light intensity than the other of said light emitting diodes.

9. A device as in claim 8, wherein said intensity adjustment means drives said first light emitting diode at an intensity of about 1.5 to 2.5 times the intensity of said second light emitting diode.

10. A device as in claim 1, wherein said detecting means comprises a photodetector placed in proximity with said body tissue of said patient so as to detect said light transmitted through said body tissue of said patient, said photodetector producing an output proportional to the intensity of light incident thereon as said detection signal.

11. A device as in claim 1, wherein said demodulating means includes a bandpass filter having a center frequency approximately equal to the frequency of said driving signal.

12. A device as in claim 11, wherein said demodulating means further comprises at an output of said bandpass filter a full wave rectifier and an envelope detector for separating detected signals at said detection frequency from detected signals at the frequency of said driving signal.

13. A device as in claim 12, wherein said demodulating means further comprises at an output of said envelope detector a low pass filter for eliminating ripple from detected signals at said detection frequency.

14. A device as in claim 13, wherein said demodulating means further comprises means for adjusting the gain of said demodulated output.

15. A device as in claim 1, wherein said driving signal is a uniform square wave at 47 kHz.

16. A device as in claim 1, wherein said frequency of said driving signal is greater than the frequency of the fourth harmonic of the frequency of greatest amplitude in said artificial light.

17. A device as in claim 1, wherein said frequency of said driving signal exceeds 1 kHz.

18. A device as in claim 17, wherein said frequency of said driving signal exceeds 5 kHz.

19. A device as in claim 18, wherein said frequency of said driving signal exceeds 10 kHz.

20. A device as in claim 19, wherein said frequency of said driving signal exceeds 40 kHz.

21. A device as in claim 1, wherein said frequency of said driving signal is in-between harmonics of said artificial light.

22. A device as in claim 1, wherein said processing means comprises means for correcting its output in accordance with the fraction of red blood cells per unit volume of whole blood of said patient.

23. A device as in claim 22, wherein said processing means comprises means for waiting for transients to settle before sampling said demodulated output.

24. A device as in claim 23, wherein said processing means comprises means for averaging samples of said demodulated output.

25. A device as in claim 1, wherein said driving signal is a non-periodic repetitive waveform.

26. A device as in claim 1, wherein said driving signal is a sum of at least two sine waves of different frequencies.

27. A method of non-invasive measurement of arterial blood constituents for use in an environment generally illuminated by artificial light, comprising the steps of:

generating a driving signal at a driving signal frequency not substantially present in said artificial light;

selectively activating a light source for predetermined time periods defined by at least two successive cycles of said driving signal;

applying said driving signal to said light source during said predetermined time periods so as to cause said light source to transmit light having respective predetermined wavelengths through a body tissue of a patient at said driving signal frequency for said at least two successive cycles of said driving signal so as to detect said arterial blood constituents;

detecting said light transmitted through said body tissue of said patient at said driving signal frequency during said predetermined time periods and outputting a detection signal;

demodulating in a narrow band receiving frequency range including said driving signal frequency said detection signal from said driving signal so as to get a demodulated output representing the concentration of said blood constituents detected by light at said respective predetermined wavelengths, said narrow band receiving frequency range excluding frequencies substantially present in said artificial light; and

determining an output proportional to the concentration of said arterial blood constituents from said demodulated output.

28. A method as in claim 27, comprising the further step of determining an output proportional to the concentration of saturated hemoglobin oxygen from said demodulated output.

29. A method as in claim 27, wherein said driving signal frequency is modulated by a patient'"'"'s pulse and said demodulating step further includes the step of demodulating said patient'"'"'s pulse from said driving signal.

30. A method as in claim 27, comprising the further step of adjusting the intensity of said light source so that light detected in said detecting step is in range of a detector.

31. A method as in claim 27, comprising the further step of bandpass filtering said detection signal in a narrow frequency band about the frequency of said driving signal.

32. A method as in claim 27, wherein said light source comprises first and second light emitting diodes and said driving signal generated in said generating step is a square wave.

33. A method as in claim 32, wherein said selectively activating step includes the step of biasing said square wave to cause said first light emitting diode to be activated when said square wave is at a first biased level and said second light emitting diode to be activated when said square wave is at a second biased level.

34. A method as in claim 33, wherein said demodulating step includes the step of separating detected light at the frequency of said first light emitting diode from detected light at the frequency of said second light emitting diode.

35. A method as in claim 34, wherein said determining step includes the step of determining the concentration of said arterial blood constituents from the demodulated output at the frequencies of said first and second light emitting diodes.

36. A method as in claim 35, comprising the further step correcting the determined concentration of said arterial blood constituents in accordance with the fraction of red blood cells per unit volume of whole blood of said patient.

37. A device for non-invasive measurement of arterial blood constituents for use in an environment generally illuminated by artificial light, comprising:

driver means for generating a driving signal at a driving signal frequency not substantially present in said artificial light;

light source means responsive to said driving signal for transmitting light, having first and second predetermined wavelengths, at said driving signal frequency through a body tissue of a patient so as to detect said arterial blood constituents;

means for establishing first and second time periods during which said light at said first and second predetermined wavelengths are to be respectively transmitted, said first and second time periods each comprising at least two successive cycles of said driving signal;

means for applying said driving signal to said light source means for causing said light source means to transmit light at said first detection frequency through said body tissue of said patient during said first time period for at least two successive cycles of said driving signal and at aid second detection frequency through said body tissue of said patient during said second time period for at least two successive cycles of said driving signal;

detecting means for detecting at least said light transmitted through said body tissue of said patient at said driving signal frequency during said first and second time periods and outputting a detection signal;

demodulating means tuned to a narrow band receiving frequency range including said driving signal frequency for demodulating said detection signal from said driving signal so as to get a demodulated output substantially independent of the level of said artificial light and representing the concentration of said blood constituents detected by light at said first and second predetermined wavelengths, said narrow band receiving frequency range excluding frequencies substantially present in said artificial light; and

processing means for producing an output proportional to the concentration of said arterial blood constituents from said demodulated output.

38. A device as in claim 37, wherein said processing means comprises means for producing at least an output which is proportional to an oxygen content of the patient'"'"'s blood.

39. A device as in claim 37, wherein the driving signal frequency is greater than the frequency of a fourth harmonic of a supply frequency to the artificial light.

40. A device as in claim 37, wherein the driving signal frequency comprises a frequency in which is located in-between a pair of adjacent harmonics of a supply frequency to the artificial light.

41. A device as in claim 37, wherein the driver means is matched to the detecting means with respect to the driving signal frequency for allowing passage of the driving signal frequency and rejecting other frequencies.

42. A device as in claim 41, wherein the detecting means comprises a filter for allowing passage of the driving signal frequency and for rejecting other frequencies.

43. A device as in claim 37, wherein the processing means comprises means for generating a correction signal which is proportional to the fraction of red blood cells per unit volume of whole blood and for correcting for an altered extinction of light due to scattering which is proportional to a reduced blood cell count.