Apparatus and method for medical monitoring, in particular pulse oximeter

• US 5,800,348 A
• Filed: 09/23/1996
• Issued: 09/01/1998
• Est. Priority Date: 08/31/1995
• Status: Expired due to Term

• Current Assignee: Koninklijke Philips N.V.
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Kaestle, Siegfried
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): YARNELL, BRYAN KEITH
• Application Number:
  US08/710,794
• Time in Patent Office: 708 Days
• Field of Search: 128/633, 128/664-7, 128/719, 356/39-41, 600/322, 600/323, 600/336, 600/339, 600/340, 600/341, 600/473, 600/476, 600/478, 600/479
• US Class Current: 600/322
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: G01N 21/3151 using two sources of radiation of different wavelengths G01N21/33 - G01N21/39 take precedence

Abstract

A method for measuring medical parameters of a patient by radiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through the sample, the following steps are carried out. First and second modulation signals are generated having equal frequencies and a first phase difference of substantially 90°. Irradiating a first electromagnetic wave of a first wavelength into the sample, under control of the first modulation signal. Irradiating the sample with a second electromagnetic wave of a second wavelength, under control of the second modulation signal. Receiving electromagnetic waves of both wavelengths which have passed through the sample and demodulating the received signals by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to the first sinusoidal signal, the first and second sinusoidal demodulation signals having the same frequency as the first and second modulation signals, such as to generate a first and a second demodulated signal. The first and the second sinusoidal demodulated signals are set so as to have a phase difference relative to the first and second modulation signals corresponding to a system phase shift.

Claims

1. Method for measuring medical parameters of a patient by irradiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through said sample, said method comprising the steps of:

(1.1) generating first and second modulation signals having equal frequencies and having a first phase difference of substantially 90°

;

(1.2) irradiating a first electromagnetic wave of a first wavelength into said sample under control of said first modulation signal;

(1.3) irradiating a second electromagnetic wave of a second wavelength different from the first one into said sample under control of said second modulation signal;

(1.4) receiving electromagnetic waves of both wavelengths which have passed through said sample;

(1.5) demodulating signals representative of the received electromagnetic waves by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to said first sinusoidal signal, said first and second sinusoidal demodulation signals having an identical frequency as said first and second modulation signals, such as to generate a first and a second demodulated signal;

wherein the first and the second sinusoidal demodulation signals have a phase difference relative to the first and second modulation signals corresponding to a system phase shift; and

(1.6) analyzing said demodulated signals.

2. Method according to claim 1, comprising the step of determining said system phase shift and the step of adjusting said phase shift of the first and second sinusoidal signals relative to the first and second modulation signals in accordance with said determined system phase shift.

3. Method according to claim 2, wherein said determining said system phase shift includes the steps of:

reducing an amplitude of one of the first and second electromagnetic waves;

measuring of amplitudes of the first and second demodulated signals; and

determining the system phase shift on the basis of the measured amplitudes of the first and the second demodulated signal.

4. Method according to claim 2, wherein said determining said system phase shift includes the steps of:

reducing an amplitude of one of the first and second electromagnetic waves; and

determining the difference in a zero-crossing between the received electromagnetic waves and the other of the first and second electromagnetic waves.

5. Method according to claim 1, wherein the frequency of said first and second modulation signals is basically f=275 Hertz.

6. Method according to claim 1, wherein said first and second modulation signals are square-wave signals.

7. Method according to claim 1, wherein said first and second modulation signals are sinusoidal signals.

8. Method according to claim 1 comprising the step of low-pass filtering the demodulated signals.

9. Method according to claim 1 comprising the step of band-pass filtering the received electromagnetic waves before the demodulation step.

10. Method according to claim 1, wherein said electromagnetic waves are waves selected from the visible and/or the adjoining spectra of light.

11. Method according to claim 1, wherein said medical parameter is oxygen saturation.

12. Method according to claim 1, wherein said electromagnetic waves are waves selected from red or infra-red light.

13. Method for measuring medical parameters of a patient by irradiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through said sample, said method comprising the steps of:

(12.1) generating first and second modulation signals having equal frequencies and having a first phase difference of substantially 90°

;

(12.2) irradiating a first electromagnetic wave of a first wavelength into said sample under control of said first modulation signal;

(12.3) irradiating a second electromagnetic wave of a second wavelength different from the first one into said sample under control of said second modulation signal;

(12.4) receiving electromagnetic waves of both wavelengths which have passed through said sample;

(12.5) demodulating signals representative of the received electromagnetic waves by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to said first sinusoidal signal, said first and second sinusoidal demodulation signals having the same frequency as said first and second modulation signals, such as to generate a first and a second demodulated signal;

(12.6) analyzing said demodulated signals taking into account a system phase shift.

14. Method according to claim 13, wherein said analysing step comprises the steps of:

determining said system phase shift; and

correcting said demodulated signals on the basis of said determined system phase shift.

15. Method according to claim 14, wherein said determining said system phase shift includes the steps of:

reducing an amplitude of one of the first and second electromagnetic waves;

measuring of amplitudes of the first and second demodulated signals; and

determining the system phase shift on the basis of the measured amplitudes of the first and the second demodulated signal.

16. Method according to claim 15, wherein the reducing of the amplitude of one of the first and second electromagnetic waves comprises turning off the amplitude of this electromagnetic wave.

17. Method according to claim 14, wherein said determining said system phase shift includes the steps of:

reducing an amplitude of one of the first and second electromagnetic waves; and

determining a difference in a zero-crossing between the received electromagnetic waves and the other of the first and second electromagnetic waves.

18. Method according to claim 13, wherein said analysing step comprises the steps of:

determining the system phase shift;

calculating a matrix C! based on the system phase shift according to ##EQU6## calculating an inverse matrix C!^-1 based on the matrix C!;

determining an output vector ##EQU7## based on the amplitudes L_R, L_IR of the demodulated signals; and

calculating a corrected output vector A!= C!^-1 ·

L!.

19. Method according to claim 13, wherein the frequency of said first and second modulation signals is basically f=275 Hertz.

20. Method according to claim 13, wherein said first and second modulation signals are square-wave signals.

21. Method according to claim 13, wherein said first and second modulation signals are sinusoidal signals.

22. Method according to claim 13, comprising the step of low-pass filtering the demodulated signals.

23. Method according to claim 13 comprising the step of band-pass filtering the received electromagnetic waves before the demodulation step.

24. Method according to claim 13, wherein said electromagnetic waves are waves selected from the visible and/or the adjoining spectra of light.

25. Method according to claim 13, wherein said medical parameter is oxygen saturation.

26. Method according to claim 13, wherein said electromagnetic waves are waves selected from red or infra-red light.

27. Method for measuring medical parameters of a patient by irradiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through said sample, said method comprising the steps of:

(25.1) generating first and second modulation signals having equal frequencies and having a first phase difference of substantially 90°

;

(25.2) irradiating a first electromagnetic wave of a first wavelength into said sample under control of said first modulation signal;

(25.3) irradiating a second electromagnetic wave of a second wavelength different from the first one into said sample under control of said second modulation signal;

(25.4) receiving electromagnetic waves of both wavelengths which have passed through said sample;

(25.5) generating delayed received electromagnetic waves by adding a phase shift to the received electromagnetic waves which yields, together with a system phase shift, a total phase shift of the received electromagnetic waves relative to the first and second electromagnetic waves of substantially an integral multiple of 360°

;

(25.6) demodulating signals representative of the delayed received electromagnetic waves by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to said first sinusoidal signal, said first and second sinusoidal demodulation signals having an identical frequency as said first and second modulation signals, such as to generate a first and a second demodulated signal; and

(25.7) analyzing said demodulated signals.

28. Method according to claim 27, wherein said phase shift is added to the received electromagnetic waves by a digital all pass filter comprising programmable filter constants.

29. Method according to claim 27, wherein said phase shift is adjusted in accordance with the steps of:

reducing an amplitude of one of the first and second electromagnetic waves;

measuring of amplitudes of the first and second demodulated signals;

determining the system phase shift on the basis of the measured amplitudes of the first and second demodulated signals; and

adjusting the phase shift to yield together with the system phase shift a phase shift of an integral multiple of 360°

.

30. Method according to claim 29, wherein the reducing of the amplitude of one of the first and second electromagnetic waves comprises turning off the amplitude of this electromagnetic wave.

31. Method according to claim 27, wherein the frequency of said first and second modulation signals is basically f=275 Hertz.

32. Method according to claim 27, wherein said first and second modulation signals are square-wave signals.

33. Method according to claim 27, wherein said first and second modulation signals are sinusoidal signals.

34. Method according to claim 27 comprising the step of low-pass filtering the demodulated signals.

35. Method according to claim 27, wherein said electromagnetic waves are waves selected from the visible and/or the adjoining spectra of light.

36. Method according to claim 27, wherein said medical parameter is oxygen saturation.

37. Method according to claim 36 comprising the step of band-pass filtering the received electromagnetic waves before the demodulation step.

38. Method according to claim 27, wherein said electromagnetic waves are waves selected from red or infra-red light.

39. Apparatus for measuring medical parameters of a patient by irradiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through said sample, said apparatus comprising:

means for generating first and second modulation signals having equal frequencies and having a first phase difference of substantially 90°

;

means for irradiating a first electromagnetic wave of a first wavelength into said sample under control of said first modulation signal;

means for irradiating a second electromagnetic wave of a second wavelength different from the first one into said sample under control of said second modulation signal;

means for receiving electromagnetic waves of both wavelengths which have passed through said sample;

means for demodulating signals representative of the received electromagnetic waves by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to said first sinusoidal signal, said first and second sinusoidal demodulation signals having the same frequency as said first and second modulation signals, such as to generate a first and a second demodulated signal;

wherein the first and the second sinusoidal demodulation signals have a phase difference relative to the first and second modulation signals corresponding to a system phase shift; and

means for analyzing said demodulated signals.

40. Apparatus according to claim 39, comprising further a means for determining said system phase shift and for adjusting said phase shift of the first and second sinusoidal signals relative to the first and second modulation signals in accordance with said determined system phase shift.

41. Apparatus according to claim 39, wherein the frequency of said first and second modulation signals is basically f=275 Hertz.

42. Apparatus according to claim 39, wherein said first and second modulation signals are square-wave signals.

43. Apparatus according to claim 39, wherein said first and second modulation signals are sinusoidal signals.

44. Apparatus according to claim 39 comprising a low pass filter through which the demodulated signals are fed.

45. Apparatus according to claim 39 comprising a bandfilter connected between said means for receiving electromagnetic waves and said demodulating means.

46. Apparatus according to claim 39, wherein said electromagnetic waves are waves selected from the visible and/or the adjoining spectra of light.

47. Apparatus according to claim 39, wherein said apparatus is a pulse oximeter, and said medical parameter is oxygen saturation.

48. Apparatus according to claim 39, wherein said irradiating means are light-emitting diodes.

49. Method according to claim 39, wherein said electromagnetic waves are waves selected from red or infra-red light.

50. Apparatus for measuring medical parameters of a patient by irradiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through said sample, said method comprising the steps of:

means for generating first and second modulation signals having equal frequencies and having a first phase difference of substantially 90°

;

means for irradiating a first electromagnetic wave of a first wavelength into said sample under control of said first modulation signal;

means for irradiating a second electromagnetic wave of a second wavelength different from the first one into said sample under control of said second modulation signal;

means for receiving electromagnetic waves of both wavelengths which have passed through said sample;

means for demodulating signals representative of the received electromagnetic waves by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to said first sinusoidal signal, said first and second sinusoidal demodulation signals having an identical frequency as said first and second modulation signals, such as to generate a first and a second demodulated signal; and

means for analyzing said demodulated signals taking into account a system phase shift.

51. Apparatus according to claim 50, wherein said means for analyzing comprises:

means for determining said system phase shift; and

means for correcting said demodulated signals on the basis of said determined system phase shift.

52. Apparatus according to claim 50, wherein said means for analyzing comprises:

means for determining the system phase shift;

means for calculating a matrix C! based on the system phase shift according to ##EQU8## means for calculating an inverse matrix C!^-1 based on the matrix C!;

means for determining an output vector ##EQU9## based on the amplitudes L_R, L_IR of the demodulated signals; and

means for calculating a corrected output vector A!= C!^-1 ·

L!.

53. Apparatus according to claim 50, wherein said means for determining said system phase shift comprises:

means for reducing an amplitude of one of the first and second electromagnetic waves;

means for measuring of amplitudes of the first and second demodulated signals; and

means for determining the system phase shift on the basis of the measured amplitudes of the first and the second demodulated signal.

54. Apparatus according to claim 53, wherein said means for reducing of the amplitude of one of the first and second electromagnetic waves comprises means for turning off the amplitude of this electromagnetic wave.

55. Apparatus according to claim 50, wherein said means for determining said system phase shift comprises:

means for reducing the amplitude of one of the first and second electromagnetic waves; and

means for determining the difference in a zero-crossing between the received electromagnetic waves and the other of the first and second electromagnetic waves.

56. Apparatus according to claim 50, wherein the frequency of said first and second modulation signals is basically f=275 Hertz.

57. Apparatus according to claim 50, wherein said first and second modulation signals are square-wave signals.

58. Apparatus according to claim 50, wherein said first and second modulation signals are sinusoidal signals.

59. Apparatus according to claim 50 comprising a low pass filter through which the demodulated signals are fed.

60. Apparatus according to claim 50 comprising a bandfilter connected between said means for receiving electromagnetic waves and said demodulating means.

61. Apparatus according to claim 50, wherein said electromagnetic waves are waves selected from the visible and/or the adjoining spectra of light.

62. Apparatus according to claim 50, wherein said apparatus is a pulse oximeter, and said medical parameter is oxygen saturation.

63. Apparatus according to claim 50, wherein said irradiating means are light-emitting diodes.

64. Method according to claim 50, wherein said electromagnetic waves are selected from red or infra-red light.

65. Apparatus for measuring medical parameters of a patient by irradiation of electromagnetic waves into a sample and for measurement and subsequent analysis of the electromagnetic waves which have passed through said sample, said apparatus comprising:

means for generating first and second modulation signals having equal frequencies and having a first phase difference of substantially 90°

;

means for irradiating a first electromagnetic wave of a first wavelength into said sample under control of said first modulation signal;

means for irradiating a second electromagnetic wave of a second wavelength different from the first one into said sample under control of said second modulation signal;

means for receiving electromagnetic waves of both wavelengths which have passed through said sample;

means for generating delayed received electromagnetic waves by adding a phase shift to the received electromagnetic waves which yields, together with a system phase shift, a total phase shift of the received electromagnetic waves relative to the first and second electromagnetic waves of substantially an integral multiple of 360°

;

means for demodulating signals representative of the delayed received electromagnetic waves by multiplying the same with a first sinusoidal demodulation signal and with a second sinusoidal demodulation signal having the first phase difference with respect to said first sinusoidal signal, said first and second sinusoidal demodulation signals having an identical frequency as said first and second modulation signals, such as to generate a first and a second demodulated signal; and

means for analyzing said demodulated signals.

66. Apparatus according to claim 65, comprising a digital all pass filter comprising programmable filter constants for adding said phase shift to the received electromagnetic waves.

67. Apparatus according to claim 65, further comprising:

means for reducing an amplitude of one of the first and second electromagnetic waves;

means for measuring of amplitudes of the first and second demodulated signals;

means for determining the system phase shift on the basis of the measured amplitudes of the first and second demodulated signals; and

means for adjusting the phase shift to yield together with the system phase shift a phase shift of an integral multiple of 360°

.

68. Apparatus according to claim 65, wherein said means for reducing an amplitude of one of the first and second electromagnetic waves comprises means for turning off the amplitude of this electromagnetic wave.

69. Apparatus according to claim 65, wherein the frequency of said first and second modulation signals is basically f=275 Hertz.

70. Apparatus according to claim 65, wherein said first and second modulation signals are square-wave signals.

71. Apparatus according to claim 65, wherein said first and second modulation signals are sinusoidal signals.

72. Apparatus according to claim 65 comprising a low pass filter through which the demodulated signals are fed.

73. Apparatus according to claim 65 comprising a bandfilter connected between said means for receiving electromagnetic waves and said demodulating means.

74. Apparatus according to claim 65, wherein said electromagnetic waves are waves selected from the visible and/or the adjoining spectra of light.

75. Apparatus according to claim 65, wherein said apparatus is a pulse oximeter, and said medical parameter is oxygen saturation.

76. Apparatus according to claim 65, wherein said irradiating means are light-emitting diodes.

77. Method according to claim 65, wherein said electromagnetic waves are waves selected from red or infra-red light.