Methods and apparatus for determining cardiac output

• US 20040158163A1
• Filed: 09/22/2003
• Published: 08/12/2004
• Est. Priority Date: 02/10/2003
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology, The Board of Trustees of Michigan State University (Michigan State University)
• Original Assignee: Massachusetts Institute of Technology, The Board of Trustees of Michigan State University (Michigan State University)
• Inventors: Cohen, Richard J., Mukkamala, Ramakrishna, Sherman, Derin A.
• Granted Patent: US 7,815,578 B2
• Time in Patent Office: Days
• US Class Current: 600/508
• CPC Class Codes: A61B 5/02007 Evaluating blood vessel condition, e.g. elasticity, compliance
• CPC Class Codes: A61B 5/02255 the pressure being controlled by plethysmographic signals, e.g. derived from optical sensors
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence
• CPC Class Codes: A61B 5/352 Detecting R peaks, e.g. for synchronising diagnostic apparatus; Estimating R-R interval
• CPC Class Codes: A61B 5/726 using Wavelet transforms

Abstract

The present invention provides methods and apparatus for determining a dynamical property of the systemic or pulmonary arterial tree using long time scale information, i.e., information obtained from measurements over time scales greater than a single cardiac cycle. In one aspect, the invention provides a method and apparatus for monitoring cardiac output (CO) from a single blood pressure signal measurement obtained at any site in the systemic or pulmonary arterial tree or from any related measurement including, for example, fingertip photoplethysmography.

According to the method the time constant of the arterial tree, defined to be the product of the total peripheral resistance (TPR) and the nearly constant arterial compliance, is determined by analyzing the long time scale variations (greater than a single cardiac cycle) in any of these blood pressure signals. Then, according to Ohm'"'"'s law, a value proportional to CO may be determined from the ratio of the blood pressure signal to the estimated time constant. The proportional CO values derived from this method may be calibrated to absolute CO, if desired, with a single, absolute measure of CO (e.g., thermodilution). The present invention may be applied to invasive radial arterial blood pressure or pulmonary arterial blood pressure signals which are routinely measured in intensive care units and surgical suites or to noninvasively measured peripheral arterial blood pressure signals or related noninvasively measured signals in order to facilitate the clinical monitoring of CO as well as TPR.

Claims

1. A method for determining a dynamical property of the systemic or pulmonary arterial tree comprising steps of:

(a) measuring a physiologic signal over a plurality of cardiac cycles;

(b) obtaining a relationship between the timing of a cardiac contraction and the evolution of the physiologic signal over a time period greater than that of a single cardiac cycle by analyzing the physiologic signal over a plurality of cardiac cycles; and

(c) using the relationship to determine the dynamical property.

2. The method of claim 1, wherein the physiologic signal is invasively or noninvasively measured at any site in the systemic or pulmonary arterial tree.

3. The method of claim 1, wherein the physiologic signal is an arterial blood pressure (ABP) signal.

4. The method of claim 1, wherein the physiologic signal is a signal related to the arterial blood pressure signal.

5. The method of claim 1, wherein the physiologic signal is an arterial-systemic filling pressure difference (ASFPD) signal.

6. The method of claim 1, further comprising the measurement of the surface electrocardiogram (ECG).

7. The method of claim 6, further comprising analysis of the ECG.

8. The method of claim 1, further comprising using the dynamical property to determine one or more parameters of the cardiovascular system.

9. The method of claim 8, wherein the parameter is cardiac output or total peripheral resistance.

10. The method of claim 9, wherein the dynamical property is used in conjunction with a physiological signal to determine the cardiac output or total peripheral resistance.

11. The method of claim 10, wherein the physiologic signal, cardiac output, or total peripheral resistance are subjected to low pass filtering, single beat averaging, or multibeat averaging.

12. The method of claim 10, wherein the physiologic signal is arterial blood pressure, a signal related to arterial blood pressure, or the arterial-systemic filling pressure difference.

13. The method of claim 1, further comprising representing cardiac contractions by a train of impulses, each of which is located at the times of a cardiac contraction and has an area equal to the ensuing arterial pulse pressure.

14. The method of claim 1, further comprising representing cardiac contractions by a train of impulses, each of which is located at the times of a cardiac contraction, wherein the impulses have equal area.

15. The method of claim 1, further comprising determining the timing of cardiac contractions from the physiologic signal of step (a), an ECG signal or another physiologic signal.

16. The method of claim 1, further comprising filtering the physiologic signal, the cardiac contraction signal, or both prior to step (b).

17. The method of claim 1, further comprising estimating an impulse response representing the response of the physiologic signal to a single cardiac contraction.

18. The method of claim 1, wherein a parametric model is employed to impose causality.

19. The method of claim 18, wherein the parametric model is an auto-regressive moving average model.

20. The method of claim 18, further comprising estimating parameters of the model using a least squares method.

21. The method of claim 1, wherein the dynamical property is the impulse response function of the systemic or pulmonary arterial tree.

22. The method of claim 1, wherein the dynamical property is a characteristic time of the sytemic or pulmonary arterial tree.

23. The method of claim 22, wherein step (b) comprises estimating the impulse response function, and wherein the characteristic time is determined by fitting a function to a portion of the estimated impulse response function.

24. The method of claim 23, wherein the portion of the estimated impulse response function begins a selected amount of time following the maximum value of the estimated impulse response function.

25. The method of claim 24, wherein the selected amount of time is predetermined.

26. The method of claim 23, wherein the characteristic time is a time constant determined by fitting an exponential function to a portion of the estimated impulse response function.

27. The method of claim 22, further comprising the step of determining cardiac output.

28. The method of claim 27, wherein the cardiac output is determined to within a proportionality constant by dividing the magnitude of the physiologic signal by the characteristic time.

29. The method of claim 28 further comprising adjusting the proportionality constant to depend on either the heart rate or arterial blood pressure.

30. The method of claim 28, wherein the physiologic signal is an arterial blood pressure signal or an arterial-systemic filling pressure difference signal.

31. The method of claim 28, wherein the proportionality constant is compliance, further comprising the step of estimating or measuring arterial compliance.

32. The method of claim 22, further comprising the step of determining total peripheral resistance to within a proportionality constant.

33. The method of claim 32 further comprising adjusting the proportionality constant to depend on either the heart rate or arterial blood pressure.

34. The method of claim 32, wherein the physiologic signal is an arterial blood pressure signal or an arterial-systemic filling pressure difference signal.

35. The method of claim 22, further comprising measuring absolute cardiac output using an alternative method.

36. The method of claim 35, further comprising determining the proportionality constant from the measurement of absolute cardiac output.

37. The method of claim 36, further comprising using the proportionality constant to determine absolute cardiac output, absolute total peripheral resistance, or both.

38. The method of claim 22, further comprising displaying cardiac output and, optionally, one or more additional cardiovascular system parameters.

39. The method of claim 22, further comprising the step of triggering an alarm if cardiac output decreases beyond a predetermined value or proportion.

40. A method of determining cardiac output to within a constant scale factor comprising steps of:

(a) measuring a physiologic signal over a plurality of cardiac contraction cycles;

(b) estimating a function that represents the response of the physiologic signal to a cardiac contraction over a time period greater than that of a single cardiac cycle;

(c) determining a characteristic time of the function;

(d) determining cardiac output to within a constant scale factor by dividing the magnitude of the physiologic signal by the characteristic time obtained in step (c).

41. The method of claim 40, wherein the cardiac output or the first physiological signal are subjected to single beat averaging, multibeat averaging, or low pass filtering.

42. The method of claim 40, wherein the physiologic signal or signals are invasively or noninvasively measured at any site in the systemic or pulmonary arterial tree.

43. The method of claim 40, wherein the physiologic signal of step (a) is an arterial blood pressure (ABP) signal.

44. The method of claim 40, wherein the physiologic signal of step (a) is a signal related to the arterial blood pressure signal.

45. The method of claim 40, wherein the physiologic signal of step (a) is an arterial-systemic filling pressure difference (ASFPD) signal.

46. The method of claim 40, further comprising the measurement of the surface electrocardiogram (ECG).

47. The method of claim 46, further comprising analysis of the ECG.

48. The method of claim 40, further comprising representing cardiac contractions by a train of impulses, each of which is located at the times of a cardiac contraction and has an area equal to the ensuing arterial pulse pressure.

49. The method of claim 40, further comprising representing cardiac contractions by a train of impulses, each of which is located at the times of a cardiac contraction, wherein the impulses have equal area.

50. The method of claim 40, further comprising determining the timing of cardiac contractions from the physiologic signal of step (a), an ECG signal or another physiologic signal.

51. The method of claim 40, further comprising filtering the physiologic signal, the cardiac contraction signal, or both prior to step (b).

52. The method of claim 40, further comprising estimating an impulse response representing the response of the physiologic signal to a single cardiac contraction.

53. The method of claim 40, wherein a parametric model is employed to impose causality.

54. The method of claim 53, wherein the parametric model is an auto-regressive moving average model.

55. The method of claim 53, further comprising estimating parameters of the model using a least squares method.

56. The method of claim 40, wherein step (b) comprises estimating the impulse response function, and wherein the characteristic time is determined by fitting a function to a portion of the estimated impulse response function.

57. The method of claim 56, wherein the portion of the estimated impulse response function begins a selected amount of time following the maximum value of the estimated impulse response function.

58. The method of claim 57, wherein the selected amount of time is predetermined.

59. The method of claim 57, wherein the selected amount of time is at least 1.5 seconds.

60. The method of claim 59, wherein the selected amount of time is approximately 2 seconds.

61. The method of claim 56, wherein the characteristic time is a time constant determined by fitting an exponential function to a portion of the estimated impulse response function.

62. The method of claim 40, wherein the cardiac output is determined to within a proportionality constant by dividing the magnitude of the physiologic signal by the characteristic time.

63. The method of claim 40, wherein the physiologic signal is an arterial blood pressure signal or an arterial-systemic filling pressure difference signal.

64. The method of claim 40, wherein the proportionality constant is compliance, further comprising the step of estimating or measuring arterial compliance.

65. The method of claim 40, further comprising the step of determining total peripheral resistance to within a proportionality constant.

66. The method of claim 40, further comprising measuring absolute cardiac output using an alternative method.

67. The method of claim 66, further comprising determining the proportionality constant from the measurement of absolute cardiac output.

68. The method of claim 67, further comprising using the proportionality constant to determine absolute cardiac output, absolute total peripheral resistance, or both.

69. The method of claim 40, further comprising displaying cardiac output and, optionally, one or more additional cardiovascular system parameters.

70. The method of claim 40, further comprising the step of triggering an alarm if cardiac output decreases beyond a predetermined value or proportion.

71. A method of determining cardiac output to within a constant scale factor comprising steps of:

(a) measuring a first physiologic signal over a plurality of cardiac contraction cycles;

(b) measuring a second physiologic signal over a plurality of cardiac contraction cycles;

(c) estimating a function that represents the response of the second physiologic signal to a cardiac contraction over a time period greater than that of a single cardiac cycle;

(d) determining a characteristic time of the function; and

(e) determining cardiac output to within a constant scale factor by dividing the magnitude of the first physiologic signal by the characteristic time obtained in step (d).

72. The method of claim 71, wherein the cardiac output or the first physiological signal are subjected to single beat averaging, multibeat averaging, or low pass filtering.

73. A method of determining total peripheral resistance to within a constant scale factor comprising steps of:

(a) measuring a physiologic signal over a plurality of cardiac contraction cycles;

(b) estimating a function that represents the response of the physiologic signal to a cardiac contraction over a time period greater than that of a single cardiac cycle; and

(c) determining a characteristic time of the function, wherein total peripheral resistance is given to within a constant factor by the characteristic time.

74. A method of determining cardiac output to within a constant scale factor comprising steps of:

(a) measuring an arterial blood pressure signal over a plurality of cardiac contraction cycles;

(b) estimating a function that represents the response of the arterial blood pressure signal to a cardiac contraction over a time period greater than that of a single cardiac cycle;

(c) fitting the function of step (b) to an exponential function over a time period that begins a selected amount of time following the maximum value of the function;

(d) estimating the time constant of the function of step (b) as the time constant of the exponential function of step (c); and

(e) determining cardiac output to within a constant scale factor by dividing arterial blood pressure by the time constant obtained in step (d).

75. The method of claim 74, wherein the selected amount of time is predetermined.

76. A method of determining total peripheral resistance to within a constant scale factor comprising steps of:

(a) measuring an arterial blood pressure signal over a plurality of cardiac contraction cycles;

(b) estimating a function that represents the response of the arterial blood pressure signal to a cardiac contraction over a time period greater than that of a single cardiac cycle;

(c) fitting the function of step (b) to an exponential function over a time period that begins a selected amount of time following the maximum value of the function;

(d) estimating the time constant of the function of step (b) as the time constant of the exponential function, wherein total peripheral resistance is given to within a constant factor by the time constant.

77. An apparatus for determining a dynamical property of the systemic or pulmonary arterial tree comprising a computer system that includes:

(a) memory means which stores a program comprising computer-executable process steps; and

(b) a processor that executes the process steps so as to

(i) accept an input representing a measurement of a physiologic signal over a plurality of cardiac cycles;

(ii) obtain a relationship between the timing of a cardiac contraction and the evolution of the physiologic signal over a time period greater than that of a single cardiac cycle by analyzing the physiologic signal over a plurality of cardiac cycles; and

(iii) use the relationship to determine the dynamical property.

78. The apparatus of claim 77, further comprising an analog-to-digital converter.

79. The apparatus of claim 77, wherein the apparatus includes a buffer system.

80. The apparatus of claim 77, wherein the apparatus includes a display device.

81. An apparatus for determining cardiac output to within a constant scale factor comprising a computer system that includes:

(a) memory means which stores a program comprising computer-executable process steps; and

(b) a processor that executes the process steps so as to;

(i) accept an input representing a measurement of a physiologic signal over a plurality of cardiac cycles;

(ii) estimate a function that represents the response of the physiologic signal to a cardiac contraction over a time period greater than that of a single cardiac cycle;

(iii) determine a characteristic time of the function or of a second function that represents the response of a different physiologic signal to a cardiac contraction over a time period greater than a single cardiac cycle; and

(iv) determine cardiac output to within a constant scale factor by dividing the magnitude of the physiologic signal by the characteristic time obtained in step (iii).

82. The apparatus of claim 81, further comprising an analog-to-digital converter.

83. The apparatus of claim 81, wherein the apparatus includes a buffer system.

84. The apparatus of claim 81, wherein the apparatus includes a display device.

85. An apparatus for determining cardiac output to within a constant scale factor comprising a computer system that includes:

(a) memory means which stores a program comprising computer-executable process steps; and

(b) a processor that executes the process steps so as to;

(i) accept an input representing a measurement of an arterial blood pressure signal over a plurality of cardiac cycles;

(ii) estimate a function that represents the response of the arterial blood pressure to a single cardiac contraction;

(iii) fit the function of step (ii) to an exponential function over a time period that begins a selected amount of time following the maximum value of the function;

(iv) estimate the time constant of the function of step (ii) as the time constant of the exponential function of step (iii); and

(v) determine cardiac output to within a constant scale factor by dividing average ABP by the time constant obtained in step (iv).

86. The apparatus of claim 85, further comprising an analog-to-digital converter.

87. The apparatus of claim 85, wherein the apparatus includes a buffer system.

88. The apparatus of claim 85, wherein the apparatus includes a display device.