Apparatus and method for determining an approximation of the stroke volume and the cardiac output of the heart

• US 20020193689A1
• Filed: 04/03/2001
• Published: 12/19/2002
• Est. Priority Date: 04/03/2001
• Status: Active Grant

• Current Assignee: Osypka Medical GmbH
• Original Assignee: Osypka Medical GmbH
• Inventors: Bernstein, Donald P., Osypka, Markus J.
• Granted Patent: US 6,511,438 B2
• Time in Patent Office: Days
• US Class Current: 600/454
• 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 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295
• CPC Class Codes: A61B 5/412 Detecting or monitoring sepsis

Abstract

The invention relates to an apparatus and a method for determining an approximate value for the stroke volume and the cardiac output of a person'"'"'s heart. The apparatus and method employ a measured electrical impedance, or admittance, of a part of a person'"'"'s body, namely, the thorax. This part of a person'"'"'s body is chosen because its electrical impedance, or admittance, changes with time as a consequence of the periodic beating of the heart. Accordingly, the measured electrical admittance or impedance can provide information about the performance of the heart as a pump.

Claims

1. An apparatus for determining an approximate value for a stroke volume SV (in milliliter) of a subject'"'"'s heart, comprising

a) means for measuring an electrical impedance Z(t) of a part of the subject'"'"'s body, wherein a value of said electrical impedance Z(t) changes with time t as a consequence of the beating of the heart;

b) means for determining a base impedance Z₀ as a part of said electrical impedance Z(t) which does not change significantly during a period of one cardiac cycle;

c) means for determining a peak magnitude

$<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br>$

of a temporal derivative

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

of said electrical impedance Z(t), indicating an absolute maximum rate of change of said electrical impedance Z(t) during a systolic period of the cardiac cycle;

d) means for determining a left ventricular ejection time, T_LVE;

e) means for determining the cardiac cycle period T_RR of the heart; and

f) means for calculating said approximate value of the stroke volume SV wherein said calculating means is adapted to evaluate a formula

$\mathrm{SV}=V_{\mathrm{EFF}}·<br><br>C_1·<br><br>{\left(\frac{<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br>}{Z_0}\right)}^n·<br><br>{\left(\frac{1}{T_{\mathrm{RR}}}\right)}^m·<br><br>T_{\mathrm{LVE}}$

wherein 0.15<

n<

0.8 and 0≦

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

2. The apparatus of claim 1, wherein m is non-zero.

3. The apparatus of claim 1, wherein 0.3≦

n≦

0.65.

4. The apparatus of claim 1, wherein 0.45≦

n≦

0.55.

5. The apparatus of claim 1, wherein m=1−

n.

6. The apparatus of claim 1, wherein C₁=1.

7. The apparatus of claim 5, wherein m=n=0.5.

8. The apparatus of claim 1, wherein

9. The apparatus of claim 8, wherein C₂=0.

10. The apparatus of claim 8, wherein C₃ is a value in the range of 0.01-15.

11. The apparatus of claim 10, wherein C₃ is approximately 13.

12. The apparatus of claim 8, wherein X is a value in the range of 0.9-1.1.

13. The apparatus of claim 12, wherein X is approximately 1.025.

14. The apparatus of claim 8, wherein N is a value in the range of 1.0-2.0.

15. The apparatus of claim 14, wherein N is approximately 1.5.

16. The apparatus of claim 8, wherein Z_C is a value in the range of 15-25 Ω

.

17. The apparatus of claim 16, wherein Z_C is approximately 20 Ω

.

18. The apparatus of claim 1, wherein V_EFF=C₃·

W^X (in milliliter), wherein W is the subject'"'"'s weight in kilogram, and wherein C₃ is a coefficient with a constant value and X is an exponent with constant value.

19. The apparatus of claim 18, wherein C₃ is a value in the range of 0.01-15.

20. The apparatus of claim 19, wherein C₃ is approximately 13.

21. The apparatus of claim 18, wherein X is a value in the range of 0.9-1.1.

22. The apparatus of claim 21, wherein X is approximately 1.025.

23. The apparatus of claim 1, wherein said means for measuring said electrical impedance Z(t) comprises:

at least two pairs of electrodes;

a current source generating an alternating current I(t) of predetermined amplitude;

wherein one pair of electrodes is adapted to be connected to said current source;

means for measuring a voltage U(t) caused by applying said alternating current;

wherein one pair of electrodes is connected to said means for measuring the voltage U(t);

means for determining said electrical impedance Z(t) from the voltage U(t) and the current I(t).

24. The apparatus of claim 1, wherein said means for determining the peak magnitude

25. The apparatus of claim 24, wherein said means for determining Δ

Z(t) is a high-pass filter.

26. The apparatus of claim 1, wherein said means for determining Z₀ is a low-pass filter.

27. The apparatus of claim 1, wherein said means for determining T_LVE determines T_LVE by determining by analysis of

28. The apparatus of claim 1, wherein said means for determining the cardiac cycle period T_RR comprises means for analyzing at least one of a group of Z(t), Δ

Z(t),

29. The apparatus of claim 1, wherein said means for determining the cardiac cycle period T_RR comprises means for measuring an electrocardiogram and means for analyzing the measured values.

30. The apparatus of claim 1, wherein at least one of said means for determining

31. The apparatus of claim 1, further comprising means for outputting a signal, which is representative of SV.

32. The apparatus of claim 1, further comprising means for visually displaying SV to a user.

33. The apparatus of claim 1, further comprising means for calculating an approximate value for a cardiac output C₀ of the subject'"'"'s heart (in liter/minute), wherein said calculating means is adapted to evaluate a formula

35. The apparatus of claim 34, wherein n=m=0.5, and wherein C₁=1.

37. The apparatus of claim 36, wherein n=m=0.5, and wherein C₁=1.

39. The apparatus of claim 38, wherein said means for calculating the cardiac cycle period T_RR is adapted to output a signal representative of T_RR to the processing unit.

40. The apparatus of claim 38 wherein n=m=0.5, and wherein C₁=1.

34. An apparatus for determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes; and

c) a processing unit receiving at least a signal representative of U(t), said processing unit being adapted to;

calculate an impedance Z(t) from the voltage U(t) and a value of the current I(t);

input Z(t) into a low-pass filter, an output of said low-pass filter being Z₀;

input Z(t) into a high-pass filter, an output of said high-pass filter being Δ

Z(t);

calculate a peak magnitude

$\left|{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}\right|\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Z(t), Δ

Z(t) and

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

determine a cardiac cycle period T_RR of the heart from at least one of Z(t), Δ

Z(t) and

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

calculate SV according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{\left|{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}\right|}{Z_0}\right)}^n·<br><br>{\left(\frac{1}{T_{\mathrm{RR}}}\right)}^m·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15≦

n≦

0.8 and 0<

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

36. An apparatus for determining an approximate value for the stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes;

c) means for measuring an electrocardiogram; and

d) a processing unit receiving at least a signal representative of U(t) and measured values of said electrocardiogram, said processing unit being adapted to;

calculate an impedance Z(t) from the voltage U(t) and a value of the current I(t);

input Z(t) into a low-pass filter, an output of said low-pass filter being Z₀;

input Z(t) into a high-pass filter, an output of said high-pass filter being Δ

Z(t);

calculate a peak magnitude of

$<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br><br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Z(t), Δ

Z(t) and

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

determine a cardiac cycle period T_RR of the heart from the measured values of said electrocardiogram;

calculate SV according to a formula

$\mathrm{SV}=V_{\mathrm{EFF}}·<br><br>{C_1\left(\frac{<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br>}{Z_0}\right)}^n·<br><br>{\left(\frac{1}{T_{\mathrm{RR}}}\right)}^m·<br><br>T_{\mathrm{LVE}}$

wherein 0.15<

n<

0.8 and 0<

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

38. An apparatus for determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes;

c) means for measuring an electrocardiogram and means for calculating a cardiac cycle period T_RR of the heart from measured values of said electrocardiogram;

d) a processing unit receiving at least a signal representative of U(t) and a signal representative of T_RR, said processing unit being adapted to;

calculate an impedance Z(t) from the voltage U(t) and a value of the current I(t);

input Z(t) into a low-pass filter, an output of said low-pass filter being Z₀;

input Z(t) into a high-pass filter, an output of said high-pass filter being Δ

Z(t);

calculate a peak magnitude

$<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br><br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Z(t), Δ

Z(t) and

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

calculate SV according to a formula

$\mathrm{SV}=V_{\mathrm{EFF}}·<br><br>{C_1\left(\frac{<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br>}{Z_0}\right)}^n·<br><br>{\left(\frac{1}{T_{\mathrm{RR}}}\right)}^m·<br><br>T_{\mathrm{LVE}}$

wherein 0.15<

n<

0.8 and 0<

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

41. An apparatus for determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes;

c) a processor unit receiving at least a signal representative of U(t), said processing unit being adapted to;

calculate an impedance Z(t) from the voltage U(t) and a value of the current I(t);

input Z(t) into a low-pass filter, an output of said low-pass filter being Z₀;

input Z(t) into a high-pass filter, an output of said high-pass filter being Δ

Z(t);

calculate a peak magnitude

$<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br><br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Z(t), Δ

Z(t) and

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

calculate SV according to a formula

$\mathrm{SV}=V_{\mathrm{EFF}}·<br><br>C_1·<br><br>{\left(\frac{<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br>}{Z_0}\right)}^n·<br><br>T_{\mathrm{LVE}}$

wherein 0.15<

n<

0.8, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

42. A method of determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising the steps of:

a) measuring an impedance Z(t) of a part of the subject'"'"'s body, wherein a value of said impedance Z(t) changes with time t as a consequence of the beating of the heart;

b) determining a mean impedance Z₀;

c) determining a peak magnitude

$<br><br>{\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MIN}}<br><br>$

of a derivative

$\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}$

of said impedance Z(t) over the time t by using the measured impedance Z(t) for at least a systolic period of one cardiac cycle;

d) determining a left ventricular ejection time, T_LVE; and

e) determining a cardiac cycle period T_RR of the heart;

f) calculating an approximate value of the stroke volume according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{|\left(\frac{<br><br>Z<br><br>\left(t\right)}{<br><br>t}\right)<br><br>{|}_{\mathrm{MIN}}}{Z_0}\right)}^n·<br><br>{\left(\frac{1}{T_{\mathrm{RR}}}\right)}^m·<br><br>T_{\mathrm{LVE}}$

wherein 0.15<

n<

0.8 and 0<

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

43. The method of claim 42, wherein m is non-zero.

44. The method of claim 42, wherein 0.3<

n<

0.65.

45. The method of claim 42, wherein 0.45<

n<

0.55.

46. The method of claim 42, wherein m=1−

n.

47. The method of claim 42, wherein C₁=1.

48. The method of claim 46, wherein m=n=0.5.

49. The method of claim 42, wherein

50. The apparatus of claim 49, wherein C₂=0.

51. The apparatus of claim 49, wherein C₃ is a value in the range of 0.01-15.

52. The apparatus of claim 51, wherein C₃ is approximately 13.

53. The apparatus of claim 49, wherein X is a value in the range of 0.9-1.1.

54. The apparatus of claim 53, wherein X is approximately 1.025.

55. The apparatus of claim 49, wherein N is a value comprised in the range of 1.0-2.0.

56. The apparatus of claim 55, wherein N is approximately 1.5.

57. The apparatus of claim 49, wherein Z_C is a value in the range of 15-25 Ω

.

58. The apparatus of claim 57, wherein Z_C is approximately 20 Ω

.

59. The apparatus of claim 42, wherein V_EFF=C₃·

W^X (in milliliter),

wherein W is the subject'"'"'s weight in kilogram, and wherein C₃ is a coefficient with a constant value and X is an exponent with constant value.

60. The apparatus of claim 59, wherein C₃ is a value in the range of 0.01-15.

61. The apparatus of claim 60, wherein C₃ is approximately 13.

62. The apparatus of claim 59, wherein X is a value in the range of 0.9-1.1.

63. The apparatus of claim 62, wherein X is approximately 1.025.

64. The method of claim 42, wherein said impedance Z(t) is measured by applying an alternating current I(t) through the part of the subject'"'"'s body, measuring a voltage drop U(t) in the body caused by the application of said alternating current, and calculating said impedance Z(t) according to a formula

65. The method of claim 42, wherein said peak magnitude

66. The method of claim 42, wherein Z₀ is determined by sending a signal representative of Z(t) through a low-pass filter, an output of said filter being Z₀.

67. The method of claim 42, wherein T_LVE is determined by

determining by analysis of dZ(t)

$\frac{}{}$

Z

(t)

t

a point in time when an aortic valve opens;

a point in time when the aortic valve closes; and

and by calculating a time difference of said closing point in time and said opening point in time.

68. The method of claim 42, wherein the cardiac cycle period T_RR is determined by analyzing at least one of the group of Z(t), Δ

Z(t),

69. The method of claim 42, wherein the cardiac cycle period T_RR is determined by

measuring an electrocardiogram, and

analyzing the measured values.

70. The method of claim 42, further comprising the step of determining an approximate value for a cardiac output C₀ of the subject'"'"'s heart, wherein C₀ (in liter/minute) is determined as the product of SV (in milliliter),

71. An apparatus for determining an approximate value for a stroke volume SV (in milliliter) of a subject'"'"'s heart, comprising

a) means for measuring an electrical admittance Y(t) of a part of the subject'"'"'s body, wherein a value of said electrical admittance Y(t) changes with time t as a consequence of the beating of the heart;

b) means for determining a base admittance Y₀ as a part of said electrical admittance Y(t) which does not change significantly during a period of one cardiac cycle;

c) means for determining a peak magnitude

${\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}$

of a temporal derivative

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

of said electrical admittance Y(t) , indicating an absolute maximum rate of change of said electrical admittance Y(t) during a systolic period of the cardiac cycle;

d) means for determining a left ventricular ejection time, T_LVE;

e) means for determining the cardiac cycle period T_RR of the heart; and

f) means for calculating said approximate value of the stroke volume SV wherein said calculating means is adapted to evaluate a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{{\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}}{Y_0}\right)}^n·<br><br>{\left(\frac{1}{T_{R<br><br>\text{\hspace{1em}<br><br>}<br><br>R}}\right)}^m·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15≦

n≦

0.8 and 0≦

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

72. The apparatus of claim 71, wherein m is non-zero.

73. The apparatus of claim 71, wherein 0.3≦

n≦

0.65.

74. The apparatus of claim 71, wherein 0.45≦

n≦

0.55.

75. The apparatus of claim 71, wherein m=1−

n.

76. The apparatus of claim 71, wherein C₁=1.

77. The apparatus of claim 75, wherein m=n=0.5.

78. The apparatus of claim 71, wherein

79. The apparatus of claim 78, wherein C₂=0.

80. The apparatus of claim 78, wherein C₃ is a value in the range of 0.01-15.

81. The apparatus of claim 80, wherein C₃ is approximately 13.

82. The apparatus of claim 78, wherein X is a value in the range of 0.9-1.1.

83. The apparatus of claim 82, wherein X is approximately 1.025.

84. The apparatus of claim 78, wherein N is a value in the range of 1.0-2.0.

85. The apparatus of claim 84, wherein N is approximately 1.5.

86. The apparatus of claim 78, wherein Y_C is a value in the range of 0.04-0.0667 Ω

<sup>−

1</sup>.

87. The apparatus of claim 86, wherein Y_C is approximately 0.05 Ω

<sup>−

1.</sup>

88. The apparatus of claim 71, wherein V_EFF=C₃·

W^X (in milliliter), wherein W is the subject'"'"'s weight in kilogram, and wherein C₃ is a coefficient with a constant value and X is an exponent with constant value.

89. The apparatus of claim 88, wherein C₃ is a value in the range of 0.01-15.

90. The apparatus of claim 89, wherein C₃ is approximately 13.

91. The apparatus of claim 88, wherein X is a value in the range of 0.9-1.1.

92. The apparatus of claim 91, wherein X is approximately 1.025.

93. The apparatus of claim 71, wherein said means for measuring said electrical admittance Y(t) comprises:

at least two pairs of electrodes;

a current source generating an alternating current I(t) of predetermined amplitude;

wherein one pair of electrodes is adapted to be connected to said current source;

means for measuring a voltage U(t) caused by applying said alternating current;

wherein one pair of electrodes is connected to said means for measuring the voltage U(t);

means for determining said electrical admittance Y(t) from the voltage U(t) and the current I(t).

94. The apparatus of claim 71, wherein said means for determining the peak magnitude

95. The apparatus of claim 94, wherein said means for determining Δ

Y(t) is a high-pass filter.

96. The apparatus of claim 71, wherein said means for determining Y₀ is a low-pass filter.

97. The apparatus of claim 71, wherein said means for determining T_LVE determines T_LVE by determining by analysis of

98. The apparatus of claim 71, wherein said means for determining the cardiac cycle period T_RR comprises means for analyzing at least one of a group of Y(t), Δ

Y(t),

99. The apparatus of claim 71, wherein said means for determining the cardiac cycle period T_RR comprises means for measuring an electrocardiogram and means for analyzing the measured values.

100. The apparatus of claim 71, wherein at least one of said means for determining

101. The apparatus of claim 71, further comprising means for outputting a signal, which is representative of SV.

102. The apparatus of claim 71, further comprising means for visually displaying SV to a user.

103. The apparatus of claim 71, further comprising means for calculating an approximate value for a cardiac output C₀ of the subject'"'"'s heart (in liter/minute), wherein said calculating means is adapted to evaluate a formula

104. An apparatus for determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes; and

c) a processing unit receiving at least a signal representative of U(t), said processing unit being adapted to;

calculate an admittance Y(t) from the voltage U(t) and a value of the current I(t);

input Y(t) into a low-pass filter, an output of said low-pass filter being Y₀;

input Y(t) into a high-pass filter, an output of said high-pass filter being Δ

Y(t);

calculate a peak magnitude

${\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Y(t), Δ

Y(t) and

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

determine a cardiac cycle period T_RR of the heart from at least one of Y(t), Δ

Y(t) and

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

calculate SV according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{{\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}}{Y_0}\right)}^n·<br><br>{\left(\frac{1}{T_{R<br><br>\text{\hspace{1em}<br><br>}<br><br>R}}\right)}^m·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15≦

n≦

0.8 and 0<

m<

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

105. The apparatus of claim 104, wherein n=m0.5, and wherein C₁=1.

106. An apparatus for determining an approximate value for the stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes;

c) means for measuring an electrocardiogram; and

d) a processing unit receiving at least a signal representative of U(t) and measured values of said electrocardiogram, said processing unit being adapted to;

calculate an admittance Y(t) from the voltage U(t) and a value of the current I(t);

input Y(t) into a low-pass filter, an output of said low-pass filter being Y₀;

input Y(t) into a high-pass filter, an output of said high-pass filter being Δ

Y(t);

calculate a peak magnitude

${\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Y(t), Δ

Y(t) and

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

determine a cardiac cycle period T_RR of the heart from the measured values of said electrocardiogram;

calculate SV according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{{\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}}{Y_0}\right)}^n·<br><br>{\left(\frac{1}{T_{R<br><br>\text{\hspace{1em}<br><br>}<br><br>R}}\right)}^m·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15<

n<

0.8 and 0<

m<

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

107. The apparatus of claim 106, wherein n=m=0.5, and wherein C₁=1.

108. An apparatus for determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes;

c) means for measuring an electrocardiogram and means for calculating a cardiac cycle period T_RR of the heart from measured values of said electrocardiogram;

d) a processing unit receiving at least a signal representative of U(t) and a signal representative of T_RR, said processing unit being adapted to;

calculate an admittance Y(t) from the voltage U(t) and a value of the current I(t);

input Y(t) into a low-pass filter, an output of said low-pass filter being Y₀;

input Y(t) into a high-pass filter, an output of said high-pass filter being Δ

Y(t);

calculate a peak magnitude

${\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Y(t), Δ

Y(t) and

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

calculate SV according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{{\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}}{Y_0}\right)}^n·<br><br>{\left(\frac{1}{T_{R<br><br>\text{\hspace{1em}<br><br>}<br><br>R}}\right)}^m·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15<

n<

0.8 and 0<

m<

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

109. The apparatus of claim 108, wherein said means for calculating the cardiac cycle period T_RR is adapted to output a signal representative of T_RR to the processing unit.

110. The apparatus of claim 108 wherein n=m=0.5, and wherein C₁=1.

111. An apparatus for determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising

a) a current source outputting an alternating current I(t) of predetermined amplitude to two electrodes;

b) means for measuring a voltage U(t) caused by said alternating current between two electrodes;

c) a processor unit receiving at least a signal representative of U(t), said processing unit being adapted to;

calculate an admittance Y(t) from the voltage U(t) and a value of the current I(t);

input Y(t) into a low-pass filter, an output of said low-pass filter being Y₀;

input Y(t) into a high-pass filter, an output of said high-pass filter being Δ

Y(t);

calculate a peak magnitude

${\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t};<br><br>$

determine a left ventricular ejection time T_LVE from at least one of Y(t), Δ

Y(t) and

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

by using predetermined criteria;

calculate SV according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{{\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}}{Y_0}\right)}^n·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15<

n<

0.8, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.

113. The method of claim 112, wherein m is non-zero.

114. The method of claim 112, wherein 0.3<

n<

0.65.

115. The method of claim 112, wherein 0.45<

n<

0.55.

116. The method of claim 112, wherein m=1−

n.

117. The method of claim 112, wherein C₁=1.

118. The method of claim 116, wherein m=n=0.5.

119. The method of claim 112, wherein

120. The apparatus of claim 119, wherein C₂=0.

121. The apparatus of claim 119, wherein C₃ is a value in the range of 0.01-15.

122. The apparatus of claim 121, wherein C₃ is approximately 13.

123. The apparatus of claim 119, wherein X is a value in the range of 0.9-1.1.

124. The apparatus of claim 123, wherein X is approximately 1.025.

125. The apparatus of claim 119, wherein N is a value comprised in the range of 1.0-2.0.

126. The apparatus of claim 125, wherein N is approximately 1.5.

127. The apparatus of claim 119, wherein Y_C is a value in the range of 0.04-0.0667 Ω

<sup>−

1</sup>.

128. The apparatus of claim 127, wherein Y_C is approximately 0.05 Ω

<sup>−

1</sup>.

129. The apparatus of claim 112, wherein V_EFF=C₃·

W^X (in milliliter),

wherein W is the subject'"'"'s weight in kilogram, and wherein C₃ is a coefficient with a constant value and X is an exponent with constant value.

130. The apparatus of claim 129, wherein C₃ is a value in the range of 0.01-15.

131. The apparatus of claim 130, wherein C₃ is approximately 13.

132. The apparatus of claim 129, wherein X is a value in the range of 0.9-1.1.

133. The apparatus of claim 132, wherein X is approximately 1.025.

134. The method of claim 112, wherein said admittance Y(t) is measured by applying an alternating current I(t) through the part of the subject'"'"'s body, measuring a voltage drop U(t) in the body caused by the application of said alternating current, and calculating said admittance Y(t) according to a formula

135. The method of claim 112, wherein said peak magnitude

136. The method of claim 112, wherein Y₀ is determined by sending a signal representative of Y(t) through a low-pass filter, an output of said filter being Y₀.

137. The method of claim 112, wherein T_LVE is determined by

determining by analysis of

$\frac{}{}$

Y

(t)

t

a point in time when an aortic valve opens;

a point in time when the aortic valve closes; and

and by calculating a time difference of said closing point in time and said opening point in time.

138. The method of claim 112, wherein the cardiac cycle period T_RR is determined by analyzing at least one of the group of Y(t), Δ

Y(t),

139. The method of claim 112, wherein the cardiac cycle period T_RR is determined by

measuring an electrocardiogram, and

analyzing the measured values.

140. The method of claim 112, further comprising the step of determining an approximate value for a cardiac output C₀ of the subject'"'"'s heart, wherein C₀ (in liter/minute) is determined as the product of SV (in milliliter),

112. A method of determining an approximate value for a stroke volume SV of a subject'"'"'s heart, comprising the steps of:

a) measuring an admittance Y(t) of a part of the subject'"'"'s body, wherein a value of said admittance Y(t) changes with time t as a consequence of the beating of the heart;

b) determining a mean admittance Y₀;

c) determining a peak magnitude

${\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}$

of a derivative

$\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}$

of said admittance Y(t) over the time t by using the measured admittance Y(t) for at least a systolic period of one cardiac cycle;

d) determining a left ventricular ejection time, T_LVE; and

e) determining a cardiac cycle period T_RR of the heart;

f) calculating an approximate value of the stroke volume according to a formula

$S<br><br>\text{\hspace{1em}<br><br>}<br><br>V=V_{E<br><br>\text{\hspace{1em}<br><br>}<br><br>F<br><br>\text{\hspace{1em}<br><br>}<br><br>F}·<br><br>C_1·<br><br>{\left(\frac{{\left(\frac{<br><br>Y<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}}{Y_0}\right)}^n·<br><br>{\left(\frac{1}{T_{R<br><br>\text{\hspace{1em}<br><br>}<br><br>R}}\right)}^m·<br><br>T_{L<br><br>\text{\hspace{1em}<br><br>}<br><br>V<br><br>\text{\hspace{1em}<br><br>}<br><br>E}$

wherein 0.15<

n<

0.8 and 0<

m≦

1.5, and wherein V_EFF is an approximate value of the subject'"'"'s volume of electrically participating tissue, and wherein C₁ is a constant.