CA2352403A1 - Apparatus and method for determining an approximation of the stroke volume and the cardiac output of the heart - Google Patents

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

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CA2352403A1
CA2352403A1 CA002352403A CA2352403A CA2352403A1 CA 2352403 A1 CA2352403 A1 CA 2352403A1 CA 002352403 A CA002352403 A CA 002352403A CA 2352403 A CA2352403 A CA 2352403A CA 2352403 A1 CA2352403 A1 CA 2352403A1
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pass filter
heart
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CA2352403C (en
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Donald P. Bernstein
Markus J. Osypka
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Osypka Medical GmbH
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/41—Detecting, measuring or recording for evaluating the immune or lymphatic systems
    • A61B5/412—Detecting or monitoring sepsis
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
    • A61B5/026—Measuring blood flow
    • A61B5/029—Measuring or recording blood output from the heart, e.g. minute volume
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
    • A61B5/026—Measuring blood flow
    • A61B5/0295—Measuring blood flow 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
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/053—Measuring electrical impedance or conductance of a portion of the body
    • A61B5/0535—Impedance plethysmography

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  • Immunology (AREA)
  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
  • Radiology & Medical Imaging (AREA)
  • Measurement And Recording Of Electrical Phenomena And Electrical Characteristics Of The Living Body (AREA)
  • Measuring Pulse, Heart Rate, Blood Pressure Or Blood Flow (AREA)

Abstract

The invention relates to an apparatus and a method for deter-mining 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 peri-odic beating of the heart. Accordingly, the measured electri-cal admittance or impedance can provide information about the performance of the heart as a pump.

Claims (140)

1. An apparatus for determining an approximate value for a stroke volume SV (in milliliter) of a subject's heart, com-prising a) means for measuring an electrical impedance Z(t) of a part of the subject's body, wherein a value of said electri-cal 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 sig-nificantly during a period of one cardiac cycle;
c) means for determining a peak magnitude of a temporal derivative 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 wherein 0.15 <= n <= 0.8 and 0 <= m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume 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 C1.45 <= 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 (in milliliter), wherein W is the subject's weight in kilogram (kg), wherein C3 is a coefficient with constant value and X and N
are exponents with constant values, wherein mZ~ =1 for Z0 >= Z C, and mZ~ for Z0 < Z c, and wherein Z c is a constant, and wherein C2 is a constant.
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 - 2Ø
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 .OMEGA..
17. The apparatus of claim 16, wherein Z c is approximately 20 .OMEGA..
18. The apparatus of claim 1, wherein V EFF =C8 .cndot.W x (in milli-liter), 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 approxi-mately 1.025.
23. The apparatus of claim 1, wherein said means for measur-ing 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 con-nected to said current source;
- means for measuring a voltage U(t) caused by applying said alternating current;
- wherein one pair of electrodes in 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 raid means for deter-mining the peak magnitude comprises:
- means for determining .DELTA.Z(t) from Z(t);
- means for calculating for at least the systolic period of one cardiac cycle;
- means for determining the maximum of an input func-tion.
25. The apparatus of claim 24, wherein said means for deter-mining .DELTA.Z(t) is a high-pass filter.
26. The apparatus of claim 1, wherein said means for deter-mining Z0 is a low-pass filter.
27. The apparatus of claim 1, wherein said means for deter-mining T LVE determines T LVE by determining by analysis of - a point in time when an aortic valve opens;
- a point in time when the aortic valve closes;
and by calculating a time difference of said closing point in time and said opening point in time.
28. The apparatus of claim 1, wherein said means for deter-mining the cardiac cycle period T RR comprises means for ana-lyzing at least one of a group of Z(t),.DELTA.Z(t), and
29. The apparatus of claim 1, wherein said means for deter-mining the cardiac cycle period T RR comprises means for meas-uring an electrocardiogram and means for analyzing the meas-ured values.
30. The apparatus of claim 1, wherein at least one of said means for determining Zo, T LVE and T RR and said means for calculating are comprised in a processing unit.
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 CO of the subject's heart (in liter/minute), wherein said calculat-ing means is adapted to evaluate a formula
34. An apparatus far 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 al-ternating current between two electrodes; and c) a processing unit receiving at least a signal repre-sentative 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 falter, an output of said low-pass filter being Zo;

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

- calculate a peak magnitude of - determine a left ventricular ejection time T LVE
from at least one of Z(t), .DELTA.Z (t) and by using predeter-mined criteria;

- determine a cardiac cycle period T RR of the heart from at least one of Z(t), .DELTA.Z(t) and by using predeter-mined criteria;

- calculate SV according to a formula wherein 0.15 <= n <= 0.8 and 0 < m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume of electrically participating tissue, and wherein C1 is a constant.
35. The apparatus of claim 34, wherein n = m = 0.5, and wherein C1 = 1.
36. An apparatus for determining an approximate value for the stroke volume SV of a subject's heart, comprising
37 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 al-ternating current between two electrode;

c) means for measuring an electrocardiogram; and d) a processing unit receiving at least a signal repre-sentative of U(t) and measured values of said electrocardio-gram, 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 Zo;

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

- calculate a peak magnitude of - determine a left ventricular ejection time T LVE
from at least one of Z(t), .DELTA.Z(t) and by using predeter-mined criteria;

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

- calculate SV according to a formula wherein 0.15 < n < 0.8 and 0 < m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume of electrically participating tissue, and wherein C1 is a constant.
38 37. The apparatus of claim 36, wherein n = m = 0.5, and wherein C1 = 1.

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 al-ternating 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 repre-sentative 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 Zo;

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

- calculate a peak magnitude of of ;

- determine a left ventricular ejection time T LVE
from at least one of Z(t), .DELTA.Z(t) and by using predeter-mined criteria;

- calculate SV according to a formula wherein 0.15 < n < 0.8 and 0 < m <= 1.5,
39 and wherein VEFF is an approximate value of the subject's vol-ume of electrically participating tissue, and wherein C1 is a constant.

39. The apparatus of claim 38, wherein said means for calcu-lating 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.
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 al-ternating current between two electrodes;

c) a processor unit receiving at least a signal repre-sentative 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 falter, an output of said low-pass filter being Zo;

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

- calculate a peak magnitude of =IMG;

- determine a left ventricular ejection time T LVE
from at least one of Z(t), .DELTA.Z(t) and by using predeter-mined criteria;

- calculate SV according to a formula wherein 0.15 < n < 0.8, and wherein V EFF is an approximate value of the subject's vol-ume 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 sub-ject'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 of a deriva-tive 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 wherein 0.15 < n < 0.8 and 0 < m < 1.5, and wherein V EFF is an approximate value of the subject's vol-ume 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 V EFF = (in milliliter) , wherein W is the subject's weight in kilogram (kg), wherein C3 is a coefficient with constant value and X and N
are exponents with constant values, wherein mZ~ =1 for Z0 >= Z c, and for Z0 < Z c, and wherein Z c is a constant, and wherein C2 is a constant.
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 com-prised in the range of 1.0 - 2Ø
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 .OMEGA..
58. The apparatus of claim 57, wherein Z c is approximately 20 .OMEGA..
59. The apparatus of claim 42, wherein V EFF = C s-W x (in mil-liliter), 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 approxi-mately 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 cur-rent, and calculating said impedance Z(t) according to a for-mula .
65. The method of claim 42, wherein said peak magnitude is determined by:
- sending a signal representative of Z(t) through a high-pass filter, an output of said filter being taken to be .DELTA.Z (t) , - calculating for at least the systolic period of one cardiac cycle;
- determining an absolute magnitude of said derivative.
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 - 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) , .DELTA.Z (t) , and for at least two consecutive cardiac cycles.
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 CO of the subject's heart, wherein CO (in liter/minute) is deter-mined as the product of SV (in milliliter) , and a con-stant:

71. An apparatus for determining an approximate value for a stroke volume SV (in milliliter) of a subject's heart, com-prising a) means for measuring an electrical admittance Y(t) of a part of the subject's body, wherein a value of said elec-trical 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 sig-nificantly during a period of one cardiac cycle;
c) means for determining a peak magnitude of a temporal derivative 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 wherein 0.15 <= n <= 0.8 and 0 <= m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume 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 V EFF = C3 ~W X ~mY~~ (in milliliter) , wherein W is the subject's weight in kilogram (kg), wherein C3 is a coefficient with constant value and X and N
are exponents with constant values, wherein mY~ =1 for Y0 <= Y0, and for Y0 > Y c.
and wherein Y c is a constant, and wherein C2 is a constant.
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 - 2Ø
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 .OMEGA.-1.
87. The apparatus of claim 86, wherein Y c is approximately 0.05 .OMEGA.-1.
88. The apparatus of claim 71, wherein V EFF -C3-W x (in mil-liliter), 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 approxi-mately 1.025.
93. The apparatus of claim 71, wherein said means for meas-uring 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 con-nected 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 deter-mining the peak magnitude comprises:
- means for determining .DELTA.Y(t) from Y(t);
- means for calculating for at least the systolic period of one cardiac cycle;
- means for determining the maximum of an input func-tion.
95. The apparatus of claim 94, wherein said means for deter-mining .DELTA.Y(t) is a high-pass filter.
96. The apparatus of claim 71, wherein said means for deter-mining Y0 is a low-pass filter.
97. The apparatus of claim 71, wherein said means for deter-mining T LVE determines T LVE by determining by analysis of - a point in time when an aortic valve opens;
- a point in time when the aortic valve closes;
and by calculating a time difference of said closing point in time and said opening point in time.
98. The apparatus of claim 71, wherein said means for deter-mining the cardiac cycle period T RR comprises means for ana-lyzing at least one of a group of Y(t), .DELTA.Y(t), and
99. The apparatus of claim 71, wherein said means for deter-mining the cardiac cycle period T RR comprises means for meas-uring an electrocardiogram and means for analyzing the meas-ured values.
100. The apparatus of claim 71, wherein at least one of said means for determining , Y0, T LVE and T RR and said means for calculating are comprised in a processing unit.
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 CO of the subject's heart (in liter/minute), wherein said calculat-ing 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 repre-sentative 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 .DELTA.Y(t);
- calculate a peak magnitude ;
- determine a left ventricular ejection time T LVE
from at least one of Y(t) , .DELTA.Y(t) and by using predeter-mined criteria;
- determine a cardiac cycle period T RR of the heart from at least one of Y (t) , .DELTA.Y (t) and by using predeter-mined criteria;
- calculate SV according to a formula wherein 0.15 <= n <= 0.8 and 0 < m <=1.5, and wherein V EFF is an approximate value of the subject's vol-ume of electrically participating tissue, and wherein C1 is a constant.
105. The apparatus of claim 104, wherein n = m = 0.5, 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 al-ternating current between two electrodes;
c) means for measuring an electrocardiogram; and d) a processing unit receiving at least a signal repre-sentative of U(t) and measured values of said electrocardio-gram, 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 .DELTA.Y(t);
- calculate a peak magnitude <IMGS;
- determine a left ventricular ejection time T LVE
from at least one of Y(t), .DELTA.Y(t) and by using predeter-mined criteria;

- determine a cardiac cycle period T RR of the heart from the measured values of said electrocardiogram;
- calculate SV according to a formula wherein 0.15 < n < 0.8 and 0 < m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume 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 al-ternating 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 repre-sentative 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 .DELTA.Y(t);

- calculate a peak magnitude of - determine a left ventricular ejection time T LVE
from at least one of Y(t), .DELTA.Y(t) and by using predeter-mined criteria;
- calculate SV according to a formula wherein 0.15 < n < 0.8 and 0 < m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume of electrically participating tissue, and wherein C1 is a constant.
109. The apparatus of claim 108, wherein said means for cal-culating 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 al-ternating current between two electrodes;
c) a processor unit receiving at least a signal repre-sentative 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 .DELTA.Y(t);
- calculate a peak magnitude of - determine a left ventricular ejection time T LVE
from at least one of Y(t), .DELTA.Y(t) and by using predeter-mined criteria;
- calculate SV according to a formula wherein 0.15 < n < 0.8, and wherein V EFF is an approximate value of the subject's vol-ume of electrically participating tissue, and wherein C1 is a constant.
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 sub-ject'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 of a deriva-tive 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 wherein 0.15 < n < 0.8 and 0 < m <= 1.5, and wherein V EFF is an approximate value of the subject's vol-ume 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 V EFF =C3.cndot.W X .cndot.mY~N (in milliliter), wherein W is the subject's weight in kilogram (kg), wherein C3 is a coefficient with constant value and X and N
are exponents with constant values, wherein mY~ =1 for Y0 <= Y c, and mY~ = for Y0 > Y c, and wherein Y c is a constant, and wherein C2 is a constant.
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 com-prised in the range of 1.0 - 2Ø
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 .OMEGA.-1.
128. The apparatus of claim 127, wherein Y c is approximately 0.05 .OMEGA.-1.
129. The apparatus of claim 112, wherein V EFF =C3 .cndot.W x (in mil-liliter), 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 approxi-mately 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 cur-rent, and calculating said admittance Y(t) according to a formula .
135. The method of claim 112, wherein said peak magnitude is determined by:
- sending a signal representative of Y(t) through a high-pass filter, an output of said filter being taken to be .DELTA.Y(t), - calculating for at least the systolic period of one cardiac cycle;
- determining an absolute magnitude of said derivative.
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 - 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 pe-riod T RR is determined by analyzing at least one of the group of Y(t), .DELTA.Y(t), for at least two consecu-tive cardiac cycles.
139. The method of claim 112, wherein the cardiac cycle pe-riod 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 CO of the subject's heart, wherein CO (in liter/minute) is deter-mined as the product of SV (in milliliter), and a con-stant:
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