Apparatus and method for measuring cardiac output

• US 5,423,326 A
• Filed: 05/14/1993
• Issued: 06/13/1995
• Est. Priority Date: 09/12/1991
• Status: Expired due to Term

• Current Assignee: Caldwell Simpson LLC
• Original Assignee: Drexel University
• Inventors: Wang, Xiang, Sun, Hun H.
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): Jastrzab, Jeffrey R.
• Application Number:
  US08/061,360
• Time in Patent Office: 760 Days
• Field of Search: 128/670-672, 128/693-696, 128/700, 128/713, 128/687, 128/734
• US Class Current: 600/526
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295
• CPC Class Codes: A61B 5/316 Modalities, i.e. specific diagnostic methods
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives
• CPC Class Codes: A61B 5/7242 using integration
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

The present invention reliably computes cardiac output with a noninvasive procedure. The system measures the electrical impedance of a patient'"'"'s body, during a time interval of interest. The system then obtains frequency transforms of various time segments of the first derivative of the impedance signal. Each such transform is integrated over a frequency range of interest, and the values of the integrals are plotted as a function of time. The graph so derived has characteristic extrema which can be used to identify critical points in the impedance derivative signal. These critical points can be used to determine parameters which are used in a calculation of stroke volume and cardiac output.

Claims

1. A method of measuring cardiac output of a patient, the method comprising the steps of:

a) measuring electrical impedance between two locations on the patient, for a predetermined first time interval, thereby obtaining an impedance signal over said first time interval,

b) differentiating said impedance signal to obtain a signal representative of a first derivative of said impedance signal over said first time interval,

c) computing a frequency transform of said first derivative of said impedance signal, for each of a plurality of discrete second time intervals, each second time interval being at least an order of magnitude shorter than said first time interval, each of said frequency transforms comprising a function of power versus frequency at one of said second time intervals,

d) computing a plurality of,definite integrals of each of said frequency transforms, the definite integrals being taken over a predetermined range of frequencies,

e) assembling a graph having an abscissa which comprises time, and an ordinate comprising one of said definite integrals which corresponds to a time represented by the abscissa,

f) identifying points in time at which extrema appear in the graph produced in step (e), and determining corresponding points in time in said first derivative of said impedance signal,

g) using said points in the first derivative of said impedance signal to determine a maximum excursion of said first derivative of said impedance signal and to determine a time interval corresponding to ventricular ejection time of the patient, and

h) calculating cardiac output according to the maximum first derivative of said impedance signal and the ventricular ejection time determined in step (g).

2. The method of claim 1, wherein the frequency transform in step (c) comprises a Fourier transform.

3. The method of claim 1, further comprising the steps of measuring an electrocardiogram of the patient, and using the electrocardiogram to determine a time at which said first time interval begins.

4. A method of measuring cardiac output, the method comprising the steps of:

a) obtaining a signal corresponding to a first derivative of electrical impedance of a patient, with respect to time,

b) deriving, for each moment in time, a function which relates intensity of the signal to its frequency, and deriving a graph corresponding to said function,

c) computing a definite integral of each graph derived in step (b), over a frequency range of interest, and plotting said computed definite integrals as a function of time to form another graph,

d) comparing extrema of the graph formed in step (c) with the signal obtained in step (a) to identify points on the signal of step (a) which determine a maximum impedance derivative and a ventricular ejection time, and

e) computing cardiac output from knowledge of the maximum impedance derivative and the ventricular ejection time

5. The method of claim 4, wherein the function derived in step (b) comprises a Fourier transform.

6. The method of claim 4, further comprising the steps of measuring an electrocardiogram of the patient, and using the electrocardiogram to select a time at which the impedance derivative signal is considered to begin.

7. A method of measuring cardiac output of a patient, the method comprising the steps of:

a) measuring electrical impedance of the patient'"'"'s body, and computing a first graph of said impedance as a function of time,

b) computing a plurality of definite integrals of a transform of a signal represented in said first graph, over a frequency range of interest, and deriving a second graph by plotting said definite integrals as a function of time, and

c) comparing said first graph with said second graph, said second graph having a recognizable pattern of extrema, wherein the extrema of the second graph identify points on the first graph which determine parameters from which one can calculate cardiac output, and using said parameters to calculate cardiac output.

8. The method of claim 7, wherein the transform of step (b) comprises a Fourier transform.

9. The method of claim 7, further comprising the steps of measuring an electrocardiogram of the patient, and using the electrocardiogram to select a time at which said first graph begins.

10. Apparatus for measuring cardiac output of a patient, the apparatus comprising:

a) means for comparing a first graph derived by measuring electrical impedance of the patient'"'"'s body, with a second graph having extrema, wherein the extrema of the second graph identify points on the first graph which determine parameters from which one can calculate cardiac output, and means for using said parameters to calculate cardiac output, and

b) means for plotting a definite integral of a transform of a signal represented in said first graph, over a frequency range of interest, as a function of time, the plotting means having an output, the plotting means including means for connecting said output to the comparing means wherein the second graph is obtained from said means for plotting.

11. The apparatus of claim 10, wherein the comparing means comprises a programmable computer.

12. Apparatus for determining cardiac output of a patient, the apparatus comprising:

a) at least one pair of electrodes connectable to the patient, a source of electric current connected to the electrodes, and means for determining instantaneous body impedance of the patient connected to the electrodes,

b) computer means for receiving and processing data from the impedance determining means,

c) wherein the computer means comprises means for calculating a plurality of frequency transforms of a first derivative of the impedance, for each of a plurality of periods of time, generating a graph formed by plotting definite integrals of said frequency transforms, over a frequency range of interest, against times corresponding to said frequency transforms, comparing the graph with the first derivative of the impedance to determine parameters necessary to calculate cardiac output, and using said parameters to calculate cardiac output.