Impedance cardiograph apparatus and method

• US 5,505,209 A
• Filed: 07/07/1994
• Issued: 04/09/1996
• Est. Priority Date: 07/07/1994
• Status: Expired due to Term

• Current Assignee: INTELLECTUAL PROPERTY, LLC
• Original Assignee: Reining International, Ltd.
• Inventors: Reining, William N.
• Primary Examiner(s): Sykes, Angela D.
• Assistant Examiner(s): NASSER, ROBERT L
• Application Number:
  US08/271,689
• Time in Patent Office: 642 Days
• Field of Search: 128/668, 128/693, 128/713, 128/734
• US Class Current: 600/547
• 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

Abstract

An impedance cardiograph which determines cardiac output from a measurement of variations of chest impedance, provides an improved method of calculating the effect of patient volume on the measurement from measurable patient height and chest circumference, and provides a correction process that identifies as a source of error variations in the first derivative of impedance. This latter source of error is minimized by preprocessing the impedance derivative value with a compression function which reduces the range of values of the impedance derivative when that value differs significantly from a norm of the population.

Claims

1. An impedance cardiograph used to evaluate cardiac output of a human patient having a height and a chest circumference and chest cross-sectional area A comprising:

means for applying an electrical excitation signal to the chest of a patient;

electrodes, adapted to be positioned on the patient with a separation distance of L thereby being responsive to the excitation signal, for producing a first electrical signal Z which varies with impedance changes in the patient;

an input device for receiving values of the height and circumference and providing corresponding second and third electrical signals H and C indicating those values;

electrical circuit means for receiving the first, second and third electrical signals and providing an indication of the patient'"'"'s cardiac stroke volume SV as a function of Z, A, and L;

where A is deduced by the following approximation;

##EQU35## where K is a predetermined constant.

2. The impedance cardiograph of claim 1 wherein the electric circuit means calculates SV according to the following formula:

##EQU36## where ρ

is the resistivity of blood;

Z₀ is an average baseline impedance;

L_vet is a left ventricular ejection interval; and

##EQU37## is a function of the time derivative of the impedance Z.

3. The impedance cardiograph of claim 2 wherein ##EQU38## is a magnitude of the minimum time derivative of Z.

4. The impedance cardiograph of claim 2 wherein ##EQU39## is a compressed value of a time derivative of Z about a predetermined normal value so that ##EQU40## has a value less than the time derivative of Z if the time derivative of Z is greater than said predetermined normal value and ##EQU41## has a value greater than the time derivative of Z if the time derivative of Z is less than the predetermined normal value.

5. The impedance cardiograph of claim 1 wherein K is substantially 0.875.

6. An impedance cardiograph used to evaluate cardiac output of a human patient comprising:

means for applying an electrical excitation signal to the chest of a patient;

electrodes, adapted to be positioned on the patient for producing a first electrical signal Z which varies with impedance changes in the patient;

an electronic processor for calculating a time derivative of Z, and providing a non-linearly non-linearly compressed derivative, having a value less than the time derivative of Z if the time derivative of Z is greater than a predetermined normal value and having a value greater than the time derivative of Z if the time derivative of Z is less than the predetermined normal value;

electrical circuit means for providing an indication of patient'"'"'s cardiac stroke volume SV as a function of the compressed time derivative.

7. The impedance cardiograph of claim 6 wherein the electronic processor produces the compressed value of the time derivative of Z according to the following formula:

##EQU42## where;

##EQU43## is a time derivative of Z ##EQU44## is the compressed time derivative of Z ##EQU45## is the predetermined normal value and is an average value of ##EQU46## for a population.

8. The impedance cardiograph of claim 6 wherein the predetermined normal value is substantially 1.73 ohms per second.

9. A method of evaluating the cardiac output of a human patient having a height H and a chest circumference C and chest cross-sectional area A comprising the step of:

(a) applying an electrical excitation signal to the chest of a patient;

(b) positioning electrodes on the patient with a separation distance of L thereby being responsive to the excitation signal to produce a first electrical signal Z which varies with impedance changes in the patient;

(c) employing an electronic computer to;

(i) receive values of the height and circumference and providing corresponding second and third electrical signals H and C indicating those values;

(ii) evaluate the patient'"'"'s cardiac stroke volume SV as a function of Z, A, L and left ventricular ejection interval L_vet ;

where A is deduced by the following approximation;

##EQU47## in which K is a predetermined constant.

10. The method of claim 9 including calculating SV using said electronic computer according to the following formula:

##EQU48## where ρ

is the resistivity of blood;

Z₀ is an average baseline impedance; and

##EQU49## is a function of the time derivative of the impedance Z.

11. The method of claim 10 wherein ##EQU50## is a magnitude of the minimum time derivative of Z.

12. The method of claim 10 wherein ##EQU51## is a compressed value of the time derivative of Z about a predetermined normal value so that ##EQU52## has a value less than the time derivative of Z if the time derivative of Z is greater than said predetermined normal value and ##EQU53## has a value greater than the time derivative of Z if the time derivative of Z is less than the predetermined normal value.

13. The method of claim 12 wherein K is substantially 0.875.

14. A method of evaluating cardiac output of a human patient comprising the steps of:

(a) applying an electrical excitation signal to the chest of a patient;

(b) positioning electrodes on the patient for producing a first electrical signal Z which varies with impedance changes in the patient;

(c) determining a compressed derivative of the impedance signal Z, having a value less than the time derivative of Z if the time derivative of Z is greater than a predetermined normal value and having a value greater than the time derivative of Z if the time derivative of Z is less than the predetermined normal value; and

(e) evaluating the patient'"'"'s cardiac stroke volume SV as a function of the compressed time derivative.

15. The method of claim 14 step (c) is according to the following formula:

##EQU54## where;

##EQU55## is the magnitude of the minimum time derivative of Z ##EQU56## is the compressed time derivative of z ##EQU57## is the predetermined normal value and is an average value of ##EQU58## for a population.

16. The method of claim 14 wherein the predetermined normal value is substantially 1.73 ohms per second.