Apparatus and method for determining cardiac output in a living subject

• US 6,636,754 B1
• Filed: 07/10/2000
• Issued: 10/21/2003
• Est. Priority Date: 07/10/2000
• Status: Expired due to Term

• Current Assignee: FUJIFILM SonoSite, Inc. (Fujifilm Holdings Corporation)
• Original Assignee: Cardiodynamics International Corporation (Fujifilm Holdings Corporation)
• Inventors: Elf, James O., Baura, Gail D.
• Primary Examiner(s): Getzow, Scott M.
• Application Number:
  US09/613,183
• Time in Patent Office: 1,198 Days
• Field of Search: 600/372, 600/382, 600/393, 600/394, 600/509, 607/28, 607/152, 607/149
• US Class Current: 600/393
• CPC Class Codes: A61B 2562/0217 Electrolyte containing
• CPC Class Codes: A61B 5/02028 Determining haemodynamic parameters not otherwise provided for, e.g. cardiac contractility or left ventricular ejection fraction
• 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 5/282 Holders for multiple electrodes for introduction into the body A61B5/287
• CPC Class Codes: A61B 5/6843 Monitoring or controlling sensor contact pressure

Abstract

An improved apparatus and method for determining the cardiac output of a living subject. The improved apparatus generally comprises one or more electrode assemblies or patches affixed to the skin of the subject in the vicinity of the thoracic cavity. The terminals of each electrode patch are in contact with an electrolytic gel, and are spaced a predetermined distance from one another within the patch. This predetermined spacing allows for more consistent measurements, and also allows for the detection of a loss of electrical continuity between the terminals of the patch and their associated electrical connectors in the clinical environment. The method generally comprises generating and passing a stimulation current through the terminals and the thoracic cavity of the subject, and measuring the impedance as a function of time. This impedance is used to determine cardiac muscle stroke volume, which is then used in conjunction with the subject'"'"'s cardiac rate (also detected via the electrode patches) to determine cardiac output. A method of detecting a loss of electrical continuity in one or more of the terminals of the electrode patch is also disclosed.

Claims

1. A system for determining the cardiac output of a living subject comprising:

a plurality of electrode assemblies, each electrode assembly having a plurality of terminals, at least two of said plurality of terminals being spaced from one another by a predetermined distance;

a current source capable of generating a substantially constant current;

a plurality of electrical leads connecting said current source with individual ones of said terminals of said electrode assemblies,

a circuit for measuring the difference in voltage at said terminals resulting from the flow of said current through said subject and said terminals under varying cardiac conditions of said subject; and

a circuit for measuring ECG potentials from at least one of said electrode assemblies.

2. The system of claim 1, further comprising a circuit for measuring the difference in the impedance of at least two of said terminals as a function of time, said difference being compared to a first value to evaluate the electrical continuity of at least one of said terminals, said first value being based at least in part on said predetermined distance.

3. The system of claim 1, wherein said electrode assemblies comprise:

a substrate, said substrate having a plurality of apertures formed therein;

a plurality of terminals disposed within respective ones of said apertures, at least a portion of each of said terminals being capable of conducting an electrical current; and

at least one gel element, said at least one gel element adapted to transfer electrical current between the skin of said subject and said at least two of said plurality of terminals.

4. The system of claim 3, wherein said circuit for measuring ECG potentials comprises a circuit adapted to measure body surface potentials between at least two of said terminals in order to identify a plurality of QRS complex events within said subject.

5. The system of claim 4, further comprising a processor in data communication with both said circuit for measuring the difference in voltage and said circuit for measuring ECG potentials, said processor being adapted to determine cardiac output based on said difference in voltage and said QRS complex events.

6. The system of claim 3, wherein said terminals each comprise:

a central axis;

a sidewall portion substantially parallel to said axis; and

a top portion, said top portion having a diameter greater of that of said vertical sidewall portion.

7. The system of claim 6, wherein said electrical leads each comprise a connector, said connector being adapted to form an electrical conduction path between said connector and a respective one of said terminals, said connector further comprising a plurality of electrically conductive arms biased against said sidewall portion of said terminal when said connector is mated with said terminal.

8. A method of measuring the cardiac output of a living subject, comprising:

providing a plurality of electrode arrays, each of said electrode arrays having a plurality of terminals, at least two of said terminals being spaced a predetermined distance apart;

positioning the electrode arrays at respective locations in relation to the thoracic cavity of the subject;

generating an electrical current, said current passing from a first electrode of at least one of said electrode arrays through said subject and to a second electrode of at least one of said arrays;

measuring the voltage at said second electrode of said at least one electrode array;

determining stroke volume from the measured voltage; and

determining cardiac output based at least in part on said stroke volume.

9. The method of claim 8, wherein the act of positioning comprises placing at least one electrode array on both the left and right sides of the subject'"'"'s abdomen.

10. The method of claim 9, further comprising placing at least one electrode array on both the left and right sides of the subject'"'"'s neck.

11. The method of claim 8, wherein the act of determining stroke volume comprises determining ventricular ejection time (VET) and the derivative of impedance, and calculating stroke volume based at least in part thereon.

12. The method of claim 11, wherein the act of determining cardiac output comprises multiplying stroke volume and cardiac rate.

13. The method of claim 12, further comprising:

measuring ECG potentials from said subject using at least one of said electrode arrays; and

determining cardiac rate based at least in part on said ECG potentials.

14. The method of claim 8, wherein the act of measuring voltage comprises measuring a differential voltage across respective ones of the terminals of at least one of said electrode arrays.

15. The method of claim 8, further comprising:

measuring a first impedance waveform at a first of said terminals;

measuring a second impedance waveform at a second of said terminals; and

evaluating the continuity of at least one electrical connection based at least in part on the relationship of at least one of said first and second impedance waveforms to a predetermined value.

16. The method of claim 15, wherein the act of evaluating comprises taking the difference of said first and second waveforms and comparing that difference to a threshold value.

17. A cardiac electrode assembly for use on a living subject, comprising:

a substrate, said substrate having a plurality of apertures formed therein, at least two of said apertures being formed a predetermined distance apart;

a plurality of terminals disposed within respective ones of said apertures, at least a portion of each of said terminals being capable of conducting an electrical current; and

at least one gel element, said at least one gel element being adapted to transfer electrical current between the skin of said subject and at least one of said plurality of terminals.

18. The electrode assembly of claim 17, wherein said terminals each comprise a substantially vertical sidewall portion, and a top portion having a diameter greater of that of said vertical sidewall portion so as to retain an electrical connector attached thereto.

19. The electrode assembly of claim 17, wherein said predetermined distance is established such that differential voltage measurements taken across said terminals fall within a predetermined voltage band when electrical continuity exists in each of said terminals.

20. A cardiac electrode assembly adapted for measuring the cardiac output of a living subject, comprising:

a flexible substrate, said substrate having at least two apertures formed therein, said at least two apertures being formed a predetermined distance apart;

at least two terminals disposed within respective ones of said apertures, at least a portion of each of said terminals being capable of conducting an electrical current; and

a first electrolytic element disposed proximate to a first of said at least two apertures, said first electrolytic element adapted to transfer an electrical potential between the skin of said subject and a first of said at least two terminals;

a second electrolytic element disposed proximate to a second of said at least two apertures, said second electrolytic element adapted to transfer an electrical potential between the skin of said subject and a second of said at least two terminals;

an attachment element, said attachment element cooperating with said substrate and the skin of said living subject to removably affix said electrode assembly to said skin.

21. A method of monitoring the electrical continuity of a plurality of electrodes in an impedance cardiography system, comprising:

providing a plurality of electrically conductive terminals;

disposing the terminals in relation to the thoracic cavity of a subject;

generating a current between a first of said terminals and a second of said terminals, said current passing through at least a portion of said thoracic cavity;

obtaining an impedance waveform from said second terminal; and

comparing said impedance waveform to a similar waveform obtained from another of said terminals;

wherein the difference between said impedance waveform and said similar waveform is d to evaluate the electrical continuity of said first terminal.

22. A system for determining the cardiac output of a living subject, comprising:

a plurality of electrode assemblies, each electrode assembly having;

a substrate having a plurality of apertures formed therein;

a plurality of electrically conductive terminals disposed within respective ones of said apertures, each terminal having a central axis, a sidewall portion substantially parallel to said axis, and a top portion, said top portion having a diameter greater of that of said vertical sidewall portion, at least two of said plurality of terminals being spaced from one another by a predetermined distance; and

at least one gel element, said at least one gel element adapted to transfer electrical current between the skin of said subject and said at least two of said plurality of terminals;

a current source capable of generating a substantially constant current;

a plurality of electrical leads connecting said current source with individual ones of said terminals of said electrode assemblies, said electrical leads each comprising a connector, said connector being adapted to form an electrical conduction path between said connector and a respective one of said terminals, said connector further comprising a plurality of electrically conductive arms biased against said sidewall portion of said terminal when said connector is mated with said terminal;

a circuit for measuring the difference in voltage at said terminals resulting from the flow of said current through said subject and said terminals under varying cardiac conditions of said subject; and

a circuit for measuring ECG potentials from at least one of said electrode assemblies.

23. The system of claim 22, further comprising a circuit for measuring the difference in the impedance of at least two of said terminals as a function of time, said difference being compared to a first value to evaluate the electrical continuity of at least one of said terminals, said first value being based at least in part on said predetermined distance.

24. The system of claim 22, wherein said circuit for measuring ECG potentials comprises a circuit adapted to measure body surface potentials between at least two of said terminals in order to identify a plurality of QRS complex events within said subject.

25. The system of claim 24, further comprising a processor in data communication with both said circuit for measuring the difference in voltage and said circuit for measuring ECG potentials, said processor being adapted to determine cardiac output based on said difference in voltage and said QRS complex events.

26. A system for determining the cardiac output of a living subject, comprising:

a plurality of electrode assemblies, each electrode assembly having a plurality of terminals, at least two of said plurality of terminals being spaced from one another by a predetermined distance;

a current source capable of generating a substantially constant current;

a plurality of electrical leads connecting said current source with individual ones of said terminals of said electrode assemblies,

a circuit for measuring the difference in voltage at said terminals resulting from the flow of said current through said subject and said terminals under varying cardiac conditions of said subject; and

a circuit for measuring ECG potentials from at least one of said electrode assemblies;

wherein at least a first portion of said plurality of electrode assemblies are adapted for placement on the neck of said subject; and

a second portion of said plurality of electrode assemblies are adapted for placement at or near the level of the xiphoid process of said subject.

27. A method of measuring the cardiac output of a living subject, comprising:

providing a plurality of electrode arrays, each of said electrode arrays having a plurality of terminals, at least two of said terminals being spaced a predetermined distance apart;

positioning the electrode arrays at respective locations on the neck and thorax xiphoid process of the subject;

generating an electrical current, said current passing from a first electrode of at least one of said electrode arrays through said subject and to a second electrode of at least one of said arrays;

measuring the voltage at said second electrode of said at least one electrode array;

determining stroke volume from the measured voltage; and

determining cardiac output based at least in part on said stroke volume.

28. A method of monitoring the electrical continuity of a plurality of electrodes in an impedance cardiography system, comprising:

providing a plurality of electrically conductive terminals;

disposing the terminals in relation to the thoracic cavity of a subject;

generating a current between a first of said terminals and a second of said terminals, said current passing through at least a portion of said thoracic cavity;

obtaining a first impedance waveform from said second terminal; and

comparing said first impedance waveform to a threshold value;

wherein the difference between said first impedance waveform and said threshold impedance value is used to evaluate the electrical continuity of said first terminal.

29. The method of claim 28, wherein said threshold value is determined by:

obtaining a second impedance waveform from at least one of said electrically conductive terminals prior to said act of obtaining said first impedance waveform;

storing at least a portion of said second waveform in a storage device; and

determining said threshold based at least in part on said stored at least portion of said second waveform.

30. The method of claim 28, wherein said threshold value is determined by:

applying a constant current between said first and second electrodes;

obtaining a voltage value from at least one of said terminals; and

deriving said threshold impedance value using at least said voltage value and said constant current.

31. A method of monitoring the electrical continuity of a plurality of electrodes in an impedance cardiography system, comprising:

providing a plurality of electrically conductive terminals;

disposing respective ones of the terminals at the neck and approximately the level of the xiphoid process of the thorax of a subject, at least two of said terminals being disposed at predetermined spacing with respect to each other;

generating a current between a first of said terminals and a second of said at least two terminals, said current passing through at least a portion of said thoracic cavity;

obtaining a first impedance waveform from said second terminal; and

comparing said first impedance waveform to a threshold value;

wherein the difference between said first impedance waveform and said threshold impedance value is used to evaluate the electrical continuity of said first terminal.

32. The method of claim 31, wherein said threshold value is determined by:

obtaining a second impedance waveform from at least one of said electrically conductive terminals prior to said act of obtaining said first impedance waveform;

storing at least a portion of said second waveform in a storage device; and

determining said threshold based at least in part on said stored at least portion of said second waveform.