Method and apparatus for electrically compensated measurement of cardiac output

• US 5,092,339 A
• Filed: 07/23/1990
• Issued: 03/03/1992
• Est. Priority Date: 07/23/1990
• Status: Expired due to Fees

• Current Assignee: Cook Pacemaker Corporation (Cook Group Incorporated), Purdue Research Foundation (The Trustees of Purdue University (d/b/a Purdue University))
• Original Assignee: Anthony C. Raghep, Frederick J. Shipko, Joe D. Bourland, Leslie A. Geddes, Neal E. Fearnot, William D. Voorhees III
• Inventors: Fearnot, Neal E., Voorhees, William D. III, Geddes, Leslie A., Bourland, Joe D., Shipko, Frederick J., Raghep, Anthony C.
• Primary Examiner(s): Jaworski, Francis
• Assistant Examiner(s): MANUEL, GEORGE C
• Application Number:
  US07/557,135
• Time in Patent Office: 589 Days
• Field of Search: 128/692, 128/693, 128/713, 128/695, 128/734, 128/673
• US Class Current: 600/505
• CPC Class Codes: A61B 5/0275 using tracers, e.g. dye dilution
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence

Abstract

Cardiac output is measured using an injected substance (indicator) that changes the conducting property (electrical resistivity) of blood, provided the indicator has a different resistivity from that of blood. A new type of electrically compensated tetrapolar conductivity cell located at the distal end of an intravascular catheter is disclosed. The conductivity cell consists of four electrodes arranged across the end of a catheter at its tip. The outer electrodes used for current injection are enlarged to reduce the electrode-electrolyte interface impedance. Capacitance compensation of the cather/tetrapolar conductivity cell provides reliable and repeatable blood resistivity measurements. The catheter further includes a calibration resistor thereby eliminating a calibration step previously necessary in using such a catheter for measuring cardiac output.

Claims

1. An indicator-dilution method for measuring cardiac output, said method comprising the steps of:

placing a conductivity cell in a flow path of blood;

compensating stray and distributed capacitances of said conductivity cell;

electrically calibrating said conductivity cell without withdrawing blood from said flow path, said calibrating being accomplished by utilizing said conductivity cell to generate an output signal indicative of baseline blood resistivity;

injecting an indicator into said blood flow path and obtaining an output signal from said conductivity cell indicative of blood resistivity as altered by said indicator; and

determining cardiac output from said output signal indicative of baseline blood resistivity and said output signal indicative of altered blood resistivity which is a dilution curve.

2. The method of claim 1 wherein said conductivity cell includes a plurality of electrodes extending outward from said conductivity cell, each of said electrodes connected to a respective electrode wire, each of said electrodes formed so that the surface area of said electrode exposed to blood flow is larger than the cross-sectional area of said electrode wires.

3. The method of claim 2 wherein said electrodes are formed so that each of said electrodes is adapted to be situated perpendicular to the direction of blood flow.

4. The method of claim 1 including electrical determination of the conductivity cell constant of said conductivity cell after said conductivity cell is placed in the flow path of blood.

5. The method of claim 4 wherein an identifier device is attached to said conductivity cell, said identifier device having a detectable, measurable electronic parameter indicative of said conductivity cell constant.

6. The method of claim 5 wherein said identifier device is a resistor.

7. A method for measuring cardiac output with a conductivity cell catheter, said catheter having a proximal end and a distal end, said method comprising the steps of:

positioning a detector on said distal end of said catheter;

providing electrical leads extending through said catheter from said detector to said proximal end of said catheter;

neutralizing stray capacitance associated with said electrical leads;

inserting said catheter into a body such that said detector is in a flow path of blood in said body;

electrically calibrating said detector without withdrawing blood from said body, said calibrating including electrically exciting said detector so that said detector provides an output signal indicative of baseline blood resistivity;

injecting an indicator into said blood flow path and electrically exciting said detector so that said detector provides an output signal indicative of blood resistivity as altered by said indicator; and

determining cardiac output from said output signal indicative of baseline blood resistivity and said output signal indicative of altered blood resistivity.

8. The method of claim 7 wherein said detector is a tetrapolar conductivity cell, and wherein said method includes electrically exciting a first pair of electrodes of said tetrapolar conductivity cell to develop a potential indicative of baseline blood resistivity on a second pair of electrodes of said tetrapolar conductivity cell.

9. The method of claim 8 wherein said electrodes extend out from the tip of the distal end of said catheter, said electrodes adapted to be situated approximately in a line normal to the direction of blood flow.

10. The method of claim 8 including electrically identifying a conductivity cell constant specific to said tetrapolar conductivity cell after said catheter is inserted into said body by measuring an electrical parameter of an electrical identifier located within said catheter and having leads extending out of the proximal end of said catheter.

11. The method of claim 10 wherein said electrical component is a resistor and said measurable electrical characteristic is resistance.

12. The method of claim 7 wherein said providing step includes providing electrical leads each having a substantially uniform cross-sectional area, and wherein said positioning step includes providing on said distal end a pair of electrodes each connected to one of said leads and each having an exposed surface area substantially larger than the cross-sectional area of the lead to which it is connected.

13. The method of claim 12 wherein said electrodes extend out from the tip of the distal end of said catheter and said electrodes are positioned approximately perpendicular to the direction of blood flow.

14. An apparatus for enabling measurement of cardiac output by determination of blood resistivity, said apparatus comprising:

a conductivity cell having a plurality of electrodes;

capacitance compensation means connected to said conductivity cell for neutralizing stray and distributed capacitances;

positioning means for positioning said conductivity cell in the flow path of blood;

wherein said positioning means includes a catheter having said conductivity cell attached to the distal end of said catheter and means for preventing said electrodes from contacting a wall defining the flow path of blood, and wherein said conductivity cell is a tetrapolar conductivity cell having four electrodes, said catheter also including four conductors extending along and within the entire length of said catheter, said four conductors each connected to one of said four electrodes; and

wherein said electrodes emerge in a line from the cross-section of the distal end of said catheter, and the outer pair of said electrodes having exposed surfaces substantially larger than the cross-sectional area of said conductors.

15. The catheter of claim 14 wherein said electrodes are positioned in a line across the distal end of said catheter normal to the direction of blood flow.

16. The catheter of claim 14 including an indicator injection lumen and an indicator injection port, said indicator injection lumen communicating with said indicator injection port, said indicator injection port located near the distal end of said catheter.

17. The catheter of claim 16 wherein said catheter includes means for preventing said electrodes from contacting a wall defining the flow path of blood and situated so that said means for preventing electrodes from contacting a wall is between said indicator injection port and said distal end of said catheter.

18. The catheter of claim 17 including a second lumen, and wherein said means for preventing said electrodes from contacting a wall is a balloon communicating with said second lumen.

19. The catheter of claim 18 wherein said catheter includes a third lumen communicating with a port located between said balloon and said distal end of said catheter.

20. An apparatus for enabling measurement of cardiac output by determination of blood resistivity, said apparatus comprising:

a catheter having a proximal and a distal end and adapted for insertion into a body so that the distal end of said catheter is in the flow path of blood in said body, said catheter having four conductors symmetrically contained within the body of said catheter;

a tetrapolar conductivity cell attached to said catheter, said conductivity cell having four electrodes arranged substantially in a line and connected to said four conductors, forming an inner pair and outer pair of electrodes, said outer pair of said electrodes having an exposed surface area larger than the cross-sectional area of each of said conductors, said electrodes extending out of said distal end of said catheter, and said electrodes positioned substantially perpendicular to the direction of blood flow;

a constant-current source connected with the outer pair of said electrodes of said conductivity cell;

isolation amplifying means connected with said conductors connected to the inner pair of said electrodes for amplifying a potential appearing on the inner pair of said electrodes and providing an isolated analog output of blood resistivity for enabling inscribing of curve corresponding to blood resistivity;

a first capacitor connected between one of said outer electrodes and one of said inner electrodes, and a second capacitor connected between the remainder of said electrodes, said first and second capacitors located at the proximal end of said catheter and electrically connected to said electrodes via said conductors;

electrical identification means for identifying the conductivity cell constant peculiar to said catheter and conductivity cell in combination, said identification means having an electrical connection at the proximal end of said catheter; and

indicating means connected with said first amplifying means for indicating resistivity.

21. The apparatus of claim 20 wherein said outer electrodes are metal tubes crimped to said conductors and contained within the distal end of said catheter, said crimped tubes being exposed at the distal end of said catheter.

22. The apparatus of claim 21 wherein epoxy is applied to said electrodes at the distal end of said catheter to fix said electrodes in position relative to one another.

23. The apparatus of claim 20 wherein said first and second capacitors are contained within said catheter and said catheter includes a connector attached to said proximal end for connecting said four conductors to said constant-current source and said isolation amplifying means.