Electrode impedance monitoring method apparatus for electrocardiography

• US 4,321,932 A
• Filed: 06/29/1979
• Issued: 03/30/1982
• Est. Priority Date: 06/29/1979
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Francis, David B.
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US06/053,637
• Time in Patent Office: 1,005 Days
• Field of Search: 128/695, 128/696, 128/734, 128/639-642, 128/710
• US Class Current: 600/509
• CPC Class Codes: A61B 5/276 Protection against electrode failure
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: G01R 27/16 Measuring impedance of element or network through which a current is passing from another source, e.g. cable, power line

Abstract

A patient'"'"'s own heart is used as a voltage source for measurement of EKG electrode impedance. The amplitude of an EKG signal obtained while the EKG electrodes are shorted together with an impedance is compared to the amplitude of an EKG signal obtained under identical conditions except that the impedance short is not present. The value of the inter-electrode impedance is then equal to one less than the ratio between the unshorted amplitude and the shorted amplitude times the value of the shorting impedance. In a preferred embodiment, the value of the shorting impedance is made equal to the maximum permitted inter-electrode impedance. Then, if the amplitude of the EKG signal taken from a patient decreases upon temporary insertion of the impedance short to a value less than half of the EKG amplitude obtained without the impedance short, the inter-electrode impedance is too high.

Claims

1. Inter-electrode impedance measuring apparatus for electrocardiography, comprising:

two EKG electrodes;

an impedance element Z_T ;

means for temporarily connecting said impedance element between said two EKG electrodes while said EKG electrodes are in position to pick up EKG signals from a subject;

means for measuring both the amplitude A_U of an EKG signal picked up by said EKG electrodes when said impedance element is not connected between said two EKG electrodes and the amplitude A_L of an EKG signal picked up by said EKG electrodes when said impedance element is connected between said two EKG electrodes, the inter-electrode impedance Z_E being related to Z_T, A_U and A_L at least to an approximation by the formula

space="preserve" listing-type="equation">Z.sub.E =Z.sub.T (A.sub.U /A.sub.L -1).

2. Apparatus as defined in claim 1 wherein said means for measuring has an input impedance which is much higher than Z_T.

3. Apparatus as defined in claim 1 wherein the impedance value of Z_T may be varied.

4. Apparatus as defined in claim 1 where said means for temporarily connecting comprises a manually operatable switch and said means for measuring comprises means for displaying EKG waveforms.

5. Apparatus as defined in claim 4 wherein said means for displaying EKG waveforms comprises a strip chart recorder.

6. Apparatus as defined in claim 1 wherein said means for measuring comprises means for finding the amplitude of the largest R-wave in an EKG signal, the amplitude of the largest R-wave picked up when said impedance element is not connected between said two EKG electrodes being A_U and the amplitude of the largest R-wave picked up when said impedance element is connected between said two EKG electrodes being A_L.

7. Apparatus as defined in claim 1 wherein said means for measuring comprises means for finding the average amplitude of the R-waves in an EKG signal, the average amplitude of the R-waves picked up when said impedance element is not connected between said two EKG electrodes being A_U and the average amplitude of the R-waves picked up when said impedance element is connected between said two EKG electrodes being A_L.

8. Apparatus for aiding in the determination of tolerable inter-electrode impedance for electrocardiography, comprising:

two EKG electrodes;

an impedance element Z_T ;

means for temporarily connecting said impedance element between said two EKG electrodes while said EKG electrodes are in position to pick up EKG signals from a subject;

means for measuring both the amplitude A_U of an EKG signal picked up by said EKG electrodes when said impedance element is not connected between siad two EKG electrodes and the amplitude A_L of an EKG signal picked up by said EKG electrodes when said impedance element is connected between said two EKG electrodes, the inter-electrode impedance Z_E being related to Z_T, A_U and A_L at least to an approximation by the formula

space="preserve" listing-type="equation">Z.sub.E =Z.sub.T (A.sub.U /A.sub.L -1).

9. Apparatus as defined in claim 8 wherein Z_T has a predetermined value related to the maximum permitted impedance value for Z_E, said predetermined value of Z_T and said maximum desired impedance value for Z_E determining in accordance with said formula a maximum permitted value for A_U /A_L.

10. Apparatus as defined in claim 9 and further comprising means for determining from the measured values for A_U and A_L whether the maximum permitted value for A_U /A_L has been exceeded.

11. Apparatus as defined in claim 9 wherein said predetermined value of Z_T is equal to the maximum permitted impedance value for Z_E, the maximum permitted ratio for A_U /A_L thereby being equal to two.

12. Apparatus as defined in claim 11 and further comprising means for determining whether the measured value of A_U is more than twice the measured value of A_L.

13. Apparatus as defined in claim 12 and further comprising means for alerting an operator that the maximum permitted value for A_E has been exceeded when the measured value of A_U is more than twice the measured value of A_L.

14. Apparatus as defined in claim 13 implemented with a digital computer.

15. Apparatus as defined in claim 9 wherein said means for measuring has an input impedance which is much higher than Z_T.

16. Apparatus as defined in claim 9 wherein said predetermined impedance value of Z_T may be varied.

17. Apparatus as defined in claim 9 where said means for temporarily connecting comprises a manually operatable switch and said means for measuring comprises means for displaying EKG waveforms.

18. Apparatus as defined in claim 17 wherein said means for displaying EKG waveforms comprises a strip chart recorder.

19. Apparatus as defined in claim 9 wherein said means for measuring comprises means for finding the amplitude of the largest R-wave in an EKG signal, the amplitude of the largest R-wave picked up when said impedance element is not connected between said two EKG electrodes being A_U and the amplitude of the largest R-wave picked up when said impedance element is connected between said two EKG electrodes being A_L.

20. Apparatus as defined in claim 9 wherein said means for measuring comprises means for finding the average amplitude of the R-waves in an EKG signal, the average amplitude of the R-waves picked up when said impedance element is not connected between said two EKG electrodes being A_U and the average amplitude of the R-waves picked up when said impedance element is connected between said two EKG electrodes being A_L.

21. A method for measuring the impedance of EKG electrodes, comprising the steps of:

positioning two EKG electrodes on a subject which is generating EKG signals;

measuring the amplitude A_U of the EKG signal picked up between the EKG electrodes;

connecting temporarily the two EKG electrodes together through an impedance Z_T so as to reduce the amplitude of the EKG signl picked up between the EKG electrodes;

measuring the reduced amplitude A_L of the EKG signal picked up between the impedance loaded EKG electrodes; and

deriving the impedance Z_E between the EKG electrodes in accordance with the formula

space="preserve" listing-type="equation">Z.sub.E =Z.sub.T (A.sub.U /A.sub.L -1).

22. A method for determining whether the impedance of EKG electrodes is sufficiently low, comprising the steps of:

positioning two EKG electrodes on a subject which is generating EKG signals;

determining the amplitude of the EKG signal picked up between the EKG electrodes;

temporarily connecting the two EKG electrodes together through an impedance having a predetermined value related to the maximum permitted inter-electrode impedance;

determining the amplitude of the EKG signal picked up between the impedance shorted EKG electrodes; and

determining whether the ratio between the amplitude of the EKG signal picked up between the impedance shorted EKG electrodes and the amplitude of the EKG signal picked up between the EKG electrodes when they are not impedance shorted is less than a predetermined value, the inter-electrode impedance being higher than permitted when said ratio is less than said predetermined value.

23. A method as defined in claim 22 wherein said predetermined value of said impedance is equal to the maximum permitted inter-electrode impedance and said ratio is one-half.