Axis shift analysis of electrocardiogram signal parameters especially applicable for multivector analysis by implantable medical devices, and use of same

• US 6,324,421 B1
• Filed: 03/29/1999
• Issued: 11/27/2001
• Est. Priority Date: 03/29/1999
• Status: Expired due to Term

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Nelson, Shannon, Stadler, Robert W.
• Primary Examiner(s): Bockelman, Mark
• Application Number:
  US09/280,203
• Time in Patent Office: 974 Days
• Field of Search: 600/509, 600/517, 600/521, 128/901, 128/923
• US Class Current: 600/509
• CPC Class Codes: A61B 5/341 Vectorcardiography [VCG]
• CPC Class Codes: A61B 5/349 Detecting specific parameters of the electrocardiograph cycle
• CPC Class Codes: A61B 5/358 Detecting ST segments
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61N 1/37 Monitoring; Protecting
• CPC Class Codes: Y10S 128/901 Suppression of noise in electric signal

Abstract

We show how to determine whether there has been an axis shift in an electrocardiogram waveform and how to use this for filtering out bad electrocardiogram information and to modify an adaptive filter that can be used to adapt the filtering of such electrocardiogram information to make it available for determining physiologic conditions even after an axis shift.

Claims

1. A method for determining whether there has been an axis shift in an electrocardiogram waveform comprising a repeated cycle of steps, each cycle comprising the steps:

a) for a given cardiac cycle, collecting into a buffer selected characteristic portions of the electrocardiogram signal including a fiducial point within said electrocardiogram signal;

b) determining a selection of samples surrounding temporally said fiducial point, and based on values of electrocardiogram amplitude of said samples producing a parameter representative of said a cardiac cycle;

c) determining if said parameter is out of an allowable range of said parameter, wherein the size of said range is adaptively determined based on previously-calculated values for said parameter; and

d) declaring an axis shift if said parameter is out of said range for more than a predetermined amount of time.

2. The method of claim 1 wherein said predetermined amount in step d comprises a predetermined number of times out of a predetermined number of cycles.

3. The method of claim 1 wherein said fiducial point is an R-peak for said a cardiac cycle, and the following steps are used to determine the R-peak point;

finding a peak positive value and a peak negative value in said buffer of samples,

finding an isoelectric sample,

employing the amplitude value of said isoelectric sample and said peak values to determine which of said peak values is furthest from said isoelectric sample value,

declaring the sample with the peak value further from said isoelectric sample value as the sample located at said R-wave peak.

4. The method of claim 3 wherein in step “

b”

said parameter value is an amplitude of an R-wave and is determined based on said R-wave peak value.

5. The method as set forth in claim 1 wherein said declaration of said parameter axis shift is used to determine ranges for acceptable values for determining a physiologic characteristic from the set of physiologic characteristics including T wave height variation, ischemic condition or QT variation.

6. The method of any of claims 1-3 further comprising the step of simultaneously performing steps a-d on a plurality of electrocardiogram vectors, and still further comprising the step of determining an axis shift combination value based on whether an axis shift is declared for more than a predetermined number of electrocardiogram vectors.

7. The method of any of claims 1-5 and further comprising the step of adjusting an alarm threshold for a physiologic parameter change based on whether an axis shift is declared.

8. The method of any of claims 1-5 further comprising the step of adapting baseline values of said measured parameter to change responsive to declaration of an axis shift.

9. The method of any of claims 1-5 further comprising the step of adapting ST change variable filter coeficients responsive to declaration of an axis shift.

10. The method of any of claims 1-5 further comprising the step of adapting a noise filter to reduce sensitivity to motion artifacts in a group of cardiac cycles surrounding the temporal location of a declaration of an axis shift.

11. A method for determining whether an axis shift has occurred in an electrocardiogram (EGM) waveform, comprising the methods of:

a) obtaining samples of the EGM waveform over a predetermined number of cardiac cycles;

b) calculating a predetermined parameter indicative of a selected characteristic of the EGM waveform;

c) repeating steps a) and b), and updating the predetermined parameter based on newly-obtained samples of the EGM waveform and on previously-calculated values of the predetermined parameter;

d) determining an allowable range of values for the predetermined parameter, the size of the allowable range being based on a current value and at least one previous value of the predetermined parameter as calculated in method c); and

e) determining that an axis shift has occurred if the predetermined parameter is outside the allowable range of values for a predetermined period of time.

12. The method of claim 11, wherein method e) includes the method of determining that an axis shift has occurred if the predetermined parameter is outside the allowable range of values for a predetermined number of cardiac cycles.

13. The method of claim 11, wherein method a) obtains samples of the EGM waveform for one or more characteristic portions of the predetermined number of cardiac cycles.

14. The method of claim 13, wherein the one or more characteristic portions include a fiducial point of a cardiac cycle, and wherein method a) includes the methods of:

determining an isoelectric signal level for the EGM waveform; and

selecting the fiducial point of the cardiac cycle as the point of the cardiac cycle wherein the signal amplitude deviates the most from the isoelectric signal level.

15. The method of claim 11, wherein the selected characteristic is the amplitude of an R-wave of the EGM waveform.

16. The method of claim 15, wherein method c) includes the method of obtaining a running mean value of the amplitude of the R-waves for multiple cardiac cycles.

17. The method of claim 11, and including the methods of:

performing methods a) through d) for multiple vectors of the EGM waveform; and

wherein method e) includes the method of determining that an axis shift has occurred if the predetermined parameter is outside the allowable range of values for a predetermined period of time for a predetermined number of the multiple vectors.

18. The method of claim 11, and further including the methods of:

calculating one or more parameters from the EGM waveform to be used to evaluate a physiological condition; and

evaluating the physiological condition using acceptable ranges for each of the one or more parameters, wherein the acceptable ranges are determined based on the axis shift determination from method e).