Surgery – Diagnostic testing – Cardiovascular
Utility Patent
1999-06-18
2001-01-02
Getzow, Scott M. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
Utility Patent
active
06169919
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to cardiology. More specifically, the invention relates to a system and method for calculating a magnitude of alternation in the T-waves of an electrocardiogram signal.
2. Background of the Invention
There is a growing interest in identifying patients with T-wave alternans as this has been shown to be a marker of electrical instability. For example, U.S. Pat. No. 5,148,812 to Richard L. Verrier and Bruce D. Nearing, the full text of which is incorporated herein by reference as if reproduced in full below, describes a method for non-invasive dynamic tracking of cardiac vulnerability to ventricular fibrillation by analysis of T-wave alternans. The '812 patent discloses a method for quantifying the magnitude of alternation in an electrocardiogram (ECG) signal that could be performed non-invasively during, for example, exercise stress testing.
The magnitude of alternans can, however, be difficult to accurately quantify. Alternans magnitudes are typically in the range of several microvolts to several hundred microvolts. These small amplitudes make the measurement and analysis of the alternans susceptible to noise. Noise sources such as white noise, motion artifacts caused by respiration or patient movement, noisy heart beats, premature beats and the like can skew alternans measurements. The inventors have therefore sought to improve upon the conventional methods for quantifying alternans.
SUMMARY OF THE INVENTION
The invention is a system and method for quantifying alternation in the T-wave and ST segment of an ECG signal. In a preferred embodiment of the invention, a digitized ECG signal (i.e., ECG data) is received for processing. The ECG data are used to calculate an odd median complex for the odd beats in the ECG data and an even median complex for the even beats in the ECG data. The odd median complex is then compared with the even median complex to obtain an estimate of the amplitude of beat-to-beat alternation in the ECG signal.
Filtering of the ECG data involves low pass filtering the ECG data to remove high frequency noise, applying a baseline wander removal filter to the ECG data to remove low frequency artifacts, removing ventricular arrhythmias from the ECG data, and eliminating noisy beats from the ECG data. The filtered data are more suitable for use in calculating an accurate estimate of alternation.
The step of applying a baseline wander removal filter to the ECG data includes determining an isoelectric value at each of a first isoelectric point (point
1
) in a first beat, a second isoelectric point (point
2
) in a second beat, and a third isoelectric point (point
3
) in a third beat of the ECG data; fitting a spline curve to the first three isoelectric values; subtracting the values of the spline curve from the corresponding values of the ECG data between the first isoelectric point and the second isoelectric point; subtracting the values of the spline curve from the corresponding values of the ECG data between the second isoelectric point and the third isoelectric point; determining an isoelectric value for a next isoelectric point (e.g., point
4
) in a next beat of the ECG data; fitting a next spline curve to the next isoelectric value (e.g., point
4
) and isoelectric values corresponding to a previous two consecutive isoelectric points (e.g., points
2
and
3
); subtracting the values of the next spline curve from the corresponding values of the ECG data between the next isoelectric point (e.g., point
4
) and the previous isoelectric point (e.g., point
3
); and repeating the process until a desired plurality of beats (e.g., points
3
,
4
and
5
; then points
4
,
5
and
6
; and so on) in the ECG data have been processed to remove low frequency artifacts from the ECG data.
The step of eliminating noisy beats includes calculating a mean value of all samples within a selected portion of a selected beat of the ECG data, calculating a difference between the mean value and each sample within the selected portion of the selected beat, calculating an average of the absolute value of the differences, comparing the average to a threshold, identifying the selected beat as a noisy beat based on the comparison of the average to the threshold, and eliminating the noisy beat from the calculation of the odd and even median complexes.
The step of calculating an odd median complex proceeds as follows. A first array (representing the odd median complex) is initialized with a plurality of odd median complex values. A second array (representing the even median complex) is initialized with a plurality of even median complex values. The samples of an odd beat of the ECG data are compared to corresponding values in the first array and, based on the comparison, the values of the first array are adjusted as follows. If a sample of the odd beat exceeds the corresponding value of the first array by a first amount but by less than a second amount, then the corresponding value is incremented by the first amount. If a sample of the odd beat exceeds the corresponding value of the first array by the second amount or by greater than the second amount, then the corresponding value is incremented by the second amount. If a sample of the odd beat is less than the corresponding value of the first array by the first amount but not the second amount, then the corresponding value is decremented by the first amount. Finally, if a sample of the odd beat is less than the corresponding value of the first array by the second amount or by greater than the second amount, then the corresponding value is decremented by the second amount. This process is repeated for other odd beats desired to be included in the calculation. This same process is then followed for the second array using the even beats.
Once the odd median complex and the even median complex have been calculated, a difference between sample points of the odd median complex and sample points of the even median complex is calculated for the region of the T-wave to obtain the estimate of the magnitude of alternation. Preferably, the maximum difference between the two complexes in the region of the T-wave is used as the estimate of the magnitude of alternation.
If desired, the ECG data may be divided into time segments with an alternans estimate being calculated for each segment. For example, a measure of alternation may be calculated for a time segment representing 15 seconds of ECG data.
These and other features and advantages of the invention are described in detail below with reference to the figures in which like reference numbers indicate like elements. Also in the figures, the left most digit of each reference number corresponds to the figure in which the reference number is first used.
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Adam, Dan R. et al., “Fluctuations in T-Wave Morphology and Susceptibility to Ventricular Fibrillation,”J. Electrocardiology, vol. 17, No. 3, 1984, pp. 209-218.
Adam, Dan R. et al., “Ventricular Fibrillation And Fluctuations In The Magnitude Of The Repolarization Vector,”Computers in Cardiology, 1982, pp. 2
Nearing Bruce D.
Verrier Richard L.
Beth Israel Deaconess Medical Center Inc.
Getzow Scott M.
Sterne Kessler Goldstein & Fox P.L.L.C.
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