Extraction of cardiac signal data

1. An apparatus comprising:

a circuit-based computer configured and arranged with executable instructions to

identify a T-wave offset point within a first time window of an ECG signal, and

provide the identified T-wave offset point as an output based upon a noise characteristic of the ECG signal in a second time window that includes at least a portion of the T-wave of the ECG signal.

2. The apparatus of claim 1 , wherein the circuit-based computer is configured and arranged with the executable instructions to identify the T-wave offset point by

identifying a location of a QRS complex in the ECG signal,

identifying the first time window based upon a location of the QRS complex,

computing a T-wave emphasis signal, and

evaluating the emphasis signal within the first time window to identify the T-wave offset point as a first baseline point after a last significant peak or valley of the emphasis signal.

3. The apparatus of claim 2 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute the T-wave emphasis signal by:

decomposing the ECG signal into subcomponents,

identifying subcomponents in the first time window as either primarily associated with noise or primarily associated with the T-wave of the underlying ECG signal, and

using at least one of the subcomponents primarily associated with the T-wave as the emphasis signal.

4. The apparatus of claim 1 , further including a non-transitory electronic data storage medium that includes the instructions stored thereupon.

5. The apparatus of claim 1 , wherein the second time window includes the identified T-wave offset point.

6. The apparatus of claim 1 , wherein the second time window has a duration of about 30% of the duration of a nominal QT interval of the ECG signal, and extends beyond the T-wave offset point by about 10% of the nominal QT interval duration.

7. The apparatus of claim 1 , wherein the second time window has a duration of about 50 msec and is centered on about the identified T-wave offset point.

8. The apparatus of claim 1 , wherein the second time window extends from about the point of a largest deflection of the T-wave from an isoelectric line of the ECG signal to about the T-wave offset point.

9. The apparatus of claim 1 , wherein the second time window includes a time period extending from about a QRS offset point in the ECG signal to about the T-wave offset point.

10. The apparatus of claim 1 , wherein the second time window includes a time period extending the full duration of a cardiac cycle in the ECG signal.

11. The apparatus of claim 1 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute the noise characteristic by:

decomposing the ECG signal into subcomponents,

identifying said subcomponents in the second time window as either primarily associated with noise or primarily associated with the T-wave of the underlying ECG signal,

computing a noise signal for the second time window by using at least one subcomponent not primarily associated with the T-wave, and

computing said noise characteristic based upon at least one of: energy of the noise signal, standard deviation of the noise signal, zero crossing density of the noise signal, a metric of noise amplitude computed using an envelope of the noise signal, and a metric of energy of the ECG signal relative to energy of the noise signal.

12. The apparatus of claim 11 , wherein the circuit-based computer is configured and arranged with the executable instructions to

compute a metric of energy of the ECG signal using at least one of the subcomponents within the second time window that are primarily associated with the T-wave.

13. The apparatus of claim 1 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute the noise characteristic by:

computing a denoised signal for the second time window using at least one of a band-pass filter, wavelet thresholding, and an adaptive filter that passes primarily T-wave energy,

capturing a residual of the step of computing the denoised signal as a noise signal for the second time window, and

computing said noise characteristic based upon at least one of: energy of the noise signal, standard deviation of the noise signal, zero crossing density of the noise signal, a metric of noise amplitude computed using an envelope of the noise signal, and a metric of energy of the denoised signal relative to energy of the noise signal.

14. The apparatus of claim 1 , wherein the circuit-based computer is configured and arranged with the executable instructions to provide the identified T-wave offset point as an output based upon a noise characteristic of the ECG signal in the second time window by comparing the noise characteristic to a threshold and providing the identified T-wave offset point based upon the comparison.

15. The apparatus of claim 1 , wherein the circuit-based computer is configured and arranged with the executable instructions to provide the identified T-wave offset point based upon a presence, in a cardiac cycle within the ECG, of at least one of: atrial fibrillation, QT dispersion in a multi-lead ECG exceeding a threshold, T-wave morphology complexity exceeding a predefined threshold, ventricular ectopy, a QT interval measurement that falls outside of a user-defined physiologic outlier value or a statistical outlier value.

16. The apparatus of claim 1 , wherein the circuit-based computer is configured and arranged with the executable instructions to assemble a time series of provided QT interval values for analysis of variability using the provided identified T-wave offset point.

17. An apparatus for providing a repolarization signal for a cardiac cycle of an ECG signal, the apparatus comprising:

a circuit-based computer configured and arranged with executable instructions to

identify the location of a QRS complex in the cardiac cycle;

identify T-wave onset and offset points based on the identified location of the QRS complex;

define the start and end of the repolarization signal for the cardiac cycle, based respectively upon the T-wave onset and offset points;

determine a noise characteristic of the ECG signal in a time window spanning from about the start to about the end of the repolarization signal, and

providing the repolarization signal as an output, based upon the determined noise characteristic.

18. The apparatus of claim 17 , wherein the circuit-based computer is configured and arranged with the executable instructions to determine the noise characteristic by:

decomposing the ECG signal into subcomponents,

identifying ones of said subcomponents of the ECG signal within the time window as primarily associated with the T-wave of the underlying ECG signal,

computing a noise signal for the time window using at least one of the subcomponents not primarily associated with the T-wave, and

computing said noise characteristic based upon at least one of: energy of the noise signal, standard deviation of the noise signal, zero crossing density of the noise signal, a metric of noise amplitude computed using an envelope of the noise signal, and a metric of energy of the ECG signal relative to energy of the noise signal.

19. The apparatus of claim 18 , wherein the circuit-based computer is configured and arranged with the executable instructions to

compute a metric of energy of the ECG signal using at least one of the subcomponents within the second time window that is primarily associated with the T-wave.

20. The apparatus of claim 17 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute an output repolarization signal by

decomposing the ECG signal into subcomponents,

identifying ones of said subcomponents of the ECG signal within the time window as primarily associated with the T-wave of the underlying ECG signal, and

computing said output repolarization signal by combining at least two of the subcomponents that are primarily associated with the T-wave.

21. The apparatus of claim 17 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute the noise characteristic by:

computing a denoised signal for the time window using at least one of a band-pass filter, wavelet thresholding, and an adaptive filter that passes primarily T-wave energy,

computing a noise signal for the time window using a residual of the step of computing the denoised signal, and

computing said noise characteristic based upon at least one of: energy of the noise signal, standard deviation of the noise signal, zero crossing density of the noise signal, a metric of noise amplitude computed using an envelope of the noise signal, and a metric of energy of the denoised signal relative to energy of the noise signal.

22. The apparatus of claim 17 , wherein the circuit-based computer is configured and arranged with the executable instructions to compare the noise characteristic to a threshold and provide the repolarization signal as an output based upon the comparison.

23. The apparatus of claim 17 , wherein the circuit-based computer is configured and arranged with the executable instructions to provide a repolarization signal as an output based upon the determined noise characteristic and the presence in the cardiac cycle of at least one of: atrial fibrillation in the ECG signal, a degree of QT dispersion exceeding a threshold when the ECG signal is a multi-lead signal, T-wave morphology complexity of the ECG signal exceeding a threshold, ventricular ectopy, and a QT interval measurement that falls outside of a user-defined physiologic outlier value or a statistical outlier value.

24. The apparatus of claim 17 , wherein the circuit-based computer is configured and arranged with the executable instructions to append the provided repolarization signal to a matrix of repolarization signals in which a dimension of the matrix corresponds to the number of cardiac cycles of the ECG signal.

25. An apparatus for providing a time series of beat-to-beat QT interval values from a digitized ECG signal of an ambulatory subject, the apparatus comprising:

a circuit-based computer configured and arranged with executable instructions to

identify the location of a QRS complex and a Q-wave onset point of a cardiac cycle of the ECG signal;

determine a first time window of the cardiac cycle in which to search for a T-wave offset point for a T-wave in the cardiac cycle, based upon one of the identified location of the QRS complex and the identified location of the Q-wave onset point;

identify the T-wave offset point within the first time window;

compute a QT interval value using the identified Q-wave onset point of the QRS complex and the identified T-wave offset point, and

provide the QT interval value in a time series of beat-to-beat QT interval values, based upon a noise characteristic of the digitized ECG signal in a second time window that includes at least a portion of the T-wave.

26. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to, prior to identifying the T-wave offset point, suppress energy in a portion of the digitized ECG signal that is not primarily associated with T-wave energy in the first time window, using at least one of MDSP denoising, wavelet threshold denoising, band-pass filtering, and adaptive filtering.

27. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute the noise characteristic by:

decomposing the digitized ECG signal into subcomponents,

identifying said subcomponents as primarily associated with either noise or a T-wave of an underlying ECG signal,

computing a noise signal by combining the subcomponents within the second time window that are primarily associated with noise,

computing a denoised signal by combining the subcomponents within the second time window that are primarily associated with the T-wave of the underlying ECG signal, and

computing said noise characteristic based upon at least one of: energy of the noise signal, standard deviation of the noise signal, zero crossing density of the noise signal, a metric of noise amplitude based upon an envelope of the noise signal, and a metric of energy of the denoised signal relative to energy of the noise signal.

28. The apparatus of claim 27 , wherein the metric of energy of the denoised signal relative to energy of the noise signal is a signal-to-noise ratio.

29. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute the noise characteristic by:

computing a denoised signal for the second time window using at least one of a band-pass filter, wavelet thresholding, and an adaptive filter, and computing a noise signal as a difference between the digitized ECG signal and the denoised signal, and

computing said noise characteristic based upon at least one of: energy of the noise signal, standard deviation of the noise signal, zero crossing density of the noise signal, a metric of noise amplitude computed using an envelope of the noise signal, and a metric of energy of the denoised signal relative to energy of the noise signal.

30. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to provide the QT interval value in the time series of beat-to-beat QT interval values based upon a comparison of the noise characteristic to a threshold value.

31. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to provide the QT interval value in the time series of beat-to-beat QT interval values based upon the noise characteristic and the presence, in the cardiac cycle, of at least one of: atrial fibrillation, ventricular ectopy, QT dispersion in a multi-lead ECG exceeding a threshold, T-wave morphology complexity exceeding a predefined threshold, ventricular ectopy, a QT interval measurement that falls outside of a user-defined physiologic outlier value or a statistical outlier value.

32. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to

repeat the steps to compute a plurality of the QT interval values, and

provide ones of the QT interval values in the time series of beat-to-beat QT interval values based upon a comparison of a noise characteristic of a corresponding ECG signal in the second window to a predefined threshold.

33. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to compute a marker of arrhythmogenic risk by computing, using the beat-to-beat QT interval values, at least one of:

QT interval alternans,

short-term variability of QT intervals,

long-term variability of QT intervals,

root mean square, standard deviation of QT interval successive differences, and

multiscale entropy.

34. The apparatus of claim 25 , wherein the circuit-based computer is configured and arranged with the executable instructions to provide the time series of beat-to-beat QT intervals consisting of QT interval values having error due to noise that is less than 2.5% of a mean QT interval of the ECG signal.

35. A computer program product, comprising:

a non-transitory computer readable medium storing executable program instructions, which when executed by a computing apparatus, cause the computing apparatus system to perform a method comprising:

identifying a T-wave offset point within a first time window of an ECG signal, and

providing the identified T-wave offset point as an output based upon a noise characteristic of the ECG signal in a second time window that includes at least a portion of the T-wave of the ECG signal.

36. The computer program product of claim 35 , wherein the non-transitory computer readable medium stores executable program instructions that, which when executed by the computing apparatus, causes the computing apparatus system to

identify the location of a QRS complex in the ECG signal, and determine the first time window based upon the identified location of the QRS complex.