Physiologic signal processing to determine a cardiac condition

1. A method of determining a cardiac condition, the method comprising:

receiving a sensed physiologic signal for a period of time including multiple cardiac cycles;

determining, by a processing module using the received physiologic data, a heart beat frequency to be used as a reference frequency;

extracting, by the processing module, a plurality of harmonics of the received physiologic signal based on the reference frequency, wherein the harmonics correspond to a plurality of alternans frequencies and include integer harmonics and fractional harmonics, and wherein the harmonics are extracted using one or more band pass filters, and further comprising assembling a blended signal comprising a sum of corresponding integer and fractional harmonics; and

determining, by the processing module, amplitudes of at least some of the extracted harmonics and using the amplitudes to determine an alternans indicator value indicative of a patient's susceptibility to a cardiac condition or to a cardiac event, and storing the alternans indicator value in an electronic storage location.

2. The method of claim 1 , wherein the sensed physiologic signal is an ECG signal.

3. The method of claim 2 , wherein the harmonics are extracted from a repolarization component of the ECG signal.

4. The method of claim 3 , wherein the repolarization component is a continuous repolarization component of the ECG signal.

5. The method of claim 4 , wherein the alternans indicator value is a T-wave alternans amplitude.

6. The method of claim 1 , further comprising computing a complex envelope of the blended signal.

7. The method of claim 1 , wherein the sensed physiologic signal is collected by an implanted device.

8. The method of claim 7 , wherein the implanted device collects the sensed physiologic signal using at least two electrodes positioned subcutaneously within a patient.

9. The method of claim 7 , wherein the implanted device collects the sensed physiologic signal using at least one intracardiac electrode.

10. The method of claim 7 , wherein the sensed physiologic signal is telemetered from the implanted device to external processing equipment.

11. The method of claim 10 , wherein the processing module is within the external processing equipment.

12. The method of claim 7 , wherein the sensed physiologic signal is analyzed within the implanted device.

13. The method of claim 7 , wherein the processing module is within the implanted device.

14. The method of claim 1 , wherein the physiologic signal is an electrocardiogram or electrogram, and wherein the alternans indicator value is a QRS alternans value.

15. The method of claim 1 , further comprising issuing a warning in response to comparing the alternans indicator value to the predetermined threshold value.

16. The method of claim 1 , wherein input noise associated with the sensed physiologic signal is measured, and wherein at least a portion of the sensed physiologic signal is rejected if the measured noise exceeds a predetermined threshold.

17. The method of claim 1 , further comprising comparing the alternans indicator value to a predetermined threshold value and storing a result of the comparison in another electronic storage location.

18. A method of determining a cardiac condition, the method comprising:

receiving a sensed physiologic signal for a period of time including multiple cardiac cycles;

determining, by a processing module using the received physiologic data, a heart beat frequency to be used as a reference frequency;

extracting, by the processing module, a plurality of harmonics of the received physiologic signal based on the reference frequency, wherein the harmonics correspond to a plurality of alternans frequencies and include integer harmonics and fractional harmonics, and wherein the harmonics are extracted using one or more band pass filters, and further comprising normalizing one or more powers of the extracted harmonics by a power of the noise band; and

determining, by the processing module, amplitudes of at least some of the extracted harmonics and using the amplitudes to determine an alternans indicator value indicative of a patient's susceptibility to a cardiac condition or to a cardiac event, and storing the alternans indicator value in an electronic storage location.

19. A method of determining a cardiac condition, the method comprising:

receiving a sensed physiologic signal for a period of time including multiple cardiac cycles;

determining, by a processing module using the received physiologic data, a heart beat frequency to be used as a reference frequency;

extracting, by the processing module, a plurality of harmonics of the received physiologic signal based on the reference frequency, wherein the harmonics correspond to a plurality of alternans frequencies, wherein each of the plurality of harmonics are fractional harmonics; and

determining, by the processing module, amplitudes of at least some of the extracted harmonics and using the amplitudes to determine an alternans indicator value by taking the square root of a summation of powers of amplitudes of the extracted fractional harmonics, wherein the alternans indicator value is indicative of a patient's susceptibility to a cardiac condition or to a cardiac event, and storing the alternans indicator value in an electronic storage location.

20. A method of determining a cardiac condition, the method comprising:

receiving a sensed physiologic signal for a period of time including multiple cardiac cycles;

determining, by a processing module using the received physiologic data, a heart beat frequency to be used as a reference frequency;

extracting, by the processing module, a plurality of harmonics of the received physiologic signal based on the reference frequency, wherein the harmonics correspond to a plurality of alternans frequencies and include integer harmonics and fractional harmonics, and wherein the harmonics are extracted using one or more band pass filters, wherein alternans signal morphology is reconstructed from the fractional harmonics; and

determining, by the processing module, amplitudes of at least some of the extracted harmonics and using the amplitudes to determine an alternans indicator value indicative of a patient's susceptibility to a cardiac condition or to a cardiac event, and storing the alternans indicator value in an electronic storage location.

21. A system, comprising:

an electrode configured to sense a physiologic signal of a patient for a period of time including multiple cardiac cycles; and

a processor module programmed to:

receive the sensed physiologic signal;

determine, using the received physiologic data, a heart beat frequency to be used as a reference frequency;

extract a plurality of harmonics of the received physiologic signal based on the reference frequency, wherein the harmonics correspond to a plurality of alternans frequencies and include integer harmonics and fractional harmonics;

assemble a blended signal comprising a sum of corresponding integer and fractional harmonics; and

determine amplitudes of at least some of the extracted harmonics and use the amplitudes to determine an alternans indicator value indicative of a patient's susceptibility to a cardiac condition or to a cardiac event.

22. The system of claim 21 , wherein the sensed physiologic signal is an ECG signal.

23. The system of claim 22 , wherein the harmonics are extracted from a repolarization component of the ECG signal.

24. The system of claim 21 , wherein the alternans indicator value is a T-wave alternans amplitude.