Ventricular far field reduction

1. A method of far field reduction, comprising the steps of detecting peaks of a QRS complex of a cardiac electrogram;

extracting unipolar beats of an intracardiac electrogram that occur within a predetermined time interval that includes respective detected peaks;

constructing a first mean unipolar beat by averaging the extracted unipolar beats;

cross correlating the extracted unipolar beats with the first mean unipolar beat by determining respective time-lags therebetween;

accepting ones of the extracted unipolar beats wherein a cross correlation thereof exceeds a predetermined correlation threshold;

constructing a second mean unipolar beat from the accepted unipolar beats;

determining a ventricular far field component by subtracting the second mean unipolar beat from the extracted unipolar beats;

subtracting the ventricular far field component from the intracardiac electrogram to distinguish a local component of the intracardiac electrogram; and

controlling a medical procedure responsively to the local component.

2. The method according to claim 1 , wherein the predetermined correlation threshold comprises a time lag that does not exceed 10 ms.

3. The method according to claim 1 , wherein the predetermined correlation threshold comprises a difference between peak amplitudes of the extracted unipolar beats and the first mean unipolar beat that does not exceed 10 percent.

4. The method according to claim 1 , wherein constructing a first mean unipolar beat further comprises computing a median derivative beat by differentiating the extracted unipolar beats, and cross correlating is performed between the extracted unipolar beats and the median derivative beat.

5. The method according to claim 1 , wherein constructing a second mean unipolar beat comprises aligning the extracted unipolar beats.

6. The method according to claim 1 , wherein constructing a second mean unipolar beat comprises:

dividing the accepted unipolar beats into a plurality of clusters; and

constructing respective cluster-specific mean unipolar beats for each of the clusters.

7. The method according to claim 6 , wherein dividing the accepted unipolar beats is performed by k-means clustering according to Euclidean distances between the accepted unipolar beats.

8. The method according to claim 6 , wherein determining a ventricular far field component comprises:

selecting an optimum one of the cluster-specific mean unipolar beats for respective accepted unipolar beats; and

subtracting the selected cluster-specific mean unipolar beats from the extracted unipolar beats.

9. An apparatus, comprising:

an intra-body probe having an electrode, the probe being configured to contact tissue in a heart;

one or more body surface electrodes adapted to placed on an outer surface of a subject's body;

a display; and

a processor, which is configured to receive a cardiac electrogram signal from the one or more body surface electrodes and which is configured to receive an electrical signal from the electrode, to distinguish a local component, due to the tissue with which the electrode is in contact, in the electrical signal from a remote-field contribution to the signal by performing the steps of:

detecting peaks of a QRS complex of the cardiac electrogram;

extracting unipolar beats of an intracardiac electrogram received via the electrode, wherein the extracted unipolar beats occur within a predetermined time interval that includes respective detected peaks;

constructing a first mean unipolar beat by averaging the extracted unipolar beats;

cross correlating the extracted unipolar beats with the first mean unipolar beat by determining respective time-lags therebetween;

accepting ones of the extracted unipolar beats wherein a cross correlation thereof exceeds a predetermined correlation threshold;

constructing a second mean unipolar beat from the accepted unipolar beats;

determining a ventricular far field component by subtracting the second mean unipolar beat from the extracted unipolar beats;

subtracting the ventricular far field component from the intracardiac electrogram to distinguish the local component of the intracardiac electrogram; and

outputting the local component to the display.

10. The apparatus according to claim 9 , wherein the predetermined correlation threshold comprises a time lag that does not exceed 10 ms.

11. The apparatus according to claim 9 , wherein the predetermined correlation threshold comprises a difference between peak amplitudes of the extracted unipolar beats and the first mean unipolar beat that does not exceed 10 percent.

12. The apparatus according to claim 9 , wherein constructing a first mean unipolar beat further comprises computing a median derivative beat by differentiating the extracted unipolar beats, and cross correlating is performed between the extracted unipolar beats and the median derivative beat.

13. The apparatus according to claim 9 , wherein constructing a second mean unipolar beat comprises aligning the extracted unipolar beats.

14. The apparatus according to claim 9 , wherein constructing a second mean unipolar beat comprises:

dividing the accepted unipolar beats into a plurality of clusters; and

constructing respective cluster-specific mean unipolar beats for each of the clusters.

15. The apparatus according to claim 14 , wherein dividing the accepted unipolar beats is performed by k-means clustering according to Euclidean distances between the accepted unipolar beats.

16. The apparatus according to claim 14 , wherein determining a ventricular far field component comprises:

selecting an optimum one of the cluster-specific mean unipolar beats for respective accepted unipolar beats; and

subtracting the selected cluster-specific mean unipolar beats from the extracted unipolar beats, respectively.