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A cardiac rhythm management system includes atrial shock timing optimization. Because an atrial tachyarrhythmia, such as atrial fibrillation typically causes significant variability in the ventricular heart rate, resulting in potentially proarrhythmic conditions. The system avoids delivering atrial

A cardiac rhythm management system includes atrial shock timing optimization. Because an atrial tachyarrhythmia, such as atrial fibrillation typically causes significant variability in the ventricular heart rate, resulting in potentially proarrhythmic conditions. The system avoids delivering atrial cardioversion/defibrillation therapy during potentially proarrhythmic conditions because doing so could result in dangerous ventricular arrhythmias. Using Ventricular Rate Regularization ("VRR") techniques, the system actively stabilizes the ventricular heart rate to obtain less potentially proarrhythmic conditions for delivering the atrial tachyarrhythmia therapy. The intrinsic ventricular heart rate is stabilized at a variable VRR-indicated rate, computed using an infinite impulse response (IIR) filter, and based on the underlying intrinsic ventricular heart rate. The system withholds delivery of atrial cardioversion/defibrillation therapy until the intervals between ventricular beats ("V--V intervals") meet certain criteria that decrease the chance that the atrial cardioversion/defibrillation therapy will induce a ventricular arrhythmia.

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What is claimed is: 1. A method comprising: computing a present indicated ventricular pacing interval, using infinite-impulse response (IIR) filtering, the filtering using as inputs: (1) a most recent V--V interval duration between ventricular events; and (2) a previously-computed value of the indi

What is claimed is: 1. A method comprising: computing a present indicated ventricular pacing interval, using infinite-impulse response (IIR) filtering, the filtering using as inputs: (1) a most recent V--V interval duration between ventricular events; and (2) a previously-computed value of the indicated ventricular pacing interval that corresponds to a V--V interval that immediately precedes the most recent V--V interval, the IIR filtering providing, as an output, the present indicated ventricular pacing interval; pacing a ventricle using the present indicated ventricular pacing interval; detecting an atrial tachyarrhythmia; determining whether potentially proarrhythmic conditions exist in the ventricle; and after the pacing the ventricle, the detecting the atrial tachyarrhythmia, and the determining whether potentially proarrhythmic conditions exist, then delivering cardioversion/defibrillation therapy to the atrium if it is determined that no potentially proarrhythmic conditions exist in the ventricle, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until it is determined that no potentially proarrhythmic conditions exist in the ventricle. 2. The method of claim 1, in which the determining whether potentially proarrhythmic conditions exist in the ventricle comprises: comparing the most recent V--V interval to a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than the first predetermined value and the most recent V--V interval exceeds or equals a preceding V--V interval less a second predetermined value; deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than a third predetermined value; and deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than the third predetermined value and the most recent V--V interval is shorter than the preceding V--V interval less the second predetermined value. 3. The method of claim 2, in which the determining whether potentially proarrhythmic conditions exist in the ventricle further comprises: deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is equal to the third predetermined value; and deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is equal to the first predetermined value. 4. The method of claim 2, in which the first predetermined value is approximately between 700 milliseconds and 1000 milliseconds. 5. The method of claim 2, in which the first predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat. 6. The method of claim 2, in which the second predetermined value is approximately between 0 milliseconds and 200 milliseconds. 7. The method of claim 2, in which second predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat. 8. The method of claim 2, in which the third predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat. 9. The method of claim 2, in which the third predetermined value is approximately between 350 milliseconds and 1000 milliseconds, and the third predetermined value is one of: (1) less than the first predetermined value, and (2) less than or equal to the first predetermined value. 10. The method of claim 1, in which the pacing the ventricle using the present indicated ventricular pacing interval is initiated by the detecting the atrial tachyarrhythmia. 11. The method of claim 1, in which the pacing the ventricle using the present indicated ventricular pacing interval is independent of the detecting the atrial tachyarrhythmia. 12. A method comprising: detecting an atrial tachyarrhythmia; stabilizing a ventricular heart rate, using an infinite impulse response filter to filter intervals between paced or sensed ventricular depolarizations to compute an indicated ventricular pacing interval on a beat-to-beat basis, in response to the detecting the atrial tachyarrhythmia; determining whether potentially proarrhythmic conditions exist; and delivering cardioversion/defibrillation therapy to the atrium, in response to the detecting the atrial tachyarrhythmia, if the determining whether potentially proarrhythmic conditions exist indicates that no potentially proarrhythmic conditions exist, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until no potentially proarrhythmic conditions exist. 13. The method of claim 12, in which the determining whether potentially proarrhythmic conditions exist comprises: comparing a most recent V--V interval to a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than the first predetermined value and the most recent V--V interval exceeds or equals a preceding V--V interval less a second predetermined value; deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than a third predetermined value; and deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than the third predetermined value and the most recent V--V interval is shorter than the preceding V--V interval less the second predetermined value. 14. The method of claim 13, in which the determining whether potentially proarrhythmic conditions exist in the ventricle further comprises: deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is equal to the third predetermined value; and deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is equal to the first predetermined value. 15. The method of claim 13, in which the first predetermined value is approximately between 700 milliseconds and 1000 milliseconds. 16. The method of claim 13, in which the first predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat. 17. The method of claim 13, in which the second predetermined value is approximately between 0 milliseconds and 200 milliseconds. 18. The method of claim 13, in which second predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat. 19. The method of claim 13, in which the third predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat. 20. The method of claim 13, in which the third predetermined value is approximately between 350 milliseconds and 1000 milliseconds, and the third predetermined value is one of: (1) less than the first predetermined value, and (2) less than or equal to the first predetermined value. 21. A cardiac rhythm management system, comprising: an atrial heart signal sensing circuit; a ventricular heart signal sensing circuit, configured to sense V--V intervals between ventricular depolarizations of a ventricle; a ventricular pacing therapy circuit; an atrial cardioversion/defibrillation therapy circuit; and a controller, communicatively coupled to the atrial heart signal sensing circuit, the ventricular heart signal sensing circuit, the ventricular pacing therapy circuit, and the atrial cardioversion/defibrillation therapy circuit, the controller including: a ventricular rate stabilization circuit, including an infinite impulse response (IIR) filter circuit outputting a variable indicated pacing interval, the IIR filter circuit using at least first input comprising a duration of a most recent sensed V--V interval and a second input comprising a preceding value of the indicated pacing interval; and an atrial cardioversion/defibrillation control circuit that (a) determines if potentially proarrhythmic ventricular conditions exist, and (b) includes an atrial therapy circuit that delivers cardioversion/defibrillation therapy to the atrium if no potentially proarrhythmic conditions exist, and otherwise withholds the delivery of cardioversion/defibrillation therapy to the atrium. 22. The system of claim 21, further comprising a programmer, remote from and communicatively coupled to the controller. 23. The system of claim 21, further comprising a leadwire adapted for being coupled between at least one of: (1) a coronary sinus, and (2) a superior vena cava and an implantable device comprising the atrial and ventricular heart signal sensing circuits, the ventricular pacing and atrial cardioversion/defibrillation circuits, and the controller. 24. The system of claim 21, in which the controller includes an atrial tachyarrhythmia detection module, and in which the ventricular pacing therapy circuit is configured to initiate pacing of the ventricle at the variable indicated pacing interval, outputted by the IIR filter circuit, in response to an atrial tachyarrhythmia detected by the atrial tachyarrhythmia detection module. 25. A method comprising: detecting an atrial tachyarrhythmia; stabilizing a ventricular heart rate, using an infinite impulse response filter to compute an indicated ventricular pacing interval, in response to the detecting the atrial tachyarrhythmia; determining whether potentially proarrhythmic conditions exist, including: comparing a most recent V--V interval to a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than or equal to a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than the first predetermined value and the most recent V--V interval exceeds or equals a preceding V--V interval less a second predetermined value; deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than or equal to a third predetermined value; and deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than the third predetermined value and the most recent V--V interval is shorter than the preceding V--V interval less the second predetermined value; and delivering cardioversion/defibrillation therapy to the atrium, in response to the detecting the atrial tachyarrhythmia, if the determining whether potentially proarrhythmic conditions exist indicates that no potentially proarrhythmic conditions exist, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until no potentially proarrhythmic conditions exist. 26. The method of claim 25, in which the using the infinite impulse response filter to compute the indicated ventricular pacing interval includes using the infinite impulse response filter to compute the indicated ventricular pacing interval on a beat-to-beat basis. 27. A method comprising: detecting an atrial tachyarrhythmia; stabilizing a ventricular heart rate, using an infinite impulse response filter to compute an indicated ventricular pacing interval, in response to the detecting the atrial tachyarrhythmia; determining whether potentially proarrhythmic conditions exist, including: comparing a most recent V--V interval to a first predetermined value, in which the first predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than the first predetermined value and the most recent V--V interval exceeds or equals a preceding V--V interval less a second predetermined value; deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than a third predetermined value; and deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than the third predetermined value and the most recent V--V interval is shorter than the preceding V--V interval less the second predetermined value; and delivering cardioversion/defibrillation therapy to the atrium, in response to the detecting the atrial tachyarrhythmia, if the determining whether potentially proarrhythmic conditions exist indicates that no potentially proarrhythmic conditions exist, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until no potentially proarrhythmic conditions exist. 28. The method of claim 27, in which the using the infinite impulse response filter to compute the indicated ventricular pacing interval includes using the infinite impulse response filter to compute the indicated ventricular pacing interval on a beat-to-beat basis. 29. A method comprising: detecting an atrial tachyarrhythmia; stabilizing a ventricular heart rate, using an infinite impulse response filter to compute an indicated ventricular pacing interval, in response to the detecting the atrial tachyarrhythmia; determining whether potentially proarrhythmic conditions exist, including: comparing a most recent V--V interval to a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than the first predetermined value and the most recent V--V interval exceeds or equals a preceding V--V interval less a second predetermined value, in which the second predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat; deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than a third predetermined value; and deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than the third predetermined value and the most recent V--V interval is shorter than the preceding V--V interval less the second predetermined value; and delivering cardioversion/defibrillation therapy to the atrium, in response to the detecting the atrial tachyarrhythmia, if the determining whether potentially proarrhythmic conditions exist indicates that no potentially proarrhythmic conditions exist, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until no potentially proarrhythmic conditions exist. 30. The method of claim 29, in which the using the infinite impulse response filter to compute the indicated ventricular pacing interval includes using the infinite impulse response filter to compute the indicated ventricular pacing interval on a beat-to-beat basis. 31. A method comprising: detecting an atrial tachyarrhythmia; stabilizing a ventricular heart rate, using an infinite impulse response filter to compute an indicated ventricular pacing interval, in response to the detecting the atrial tachyarrhythmia; determining whether potentially proarrhythmic conditions exist, including: comparing a most recent V--V interval to a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than a first predetermined value; deeming no potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than the first predetermined value and the most recent V--V interval exceeds or equals a preceding V--V interval less a second predetermined value; deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is shorter than a third predetermined value, in which the third predetermined value is different when the most recent V--V interval is initiated by a sensed beat than when the most recent V--V interval is initiated by a paced beat; and deeming potentially proarrhythmic conditions to exist if the most recent V--V interval is longer than the third predetermined value and the most recent V--V interval is shorter than the preceding V--V interval less the second predetermined value; and delivering cardioversion/defibrillation therapy to the atrium, in response to the detecting the atrial tachyarrhythmia, if the determining whether potentially proarrhythmic conditions exist indicates that no potentially proarrhythmic conditions exist, otherwise withholding the delivery of cardioversion/defibrillation therapy to the atrium until no potentially proarrhythmic conditions exist. 32. The method of claim 31, in which the using the infinite impulse response filter to compute the indicated ventricular pacing interval includes using the infinite impulse response filter to compute the indicated ventricular pacing interval on a beat-to-beat basis.