Optimal Programming of Antitachycardia Pacing Functions

The majority of ventricular tachyarrhythmias for which implantable cardioverter de~brillators are implanted is due to reentry. In order for paced beats to entrain or terminate a tachycardia, the arrhythmia must have an excitable gap, i.e., tissue in the circuit between the activation wavefront and the tail of refractoriness that can be depolarized by an electrical impulse entering the tachycardia circuit. When reentry is the mechanism, ventricular tachycardia can often be entrained and/or terminated by pacing [1]. Entrainment occurs when impulses enter the circuit and propagate in the antegrade direction but collide with the previous impulse in the retrograde direction. Antitachycardia pacing algorithms in implantable cardioverter de~brillators are based on the principles of entrainment [2]. The tachycardia terminates when an impulse collides in the retrograde direction and blocks in the antegrade direction. Antitachycardia pacing has several advantages over shock therapy for ventricular tachycardia. First, because it does not require that capacitors be charged, antitachycardia pacing can be delivered more quickly. This decreases the likelihood that the patient will develop hypotension or myocardial ischemia from the arrhythmia. Second, antitachycardia pacing is better tolerated than shock therapy. Direct current countershocks are often associated with pain, and, when they occur frequently, can cause signi~cant patient anxiety. In contrast, antitachycardia pacing may be associated with brief palpitation or dizziness, but is rarely associated with discomfort. Many patients are not aware of antitachycardia pacing events at all. Because it is so well tolerated, antitachycardia pacing may, in selected patients, obviate the need for antiarrhythmic drugs which are sometimes necessary to decrease the frequency of ventricular tachycardia and the attendant shocks. Third, antitachycardia pacing, when successful, uses far less energy than a direct current counter shock. In some patients this may promote longer implantable cardioverter de~brillator battery life. For all of its potential bene~ts, antitachycardia pacing is not without risk. The major risk associated with antitachycardia pacing is that of proarrhythmia. Antitachycardia pacing has the potential to accelerate an hemodynamically well tolerated ventricular tachycardia into one that is associated with hemodynamic collapse or one that is more dif~cult to terminate, even with a shock [3]. Studies suggest that this may occur in 2 to 21% of cases [4]. Proarrhymia may also occur if an implantable cardioverter de~brillator misclassi~es a supraventricular tachyarrhythmia as ventricular tachycardia [3]. In such a case, antitachycardia pacing may induce ventricular tachycardia. This is most likely to occur in patients with slow ventricular tachycardia, the rate of which overlaps with that of sinus tachycardia or other atrial arrhythmias. Finally, antitachycardia pacing, if it is unsuccessful, may delay the delivery of de~nitive shock therapy for an arrhythmia. This has the potential to subject a patient to myocardial ischemia and may limit the ef~cacy of the countershock.