Significance of transient entrainment in pacing treatment of tachyarrythmias.

Rapid pacing techniques can be used to interrupt reentrant tachyarrhythmias whose reentry circuit includes an excitable gap. These rhythms include classic atrial flutter (Type I), ectopic atrial tachycardia, AV nodal reentrant tachycardia, AV junctional reentrant tachycardia involving an AV bypass pathway, ventricular tachycardia, and probably sinus node reentrant tachycardia. When utilizing rapid pacing techniques to interrupt any of these arrhythmias, it is necessary to pace at a critically rapid rate, and then for a critical duration of time at that rapid rate in order to interrupt the tachycardia. Pacing at rates short of the critically rapid rate, or short of the critical duration, will simply transiently entrain the tachycardia to the pacing rate without interrupting it. Therefore, when such pacing is terminated, the tachycardia will resume promptly at its previous spontaneous rate. Furthermore, for some arrhythmias, pacing may be achieved at an appropriately rapid rate to interrupt the tachycardia and the pacing may be maintained for an appropriate period of time. However, in the latter instances, the phenomenon of termination and reinitiation of the arrhythmia sometimes may occur, in which case cessation of pacing is not associated with interruption of the arrhythmia. Knowledge of this phenomenon is important when using either permanent or temporary pacing techniques to interrupt reentrant arrhythmias.