The induction of ventricular arrhythmias in acute myocardial ischemia by atrial pacing with long-short cycle sequences.

Recent studies emphasize the importance of heart rate in the genesis of ventricular arrhythmias during myocardial ischemia. The role of alterations in rhythm has not previously been systematically investigated. In 20 dogs subjected to acute myocardial ischemia and crushing of the sinus node, standard electrocardiographic leads were recorded, as well as His bundle and epicardial electrograms from the normal and ischemic areas. Abrupt pauses in regular atrial pacing did not cause arrhythmias prior to the onset of ischemia; however, during ischemia, atrial pacing with intermittent abrupt pauses resulted in the induction of ventricular arrhythmias beginning after the second conducted beat following each pause (ventricular premature beats, 20/20; ventricular tachycardia, 19/20; and ventricular fibrillation, 8/20). Onset of the arrhythmia was associated with increased delay in activation of ischemic epicardium and fractionation of the electrogram potential of the second conducted impulse. Typical Gouaux-Ashman phenomenon was an incidental observation. Unlike the Gouaux-Ashman phenomenon, which is restricted to the His-Purkinje system, the phenomenon we observed orginated within ischemic myocardium. In vitro studies indicate that the underlying mechanism may be related to postrepolarization refractoriness induced by ischemia.