Retrograde invasion of the bundle branches producing aberration of the QRS complex during supraventricular tachycardia studied by programmed electrical stimulation.

Four patients with paroxysmal supraventricular tachycardia have been studied using programmed electrical stimulation of the heart and intracardiae recordings. One patient had an atrioventricular (A-V) junctional tachycardia, possibly due to a rapidly discharging protected ectopic focus, and three patients had anomalous atrioventricular connections. The ECG recordings in all four patients showed a bundle branch block pattern during tachycardia, and normal intraventricular conduction could be produced by suitably timed right ventricular premature beats. The cycle length of the tachycardia was shorter with normal intraventricular conduction than in the presence of a bundle branch block pattern in two patients. Evidence is presented indicating that the bundle branch block during tachycardia was likely to be due to concealed retrograde conduction of the appropriate bundle branch, and in the presence of an anomalous bypass, the cycle length of the tachycardia depended on the anatomical relationship of the anomalous bypass to the blocked bundle branch.