Effects of isoproterenol and amiodarone and the role of exercise in initiation of circus movement tachycardia in the accessory atrioventricular pathway.

The effects of isoproterenol on induction of circus movement tachycardia were studied in 21 patients with an accessory atrioventricular pathway. Forty-six studies were performed. Thirteen patients were studied before and during administration of isoproterenol without antiarrhythmic drugs (group A). Ten patients (including 2 from group A) were studied before and during administration of isoproterenol while receiving oral treatment with amiodarone (group B). Ability to initiate circus movement tachycardia before or during administration of isoproterenol by programmed stimulation was correlated with the relation of circus movement tachycardia to exercise in these patients. (Seven patients in group A and 7 in group B had circus movement tachycardia related to exercise.) Isoproterenol significantly shortened sinus cycle length, duration of the QRS complex during sinus rhythm, anterograde effective refractory period of the accessory pathway, and circus movement tachycardia cycle length, owing to shortening of the AH interval during the arrhythmia. Isoproterenol made initiation of circus movement tachycardia possible in patients in whom the arrhythmia could not be initiated before. However, this effect did not correlate with the relation of the spontaneously occurring circus movement tachycardia to exercise. The electrophysiologic effects produced by isoproterenol did not differ between patients with and without exercise-related tachycardia. In all patients in whom circus movement tachycardia was initiated before administration of isoproterenol, the tachycardia was still inducible during administration of that drug. It is concluded that isoproterenol facilitates initiation of circus movement tachycardia in patients with an accessory pathway, mainly by facilitating anterograde conduction over the atrioventricular node.(ABSTRACT TRUNCATED AT 250 WORDS)