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Four patients are reported in whom alternation ofRR cycle length before reversion to sinus rhythm occurred during the administration of intravenous verapamil. In 2 of these 4 patients, alternation of cycle lengths was associated with a changing QRS morphology. It is suggested that one patient had a reciprocating tachycardia using dual AV nodal pathways. The alternating cycle lengths in response to verapamil are explained on the basis of a 2:i block in the antegrade pathway unmasking a third pathway, thus providing two pathways with differing antegrade conduction. In the second patient the mechanism of the supraventricular tachycardia appears to be atrial or AV nodal extrasystolic tachycardia. We attribute the alternating cycle length to a 3:2 Wenckebach block before reversion to sinus rhythm. However, a reciprocating mechanism in this and the other two patients cannot be excluded. The possible mechanismsfor verapamil-induced alternation are discussed.

Cycle length alternation in supraventricular tachycardia after administration of verapamil.