Procainamide-induced left anterior hemiblock of the 2:1 type (pseudoelectrical alternans).

A selective defect in cardiac conduction in the anterior division of the left bundle branch was induced in a patient by therapy with procainamide hydrochloride. The defect in conduction, when intermittent, resulted in 2:1 Wencklebach sequences and produced a pattern similar to classic electrical alternans. The prognosis in drug-induced alternans is good, and it is important to differentiate it from true electrical alternans.