Atrioventricular block precipitated by isoproterenol.

An isoproterenol infusion was administered during an electrophysiologic study (EPS) in a patient with a history of near syncope, left bundle branch block, and no documented atrioventricular (AV) block. Isoproterenol precipitated classic 2:1 Infra-Hisian AV block most probably proximal to the site of recording a His-Purkinje potential consistent with right bundle branch activity. Paroxysmal AV block also occurred during isoproterenol washout at a different site located distal to the presumed right bundle branch potential. Isoproterenol may be valuable diagnostically in an occasional patient suspected of AV block in whom an EPS is unrevealing and a drug challenge is negative.