Atrioventricular block precipitated by isoproterenol

Area of Science:

• Cardiology
• Electrophysiology
• Pharmacology

Background:

• A patient with a history of near syncope and left bundle branch block underwent an electrophysiologic study (EPS) due to suspected atrioventricular (AV) block.
• No pre-existing AV block was documented prior to the study.

Observation:

• Isoproterenol infusion was administered during the EPS.
• The infusion precipitated 2:1 infra-Hisian AV block, likely proximal to the recorded His-Purkinje potentials indicating right bundle branch activity.
• Additional paroxysmal AV block occurred during drug washout, distal to the presumed right bundle branch potential.

Findings:

• Isoproterenol can unmask or precipitate infra-Hisian AV block in susceptible individuals.
• The drug's effects on AV conduction can manifest at different levels (proximal and distal to His-Purkinje system).
• The timing of AV block during infusion and washout provides diagnostic information.

Implications:

• Isoproterenol infusion may serve as a valuable diagnostic tool for suspected AV block when electrophysiologic studies are inconclusive.
• This drug challenge can help identify patients with occult conduction system disease.
• Careful monitoring during and after isoproterenol administration is crucial for accurate diagnosis of AV block.