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Address reprint requests and correspondence: Dr. Daniel R. Frisch, Beth Israel Deaconess Medical Center, Department of Medicine, Cardiology Division, Electrophysiology Section, 1 Deaconess Road, Baker 4, Boston, Massachusetts 02215.
A 68-year-old woman with a history of paroxysmal atrial fibrillation was referred
for pulmonary vein isolation. After the pulmonary veins were isolated, burst atrial
pacing during infusion of intravenous isoproterenol induced atrial flutter at a cycle
length of 230 ms, with variable but rapid AV conduction. An alternating pattern of
left bundle branch block (LBBB) and right bundle branch block (RBBB) was observed
during the tachycardia (Figure 1). What is the mechanism of the alternating bundle branch block?
Figure 1ECG leads V1–V3 showing alternating left bundle branch block with right bundle branch block during
atrial flutter with rapid AV conduction.
