Ventricular parasystolic rhythm masquerading as bilateral bundle branch block

A patient with sinus rhythm, complete left bundle branch block and 2:1 A-V conduction is described. A ventricular parasystolic rhythm, probably arising from the main left bundle branch system, occasionally gave rise to periods that simulated 3:2 A-V Wenckebach conduction with alternating bundle branch block when the parasystolic rate was approximately one third of the sinus rate. Fusion impulses with complete normalization of the QRS complex were observed. The value of analysis of long rhythm strips and repeated observations is stressed for unraveling complex disorders of cardiac rhythm.