Wenckebach-type exit block from an ectopic focus as a cause of variable coupling.

Intermittent bigeminal and trigeminal ventricular premature beats were recorded in an otherwise healthy 14 year old male. Coupling intervals progressively lengthened until an ectopic beat was dropped. Odd numbers of sinus beats occurred between bigeminal runs. This rhythm is interpreted as being due to Wenckebach-type block in an exit pathway from the ectopic focus, resulting in concealment of the persistently active extrasystolic mechanism.