Supernormal phase of AV conduction. A study during heart block and endocardial pacing.

A distinct, well-defined phase has been identified in a patient with heart block during which sinus impulses were conducted to the ventricles. This phase persisted during pacing but its onset and termination were delayed. No significant difference was found at varying pacing rates. On four occasions while pacing, two consecutive sinus impulses were conducted to the ventricles but this was not seen during the long recordings taken when in spontaneous idioventricular rhythm. This phase of conduction cannot be satisfactorily explained either by a vagal mechanism or refractory variations within the AV junctional tissue, and lends strong support to the recently challenged concept of a supernormal phase of conduction.