Experimental Studies on the Facilitation of AV Conduction by Ectopic Beats in Dogs and Rabbits

Experiments were performed on the hearts of open-chest anesthetized dogs and on isolated rabbit hearts. Intracellular microelectrodes and bipolar surface electrodes were used for recording from and for stimulating the atria, AV node, bundle of His, bundle branches and ventricles. Atrial or ventricular premature beats which were usually blocked or delayed within the AV node were facilitated when ectopic activity caused preexcitation and early recovery of the site of block or delay. Ectopic atrial, ventricular, nodal, and bundle of His responses allowed previously blocked or delayed antegrade and retrograde beats to be conducted only when the ectopic activity arrived at an appropriate time at the site of block within the AV node. Therefore, apparent supernormal AV conduction may occur in the human heart by this mechanism, and some reported cases of supernormal conduction may be due to the preexcitation and early recovery of a refractory barrier by ectopic activity.