Effects of activation sequence on the local recovery of ventricular excitability in the dog.

We measured refractory periods at ventricular sites in the dog heart during drive from ventricular test sites and during combined drive from the test sites and other ventricular sites or the atrium. We found that ventricular activation sequence caused significant alterations on refractory period. Combined drive from two sites which resulted in fusion complexes was associated with shorter refractory periods at the test sites than those observed during drive at the test sites only. Our findings are explicable because earlier excitation and consequent earlier recovery of surrounding areas during fusion beats exerts an electrotonic influence on test sites.