Electrotonic interaction during canine ventricular repolarization

Canine ventricular refractory periods were measured during test site drive, during drive of single ectopic sites, during fusion drive from two ectopic sites, and during fusion drive from an ectopic site and the test site. Refractory period duration was dependent on the driving modes employed. Refractory periods were 2.63 +/- 0.73%, 3.42 +/- 0.87%, 3.54 +/- 100%, and 4.68 +/- 1.36% (mean +/- SD) shorter during drive of single ectopic sites 2, 4, 6, and 40-60 mm, respectively, from the test site than during test site drive. During fusion drive from two ectopic sites, refractory periods were an average of 2.44 +/- 1.04 msec (mean +/- SD) less than during drive from a single ectopic site (P less than 0.005). When fusion of activation, induced by ectopic and test site drive, was located within 4 mm or less of the test site, refractory periods during fusion drive were also significantly shorter than during test site drive (P less than 0.05). Refractory periods were as much as 10 msec shorter when fusion occurred within 1 mm of the test site than their durations during test site drive. The differences in refractory periods measured during various driving modes were most likely due to electrotonic interactions during ventricular repolarization and are explicable on the basis of the intracellular distribution of potentials to be expected with each driving mode.