Studies on overdrive stimulation of canine cardiac Purkinje fibers: maximal diastolic potential as a determinant of the response

The effects of overdrive stimulation were studied on preparations of isolated canine cardiac Purkinje fibers using standard microelectrode techniques. Preparations were made from false tendons, the subendocardial right bundle branch and 24 hour infarct zone Purkinje fibers. Three types of automaticity were recognized: high potential automaticity occurred in fibers with maximal diastolic potentials greater than -70 mV, intermediate potential automaticity occurred in fibers with maximal diastolic potentials between -61 and -70 mV and low potential automaticity occurred in fibers with maximal diastolic potentials less than -60 mV. Short periods of overdrive (15 seconds or 30 beats) resulted in marked suppression of high potential automaticity, slight suppression of intermediate potential automaticity and little or no suppression of low potential automaticity. The extent of postoverdrive suppression of intermediate potential automaticity was related to the rate and duration of the overdrive train and the amount of hyperpolarization that occurred in the pacemaker fiber. Finally, in three experiments on Purkinje fibers from 24 hour infarct zones, delayed afterdepolarizations occurred. In two of these preparations, overdrive stimulation resulted in biphasic responses (postoverdrive enhancement followed by postoverdrive suppression). The results of this study suggest that it may be possible to design simple electrophysiologic tests to determine whether an automatic arrhythmia is being caused by normal or partially depolarized ectopic pacemakers.