Absence of supernormal conduction in canine myocardium

Transmural conduction time, measured as the difference in arrival time of impulses from a distant stimulating site at endocardial and epicardial electrodes near the left ventricular apex, has been reported to decrease when closely coupled extrasystoles are interpolated, indicating that muscle conduction could be supernormal. We have now determined that reduction in transmural conduction time is accounted for completely by the relatively late arrival time of the extrasystolic wave front at the endocardial recording site. The endocardial recording site was activated later than an immediately adjacent site within the wall in two out of eight animals, which could be interpreted as retrograde conduction. No evidence for supernormal conduction within the ventricular wall could be obtained by multiple electrode recordings. Supernormal conduction throughout the myocardial wall could not be demonstrated on stimulation of the endocardial site. We conclude that supernormal conduction in myocardial muscle cannot be demonstrated and that changes in transmural conduction time do not always measure myocardial conduction velocity.