[The supernormal phase of cardiac conduction. An experimental study in the dog]

The existence of a cardiac phase of supernormal excitability is not fully proved at the cellular level. As far as the whole heart is concerned, the phenomenon of supernormal conduction is still under discussion. Intracardiac conduction has been studied both in the right atrium (13 dogs) and in the right ventricle (14 dogs) by programmed stimulation (extrastimulus technique), while monophasic action potential (MAP) was recorded. The phenomenon of supernormal conduction was noted in 67.7% of cases, starting at 59.0% and ending at 78.0% of the basic cycle, hence occuring shortly after phase 3 of the MAP. In the ventricle, a phase of supernormal conduction was present in 52.4% of cases, starting at 66.1% and ending at 77.9% of the basic cycle. After ajmaline, the supernormal conduction was moved towards the end of the cycle. The mechanism of supernormal conduction, and its implications in the study of arrhythmias are discussed.