Supernormal conduction in the canine bundle of His and proximal bundle branches

We used close bipolar intramural electrodes and catheter electrodes to study the characteristics of conduction in the bundle of His and proximal bundle branches during premature atrial beats in 17 open-chest anesthetized dogs. The electrophysiological properties of the proximal conducting system were heterogenous. The shortest interval between a normal His bundle response and a premature response that was not accompanied by changes in conduction time (the total recovery time) was 258.8 +/- 23.9 (SD) ms for the proximal His and 310.7 +/- 30.6 ms for the distal His and proximal bundle branches. A period of supernormal conduction, in which the conduction times of premature beats were faster than during earlier or later beats, was localized to the distal portion of the bundle of His and proximal bundle branches. The minimal conduction time during the supernormal period was decreased by 9.6 +/- 4.6% below control diastolic conduction times, and the supernormal period was 61.0 +/- 25.7 ms in duration. The characteristics of the period of supernormal conduction in the distal bundle of His and proximal bundle branches were very similar to those previously found in the peripheral bundle branch-Purkinje system. The mechanism of supernormal conduction in the bundle of His is most probably due to a period of supernormal excitability.