Nature of His Potential in His Electrogram

Unipolar His bundle electrograms were recorded with the muscle preparation isolated from the rabbit or the dog heart at as various sites as possible on and near the A-V conduction system and changes in appearance time and configuration of the so-called His (H) potential were examined. When the exploring electrode was shifted from the coronary sinus orifice down to the transition between the A-V node and the His bundle, the H potential was small but observed distinctly and its appearance time was rather fixed. The earliest excitation of the His bundle is thus assumed to be not much earlier than 1/3 or 1/4 the P-R interval before the Q wave. The nodal (N) potential appeared in addition to the H potential at variable time points while the electrode was shifted on the A-V node and disap-peared when it was reached the His bundle. The appearance did not give the impression that the N potential changed to the H potential accord-ing to the electrode shift. Over the His bundle the H potential changed its appearance time successively. The H potential was also compared with the action potential obtained from the points as close as possible to the ex-ploring electrode or with the other reference electrogram. It took the electrical field generated by the electrical activity of the whole of the His bundle, but it was influenced more by the local electrical activity than in the case of the electrocardiogram of the whole heart in situ.