His bundle electrograms of dog. Correlation with intracellular recordings.

Transmembrane potentials from the penetrating and branching portions of the bundle of His were simultaneously recorded along with bipolar extracellular electrograms. Recordings were obtained during periods of normal antegrade and retrograde conduction and also during various degrees of conduction delay and block within and distal to the bundle of His. The onset and ternination of extracellular electrogram coincided with the upstrokes of the transmembrane action potentials recorded from the penetrating and branching portions respectively and the duration of the extracellular electrogram equaled the interelectrode conduction time. Thus, the extracellular electrogram accurately reflected the timing and duration of the electrical activity of the entire bundle of His (penetrating and branching portions). Intra-His bundle conduction delay as determined by an increase in interelectrode conduction time and a decrease in the upstroke velocity of phase 0 of the action potentials, resulted in a decrease in the amplitude and an increase in the duration of the extracellular recording. Significant intra-His bundle conduction delay resulted in two His bundle deflections (H and H') in the extracellular electrogram recordings. The onset of the two His deflections coincided with the upstrokes of the two transmembrane potential recordings. When intra-His bundle block occurred the action potential distal to the block and the corresponding deflection on the extracellular recording disappeared. The results of this study provide further evidence supporting the validity of clinical electrode-catheter recordings of His bundle activity.