The Dominant Pacemaker of the Human Heart

In 25 cases at open heart surgery, atrial and ventricular close bipolar electrograms, an atrial unipolar electrogram, and a lead-II electrocardiogram were simultaneously monitored throughout each procedure. In six cases the ventricles or atria, or both, were paced by electrical stimulation. Although atrial activity could frequently not be clearly delineated on the lead II electrocardiogram, the time relationship between atrial and ventricular depolarization could at all times be determined from the electrograms. During ventricular premature beats and ventricular rhythms, retrograde A-V conduction and retrograde activation of the atria were common, and unidirectional retrograde A-V block was infrequent. In spontaneous and induced cardiac arrhythmias the fastest pacemaker of the heart, atrial, His bundle, or ventricular, usually activated the entire heart.