The dominant pacemaker of the human heart. Antegrade and retrograde activation of the heart.

SUMMARY In 25 cases at open heart surgery, atrial and ventricular close bipolar electrograms, an atrial unipolar electrogram, and a lead-IL 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 retro-grade 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. THAT the fastest pacemaker of the heart will activate the entire heart is an axiom in cardiac physiology. Yet, in the literature, there is a controversy as to the incidence of ventriculo-atrial conduction during ventricu-lar rhythms in man.'-" Most observations on cardiac arrhythmias have been made from the standard lead elec-trocardiogram. During arrhythmias, it is often difficult or impossible to delineate the relationship between atrial and ventricular depolariz-ation from standard lead electrocardiographic tracings. In order to depict clearly atrial and ventricular depolarization during spontaneous and induced arrhythmias, electrograms from the atria and ventricles of the human heart 22 Cardiac arrhythmias were simultaneously recorded. The purpose of this report is to demonstrate the incidence of retrograde activation of the heart and to reaffirm the concept, that, in man, the fastest pacemaker of the heart will usually activate the entire heart. Methods Simultaneous close bipolar atrial and ventricu-lar electrograms, an atrial unipolar electrogram, and a lead IL electrocardiogram were monitored during open heart surgery in 25 cases. All patients were in sinus rhythm. The method used in this study for recording epicardial electrograms has been previously described in detail.12-17 Acrylic plaque electrodes containing five silver contacts 2 to 5 mm apart were sutured to the epicardial surface of the right atrium near the sinus node, to the free wall of the left atrium, and to the epicardial surface of the right ven-tricle. In four cases with an interatrial septal defect, the right atrial electrode plaque was positioned just above the A-V (atrioventricular) groove, and the left atrial electrode plaque was positioned superiorly in the area bounded by the right pulmonary artery superiorly, the ascending aorta medially, and the superior vena