Activation of the sulcus terminalis observed in the morphology of the canine P wave

A smalll positive component has been observed at the initial inscription of the P wave recorded in leads II and III at high gain and sweep speed. The average duration of this deflection for pacemakers localized in the region of the anatomical SA node was 18 msec. with a range from 12 to 24 msec. The duration of this initial component for pacemakers localized in the middle region of the SA node coincided with activation of the upper 2/3 of the sulcus terminalis including the epicardial tissue of the SA node and the first centimeter of the interatrial band (Bachmann's bundle). The configuration of this initial deflection and the atrial tissue activated during its time course changed with shifts in pacemaker localization. Pacemaker localization in the rostral region of the upper third of the sulcus produced a smooth, rounded positive initial deflection. Pacemaker localization in the caudal region of the upper third of the sulcus terminalis produced an initial positive component that was bifid. The duration of this bifid deflection corresponded to activation of most of the sulcus terminalis. The configuration of this component as well as the other morphological characteristics of the P waves in leads II and III will provide a more precise means of determining pacemaker localization along the sulcus terminalis from the ECG.