Changes in canine P wave morphology observed with shifts in intra-SA nodal pacemaker localization

Abstract

Changes in P wave morphology were correlated with neurally produced intra-SA nodal pacemaker shifts in the open-and closed-chest anesthetized dog. The three standard limb leads were recorded at high gain and sweep speed. The anatomical SA node was arbitrarily divided into rostral, middle, and caudal regions with intra-SA nodal pacemaker localization inferred from the initial activation of three bipolar electrodes sutured over the three regions. Additional bipolar electrograms were recorded from the distal sulcus terminalis and the medial surface of the left atrial appendage at the termination of the interatrial band. The morphological features of the P waves in leads II and III were identifical in open and closed chest preparations. A significant increase in amplitude was observed with opening the chest for all intra-SA nodal pacemaker localizations. Pacemaker localization in the rostral region of the SA node produced P waves in leads II and III with a small positive component at its initial inscription, a bifid peak, and a smooth recovery to baseline. Middle intra-SA nodal pacemaker localization produced P waves in leads II and III which also exhibited a small positive deflection at its initial inscription, a bifid peak with one of the peaks larger than the other, and a notch or plateau in the upper third to half of the descending limb of the wave. This notch upon recovery to baseline in leads II and III consistently differentiated the P waves associated with a middle SA nodal pacemaker from those of a rostral origin. In addition, the amplitude of the P wave observed with a middle SA nodal pacemaker tended to be larger in both closed and open chest preparations than that observed with a rostral SA nodal pacemaker. The initial positive deflection at the inscription of the P wave may provide a means of inferring SA nodal pacemaker localization from the ECG.