Role of the low amplitude potential in the initial P wave signal-averaged electrocardiogram [corrected] in sick sinus syndrome.

BACKGROUND: The cause of abnormally low amplitude of the initial P wave signal-averaged electrocardiogram (P-SAECG) in patients with sick sinus syndrome (SSS) is unknown. METHODS AND RESULTS: Thirteen normal patients (Group C) and 33 with SSS (Group S) were examined. The root mean square amplitude for the initial 30 ms (EP30) and the duration of below-4 microV signals of the filtered P waves (ED4) were measured using the P-SAECG. The interval from an atrial potential on the sinus-node electrogram (SNE) to P wave onset (AS-P), and the interval from the P wave to the atrial potential on the His-bundle-electrogram (P-AH) were measured in the electrophysiological study. The sino-atrial conduction time was measured by a conventional method (indirect sino-atrial conduction time (SACTi)) and using SNE (direct sino-atrial conduction time (SACTd)). The EP30 was significantly lower and the ED4 significantly longer in Group S. The AS-P was significantly longer in Group S (p<0.01), but the P-AH was not different. In Group S, the AS-P was significantly correlated with EP30 and ED4 (p<0.01), but the P-AH was uncorrelated. The SACTi was significantly correlated with EP30 and ED4 (p<0.05), but the SACTd was uncorrelated. CONCLUSION: The abnormality of the initial portion of the P-SAECG observed in SSS appears to be due to disturbed conduction through the atrial myocardium around the sino-atrial node.