[The sinus node electrogram in the evaluation of atrial pauses].

The aim of this study was to establish the electrophysiological mechanism of atrial pauses, defined as a sudden lengthening of atrial cycle greater than or equal to 10% of its basal value. The sinus node electrogram (SNE) was recorded during electrophysiological study in 20 subjects. Satisfactory recordings of 64 pauses were obtained, 25 of which were spontaneous, 16 were induced by short periods of atrial pacing (AP) in normal subjects, whereas 23 atrial pauses were induced with the premature stimulation method. The basal sinus cycle and the one during the spontaneous pauses were measured from the upstroke slope on the SNE. The electrically induced pauses of the sinus cycle were measured from the artifact of the stimulus to the upstroke slope of the SNE of the first post-pacing beat. We have found that: 1-spontaneous atrial pauses correspond to a moderate slowing of the sinus cycle and to a depression of the sinoatrial conduction occasionally up to a second degree sinoatrial block. Only in subjects with sick sinus disease, the atrial pauses are induced by depression of the sinus automatism; 2-in normal subjects the AP results in a sinoatrial conduction delay; 3-the single extrastimulus regularly produces a moderate increase of the sinoatrial conduction time with variable but minimal effects on the sinus node automatism.