The sinus node electrogram in patients with and without sick sinus syndrome: techniques and correlation between directly measured and indirectly estimated sinoatrial conduction time.

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Abstract
Sinus node electrograms (SNE) were recorded in 44 patients with a standard quadripolar electrode catheter (USCI # 6) with 10-mm interelectrode distance. In 23 patients, the catheter was positioned at the junction of the superior vena cava (SVC) and right atrial (RA) wall so that the concave curve of the catheter was facing the concave surface of the RA wall. The distal poles of the catheter were close to, but not in direct contact with, the RA endocardium underlying the anatomic location of the sinus node (method 1). In 21 patients, the catheter was looped in the RA and advanced to the junction of the SVC-RA wall so that the distal poles of the catheter were in direct contact with the RA endocardium underlying the area of the sinus node (method 2). Stable SNE could be obtained in 10 of 23 patients (43%) by method 1 and in 18 of 21 (86%) by method 2. SNE by both methods showed 2 negatively directed deflections (diastolic and upstroke slope of the sinus mode) of low frequency and amplitude occurring after the deflections of the T and U waves but before the P wave and intraatrial electrograms from various atrial sites. Unlike method 1, atrial activation by method 2 was characterized by an atrial injury potential. SNE by method 2 were more stable and less prone to baseline drifts. Carotid sinus massage performed in 10 patients resulted in prolongation of the onset of the upstroke slope to atrial activation. In 1 patient, it resulted in marked 1st-degree sinoatrial block. SNE were recorded in 1 patient during and after a run of spontaneous atrial flutter. After termination of atrial flutter, sinoatrial conduction time (SACT) prolonged for the 1st postflutter sinus beat and normalized by the 3rd sinus beat. Direct SACT was assessed on the SNE in all 28 patients, 12 of whom had sick sinus syndrome (SSS). SACT was also estimated indirectly by continuous pacing in 28 patients and by premature stimulation in 20 patients. Direct SACT was significantly (P < 0.001) longer in patients with SSS (135 .+-. 30 ms, mean .+-. SD) than in patients without SSS (87 .+-. 12 ms). There was a good correlation between direct SACT and SACT by continuous pacing (r = 0.843, n = 28) and direct SACT and SACT by premature stimulation (r = 0.778, n = 18). In certain patients, there were appreciable differences and direct SACT was longer than estimated SACT by continuous pacing or premature stimulation. Apparently, the success rate of stable SNE with no appreciable baseline drift is increased with direct contact of the catheter with atrial endocardium underlying the sinus node area (method 2); carotid sinus massage results in depression of sinus node automaticity and prolongation of SACT; direct SACT is significantly longer in patients with SSS and can separate most patients with from those without SSS; and there is a good correlation between direct SACT and indirectly estimated SACT, although the latter underestimates the actual SACT.