First-degree sinoatrial heart block: sinoatrial block in the sick-sinus syndrome.

Sinoatrial conduction time (SACT) was estimated from the delay in the atrial recovery period after premature depolarization applied in that portion of atrial diastole when increasing prematurity resulted in a constant recovery interval. In 20 normal patients SACT was 169 msec. +/- 91 (2 S.D.). At least nine of 19 patients with "sick-sinus syndrome" (SSS) demonstrated SACT that were longer than seen in these normal subjects. SACT was prolonged in seven of nine SSS patients with abnormal A-V nodal conduction. Among 10 SSS patients with normal A-V conduction, only two had prolonged SACT. This study identifies first-degree sinoatrial block as a frequent manifestation of SSS associated with the presence of A-V node conduction abnormalities.