Circadian rhythm of sinoatrial conduction time assessed by 24 hours ECG monitoring.

: SACT was evaluated in 50 subjects during 24 hours ECG monitoring. All of these subjects presented frequent atrial beats with no evidence of bradyarrhythmias. SACT was measured in accordance with Strauss' formula (SACT = P2P3 - P1P1/2). Twenty-four hours recording were divided in to 4 time intervals, 6 hour each, starting 7 o'clock. The SACT is higher during nighttime than during daytime. The phenomenon is, presumably, attributable to predominance of the para-sympathetic tone during the night.