Leg versus forearm flow: 24 h monitoring in 14 normotensive subjects and in 14 age-matched hypertensive patients confined to bed.

Abstract OBJECTIVE: A circadian rhythm of blood pressure has been demonstrated both in subjects who are physically active during the day and in those confined to bed. The study of the circadian rhythm of arterial flow and peripheral resistance, on the other hand, is limited to pioneer experiments. This paper is aimed at demonstrating that leg peripheral resistance has circadian fluctuations which are modulated by spinal neural traffic. METHODS: Eleven normal (able-bodied) human subjects and 11 patients with spinal transection due to spinal cord injury (SCI) were studied. They were confined to bed for 24 h. Blood pressure and heart rate were monitored every 15 min with an automatic device and leg flow with an automatic strain-gauge plethysmograph synchronised to the pressurometer. Peripheral resistance was calculated at the same intervals. RESULTS: In able-bodied subjects leg resistance was significantly higher during waking hours (when the sympathetic system is more activated) than during sleep, while in subjects with spinal cord injury no difference was detected between day-time and night-time. CONCLUSIONS: The circadian rhythm is controlled by adrenergic fibres transmitted via the spinal cord.