Sleep-induced limb vasodilation in individuals confined to bed for 24 h

Abstract

The sleep/wake rhythm in limbs has been scarcely studied, especially due to the difficulty associated with continuous monitoring of arterial flow to the forearm and leg for a 24-h period. Addressing this constraint, we employed indium-gallium-in-silicone strain-gauge venous-occlusion plethysmography, an automated method facilitating the measurement of 24-h limb arterial flow in bed-confined subjects without disturbing their natural sleep. This article presents the state of the art in this field. Our examination of 60 healthy normotensive individuals revealed a distinctive sleep/wake rhythm in limb arterial flow, characterized by elevated values during sleep (32.7% in the forearm, P < 0.0001; 39.1% in the leg, P < 0.0001). Correspondingly, limb resistance mirrored the trend of flow (-32.7%, P < 0.0001; -33.5%, P < 0.0001), with these variations attributed to sleep-induced limb vasodilation. Sleep-associated vasodilation was also evident in 21 hypertensive individuals (leg resistance: -33.1%, P < 0.0001) and 13 heart transplant recipients lacking vagal and sympathetic cardiac innervation (resistance: -33.6%, P < 0.0001). On the contrary, among 11 subjects with an interrupted spinal cord, we observed forearm vasodilation (resistance: -36.6%, P < 0.0001) but observed no leg vasodilation if the spinal lesion was under T2 (innervating the leg). Furthermore, a loss of sleep-induced vasodilation occurred in both the forearm and leg if the injury was above C7 (innervating both forearm and leg). Our conclusion posits the existence of sleep-induced limb vasodilation, a phenomenon attributed to signals traveling along the spinal cord, with the heart playing no discernible role in this rhythmic process, and arterial hypertension deemed irrelevant. Comprehensive further studies are imperative to elucidate the precise triggers of limb vasodilation during sleep.