ObjectiveTo examine the relationships among the circadian rhythms of blood pressure, autonomic nervous function, and physical activity of patients with varying levels of spinal cord injury.Design and methodsWe studied 19 patients with spinal cord injury [10 tetraplegic patients with cervical cord injury (C4–C7), and nine paraplegic patients with thoracic cord injury (Th6–Th12)] compared with 16 control subjects. A new multibiomedical recorder was used to measure blood pressure (every 30 min), cardiac vagal activity (hourly frequency of R-R50), and physical activity (integrated acceleration/min) for 24 h under hospital conditions. Systemic sympathetic nervous activity and sympathoadrenal functioning were assessed by examination of hormone levels in the blood.ResultsDaytime and night-time values were compared; the variations in systolic and diastolic blood pressures and heart rate were slight in members of the tetraplegia group, but almost normal differences were observed in members of the paraplegia group. The circadian profile of cardiac vagal activity was normal for both patient groups, suggesting that an alteration in the sympathetic nervous rhythm had occurred in the tetraplegic patients. The plasma norepinephrine level was lower in members of the tetraplegia group than it was in members of the control group (P< 0.001), but was normal in members of the paraplegia group. The plasma level of epinephrine was lower in members of the tetraplegia (P< 0.05) and the paraplegia (P< 0.1) groups than it was in members of the control group. Daytime physical activity of members of both groups of patients was lower than that of subjects in the control group (P< 0.001 for both).ConclusionThe central sympathoexcitatory pathway to the upper thoracic cord plays a critical role in the maintenance of normal circadian blood pressure rhythm in humans. Motor nerve functioning and sympathoadrenal secretion are not essential to this regulation.