Sympathetic nervous system activity and cardiovascular homeostasis during head-up tilt in patients with spinal cord injuries

The relationship between sympathetic nervous system activity and cardiovasc ular responses to head-up tilt in patients with spinal cord injuries and in able-bodied subjects was studied. Twenty-seven adults, nine in each of the three groups (tetraplegia, paraplegia, and able-bodied subjects) were tilt ed 70 degrees, head up, for 12 minutes after 20 minutes supine rest. Differ ences between steady-state measurements of mean arterial pressure, stroke v olume, and sympathetic nervous system activity were estimated in both posit ions. Sympathetic nervous system activity was reflected by the low-frequenc y peak of the blood pressure variability spectrum. From supine rest to head -up tilt, low-frequency power increased in able-bodied subjects (median, 0. 42 mm Hg-2, p = 0.003), which was different (p = 0.015) from patients with tetraplegia and paraplegia (-0.15 and -0.10 mm Hg-2, respectively). Stroke volume and mean arterial pressure decreased in patients with tetraplegia (- 40% and -9 mm Hg, respectively; p = 0.008, both variables) more than in abl e-bodied subjects (-33%, 11 mm Hg, respectively) or patients with paraplegi a (-24%, 8 mm Hg, respectively). Results indicated increased sympathetic ne rvous system activity during head-up tilt in able-bodied subjects, but not in patients with paraplegia or tetraplegia, whereas patients with tetrapleg ia, but not paraplegia, showed poorer cardiovascular homeostasis than able- bodied subjects, This suggests that patients with paraplegia maintained car diovascular homeostasis during head-up tilt without increased sympathetic n ervous system activity.