Effect of different sympathetic stimuli-autonomic dysreflexia and head-up tilt-on leg vascular resistance in spinal cord injury.

Abstract

OBJECTIVE: To compare the effect of different sympathetic stimuli, that is, exaggerated sympathetic activity and orthostatic challenges, on the increase in leg vascular resistance in persons with spinal cord injury (SCI) without and controls with supraspinal sympathetic control. DESIGN: Case-control intervention study. SETTING: Physiology research laboratory. PARTICIPANTS: Persons with SCI (N=9; motor and sensory complete spinal cord lesion above the sixth thoracic spinal segment) and able-bodied controls (N=9). INTERVENTIONS: In persons with SCI, exaggerated sympathetic activity was evoked by autonomic dysreflexia, and in controls, by using a cold pressor test (CPT). A 30 degrees head-up tilt (HUT) was performed in both groups. MAIN OUTCOME MEASURE: Leg blood flow was measured by using venous occlusion plethysmography during the different sympathetic stimuli. Leg vascular resistance was calculated as the arterial-venous pressure gradient divided by blood flow. RESULTS: In persons with SCI, leg vascular resistance significantly increased during autonomic dysreflexia and 30 degrees HUT (25+/-20 and 24+/-13 arbitrary units [AU], respectively), with no difference (P=.87) between stimuli. In controls, leg vascular resistance significantly increased during CPT and 30 degrees HUT (15+/-13 and 29+/-12AU, respectively) with no difference (P=.03) between stimuli. There were no differences (P=.22) in increase in leg vascular resistance during the different sympathetic stimuli between persons with SCI and controls. CONCLUSIONS: The increase in leg vascular resistance during autonomic dysreflexia in persons with SCI is not different from that during 30 degrees HUT, which might be caused by a limited vasoconstrictor reserve. Despite the lack of supraspinal sympathetic control in persons with SCI, the increase in leg vascular resistance during exaggerated sympathetic activity was not different from controls.