Leg vascular resistance increases during head-up tilt in paraplegics

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Abstract
Despite loss of centrally mediated sympathetic vasoconstriction to the legs, spinal cord-injured individuals cope surprisingly well with an orthostatic challenge. This study assessed changes in leg vascular resistance following head-up tilt in healthy (C) and in paraplegic (P) individuals. After 10 min of supine rest, subjects were tilted 30° head-up. Mean arterial pressure (MAP) and total peripheral resistance (TPR) increased in C (MAP from 76.7±6.6 mmHg to 80.6±8.2 mmHg; TPR from 1.12±0.26 AU to 1.19±0.31 AU) while both remained unchanged in P. Echo Doppler ultrasound determined red blood cell velocity in the femoral artery, which decreased (P from 18.9±6.2 cm/s to 12.5±4.5 cm/s, P=0.001; C from 16.3±6.2 cm/s to 10.8±5.0 cm/s, P=0.001) and leg vascular resistance, which increased (P from 402±137 AU to 643±274 AU, P=0.001; C from 238±68 AU to 400±122 AU, P=0.003) from supine to upright. The present study shows that independent of supraspinal sympathetic control, humans are able to increase leg vascular resistance and maintain blood pressure during head-up tilt.