Leg vascular resistance increases during head-up tilt in paraplegics.

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 degrees 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.