Orthostatic vasomotor response in spinal man

The cardiovascular adaptation of tetraplegics to the upright position has been previously demonstrated to be deficient. Presumably this is due to the interruption of the spinal pathways linking supraspinal control centres with the peripheral sympathetic motor neurons. Review of previous studies of this phenomenon reveals that vasomotor responses have been determined primarily from blood flow measurements in the extremities. Contradictory conclusions have been drawn. Study of the visceral circulation, in particular renal blood flow, could shed more light on this poorly understood area. Renal clearance tests were carried out on seven healthy controls and eight chronic, clinically complete tetraplegic patients. Renal blood flow, mean arterial pressure, and total renal vascular resistance in both supine and passive head-up tilt positions were calculated from collected data. Renal blood flow and total renal vascular resistance showed significant decrease and increase respectively during tilting in controls and tetraplegic subjects. Although the renal circulation is autoregulated, postural change causes profound alteration of the renal blood flow mediated through the haemodynamic effects of the renal nerves. Sympathetic renal vasoconstriction is mediated by the carotid sinus reflex through the vasomotor centre in the brainstem. In the absence of supraspinal influence the renal vasculature is shown to respond to an orthostatic stimulus with a vigorous vasoconstriction. The adaptation of spinal man to the upright position may involve the recovery of a spinal vasomotor reflex involving the splanchnic circulation.