Objective
To investigate the changes of cerebral and cardiovascular hemodynamics in response to postural change in tetraplegics after cervical spinal cord injury.

Method
We studied 5 healthy volunteers and 14 cervical cord injured patients with orthostatic hypotension. We continuously monitored heart rate (HR), blood pressure (BP) by volume clamp photoplethysmography, and cerebral blood flow velocity (BFV), pulsatility index (PI) of middle cerebral artery (MCA) by transcranial Doppler sonography at rest and during head-up-tilt (HUT). Tilt table set at 30^o initially and then increased gradually 10^o every 5 minutes up to 80^o.

Results
In the control group, BP and BFV of MCA remained unchanged during HUT. Although a decrease of BFV observed in all patients during both systolic and diastolic phases, the degree of BFV drop during diastolic phase, especially early diastolic phase was much greater than that during systolic phase. The change of BFV of MCA was significantly correlated with that of systemic hemodynamic parameters, especially systolic BP. After rehabilitative therapy for one month, there was no definite evidence of the change in cerebral autoregulation.

Conclusion
This study suggests that we can use systemic hemodynamic parameters for predicting changes of cerebral blood flow in response to orthostatic hypotension, but we fail to observe any compensatory mechanism of cerebrovascular system to maintain cerebral blood flow against systemic hemodynamic collapse.