Near-infrared spectrophotometry determined brain oxygenation during fainting

During orthostatic hypotension we evaluated whether presyncopal symptoms relate to a reduced brain oxygenation. Nine subjects performed 50°head-up tilt for 1 h and eight subjects were followed during 2 h of supine rest and during 1 h of 10°head-down tilt. Cerebral perfusion was assessed by transcranial Doppler determined middle cerebral artery blood velocity (MCA (V(mean)), while brain blood oxygenation was assessed by near-infrared spectrophotometry determined concentration changes for oxygenated (ΔHbO₂) and deoxygenated haemoglobin and brain cell oxygenation by the oxidized cytochrome c concentration (ΔCytO₂). During head-up tilt, six volunteers developed presyncopal symptoms and mean arterial pressure (88 (78 and 103) to 68 (57-79) mmHg; median and range), heart rate (96 (72-111) to 65 (50-107) beats min^-1), MCA V(mean) (59 (51-82) to 41 (29-56) cm s^-1), ΔHbO₂ (by -5.3 (-3.0 to -14.8) μmol L^-1) and ΔCytO₂ were reduced (by -0.2 (-0.1 to -0.4) μmol l^-1; P < 0.05). During tilt down the cardiovascular variables recovered immediately and ΔHbO₂ increased to 2.2 (-0.9-12.0) mmol L^-1 above the resting value and also ΔCytO₂ recovered. In the nonsyncopal head-up tilted subjects as in the controls, blood pressure, heart rate, MCA v(mean) and brain oxygenation indices remained stable. The results suggest that during orthostasis, presyncopal symptoms relate not only to cerebral hypoperfusion but also to reduced brain oxygenation.