THRESHOLDS FOR HYPOXIC CEREBRAL VASODILATION IN VOLUNTEERS

The effects of hypoxemia on the cerebral vasculature have been describ ed in animal models, but data from human studies are limited and have often relied on invasive methodology. The ability to detect a threshol d for hypoxic vasodilatation in humans may be clinically useful, parti cularly in patients with impaired intracranial compliance. Because phy siology may differ among subjects, it would be advantageous to measure such thresholds using noninvasive bedside techniques that could be re adily applied to individual patients. We have attempted to identify th resholds of hypoxic vasodilatation in volunteers using noninvasive met hods of measurement. Thirteen healthy volunteers were studied using tr anscranial Doppler sonography. Time-averaged middle cerebral artery ma ximal flow velocity (MCA FVx), mean arterial blood pressure, periphera l oxygen saturation (SpO(2)), and partial pressure of end-tidal CO2 we re measured at baseline and during graded reduction in arterial SpO(2) to 85%, at normocapnia. Flow velocity and estimated cerebrovascular r esistance (CVRe) were indexed and plotted against SpO(2). There was no significant change in mean arterial pressure during desaturation to 8 5%, although the mean heart rate increased from 65 to 82 bpm. An incre ase in MCA FVx and reciprocal decrease in CVRe were observed when SpO( 2) decreased to below 90%. These results suggest that hypoxemic cerebr al vasodilatation may be measured noninvasively and that in normal hum an volunteers, the threshold is at an SpO(2) of 90%, which is greater than that previously reported. Implications: The ability to detect the point at which cerebral blood vessels dilate using noninvasive techni ques is clinically advantageous. We have attempted to do this using tr anscranial Doppler sonography in volunteers whose inspired oxygen conc entrations were gradually decreased. Thresholds of cerebral vasodilata tion were found to be higher than those previously reported. These thr esholds are important with regard to adequate perfusion in various pat hologic states.