RESPONSE OF HUMAN CEREBRAL BLOOD-FLOW TO +G(Z) ACCELERATIONS

Intolerance symptoms associated with high sustained +G(z) (head to foo t) accelerations are attributed to lack of cerebral perfusion. To dete rmine the response of cerebral circulation to +G(z) stress, cerebral b lood flow (CBF) was measured in humans with the transcranial Doppler m ethod while cephalic arterial blood pressure was calculated simultaneo usly using a photoplethysmographic technique. Nine volunteers performe d four randomized centrifuge runs at +2 to +5 G(z) with a 0.4-G/s onse t rate for 30 s. Compared with the control values, for +2-, +3-, +4-, and +5-G(z) profiles, CBF was reduced by 19 +/- 7, 26 +/- 8, 49 +/- 26 , and 61 +/- 29% (SD), respectively, at the end of the onset and by 18 +/- 4, 21 +/- 11, 27 +/- 7, and 47 +/- 29%, respectively, in the last 20 s of the plateau of acceleration. At the end of the onset and duri ng the plateau of +G(z) acceleration, CBF was less reduced than cephal ic arterial blood pressure, suggesting that some mechanisms would occu r to maintain cerebral perfusion under +G(z) stress. These protective mechanisms are likely due to a siphon effect and/or an autoregulatory compensation.