Cerebral vasoconstriction precedes orthostatic intolerance after parabolic flight

Abstract The effects of brief but repeated bouts of micro- and hypergravity on cerebrovascular responses to head-up tilt (HUT) were examined in 13 individuals after (compared to before) parabolic flight. Middle cerebral artery mean flow velocity (MCA MFV; transcranial Doppler ultrasound), eye level blood pressure (BP) and end tidal CO 2 (P ET CO 2 ) were measured while supine and during 80° HUT for 30 min or until presyncope. In the postflight tests subjects were classified as being orthostatically tolerant (OT) ( n = 7) or intolerant (OI) ( n = 6). BP was diminished with HUT in the OT group in both tests ( p ET CO 2 with HUT ( p p p ET CO 2 . The decrease in MCA MFV that occurred during HUT in both groups preflight ( p p ET CO 2 during the same period ( R = 0.20, p = 0.42). Although cerebral vascular resistance (CVR) also increased in the OI group during the last 30 s of HUT postflight ( p ET CO 2 .