Cerebral vasoconstriction precedes orthostatic intolerance after parabolic flight

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 degrees 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 < 0.05) whereas postflight BP was not different from supine in the OI group. Postflight compared to preflight, the reduction in P(ET)CO(2) with HUT (p < 0.05) increased in both groups, although significantly so only in the OI group (p < 0.05). The OI group also had a significant decrease in supine MCA MFV postflight (p < 0.05) that was unaccompanied by a change in supine P(ET)CO(2). The decrease in MCA MFV that occurred during HUT in both groups preflight (p < 0.05) was accentuated only in the OI group postflight, particularly during the final 30 s of HUT (p < 0.05). However, this accentuated decrease in MCA MFV was not correlated to the greater decrease in 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 < 0.05), the dynamic autoregulatory gain was not simultaneously changed. Therefore, we conclude that in the OI individuals, parabolic flight was associated with cerebral hypoperfusion following a paradoxical augmentation of CVR by a mechanism that was not related to changes in autoregulation nor strictly to changes in P(ET)CO(2).