Cerebral vasoconstriction precedes orthostatic intolerance after parabolicflight

The effects of brief but repeated bouts of micro-and hypergravity on cerebr ovascular responses to head-up tilt (HUT) were examined in 13 individuals a fter (compared to before) parabolic flight, Middle cerebral artery mean flo w velocity (MCA MFV; transcranial Doppler ultrasound), eye level blood pres sure (BP) and end tidal CO2 (PETCO2) were measured while supine and during 80 degrees HUT for 30 min or until presyncope. In the postflight tests subj ects were classified as being orthostatically tolerant (OT) (n = 7) or into lerant (OI) (n = 6), BP was diminished with HUT in the CT group in both tes ts (p < 0.05) whereas postflight BP was not different from supine in the OI group. Postflight compared to preflight, the reduction in PETCO2 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 MRI postflight (p < 0.05) that was unaccompanied by a change in supine PETCO2. The decrease in MCA MFV that occurred during HUT in both groups pr eflight Go < 0.05) was accentuated only in the OI group postflight, particu larly during the final 30 s of HUT (p < 0.05), However, this accentuated de crease in MCA MFV was not correlated to the greater decrease in PETCO2 duri ng the same period (R = 0.20, p = 0.42), Although cerebral vascular resista nce (CVR) also increased in the OI group during the last 30 s of HUT postfl ight (p < 0.05), the dynamic autoregulatory gain was not simultaneously cha nged. Therefore, we conclude that in the OI individuals, parabolic flight w as associated with cerebral hypoperfusion following a paradoxical augmentat ion of CVR by a mechanism that was not related to changes in autoregulation nor strictly to changes in PETCO2. (C) 2000 Elsevier Science Inc.