Cerebral vasoconstriction precedes orthostatic intolerance after parabolic flight

• Published in: Brain research bulletin 2000-09, Vol.53 (1), p.113-120
• Main Authors: Serrador, J.M, Shoemaker, J.K, Brown, T.E, Kassam, M.S, Bondar, R.L, Schlegel, T.T
• Format: Article
• Language: English
• Subjects: Adult; Aerospace Medicine; Aircraft; Blood Pressure - physiology; Cerebrovascular Circulation - physiology; Female; Head-up tilt; Homeostasis - physiology; Humans; Hypotension, Orthostatic - etiology; Hypotension, Orthostatic - physiopathology; Hypoxia-Ischemia, Brain - complications; Hypoxia-Ischemia, Brain - etiology; Hypoxia-Ischemia, Brain - physiopathology; Male; Microgravity; Middle Aged; Middle cerebral artery; Middle Cerebral Artery - physiology; Posture - physiology; Space life sciences; Space Motion Sickness - etiology; Space Motion Sickness - physiopathology; Transcranial Doppler; Vasoconstriction - physiology; Weightlessness - adverse effects
• ISSN: 0361-9230; 1873-2747
• DOI: 10.1016/S0361-9230(00)00315-4

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 ETCO 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 < 0.05) whereas postflight BP was not different from supine in the OI group. Postflight compared to preflight, the reduction in P ETCO 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 ETCO 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 ETCO 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 ETCO 2.