Impairment of cerebral blood flow regulation in astronauts with orthostatic intolerance after flight.

Abstract

We investigated cerebral blood flow regulation in astronauts before and after flights. We hypothesized that autoregulation would be different before flight and after flight between nonfinishers and the finishers of a stand test. Twenty-seven astronauts from shuttle missions lasting 8 to 16 days underwent a 10-minute stand test: 10 days before flight, 1 to 2 hours and 3 days after landing. Mean blood flow velocity of the middle cerebral artery (MCA) was measured using transcranial Doppler; Mean arterial pressure was measured using a Finapres (Ohmeda, Englewood, CO) and was adjusted to the level of the MCA (BP(MCA)). Cross-spectral power, gain, phase, and coherence were determined for the relation between BP(MCA) and the cerebrovascular resistance index mean blood flow velocity/BP(MCA). BP(MCA) was reduced with stand (P<0.001). Differences between finishers and nonfinishers (P=0.011) and over test days (P=0.004) were observed. Cerebrovascular conductance was affected by stand (P<0.001), by group (P<0.001) with a group by stand, and test day interaction (P<0.01). Preflight data suggest that the nonfinishers were operating at a higher cerebral vasodilation than finishers for a given BP(MCA), and on landing day the nonfinishers had a greater decrease in mean blood flow velocity as a function of BP(MCA) with standing compared to finishers and preflight. There was a significant interaction effect of gender over the test days and from supine to stand (P=0.035). Our results indicate that the cause of presyncope in astronauts may be related to a mismatch of cerebral blood flow with blood pressure. Astronaut gender may also play a role in susceptibility to orthostatic intolerance after flight.