REGULATION OF CEREBRAL BLOOD FLOW AFTER SIMULATED MICROGRAVITY 1033

Abstract

Orthostatic intolerance is a frequent consequence of the cardiovascular adaptation to microgravity. Symptoms of near or true syncope ultimately will result if cerebral blood flow (CBF) falls, either due to reduced perfusion pressure (hypotension), or a failure of autoregulation. To test the hypothesis that simulated microgravity impairs regulation of CBF during orthostatic stress, we measured blood flow velocity in the middle cerebral artery(VMCA) using transcranial Doppler, along with blood pressure(Finapres), and end-tidal CO2 (mass spectrometry) in 12 healthy subjects (24±5 yrs) before and after 2 weeks of -6° head-down tilt(HDT). Maximal orthostatic stress was induced using a ramped protocol of lower body negative pressure (LBNP: -15, -30, and -40 mmHg × 5min, then -10 every 3 min to pre-syncope); Change in VMCA and magnitude-squared coherence function (MSC) estimated from beat-to-beat variation of mean BP and VMCA were used to evaluate cerebral autoregulation under graded orthostatic stress. Results: Mean BP did not change significantly from rest to -50mmHg LBNP before or after HDT. Before HDT: Mean VMCA decreased significantly by 14%, 20% and 45% at -40mmHg, -50mmHg and maximum LBNP (p<0.05) respectively. Coherence between BP and VMCA at low frequency (CohLF, 0.05-0.15Hz), which suggests impairment of cerebral autoregulation, increased by 61% and 66% at -40mmHg and -50mmHg LBNP(p<0.05). After HDT: VMCA fell at lower levels of LBNP with a significant decrease of 16% at -30mmHg LBNP (p<0.05). Coincident with the decrease in VMCA, CohLF increased earlier by 33% at -30mmHg LBNP after HDT (p<0.05). These results suggest that a shift in the autoregulatory relationship between arterial pressure and cerebral blood flow may contribute to orthostatic intolerance after adaptation to microgravity.