Introduction: The preservation of cerebral blood flow with orthostatic hypotension (e.g., following prolonged bed rest or microgravity exposure) is vital for the attenuation of symptoms and the maintenance of consciousness. We tested the hypothesis that decreasing mean arterial pressure (MAP) by > 30% is associated with compromised cerebral autoregulation and orthostatic symptoms during a squat-stand test (SST).
Methods: There were 19 subjects who performed an SST. MAP and middle cerebral artery blood flow velocity (CBFV) were recorded continuously. Subjects were divided retrospectively into those who reported: (1) at least one orthostatic symptom (Sx; n=9); or (2) no orthostatic symptoms (NSx; n=10). Cerebral autoregulation was assessed via the calculation of time to nadir and time to recovery for MAP and CBFV and linear regression analysis of the dynamic changes in MAP and CBFV (within 10 s of standing).
Results: On standing, MAP decreased by 37 +/- 2% (NSx) and 42 +/- 4% (Sx) (p = 0.100). CBFV fell by 6% more in the Sx group than in the NSx group (NSx, -33 +/- 1% vs. Sx, -39 +/- 3%, p = 0.032). Cerebral autoregulation remained intact in both groups as indicated by: (1) a faster time to nadir for CBFV compared with MAP; (2) a faster time to recovery for CBFV compared with MAP; and 3) a poor correlation between CBFV and MAP responses on standing (NSx R2 = 0.43; Sx R2 = 0.60).
Conclusion: Lower cerebral blood flow during severe hypotension may account for the reporting of orthostatic symptoms, despite the maintenance of cerebral autoregulation.