Cerebral blood flow response and its association with symptoms during orthostatic hypotension.
BACKGROUND
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).


CONCLUSIONS
Lower cerebral blood flow during severe hypotension may account for the reporting of orthostatic symptoms, despite the maintenance of cerebral autoregulation.
U.S. Army Institute of Surgical Research, Fort Sam Houston, TX 78234, USA.
Caroline A
CA
Rickards
Kenneth D
KD
Cohen
Lindsey L
LL
Bergeron
B Lubrina
BL
Burton
Prateek J
PJ
Khatri
Christopher T
CT
Lee
Kathy L
KL
Ryan
William H
WH
Cooke
Donald F
DF
Doerr
Victor A
VA
Convertino
D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology*; D016014:Linear Models; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D011187:Posture / Q000502:physiology; D020127:Recovery of Function / Q000502:physiology; D013997:Time Factors