Prior head-down tilt does not impair the cerebrovascular response to head-up tilt.
The hypothesis that cerebrovascular autoregulation was not impaired during head-up tilt (HUT) that followed brief exposures to varying degrees of prior head-down tilt (HDT) was tested in 10 healthy young men and women. Cerebral mean flow velocity (MFV) and cardiovascular responses were measured in transitions to a 60-s period of 75° HUT that followed supine rest (control) or 15 s HDT at -10°, -25°, and -55°. During HDT, heart rate (HR) was reduced for -25° and -55°, and cardiac output was lower at -55° HDT. MFV increased during -10° HDT, but not in the other conditions even though blood pressure at the middle cerebral artery (BPMCA) increased. On the transition to HUT, HR increased only for -55° condition, but stroke volume and cardiac output transiently increased for -25° and -55°. Total peripheral resistance index decreased in proportion to the magnitude of HDT and recovered over the first 20 s of HUT. MFV was significantly less in all HDT conditions compared with the control in the first 5-s period of HUT, but it recovered quickly. An autoregulation correction index derived from MFV recovery relative to BPMCA decline revealed a delay in the first 5 s for prior HDT compared with control but then a rapid increase to briefly exceed control after -55° HDT. This study showed that cerebrovascular autoregulation is modified by but not impaired by brief HDT prior to HUT and that cerebral MFV recovered quickly and more rapidly than arterial blood pressure to protect against cerebral hypoperfusion and potential syncope.
School of Aerospace Medicine, Fourth Military Medical University, Xi'an, China; and Schlegel-University of Waterloo Research Institute for Aging, Waterloo, Ontario, Canada.
Changbin
C
Yang
School of Aerospace Medicine, Fourth Military Medical University, Xi'an, China; and.
Yuan
Y
Gao
Schlegel-University of Waterloo Research Institute for Aging, Waterloo, Ontario, Canada.
Danielle K
DK
Greaves
Schlegel-University of Waterloo Research Institute for Aging, Waterloo, Ontario, Canada.
Rodrigo
R
Villar
Schlegel-University of Waterloo Research Institute for Aging, Waterloo, Ontario, Canada.
Thomas
T
Beltrame
Schlegel-University of Waterloo Research Institute for Aging, Waterloo, Ontario, Canada.
Katelyn S
KS
Fraser
Schlegel-University of Waterloo Research Institute for Aging, Waterloo, Ontario, Canada hughson@uwaterloo.ca.
Richard L
RL
Hughson
D000222:Adaptation, Physiological; D000328:Adult; D062186:Arterial Pressure; D001783:Blood Flow Velocity; D002560:Cerebrovascular Circulation; D005260:Female; D018475:Head-Down Tilt; D006339:Heart Rate; D006439:Hemodynamics; D006706:Homeostasis; D006801:Humans; D008297:Male; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D011187:Posture; D011897:Random Allocation; D012039:Regional Blood Flow; D013318:Stroke Volume; D018667:Tilt-Table Test; D013997:Time Factors; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance
cerebrovascular autoregulation; push-pull effect; transcranial Doppler ultrasound