Abstract
BackgroundHumans demonstrate a number of unique adaptations that allow for the maintenance of blood pressure and brain blood flow when upright. While several physiological systems, including cerebral autoregulation, are involved in this adaptation the unique role the vestibular system plays in helping to maintain brain blood flow is just beginning to be elucidated. In this study, we tested the hypothesis that stimulation of the vestibular system, specifically the otoliths organs, would result in changes in cerebral blood flow.ResultsTo test our hypothesis, we stimulated the vestibular organs of 25 healthy subjects by pitch tilt (stimulates both canals and otoliths) and by translation on a centrifuge (stimulates otoliths and not the canals) at five frequencies: 0.5, 0.25, 0.125 and 0.0625 Hz for 80 sec and 0.03125 Hz for 160 sec. Changes in cerebral flow velocity (by transcranial Doppler) and blood pressure (by Finapres) were similar during both stimuli and dependent on frequency of stimulation (P < 0.01). However, changes in cerebral blood flow were in opposition to changes in blood pressure and not fully dependent on changes in end tidal CO2.ConclusionThe experimental results support our hypothesis and provide evidence that activation of the vestibular apparatus, specifically the otolith organs, directly affects cerebral blood flow regulation, independent of blood pressure and end tidal CO2 changes.
Highlights
Humans demonstrate a number of unique adaptations that allow for the maintenance of blood pressure and brain blood flow when upright
Both centrifugation and tilt resulted in a modulation of Cerebral flow velocity (CFV) as measured by transcranial Doppler that were linked to stimulation frequency (Figure 3)
There was a significant effect on blood pressure at brain level of both frequency of stimulation (P < 0.01) and position within cycle (P < 0.01) that differed by frequency (Frequency × Cycle interaction, P < 0.01)
Summary
Humans demonstrate a number of unique adaptations that allow for the maintenance of blood pressure and brain blood flow when upright. While several physiological systems, including cerebral autoregulation, are involved in this adaptation the unique role the vestibular system plays in helping to maintain brain blood flow is just beginning to be elucidated. To maintain arterial pressure when upright, humans must respond to a translocation of blood from the upper into the lower body Responses to this translocation include baroreflex-mediated increases in heart rate and peripheral vasoconstriction, compensating for reduced venous return and minimizing pooling of blood in the lower body. While a role for the vestibular system in the autonomic response to the upright position has been documented, there have been no studies demonstrating a direct effect of otolith stimulation on cerebral blood flow.
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