Effect of progressive normobaric hypoxia on dynamic cerebral autoregulation

Cerebral blood flow is maintained at relatively constant levels over a wide range of perfusion pressures via cerebral autoregulation (CA). Although acute hypoxia reduces dynamic CA, it is unclear what level of hypoxia is necessary to exert this effect. We evaluated dynamic CA during progressive normobaric hypoxia (∼1 hour each at 21%, 18%, 15%, and 12% O₂) using duplex ultrasonography to measure volumetric changes in common carotid artery blood flow of 11 healthy young males. Dynamic CA was evaluated by the thigh-cuff method and represented as rate of regulation (RoR) in vascular conductance. On a separate occasion, symptoms of acute mountain sickness (AMS) were evaluated over 6 hours of prolonged hypoxia (FiO₂ = 14.1%) using the Lake Louise Questionnaire. Repeated measures ANOVA with linear trend analysis indicated that dynamic CA decreased progressively as FiO₂ was reduced (P < 0.001). Spearman rank order analysis revealed that AMS symptoms were related to changes in RoR from 21% to 15% (r = -0.869, P = 0.006) and from 21% to 12% O₂ (r = -0.648, P = 0.040), respectively. These results suggest that dynamic CA worsens with progressive hypoxia and that reductions in dynamic CA during moderate-to-severe hypoxia (<15% O₂) may be related to the severity of AMS.

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