Objectives: Acute hypoxia may impair dynamic cerebral autoregulation. However, previous studies have been controversial. The difference in methods of estimation of dynamic cerebral autoregulation is reported to be responsible for conflicting reports. We, therefore, conducted this study using two representative methods of estimation of dynamic cerebral autoregulation to test our hypothesis that dynamic cerebral autoregulation is impaired during acute exposure to mild hypoxia.
Methods: Eleven healthy men were exposed to 15% oxygen concentration for two hours. They were examined under normoxia (21% O2) and hypoxia (15% O2). The mean arterial pressure (MAP) in the radial artery was measured by tonometry, and cerebral blood flow velocity (CBFv) in the middle cerebral artery was measured by transcranial Doppler ultrasonography. Dynamic cerebral autoregulation was assessed by spectral and transfer function analyses of beat-by-beat changes in MAP and CBFv. Moreover, the dynamic rate of regulation and percentage restoration of CBFv were estimated when a temporal decrease in arterial pressure was induced by thigh-cuff deflation.
Results: Arterial oxygen saturation decreased significantly during hypoxia (97±0% to 88±1%), whereas respiratory rate was unchanged, as was steady-state CBFv. With 15% O2, the very-low-frequency power of CBFv variability increased significantly. Transfer function coherence (0.40±0.02 to 0.53±0.05) and gain (0.51±0.07 cm/s/mmHg to 0.79±0.11 cm/s/mmHg) in the very-low-frequency range increased significantly. Moreover, the percentage restoration of CBF velocity determined by thigh-cuff deflation decreased significantly during hypoxia (125±25% to 65±8%).
Conclusions: Taken together, these results obtained using two representative methods consistently indicate that mild hypoxia impairs dynamic cerebral autoregulation.
