Nishimura N, Iwasaki K, Ogawa Y, Shibata S. Oxygen administration, cerebral blood flow velocity, and dynamic cerebral autoregulation. Aviat Space Environ Med 2007; 78:1121–7. Introduction: Hyperoxia is reported to decrease steady-state cerebral blood flow (CBF). In addition, dynamic cerebral autoregulation would be altered. Hyperoxia may improve dynamic cerebral autoregulation, contrary to hypoxia. However, no previous studies have examined changes in steady-state CBF velocity (CBFV) and alterations of dynamic cerebral autoregulation during acute exposure to hyperoxia. We, therefore, evaluated dynamic cerebral autoregulation simultaneously with steady-state CBFV during stepwise hyperoxia under oxygen administration. Methods: There were eight healthy volunteers who were examined under normoxic (21% O₂) and hyperoxic conditions in stepwise fashion to 40%, 70%, and 100% O₂. Mean blood pressure (MBP) in the radial artery was measured via tonometry, and CBFV in the middle cerebral artery was measured by transcranial Doppler ultrasonography. Dynamic cerebral autoregulation was assessed by spectral and transfer function analysis of beat-to-beat changes in MBP and CBFV. Results: End-tidal CO₂ decreased significantly at 70% and 100% O₂. Steady-state CBFV decreased significantly at F_Io₂≥ 40%, while MBP was unchanged. Associated with these changes, cerebral vascular resistance index increased at 70% and 100% O₂. Transfer function gain and coherence remained unchanged at all levels of F_Io₂. Discussion: These results suggest that hyperoxemia and hypocapnia reduce steady-state CBFV and increase cerebral vascular resistance during oxygen administration. This reduction in steady-state CBFV occurs even during mild hyperoxia ≤ 40% O₂ and becomes obvious at 70% O₂ with hypocapnia. However, dynamic cerebral autoregulation may remain unchanged during hyperoxia, even with apparent changes in steady-state CBFV.