31P magnetic resonance spectroscopy study of the human visual cortex during stimulation in mild hypoxic hypoxia

Effects of mild hypoxic hypoxia on cerebral energy state, as assessed by phosphocreatine (PCr)/γ-ATP and inorganic phosphate (Pi)/ (Pi + PCr) ratios and intracellular pH (pHi) in the human visual cortex, were studied using 31P nuclear magnetic resonance spectroscopy (MRS) at 3 T. The working hypothesis that, during compromised O2 availability obtained by hypoxic hypoxia, both cerebral energy state and pHi decline due to insufficient O2 supply for energy metabolism was addressed. Under baseline hypoxic hypoxia, with blood O2 saturation ranging from 0.95 to 0.83, neither the PCr/γ-ATP and Pi/(Pi + PCr) ratios nor pHi was affected, thus, showing that cerebral energy metabolism was maintained. Contrary to the formulated hypothesis, visual stimulation during hypoxic hypoxia influenced neither the indicator ratios for energy state nor pHi in the occipital cortex. Taking these results, together with previous observations showing that cerebral blood flow responses are the same in size both in euoxia and in hypoxia at this depth (Mintun et al. in Proc Natl Acad Sci USA 98:6859-6864, 2001; Tuunanen et al. in J Cereb Blood Flow Metab 26:263-273, 2006a), it is concluded that O2 delivery to the brain during mild hypoxic hypoxia meets the demand by the energy metabolism both under baseline and stimulated states. © 2008 Springer-Verlag.