Effects of hyperoxia and hypocapnia on regional venous oxygen saturation in the primary visual cortex in conscious humans

Abstract

Hyperoxia can improve oxygen delivery in patients exposed to hypocapnia for neurosurgical procedures but this effect may be modified by regional differences in the degree of hypocapnic vasoconstriction. Using functional magnet resonance imaging (fMRI), we have investigated the influence of hyperoxia on blood flow and blood oxygenation in the primary visual cortex in hypocapnic volunteers. Consecutive fMRI measurements were performed in 10 awake, male volunteers during hypocapnia (mean PE'(CO2) 3.3 (SD 0.1) kPa) and normocapnia (PE'(CO2) 5.3 (0.1) kPa) at FI(O2) values of 0.21 and 1.0, respectively. Hypocapnia significantly reduced the pixel count in the primary visual cortex (median 169 (quartiles 34-246) vs 21 (0-40) pixels at an FI(O2) of 0.21). Additional hyperoxia had no influence on this reduction in pixel count (16 (0-28) pixels at FI(O2) 1.0 vs 21 (0-40) pixels at FI(O2) 0.21). Hyperoxia did not influence hypocapnic vasoconstriction in the primary visual cortex. These data suggest that in the primary visual cortex, administration of oxygen alone may not be sufficient to improve oxygen delivery under hypocapnic conditions.