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

Hyperoxia can improve oxygen delivery in patients exposed to hypocapnia for neurosurgical procedures but this effect may be modified by regional diffe rences in the degree of hypocapnic vasoconstriction. Using functional magne t resonance imaging (fMRI), we have investigated the influence of hyperoxia on blood flow and blood oxygenation in the primary visual cortex in hypoca pnic volunteers. Consecutive fMRI measurements were performed in 10 awake, male volunteers during hypocapnia (mean PECO2' 3.3 (SD 0.1) kPa) and normoc apnia (PECO2' 5.3 (0.1) kPa) at FIO2 values of 0.21 and 1.0, respectively. Hypocapnia significantly reduced the pixel count in the primary visual cort ex (median 169 (quartiles 34-246) vs 21 (0-40) pixels at an FIO2 of 0.21). Additional hyperoxia had no influence on this reduction in pixel count (16 (0-28) pixels at FIO2 1.0 VS 21 (0-40) pixels at FIO2, 0.21). Hyperoxia did not influence hypocapnic vasoconstriction in the primary visual cortex. Th ese data suggest that in the primary visual cortex, administration of oxyge n alone may not be sufficient to improve oxygen delivery under hypocapnic c onditions.