Hypocapnia attenuates, and nitrous oxide disturbs the cerebral oximetric response to the rapid introduction of desflurane

The aim of this study was to develop a nonlinear mixed-effects model for the increase in cerebral oximetry (rSO₂) during the rapid introduction of desflurane, and to determine the effect of hypocapnia and N₂O on the model. Twelve American Society of Anesthesiologist physical status class 1 and 2 subjects were allocated randomly into an Air and N₂O group. After inducing anesthesia, desflurane was then increased abruptly from 4.0 to 12.0%. The PET_CO2, PET_DESF and rSO₂ were recorded at 12 predetermined periods for the following 10 min. The maximum increase in rSO₂ reached +24-25% during normocapnia. The increase in rSO₂ could be fitted to a four parameter logistic equation as a function of the logarithm of PET_DESF. Hypocapnia reduced the maximum response of rSO₂, shifted the EC₅₀ to the right, and increased the slope in the Air group. N₂O shifted the EC₅₀ to the right, and reduced the slope leaving the maximum rSO₂ unchanged. The N₂O-effects disappeared during hypocapnia. The cerebrovascular reactivity of rSO₂ to CO₂ is still preserved during the rapid introduction of desflurane. N₂O slows the response of rSO₂. Hypocapnia overwhelms all the effects of N₂O.