Pharmacokinetics of isoflurane: Uptake in the brain

We studied the effect of the inspired isoflurane concentration (C _Iiso) on the pharmacokinetics of isoflurane uptake in the brain by comparing the isoflurane concentration in internal jugular-bulb blood (Jiso) with that in arterial blood (Aiso), and analyzed this by gas chromatography. Sixteen patients (aged 43-76 years) undergoing colorectal surgery were enrolled, and anesthesia was maintained with a constant C_Iiso of either 1% (group 1, n = 8) or 2% (group 2, n = 8) during the 1st hour of isoflurane anesthesia. Under constant volume-controlled ventilation, we measured the C _Iiso and the end-tidal isoflurane concentration (C_Eiso) at the mouthpiece by infrared analysis. Our results demonstrate that it takes 40 min for the brain tissue concentration to equal Aiso for 1% C_Iiso, and 50 min for 2% C_Iiso. The Aiso (and/or Jiso) for 2% C _Iiso was approximately double when compared to that for 1% C _Iiso. Except during the initial wash-in period of the functional residual capacity in the first 3 min, the differences between C_Iiso and C_Eiso revealed that the body uptake of isoflurane for 2% C _Iiso was twice that for 1% C_Iiso. These results demonstrate that the pharmacokinetics of isoflurane uptake in the brain is time-dependent for Jiso to equal Aiso, and the midpoint between Aiso and Jiso (likely representing the isoflurane concentration in brain tissue) was dependent on C_Iiso.