Pharmacokinetics of isoflurane: uptake in the brain

We studied the effect of the inspired isoflurane concentration (C(I)iso) 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(I)iso 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(I)iso and the end-tidal isoflurane concentration (C(E)iso) 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(I)iso, and 50 min for 2% C(I)iso. The Aiso (and/or Jiso) for 2% C(I)iso was approximately double when compared to that for 1% C(I)iso. Except during the initial wash-in period of the functional residual capacity in the first 3 min, the differences between C(I)iso and C(E)iso revealed that the body uptake of isoflurane for 2% C(I)iso was twice that for 1% C(I)iso. 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(I)iso.