Intracranial Space–occupying Lesion Inducing Intracranial Hypertension Increases the Encephalographic Effects of Isoflurane in a Swine Model

BACKGROUND: Patients with a brain tumor are susceptible to the hypnotic effect of anesthetics depending on the tumor's size. We investigated whether intracranial space-occupying lesions (ICSOLs) inducing intracranial hypertension increase isoflurane's effect on electroencephalographic (EEG) results. MATERIALS AND METHODS: After anesthetic induction with isoflurane, 11 swine were studied with regard to isoflurane's effect on EEGs at 0.5% to 2.0% inhalational concentration at sequential stages: baseline 1, ICSOL 1, baseline 2, ICSOL 2, baseline 3. At each ICSOL stage, an intracranial epidural balloon catheter was inflated and the intracranial pressure maintained at twice the baseline pressure. The balloon was deflated after each ICSOL stage (baselines 2 and 3). A 95% spectral edge frequency (SEF), which correlates with anesthetic hypnosis, was used to measure isoflurane's effect. Pharmacodynamics was characterized using a sigmoidal inhibitory maximum effect model for the SEF versus end-tidal concentration. RESULTS: ICSOL shifted the relations between SEF and the effect-site concentration (Ce) downward. Baseline and 50% of the maximum spectral edge effect levels significantly decreased during balloon inflation. The Ce that produced SEF=15 was 1.12 (1.04-1.20) (mean [95% confidence interval])% for baseline 1; 0.92 (0.81-1.03) for ICSOL 1; 1.02 (0.94-1.11) for baseline 2; 0.88 (0.82-0.94) for ICSOL 2; 1.05 (0.93-1.17) for baseline 3. Isoflurane's effect on EEGs increased during balloon inflation, with the alteration tending to recover after balloon deflation. CONCLUSIONS: ICSOLs inducing intracranial hypertension increase the EEG effect of isoflurane, and external compression from the brain surface enhances the anesthetic hypnosis despite minimum brain injury.