Brain tumors do not affect thiopental dosing requirements

We used the biphasic electroencephalographic (EEG) response to increasing concentrations of thiopental to measure regional brain responses to thiopental. Eight patients with cortical parietal brain tumors, 3.3 (SD 1.3) cm in diameter, and eight control patients with lung cancer and normal brain computed tomography scans received thiopental by infusion (50-75 mg/min) until burst suppression (50% isoelectric activity) on the EEG occurred. Infusion lasted 10.7 (SD 2.4) min, and the average dose of thiopental administered was 810 (SD 170) mg [11.2 (SD 1.9) mg/kg]. During infusion the EEG was continuously recorded from the F3, F4, P3, and P4 electrodes. On-line power spectral analysis was performed, and data were saved for later analysis. Four EEG parameters [log beta (15-30 Hz) power, percent beta power, spectral edge 95% and spectral edge 70%] were plotted against calculated brain concentration of thiopental [using an assumed plasma-effect site rate constant (ke0) of 0.58] for each individual. Three points were measured on each curve (50% upslope, peak, and zero intercept) to quantitate the EEG response. Statistical comparisons were performed between the following sets of data: EEG response at electrode closest to brain tumor versus electrode farthest from tumor (in the same patient); and electrodes closest to brain tumors (parietal P3 and P4) versus same electrode pair in control patients (patients with thoracic tumors) using analysis. No differences were found in any comparison. Thus, the presence of a brain tumor does not affect the response of the brain in this region to thiopental as measured using EEG.