Thiopental intravenous injections before temporary clipping and mild
hypothermia have protective effects in the setting of
cerebral ischemia, and are used clinically in some centers. However, it is not known whether mild
hypothermia affects
thiopental-induced electroencephalogram (EEG) burst suppression. In this study, the authors compared the onset and duration of EEG suppression by
thiopental in normothermic (n=10) and mildly hypothermic (n=10) patients undergoing
cerebral aneurysm surgery. Spectral analysis was used to compare the prethiopentonal continuous EEG patterns in normothermic and mild hypothermic patients. The patients' body temperatures were controlled by a circulating water mattress and intravenous fluids (normothermia = 36.4+/-0.1 degrees C, mild
hypothermia = 33.3+/-0.1 degrees C). Immediately before temporary clipping,
thiopental sodium (5 mg/kg) was administered intravenously. Onset time (the amount of time from
thiopental injection to the first complete EEG suppression), duration of suppression (the amount of time from the first complete EEG suppression to recovery on continuous EEG from burst suppression), and maximum duration of isoelectric EEG (the longest time interval between two bursts during burst suppression) were measured. Onset time was shortened (25.8+/-1.4 versus 43.5+/-5.6 seconds), and duration of suppression (531.0+/-56.6 versus 165.0+/-16.9 seconds) and the maximum duration of isoelectric EEG (47.7+/-5.8 versus 22.8+/-2.0 seconds) were prolonged in the patients with mild
hypothermia. In two normothermic patients, the standard dose of
thiopental did not produce burst suppression, but only a mild decrease in spectral edge frequency. The authors concluded that the effects of mild
hypothermia on
thiopental-induced EEG suppression are not simply additive, but synergistic.