Electroencephalographic mapping during isoflurane anesthesia for treatment of mental depression

Seven patients undergoing isoflurane anesthesia were studied using electroencephalographic (EEG) mapping, a computer-assisted technique of EEG processing that permits the analysis of the spatial distribution of frequency components of the human EEG. After induction with thiopental, the patients were hyperventilated (carbon dioxide tension, 25 to 28 mm Hg) with 4% isoflurane in oxygen until a burst-suppression EEG appeared. Brain electrical activity was compared before anesthesia, at 0.5 to 0.8% and 1.8 to 2.1% isoflurane, during bursts, and after anesthesia. Starting from an alpha EEG with occipital predominance, a distinct alpha activation appeared over the whole cortex during 0.5 to 0.8% isoflurane. Decreased alpha amplitude with a maximum over the frontotemporal regions was observed during 1.8 to 2.1% isoflurane. Bursts consisted of high amplitude in all frequency bands. In the delta and the theta bands, voltage was very low before isoflurane administration, and increased in relation to depth of anesthesia. A minimum in delta and theta amplitudes occurred over both central regions. The dominant frequency, defined as the frequency component with the highest amplitude in the frequency spectrum, decreased from 8 to 4 and finally to 1 Hz, while its amplitude increased from 3 to 13 microV. As anesthesia deepened, the dominant frequency shifted from the occiput to the frontal cortex. Brain mapping transfers an immense amount of electroencephalographic (EEG) data into a comprehensive, easily readable image of brain function during anesthesia for therapy of mental depression. To make use of this technique in the operating room, further investigation and development toward on-line mapping are necessary.