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ONE of the main effects of general anesthesia is the suppression of consciousness, i.e., prevention of awareness. This must not be confused with absence of recall. A patient who does not have recall for events during general anesthesia may still have been conscious during the procedure. Recently, a case report in ANESTHESIOLOGY described a patient who had awareness without recall during electroconvulsive therapy. In several studies, Veselis et al. identified amnestic effects at subanesthetic concentrations of anesthetics and the according anatomical structures. In the current issue of ANESTHESIOLOGY, two research groups try to identify the effect sites of anesthesia-induced loss of consciousness. At first glance, results of the two studies seem to be in conflict with one another. Velly et al. studied patients undergoing intracerebral electrode placement for deep brain stimulation. Scalp electrodes and a deep brain electrode were used for recording of electrical activity of cortical and subcortical structures. During general anesthesia, the first effect was slowing of cortical electroencephalogram with prominent amplitudes in the range, whereas the signal obtained from the deep brain electrode showed faster spindle activity with less prominent slowing of the power spectrum. As cortical slowing of electroencephalographic activity was more prominent and occurred earlier, the authors concluded that the main effect site of anesthesia-induced unconsciousness is the cerebral cortex. In contrast, Alkire et al. identified subcortical structures as one of the main effect sites of anesthesia-induced unconsciousness in animals. In their study, microinjection of nicotine into the central medial thalamus, a subcortical structure, reversed loss of righting reflex induced by sevoflurane. Given the specificity of both drug effect and localization, the authors conclude that general anesthesia blocks endogenous cholinergic arousal mechanisms in the central medial nucleus of the thalamus, i.e., the main targets of general anesthesia are thalamic structures. Therefore, these reports provide evidence in support of both cortical as well as for subcortical structures as target sites of general anesthesia (fig. 1).

The Search for Structures and Mechanisms Controlling Anesthesia-induced Unconsciousness