Isoflurane anesthesia blunts cerebral responses to noxious and innocuous stimuli: a fMRI study.

We used functional magnetic resonance imaging to determine how isoflurane affected cerebral neuronal activation resulting from noxious and innocuous stimuli. Five male volunteers were subjected to mild electrical shock and tactile stimuli applied to the hand. During low (0.7%) and moderate (1.3%) isoflurane anesthesia the stimuli were repeated and a supramaximal electrical shock was also applied. Tactile stimulation activated bilateral SI and SII, but resulted in no significant activation at low or moderate anesthesia. Electrical shock activated contralateral SI and bilateral SII; low anesthesia completely abolished this response. The supramaximal stimulus activated the caudate nucleus and bilateral thalamus at low anesthesia; these responses were diminished at moderate anesthesia. Isoflurane anesthesia blunts cerebral responses to somatosensory stimuli, and the absence of cortical activation during supramaximal stimulation suggests that noxious-induced movement is generated in lower CNS structures.