Figure 21-2 Cortical somatosensory evoked responses to median nerve stimulation in humans before induction and during anesthesia with thiopental and isoflurane/N₂ O. In spite of an equivalent or greater degree of reduction in cerebral metabolic rate with thiopental, cortical evoked responses are better preserved^[30] than during anesthesia with isoflurane, ^[31] which suggests that the electroencephalographic suppression achieved with different anesthetic agents should not be assumed to be equivalent electrophysiologic states. The cumulative thiopental doses and expired concentrations of isoflurane and N₂ O are indicated.