Values of the bispectral index do not parallel the hemodynamic response tothe rapid increase in isoflurane concentration

Purpose: To investigate the changes in hemodynamic variables and bispectral index (BIS) in response to a rapid increase in isoflurane or sevoflurane c oncentration. Method: Thirty adult patients were anesthetized with either isoflurane (iso flurane group) or sevoflurane (sevoflurane group). Two minutes after induct ion of anesthesia with thiamylal, the inspired concentrations of isoflurane and sevoflurane were rapidly increased from 0.5 minimum alveolar anaesthet ic concentration (MAC) to 3 MAC and maintained for five minutes. Heart rate (HR), mean arterial pressure (MAP), and BIS were measured every minute. Results: An increase in the anesthetic concentration caused increases in HR and MAP in the isoflurane group and a decrease in MAP in the sevoflurane g roup. Consequently, HR and MAP in the isoflurane group were significantly h igher than those in the sevoflurane group. After inhalation of high concent rations, BIS significantly and progressively decreased in both groups. Conclusion: BIS values decrease after a step increase in volatile agent con centration, whether or not a hyperdynamic action occurs.