Bispectral index values at sevoflurane concentrations of 1% and 1.5% in lower segment cesarean delivery

Abstract

Inadequate hypnosis in the absence of opioid analgesia may account for the increased incidence of awareness in cesarean delivery. An end-tidal concentration of 0.5 MAC isoflurane in 50% nitrous oxide (N(2)O) during cesarean delivery resulted in bispectral index (BIS) values >60, the threshold below which consciousness is unlikely. Our aim was to determine the BIS values achieved with the equivalent end-tidal concentration of sevoflurane and to determine if a larger concentration would consistently maintain BIS values <60. Twenty ASA physical status I-II parturients were randomized to receive an end-tidal concentration of either 1% sevoflurane or 1.5% sevoflurane delivered in 50% N(2)O throughout surgery. Thiopental 4 mg/kg was used for anesthetic induction. Morphine 0.1-0.15 mg/kg was administered only after delivery. Mean BIS values in the period between skin incision and neonatal delivery were 61 (95% confidence interval, 57-64) in the 1% sevoflurane group, versus 42 (95% confidence interval, 37-47) in the 1.5% sevoflurane group. BIS values were significantly different between groups at skin incision, uterine incision, delivery, and 10 min after delivery, but not thereafter. Indices of maternal and neonatal outcome were similar between groups.

Implications: Bispectral index (BIS) values <60 are consistent with a high probability of unconsciousness. An end-tidal concentration of 1.5% sevoflurane maintained BIS values <60 during cesarean delivery, whereas 1% did not. Adverse effects were not seen with the use of larger concentrations of sevoflurane.