An end-tidal concentration of 1% sevoflurane with 50% nitrous oxide (N₂O) during a Cesarean section resulted in bispectral index (BIS) values > 60, which are considered at risk for awareness. The present study aimed to determine whether the presence or absence of labor pain prior to the Cesarean section would affect the BIS value.

Sixty women scheduled to undergo Cesarean section under general anesthesia, were allocated to three groups of 20 patients: women undergoing elective surgery without labor pain (group 1, control), or emergency surgery without (group 2) or with (group 3) active labor pain. After endotracheal intubation, anesthesia was maintained with end-tidal 1% sevoflurane and 50% N₂O in oxygen throughout the surgery. The BIS value, systolic blood pressure and heart rate were measured before (baseline) and during the induction of anesthesia, intubation, skin incision, uterine incision, delivery and at 1, 3, 5 and 10 min after delivery. Neonatal effects were assessed using Apgar scores at 1 and 5 min after delivery.

BIS values were significantly lower in group 3 than in groups 1 and 2 throughout the study, except at baseline and induction (P < 0.05). However, the systolic blood pressure, heart rate and Apgar scores did not differ among the three groups.

These results demonstrate that 1.0% sevoflurane combined with 50% N₂O results in BIS values < 60 during Cesarean delivery in women with active labor pain but not in those without active labor pain, consistent with an adequate depth of anesthesia to prevent recall.