Background: We have conducted this study to investigate whether unilateral or bilateral spinal anesthesia with bupivacaine induces different sensitivity to intravenous (i.v.) midazolam for sedation.
Methods: Forty-two patients undergoing various elective unilateral lower extremity surgeries were allocated into two groups: (1) unilateral spinal anesthesia group (Group US, n=21; heavy bupivacaine 5 mg/ml, 9 mg) and (2) bilateral spinal anesthesia group (Group BS, n=21; heavy bupivacaine 5 mg/ml, 9 mg). One milligram of midazolam was injected i.v. at 30-s intervals until the patients did not respond to the hand grasp test beginning 15 min after spinal anesthesia. The concentration of plasma bupivacaine was evaluated every 15 min for the first 75 min after the start of the spinal anesthesia, and the bispectral index was monitored continuously.
Results: The mean venous plasma concentration of bupivacaine was not significantly different between Group US and BS. The dose of midazolam required to abolish responses to verbal commands was significantly lower in Group BS (mean 5.9+/-1.2 mg) vs. Group US (mean 9.0+/-1.4 mg).
Conclusions: A higher dosage of midazolam is required for loss of response to verbal stimulation during unilateral spinal anesthesia than during bilateral spinal anesthesia.