Determining minimum effective anesthetic concentration of hyperbaric bupivacaine for spinal anesthesia

We determined the minimum effective anesthetic concentration (MEAC) of bupivacaine for spinal anesthesia, defined as the median effective concentration at which a spinal anesthetic produces surgically equivalent anesthesia within 20 min of administration in 50% of human subjects. Two doses of spinal bupivacaine (7.5 mg and 10 mg) were administered to 45 volunteers (19-39 yr) in a randomized, double-blinded fashion. Hyperbaric bupivacaine solutions of 0.1% to 0.75% containing 8.25% dextrose were administered intrathecally and MEAC established by using the Dixon's up-and-down method. Complete anesthesia was defined as: 1) pinprick anesthesia at or higher than T12; 2) anesthesia to transcutaneous tetanic electric stimulation (50 Hz at 60 mA for 5 s) in the knees; and 3) complete leg paralysis, all occurring in both lower extremities within 20 min of intrathecal injection. We found that the MEAC of spinal bupivacaine was 0.43% (95% confidence interval 0.24-0.62) when 10 mg was administered. At this dose, a concentration as low as 0.1% could provide complete anesthesia, but consistent blockade was obtained only with the 0.7% solution. The 7.5-mg dose failed to provide complete anesthesia consistently, even in the presence of 0.75% (maximum). The current commercially available 0.75% concentration of hyperbaric bupivacaine seems to be clinically optimal when 10 mg is used if complete bilateral lower extremity blockade is desired.