Determining minimum effective anesthetic concentration of hyperbaric bupivacaine for spinal anesthesia

We determined the minimum effective anesthetic concentration (MEAC) of bupi vacaine for spinal anesthesia, defined as the median effective concentratio n at which a spinal anesthetic produces surgically equivalent anesthesia wi thin 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 y r) in a randomized, double-blinded fashion. Hyperbaric bupivacaine solution s of 0.1% to 0.75% containing 8.25% dextrose were administered intrathecall y and MEAC established by using the Dixon's up-and-down method. Complete an esthesia was defined as: 1) pinprick anesthesia at or higher than T12; 2) a nesthesia to transcutaneous tetanic electric stimulation (50 Hz at 60 mA fo r 5 s) in the knees; and 3) complete leg paralysis, all occurring in both l ower 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) wh en 10 mg was administered. At this dose, a concentration as low as 0.1% cou ld provide complete anesthesia, but consistent blockade was obtained only w ith the 0.7% solution. The 7.5-mg dose failed to provide complete anesthesi a consistently, even in the presence of 0.75% (maximum). The current commer cially 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.