BACKGROUND AND OBJECTIVES: Minimum analgesic concentration of a local anesthetic (MAC-LA) is the effective concentration for 50% of patients (EC50) during the first stage of labor. It may be used to determine relative analgesic potency and to estimate the effects of co-administered epidural analgesics. This study aimed at determining epidural bupivacaine's MAC-LA for orthopedic surgery. METHODS: A double-blind non randomized sequential allocation method for MAC calculation was applied to 23 adult patients undergoing orthopedic leg, ankle or foot surgeries. Anesthesia was obtained with lumbar spinal hyperbaric bupivacaine. An epidural catheter placed at L4-L5 level was inserted 3 to 5 cm in the cephalad direction. Postoperatively, a 20 ml epidural bupivacaine bolus followed by 0.15 ml.kg-1.h-1 infusion were administered at the appropriate concentration. Pain and Bromage scores were recorded after 4, 8 and 12 hours. Bupivacaine concentration was considered effective when visual analog pain scores were below 10 mm in all evaluations. Initial concentration was 0.3% and was subsequently decreased or increased by 0.1% for next patient when previous response was effective or ineffective, respectively. MAC-LA was calculated by Dixon and Massey's formula. RESULTS: Bupivacaine's MAC-LA (95% confidence limits) was 0.16% (0.11% and 0.21%). Intense motor blockade was observed in most patients. CONCLUSIONS: For a 0.15 ml.kg-1.h-1 infusion rate, bupivacaine's MAC-LA was 0.16%. However, the model may have not been suitable for the evaluation of motor effects of tested concentrations.
ANALGESIA; ANESTHETICS, Local; ANESTHETIC TECHNIQUES, Regional; ANESTHETIC TECHNIQUES, Regional
