SUBARACHNOID BUPIVACAINE BLOCKADE DECREASES MIDAZOLAM AND THIOPENTAL HYPNOTIC REQUIREMENTS

Study Objective: To test the hypothesis that subarachnoid bupivacaine blockade decreases hypnotic requirements for thiopental sodium and mid azolam. Design: Randomized, double-blind placebo-controlled study. Set ting: Teaching hospital. Patients: 53 nonpremedicated ASA physical sta tus I and II adult male patients scheduled for elective lower abdomina l, pelvic, or lower limb surgery. Interventions: Intravenous injection s of midazolam or thiopental were administered with or without subarac hnoid bupivacaine blockade (12.5 mg) at the L(3)-L(4), level. Thiopent al or midazolam hypnotic requirements were determined using loss of ab ility to open eyes in response to verbal command as an endpoint. The t hiopental requirements were determined by titration; the midazolam req uirements were determined from dose-response curves obtained with bolu s injections of predetermined doses of the drug. Measurements and Main Results: Subarachnoid bupivacaine blockade decreased the hypnotic dos e of thiopental from 3.40 +/- 0.68 mg/kg (mean +/- SD) with a dose ran ge of 2.3 to 4.5 mg/kg (intramuscular saline) to 2.17 +/- 0.48 mg/kg w ith a dose range of 1.3 to 2.8 mg/kg (p < 0.005 for the difference). T he ED(50) value of midazoloam decreased with the bupivacaine blockade, from 0.23 mg/kg (95% confidence limits: 0.08 to 0.38 mg/kg) to 0.06 m g/kg (0.01 to 0.14 mg/kg), with p < 0.0001 for the difference. Conclus ion: Subarachoid bupivacaine blockade decreases hypnotic requirements for both thiopental and midazolam. The results suggest that the reduct ion in hypnotic requirements is due to the decrease in afferent input induced by spinal anesthesia.