The Interaction Between Emulsified Isoflurane and Lidocaine Is Synergism in Intravenous Regional Anesthesia in Rats

BACKGROUND: Our previous studies have demonstrated that emulsified isoflurane (EI) produced epidural anesthesia and blockade of nerve conduction. We designed this study to observe whether EI could produce an anesthetic effect in IV regional anesthesia (IVRA) and to investigate the underlying interaction between EI and lidocaine when they were combined in IVRA. METHODS: IVRA was evaluated using tail-flick and tail-clamping tests in a rat model. In experiment 1, Sprague-Dawley rats were assigned to 4 groups (n = 10 per group), receiving 0.5 mL of 8% EI, 0.5% lidocaine, 30% Intralipid, or normal saline to observe whether EI could produce an anesthetic effect in IVRA. In experiment 2, for tail IVRA, EC50 (median effective concentration) of EI alone and EC50 of lidocaine alone, as well as EC50 of lidocaine with the addition of Intralipid (0.0% EI) or with the addition of EI at different concentrations (0.4%, 0.8%, and 1.6%) were determined using an up-and-down method. Isobolographic analysis was used to evaluate the interaction between EI and lidocaine. RESULTS: For experiment 1, successful IVRA was observed in 8 of 10 rats with 8% EI, 10 of 10 rats with 0.5% lidocaine, and 0 of 10 rats with 30% Intralipid or normal saline. The anesthetic effect was not different in onset time (1.5 +/- 0.9 vs 1.0 +/- 0.0 minutes, P = 0.104) or recovery time (15 +/- 9 vs 18 +/- 12 minutes, P = 0.394) between 8% EI and 0.5% lidocaine. For experiment 2, EC50 of EI was 4.467% +/- 0.375% and EC50 of lidocaine was 0.183% +/- 0.072%; EC50 of lidocaine was 0.173% +/- 0.036% with the addition of Intralipid, and 0.064% +/- 0.008%, 0.035% +/- 0.005%, and 0.028% +/- 0.006% with the addition of 0.4%, 0.8%, and 1.6% EI, respectively. With the addition of EI, the requirement for lidocaine was reduced in a synergistic manner. CONCLUSIONS: EI produced IVRA, and a synergistic interaction was found between EI and lidocaine for IVRA in a rat tail model. (Anesth Analg 2011;113:245-50)