title: The interaction between emulsified isoflurane and lidocaine is synergism in intravenous regional anesthesia in rats.
abstract: 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, EC(50) (median effective concentration) of EI alone and EC(50) of lidocaine alone, as well as EC(50) 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, EC(50) of EI was 4.467% ± 0.375% and EC(50) of lidocaine was 0.183% ± 0.072%; EC(50) 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.
authors: Department of Pharmacology, West China School of Pharmacy, Sichuan University, Chengdu, Sichuan, People's Republic of China.
authors: Cheng
authors: C
authors: Zhou
authors: Jing
authors: J
authors: Gan
authors: Jin
authors: J
authors: Liu
authors: Wen-Jun
authors: WJ
authors: Luo
authors: Wen-Sheng
authors: WS
authors: Zhang
authors: Yun-Fei
authors: YF
authors: Chai