Concentration-Dependence of Halothane Metabolism in Rabbits

Effect of end-expiratory halothane concentration on its aerobic and anaerobic metabolism was studied in 20 male rabbits. Biliary excretion of trifluoroacetic acid (TF AA), an aerobic metabolite of halothane, and the pulmonary excretion of CF2CHCl (CDFF) and CF3CH2Cl (CTFE), anaerobic metabolites of halothane, were measured simultaneously during 5 hr of inhalation at various concentrations of halothane (0.02-1.5%). The total amount of biliary TF AA excreted for 5 hr remained unchanged (38.4μmol- 44.lμmol) at each concentration except 0.02%. However, the total amount of CDFE and CTFE excreted for 5 hr increased in a concentration-dependent manner up to 0.5% end-expiratory halothane, and remained unchanged (47.2μmol-64.7μmol) at more than 0.5%. Total biliary excretion of TF AA reached its maximum at a 0.05%. These results suggest that the primary route of halothane metabolism at a subanesthetic concentration is the aerobic pathway which produces TF AA. However, at anesthetic concentration exceeding 0.5%, the anaerobic route which produces volatile CDFE and CTFE plays a role also as an aerobic route.This study was supported in part by Science Research Grants from the Ministry of Education, Science and Culture of Japa