Time constants of xenon elimination after anaesthesia

Introduction: Biologic half life and pharmacokinetics of xenon are known fr om application of xe(133) in nuclear medicine. In these investigations wash out time constants were calculated after relatively short exposure times of 10-20 minutes. Investigations using xenon concentrations and exposure time s as used in anaesthesia have not yet been carried out. We have investigate d xenon uptake, distribution and elimination under conditions of experiment al anaesthesia. Materials and methods: 7 pigs were anaesthetized with a combination of intr avenous pentobarbital and buprenorphine and an inspiratory concentration of xenon 70%. Radioactive xenon(133) was used as tracer for inert xenon(134). Washin and washout of xenon was observed under a gamma camera. Washin time was 4 hours, washout time was 2 hours. Kinetic curves for all body compart ments revealing different activity curves as the whole body were calculated separately. Statistics were calculated using an empiric regression model. Mean residence times (MRT), terminal half lives (t 1/2), biologic half live s (t(50%)) and t(90%) were calculated for the compartments whole body, lung , fatty tissue and bowel. Results: As known from previous investigations, the fastest compartment was the lung. The slowest compartments were fatty tissue and bowel, respective ly. No other compartments revealed delays of excretion as compared to whole body. Discussion: Time constants like mean residence times and half lives were di fferent as reported from investigations from nuclear medicine. The slowest compartments were found to be fatty tissue and bowel. The reason for that f inding is that in short exposures slow compartments are not saturated. Wash out time constants calculated after short exposures cannot be easily transf erred to conditions of xenon anaesthesia.