Time constants of xenon elimination after anaesthesia
Thomas Marx, Jörg Kotzerke, Sabine Musati, Christina Ring, Michael Schmidt, Helmut Reinelt, Gebhard Fröba

Introduction: Biologic  half life and pharmacokinetics of xenon are known from application of xe¹³³ in nuclear medicine. In these investigations  washout time constants were calculated after relatively short exposure times of 10 – 20 minutes. Investigations using xenon concentrations and exposure times as used in anaesthesia have not yet been carried out.  We have investigated xenon uptake, distribution and elimination under conditions of experimental anaesthesia. 

Materials and methods: 7 pigs were anaesthetized with a combination of intravenous pentobarbital and buprenorphine and an inspiratory concentration of xenon 70%. Radioactive xenon¹³³ was used as tracer for  inert xenon¹³¹. 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 compartments 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 ½), biologic half lives  (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, respectively. No other compartments revealed delays of excretion as compared to whole body. 

Discussion: Time constants like mean residence times and half lives were different as reported from investigations from nuclear medicine. The slowest compartments were found to be fatty tissue and bowel. The reason for that finding is that in short exposures slow compartments are not saturated. Washout time constants calculated after short exposures cannot be easily transferred to conditions of xenon anaesthesia.

Keywords: anaesthesia, inhalation, pharmacokinetic

Applied Cardiopulmonary Pathophysiology 9: 91-96, 2000

Thomas Marx, M.D.
Dept. Cardiac Anaesthesia
University of Ulm
Steinhövelstr. 9
D-89070 Ulm
Germany
E-mail: Thomas.Marx@medizin.uni-ulm.de