Pharmacokinetics of lignocaine and bupivacaine in surgical patients following epidural administration. Simultaneous investigation of absorption and disposition kinetics using stable isotopes.

The pharmacokinetics of lignocaine (lidocaine) and bupivacaine following epidural administration were studied in 12 surgical patients using a stable isotope method. Shortly after epidural administration of the agent to be evaluated, a deuterium-labelled analogue was administered intravenously. Plasma concentrations of the unlabelled and the deuterium-labelled local anaesthetics were determined using gas chromatography and mass fragmentography. The pharmacokinetic behaviour of both agents was consistent with a 2-compartment open model and two parallel first-order absorption processes. The mean distribution and elimination half-lives were 12 minutes and 100 minutes for lignocaine, and 22 minutes and 143 minutes for bupivacaine. The mean volumes of the central compartment and the mean steady-state volumes of distribution were: lignocaine, 43L and 99L; bupivacaine, 33L and 68L. Total plasma clearances averaged 0.95 L/min (57 L/h) for lignocaine and 0.52 L/min (31.2 L/h) for bupivacaine. The half-lives, characterising the fast and slow absorption processes, were 9.3 and 82 minutes for lignocaine, and 7.0 minutes and 362 minutes for bupivacaine; the fractions of the doses absorbed in the fast and slow processes were lignocaine 0.38 and 0.58, bupivacaine 0.28 and 0.66, respectively. The results indicate that the local anaesthetics are completely absorbed from the epidural space into the general circulation. The initial absorption rates of both local anaesthetics appear to be similar, but, later, the absorption of bupivacaine proceeds much more slowly than the absorption of lignocaine.