Pharmacokinetics and pharmacodynamics of suxamethonium

A study was undertaken to determine the time constants of elimination and effect compartment equilibration of suxamethonium and for the slope exponent of the Hill equation. Twelve patients were anaesthetized with thiopentone, fentanyl, and isoflurane in nitrous oxide and oxygen. After allowing conditions to become stable, they were administered three small doses of suxamethonium by rapid intravenous injection. The responses to supramaximal stimulation of the ulnar nerve were recorded by EMG in one and by accelerometry in eleven subjects. Because of failure to recover to control conditions, one subject was deleted from analysis. The recorded drug effect was used in a non-linear curve fitting technique to derive estimates of the pharmacokinetic and pharmacodynamic parameters. The plasma concentration of suxamethonium was adequately represented by a single compartment model. The mean half-life of elimination was 47 s with a 95% confidence interval of 24 to 70 s; that of effect compartment equilibration, 211 s with a 95% confidence range of 139 to 282 s. The average slope exponent was 6.4 and its 95% confidence range was 4.6 to 8.2. The data from the first two doses were used to predict the time taken for the third dose to recover 50%. The predictions showed a mean bias of < 1% (NS) and a mean error of 21%, with a confidence interval of 5 to 37%. In only two out of eleven subjects was the prediction error greater than 30%.