Comparison between in vivo and in vitro pharmacokinetics of succinylcholine in humans

Purpose. To compare the in vivo and in vitro pharmacokinetics of succinylcholine (SCh) in humans. Methods. A bolus of SCh 1 mg · kg^-1 (n = 7) or 2 mg · kg^-1 (n = 11) was given to 18 patients anesthetized with thiopental. Arterial blood samples for determination of in vivo SCh concentrations were collected every 30 s for 5 min. Another 20-ml blood sample was obtained before induction of anesthesia for determination of in vitro SCh. Concentrations of SCh were measured by high-performance liquid chromatography. In vivo and in vitro concentrations of SCh vs time data were analyzed by the one-compartment model. Results. The respective in vivo and in vitro pharmacokinetic parameters (SCh 1 mg · kg^-1 vs SCh 2 mg · kg^-1) were as follows: Plasma clearance was 4.17 ± 2.37 and 1.85 ± 0.281 · min^-1, P < 0.05, vs 2.91 ± 2.01 and 1.27 ± 0.431 · min^-1, P < 0.05. Elimination half-life was 25.4 ± 10.6 and 47.4 ± 5.4 s, P < 0.002 vs 26.3 ± 10.0 and 75.2 ± 21.8 s, P < 0.00005. Conclusion. These results suggest that the rapid disappearance of SCh from the circulation is due to diffusion out of the blood vessels rather than to enzymatic hydrolysis.