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

Purpose. To compare the in vivo and in vitro pharma-cokinetics 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 anes-thesia for determination of in vitro SCh. Concentrations of SCh were measured by high-performance liquid chromato-graphy. 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 ± 0.28 l·min⁻¹, P < 0.05, vs 2.91 ± 2.01 and 1.27 ± 0.43 l·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 disap-pearance of SCh from the circulation is due to diffusion out of the blood vessels rather than to enzymatic hydrolysis. Received for publication on August 31, 1998; accepted on May 11, 1999