• Thomas K Henthorn, Michael J Avram, Albert Dahan, Lars L Gustafsson, Jan Persson, Tom C Krejcie, and Erik Olofsen.
• From the Department of Anesthesiology, University of Colorado School of Medicine, Aurora, Colorado (T.K.H.) the Department of Pharmaceutical Sciences, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Aurora, Colorado (T.K.H.) the Department of Anesthesiology, Northwestern University Feinberg School of Medicine, Chicago, Illinois (M.J.A., T.C.K.) the Department of Anesthesiology, Leiden University Medical Center, Leiden, The Netherlands (A.D., E.O.) the Department of Laboratory Medicine, Division of Clinical Pharmacology (L.L.G.) the Department of Clinical Science, Intervention and Technology, Division of Anesthesiology (J.P.), Karolinska Institute at Karolinska University Hospital, Stockholm, Sweden.
• Anesthesiology. 2018 Aug 1; 129 (2): 260-270.

What We Already Know About This TopicWHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: The pharmacokinetics of infused drugs have been modeled without regard for recirculatory or mixing kinetics. We used a unique ketamine dataset with simultaneous arterial and venous blood sampling, during and after separate S(+) and R(-) ketamine infusions, to develop a simplified recirculatory model of arterial and venous plasma drug concentrations.MethodsS(+) or R(-) ketamine was infused over 30 min on two occasions to 10 healthy male volunteers. Frequent, simultaneous arterial and forearm venous blood samples were obtained for up to 11 h. A multicompartmental pharmacokinetic model with front-end arterial mixing and venous blood components was developed using nonlinear mixed effects analyses.ResultsA three-compartment base pharmacokinetic model with additional arterial mixing and arm venous compartments and with shared S(+)/R(-) distribution kinetics proved superior to standard compartmental modeling approaches. Total pharmacokinetic flow was estimated to be 7.59 ± 0.36 l/min (mean ± standard error of the estimate), and S(+) and R(-) elimination clearances were 1.23 ± 0.04 and 1.06 ± 0.03 l/min, respectively. The arm-tissue link rate constant was 0.18 ± 0.01 min, and the fraction of arm blood flow estimated to exchange with arm tissue was 0.04 ± 0.01.ConclusionsArterial drug concentrations measured during drug infusion have two kinetically distinct components: partially or lung-mixed drug and fully mixed-recirculated drug. Front-end kinetics suggest the partially mixed concentration is proportional to the ratio of infusion rate and total pharmacokinetic flow. This simplified modeling approach could lead to more generalizable models for target-controlled infusions and improved methods for analyzing pharmacokinetic-pharmacodynamic data.

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