Different behaviour of target-controlled infusion pumps despite apparently using the same Schnider pharmacokinetic model

Abstract

BACKGROUND Target-controlled infusion (TCI) pumps deliver an initial bolus of drug to achieve a preset drug target concentration quickly. OBJECTIVE We investigated whether differences exist in the initial bolus dose between different TCI pumps despite apparently using the same pharmacokinetic (PK) model. DESIGN Observational study. SETTING Laboratory data from a German University Hospital. Experiments were performed between February 2019 and September 2020. PARTICIPANTS Four TCI pumps set to the Schnider model in effect-site mode with a target propofol concentration of 3.5 μg ml−1 were studied. The algorithms of two pumps employed a fixed equilibration rate constant (k e0) of 0.456 min−1 (Perfusor® Space® and Injectomat TIVA Agilia®), while the other two used a fixed time to peak concentration (t peak) of 1.6 min (Alaris® PK and Syramed® μSP6000). Pump performance was assessed on 13 virtual patients with different combinations of age, sex, height and weight: the initial propofol dose injected was measured on a high precision scale. MAIN OUTCOME MEASURES Propofol bolus dose, as if delivered for anaesthesia induction. RESULTS A small difference in propofol bolus doses was observed for some simulations: for example, 56.4 ± 1.2 and 59.7 ± 0.2 mg with a Syramed μSP6000 and an Alaris pharmacokinetic pump, respectively, in a virtual 54-year old man with a height of 170 cm and a weight of 74 kg. In contrast, significant differences were found in other simulated patients: for example, 72.0 ± 1.0 and 111.9 ± 1.2 mg (P < 0.001) using a Perfusor Space and an Alaris TCI pump in a virtual 60-year-old, obese man with a height of 182 cm and a weight of 139 kg. CONCLUSIONS Discrepancies in the initial bolus dose can be explained by differences in k e0. Anaesthetists should be aware that differences exist in the implementation of the Schnider model between manufacturers, which could lead to disparities in the initial propofol bolus in some patients.