Assessment of the performance of the Marsh model in effect site mode for target controlled infusion of propofol during the maintenance phase of general anaesthesia in an unselected population of neurosurgical patients:

Effect of Hemodynamic Changes in Plasma Propofol Concentrations Associated with Knee-Chest Position in Spinal Surgery: A Prospective Study

Background

Anesthesia induction and maintenance with propofol can be guided by target-controlled infusion (TCI) systems using pharmacokinetic (Pk) models. Physiological variables, such as changes in cardiac output (CO), can influence propofol pharmacokinetics. Knee-chest (KC) surgical positioning can result in CO changes.

Objectives

This study aimed to evaluate the relationship between propofol plasma concentration prediction and CO changes after induction and KC positioning.

Methods

This two-phase prospective cohort study included 20 patients scheduled for spinal surgery. Two different TCI anesthesia protocols were administered after induction. In phase I (n = 9), the loss of consciousness (LOC) concentration was set as the propofol target concentration and CO changes following induction and KC positioning were quantified. In phase II (n = 11), based on data from phase I, two reductions in the propofol target concentration on the pump were applied after LOC and before KC positioning. Propofol plasma concentrations were measured at different moments in both phases: after induction and after KC positioning.

Results

Schnider Pk model showed a good performance in predicting propofol concentration after induction; however, after KC positioning, when a significant drop in CO occurred, the measured propofol concentrations were markedly underestimated. Intended reductions in the propofol target concentration did not attenuate HD changes. In the KC position, there was no correlation between the propofol concentration estimated by the Pk model and the measured concentration in plasma, as the latter was much higher (P = 0.013) while CO and BIS decreased significantly (P < 0.001 and P = 0.004, respectively).

Conclusions

Our study showed that the measured propofol plasma concentrations during the KC position were significantly underestimated by the Schnider Pk model and were associated with significant CO decrease. When placing patients in the KC position, anesthesiologists must be aware of pharmacokinetic changes and, in addition to standard monitoring, the use of depth of anesthesia and cardiac output monitors may be considered in high-risk patients.

Measuring the accuracy of propofol target-controlled infusion (TCI) before and after surgery with major blood loss

Target-controlled infusion (TCI) is based on pharmacokinetic models designed to achieve a desired drug level in the blood. TCI's predictive accuracy of plasma propofol levels at the end of surgery with major blood loss has not been well established. This prospective observational study included adult patients (BMI 20-35 kg/m²) undergoing surgery with expected blood loss ≥ 1500 mL. The study was conducted with the Schnider TCI propofol model (Alaris PK Infusion Pump, CareFusion, Switzerland). Propofol levels were assessed in steady-state at the end of anaesthesia induction (T_initial) and before the end of surgery (T_final). Predicted propofol levels (C_TCI) were compared to measured levels (C_blood). Twenty-one patients were included. The median estimated blood loss was 1600 mL (IQR 1000-2300), and the median fluid balance at T_final was + 3200 mL (IQR 2320-4715). Heart rate, mean arterial blood pressure, and blood lactate did not differ significantly between T_initial and T_final. The median bispectral index (0-100) was 50 (IQR 42-54) and 49 (IQR 42-56) at the two respective time points. At T_initial, median C_TCI was 2.2 µmol/L (IQR 2-2.45) and C_blood was 2.0 µmol/L (bias 0.3 µmol/L, limits of agreement - 1.1 to 1.3, p = 0.33). C_TCI and C_blood at T_final were 2.0 µmol/L (IQR 1.6-2.2) and 1 µmol/L (IQR 0.8-1.4), respectively (bias 0.6 µmol/L, limits of agreement - 0.89 to 1.4, p < 0.0001). Propofol TCI allows clinically unproblematic conduct of general anaesthesia. In cases of major blood loss, the probability of propofol TCI overestimating plasma levels increases.Trial registration German Clinical Trials Register (DRKS; DRKS00009312).

Model-based drug administration: current status of target-controlled infusion and closed-loop control

PURPOSE OF REVIEW

Drug administration might be optimized by incorporating pharmacokinetic-dynamic (PK/PD) principles and control engineering theories. This review gives an update of the actual status of target-controlled infusion (TCI) and closed-loop computer-controlled drug administration and the ongoing research in the field.

RECENT FINDINGS

TCI is becoming mature technology clinically used in many countries nowadays with proven safety. Nevertheless, changing populations might require adapting the established PK/PD models. As TCI requires accurate PK/PD models, new models have been developed which should now be incorporated into the pumps to allow more general use of this technology. Closed-loop administration of hypnotic drugs using an electro-encephalographic-derived-controlled variable has been well studied and has been shown to outperform manual administration. Computer administration for other drugs and fluids have been studied recently. Feasibility has been shown for systems controlling multiple components of anaesthesia, but more work is required to show clinical safety and efficiency.

SUMMARY

Evidence in the literature is increasing that TCI and closed-loop technology could assist the anaesthetists to optimize drug administration during anaesthesia.

Performance of the Minto model for the target-controlled infusion of remifentanil during cardiopulmonary bypass

We studied the predictive performance of the Minto pharmacokinetic model during cardiopulmonary bypass in patients undergoing cardiac surgery. Patients received remifentanil target-controlled infusion using the Minto model during total intravenous anaesthesia with propofol. From 56 patients, 275 arterial blood samples were drawn before, during and after bypass to determine the plasma concentration of remifentanil, and the predicted concentrations were recorded at each time. For pooled data, the median prediction error and median absolute prediction error were 21.3% and 21.8%, respectively, and 22.1% and 22.3% during bypass. Both were 148.4% during hypothermic circulatory arrest and measured concentrations were more than three times greater than predicted (26.9 (17.0) vs. 7.1 (1.6) ng.ml^-1 ). The Minto model showed considerable bias but overall acceptable precision during bypass. The target concentration of remifentanil should be reduced when using the Minto model during hypothermic circulatory arrest.