Performance of TCI Propofol Using the Schnider Model for Cardiac Surgery on Cardiopulmonary Bypass-A Pilot Study.

OBJECTIVE

This pilot study aimed to evaluate the performance of target-controlled infusion (TCI) of propofol using the Schnider pharmacokinetic model in patients undergoing cardiac surgery requiring cardiopulmonary bypass.

DESIGN

This was a prospective pharmacokinetic study.

SETTING

A tertiary care hospital.

PARTICIPANTS

This study is comprised of 10 patients, aged between 46 and 81, who underwent elective cardiac surgery requiring the use of cardiopulmonary bypass.

INTERVENTIONS: Anesthetic technique was standardized. Hypnosis was maintained using TCI of propofol, titrated to achieve a bispectral index of 30 to 60. Calculated plasma propofol concentrations were recorded at 5 time points in total, before, during, and after cardiopulmonary bypass. Blood propofol concentration was measured at each of these time points.

MEASUREMENTS AND MAIN

RESULTS

The prediction errors and absolute prediction errors were calculated for each sample. From these, the median prediction error (MDPE) and its absolute value (MDAPE) were derived. Agreement between predicted and measured propofol concentrations was assessed using a Bland-Altman plot. Mean prediction errors were also compared pre-, on, and post-bypass using the generalized linear latent and mixed model. The MDPE and MDAPE were both found to be 45%, indicating significant bias toward under-prediction in the Schnider pharmacokinetic model. This bias was increased at an average propofol concentration of 4.5 μg/mL and above. A significant decrease in mean prediction error was noted while on bypass (45.6%, 95% confidence intervals 9.2-82.1).

CONCLUSIONS

The performance of the Schnider pharmacokinetic model for TCI propofol was poor, with a tendency toward under-prediction of blood propofol concentration, especially at higher average concentrations of propofol. While mitigating the risk of awareness, the risk of other adverse effects like hypotension and cardiorespiratory depression is increased. Patients should therefore be adequately monitored, and predicted plasma propofol concentrations taken in context with other patient parameters. A lower target concentration of propofol is probably sufficient to maintain an adequate depth of anesthesia as measured by BIS.