Evaluation and optimisation of propofol pharmacokinetic parameters in cats for target-controlled infusion

Development and optimization of a fentanyl pharmacokinetic model for target-controlled infusion in anaesthetized dogs

OBJECTIVE

To investigate pharmacokinetics (PK) of fentanyl administered by target-controlled infusion (TCI), and to develop a PK model optimized by covariates for TCI in anaesthetized dogs.

STUDY DESIGN

Prospective clinical study.

ANIMALS

A group of 20 client-owned dogs with spinal pain undergoing anaesthesia for magnetic resonance imaging.

METHODS

Fentanyl was administered as an infusion to 20 anaesthetized dogs using a TCI system incorporating a previously described fentanyl two-compartment PK. Arterial blood samples were collected at specific time points during the infusion and over 60 minutes post-infusion for measurement of fentanyl plasma concentrations. The predictive performance of the Sano PK model was assessed by comparing predicted and measured plasma concentrations. A population PK analysis was then performed using a nonlinear mixed-effect modelling approach, allowing inter- and intra-individual variability estimation. Finally, a quantitative stepwise evaluation of the influence of various covariates such as weight, body condition score, size, size-related age, sex and type of premedication on the PK model was considered.

RESULTS

Overall predictive performance of the Sano PK set of variables was not clinically acceptable in anaesthetized dogs. Fentanyl PK was best described by a three-compartment model. Weight and sex were found to affect the volume of distribution of the central compartment. Addition of these two covariate/variable associations resulted in a reduction of the objective function value (OFV) from -340.18 to -448.34, and of the median population weighted residual and the median population absolute weighted residual from 16.1% and 38.6% to 3.9% and 20.3%, respectively. Fentanyl infusions at measured concentrations up to 5.4 ng mL^-1 in sevoflurane-anaesthetized dogs resulted in stable anaesthesia and smooth recoveries without complications.

CONCLUSIONS AND CLINICAL RELEVANCE

A population three-compartment PK model for fentanyl TCI in anaesthetized dogs was developed. Weight and sex have been detected and incorporated as significant covariates.

A pharmacokinetic model optimized by covariates for propofol target-controlled infusion in dogs

OBJECTIVE

To develop a population pharmacokinetic model for propofol target-controlled infusion (TCI) in dogs and to evaluate its performance for use in the clinical setting.

STUDY DESIGN

Prospective clinical study.

ANIMALS

A group of 40 client-owned dogs undergoing general anaesthesia for magnetic resonance imaging.

METHODS

Propofol was administered to 26 premedicated dogs and arterial blood samples were collected during the infusion and over 240 minutes after terminating the infusion. Propofol concentrations were measured by high-performance liquid chromatography. A population pharmacokinetic analysis was performed using a nonlinear mixed-effects modelling approach, allowing inter- and intra-individual variability estimation and quantitative evaluation of the influence of the following covariates: weight, body condition score, age, size-related age (Age_size), sex, premedication type, size and contrast agent administration. A final model was obtained using a stepwise approach in which individual covariate effects on each pharmacokinetic variable were incorporated. The performance of the developed TCI model was subsequently evaluated while inducing and maintaining anaesthesia in 14 premedicated dogs and assessed by comparing predicted and measured concentrations at specific time points.

RESULTS

Propofol pharmacokinetics was best described by a three-compartment model. Weight, Age_size, premedication and sex showed significant pharmacokinetic effects. Addition of the significant covariate/variable associations to the final model resulted in a reduction of the objective function value from 285.53 to -22.34. The median values of prediction error and absolute performance error were 3.1% and 28.4%, respectively. Induction targets between 4.0 and 6.5 μg mL^-1 allowed intubation within 5.0 ± 0.9 minutes. Anaesthesia was achieved with targets between 3.0 and 6.5 μg mL^-1. Mean time to extubation was 9.7 ± 2.6 minutes. All dogs recovered smoothly and without complications.

CONCLUSIONS AND CLINICAL RELEVANCE

Overall predictive performance of the pharmacokinetic model-driven infusion developed was clinically acceptable for administering propofol to dogs in routine anaesthesia.

The influence of storage time and temperature on propofol concentrations in canine blood and plasma

Propofol is an intravenous anesthetic commonly used due to its favorable pharmacokinetic and pharmacodynamic profile. There are discrepancies in the literature about the most appropriate sample for determining propofol concentrations. Although plasma has been used for determining propofol concentrations, whole blood has been the preferred sample. There is also a lack of consistency in the literature on the effect of storage time and temperature on propofol concentrations and this may lead to errors in the design of pharmacokinetic/pharmacodynamics studies. The purpose of this study was to determine the difference in propofol concentrations in whole blood versus plasma and to evaluate the influence of storage time (56 days) and temperature (4 °C, −20 °C, −80 °C) on the stability of propofol concentrations in blood and plasma samples. Results from the study indicate that whole blood and plasma samples containing propofol stored at −80 °C have concentrations as high as or higher than those stored at 4 °C or −20 °C for 56 days; thus, −80 °C is an appropriate temperature for propofol sample storage. Plasma propofol concentrations were consistently higher than whole blood for all three storage temperatures. Consequently, plasma is the most appropriate sample for propofol analysis due to its consistent determinations.