Predictive performance of a new pharmacokinetic model for propofol in underweight patients during target-controlled infusion

Predictive pharmacodynamic performance of the Eleveld pharmacokinetic-pharmacodynamic model for propofol: comparison of predicted and measured bispectral index

BACKGROUND

The Eleveld pharmacokinetic-pharmacodynamic model for propofol predicts bispectral index (BIS) processed electroencephalogram values from estimated effect-site concentrations. We investigated agreement between measured and predicted BIS values during total intravenous anaesthesia (TIVA).

METHODS

Forty participants undergoing lower limb surgery received TIVA using remifentanil target-controlled infusions and propofol by manually controlled, target-guided infusions based upon the Eleveld model and directed by two pharmacokinetic computer simulation applications: PKPD Tools and StelSim. We evaluated the predictive performance of the Eleveld model by calculating median prediction errors (BIS units) and by Bland-Altman analyses. We also performed |Bland-Altman analysis of supplementary data provided by the authors of the Eleveld model.

RESULTS

Whereas median prediction errors were small (MDPE -1.9, MDAPE 10), the ranges were wide (-18.5 to 24.3 and 1.7 to 24.3). The proportion of MDAPE >10 BIS units was 47.8%. Bland-Altman analysis showed a small mean bias (-0.52 BIS units) with wide limits of agreement (-27.7 to 26.2). Each participant's limits of agreement did not meet the requirements for declaring interchangeability between the two measurements. The measurement differences depended on the BIS values, as indicated by the positive slopes of the differences vs BIS values. Bland-Altman analysis of the Eleveld model supplementary data revealed similar results.

CONCLUSION

BIS predictions by the Eleveld model should be interpreted with caution. In spite of the acceptable MDPE and MDAPE, there are unacceptable degrees of both within-subject and between-subject variation during propofol target-controlled infusions. This limits the use of adjusting targeted concentrations to achieve desired simulated BIS values with confidence.

Comparative Analysis of the Performance of Electroencephalogram Parameters for Monitoring the Depth of Sedation During Remimazolam Target-Controlled Infusion

BACKGROUND

The changes in hypnotic indicators in remimazolam sedation remain unclear. We investigated the correlation of the electroencephalogram (EEG) parameters with the effect-site remimazolam concentration and the depth of sedation in patients receiving a target-controlled infusion of remimazolam.

METHODS

This prospective observational study enrolled 35 patients (32 analyzed) who underwent lower extremity varicose vein surgery or lower extremity orthopedic surgery under spinal anesthesia. We administered remimazolam by target-controlled infusion using the pharmacokinetic model introduced by Schüttler et al. The EEG data were continuously recorded, including the bispectral index (BIS), patient state index (PSI), spectral edge frequency (SEF), and raw EEG signals. The relative beta ratio (RBR), defined as log (spectral power [30-47 Hz]/spectral power [11-20 Hz]), was obtained by analyzing raw EEG. The level of sedation corresponding to each effect-site remimazolam concentration was assessed using the Modified Observer's Assessment of Alertness/Sedation (MOAA/S). The prediction probability (Pk) and Spearman's correlation coefficients (R) were calculated between effect-site remimazolam concentration, MOAA/S, and EEG parameters.

RESULTS

BIS and PSI showed significantly higher Pk for effect-site remimazolam concentration (Pk = 0.76 [0.72-0.79], P < .001 for BIS; Pk = 0.76 [0.73-0.79], P < .001 for PSI) compared to RBR (Pk = 0.71 [0.68-0.74], P < .001) and SEF (Pk = 0.58 [0.53-0.63], P = .002). BIS, PSI, and RBR showed significantly higher correlation coefficients for effect-site remimazolam concentration (R = -0.70 [-0.78 to -0.63], P < .001 for BIS; R = -0.72 [-0.79 to -0.66], P < .001 for PSI; R = -0.61 [-0.69 to -0.54], P < .001 for RBR) compared to SEF (R = -0.22 [-0.36 to -0.08], P = .002). BIS and PSI also had significantly higher Pk and correlation coefficients for MOAA/S (Pk = 0.81 [0.79-0.83], P < .001; R = 0.84 [0.81-0.88], P < .001 for BIS) (Pk = 0.80 [0.78-0.83], P < .001; R = 0.82 [0.78-0.87], P < .001 for PSI) compared to RBR (Pk = 0.74 [0.72-0.77], P < .001; R = 0.72 [0.65-0.78], P < .001) and SEF (Pk = 0.55 [0.50-0.59], P = .041; R = 0.13 [-0.01 to 0.27], P = .067).

CONCLUSIONS

BIS, PSI, and RBR showed an acceptable correlation with the effect-site remimazolam concentration and depth of sedation in this study, suggesting that these EEG-derived parameters are potentially reliable hypnotic indicators during remimazolam sedation. BIS and PSI showed superior performance as hypnotic indicators to RBR and SEF in patients receiving target-controlled infusion of remimazolam.

External validation of a pharmacokinetic model for target-controlled infusion of cefazolin as a prophylactic antibiotic

AIMS

This study aimed to evaluate the predictive performance of previously constructed cefazolin pharmacokinetic models and determine whether cefazolin administration via the target-controlled infusion (TCI) method may be possible in clinical practice.

METHODS

Twenty-five gastrectomy patients receiving cefazolin as a prophylactic antibiotic were enrolled. Two grams of cefazolin was dissolved in 50 mL of normal saline to give a concentration of 40 mg mL-1 . Before skin incision, cefazolin was administered using a TCI syringe pump, and its administration continued until the end of surgery. The target total plasma concentration was set to 100 μg mL-1 . Total and unbound plasma concentrations of cefazolin were measured in three arterial blood samples collected at 30, 60 and 120 min after the start of cefazolin administration. The predictive performance of the TCI system was evaluated using four measures: inaccuracy, divergence, bias and wobble.

RESULTS

Total (n = 75) and unbound (n = 75) plasma concentration measurements from 25 patients were included in the analysis. The pooled median (95% confidence interval) biases and inaccuracies were 6.3 (4.0-8.5) and 10.5 (8.6-12.4) for the total concentration model and -10.3 (-16.8 to -3.7) and 22.4 (18.2-26.7) for the unbound concentration model, respectively. All unbound concentrations were above 10 μg mL-1 .

CONCLUSION

Administration of cefazolin by the TCI method showed a clinically acceptable performance. Applying the TCI method by setting the total concentration as the target concentration rather than the unbound concentration is effective in maintaining a constant target concentration of cefazolin.

General purpose models for intravenous anesthetics, the next generation for target-controlled infusion and total intravenous anesthesia?

PURPOSE OF REVIEW

There are various pharmacokinetic-dynamic models available, which describe the time course of drug concentration and effect and which can be incorporated into target-controlled infusion (TCI) systems. For anesthesia and sedation, most of these models are derived from narrow patient populations, which restricts applicability for the overall population, including (small) children, elderly, and obese patients. This forces clinicians to select specific models for specific populations.

RECENT FINDINGS

Recently, general purpose models have been developed for propofol and remifentanil using data from multiple studies and broad, diverse patient groups. General-purpose models might reduce the risks associated with extrapolation, incorrect usage, and unfamiliarity with a specific TCI-model, as they offer less restrictive boundaries (i.e., the patient "doesn't fit in the selected model") compared with the earlier, simpler models. Extrapolation of a model can lead to delayed recovery or inadequate anesthesia. If multiple models for the same drug are implemented in the pump, it is possible to select the wrong model for that specific case; this can be overcome with one general purpose model implemented in the pump.

SUMMARY

This article examines the usability of these general-purpose models in relation to the more traditional models.

Population pharmacokinetic and pharmacodynamic model of propofol externally validated in Korean elderly subjects

The Eleveld propofol pharmacokinetic (PK) model, which was developed based on a broad range of populations, showed greater bias (- 27%) in elderly subjects in a previous validation study conducted by Vellinga and colleagues. We aimed to develop and externally validate a new PK-pharmacodynamic (PK-PD) model of propofol for elderly subjects. A population PK-PD model was constructed using propofol plasma concentrations and bispectral index (BIS) values that were obtained from 31 subjects aged 65 years older in previously published phase I studies. The predictive performance of the newly-developed PK-PD model (Choi model) was assessed in a separate Korean elderly population and compared with that of the Eleveld model. A three-compartment mammillary model using an allometric expression and a sigmoid E_max model well-described the time courses of propofol concentrations and BIS values. The V₁, V₂, V₃, Cl, Q₁, Q₂, E₀, E_max, Ce₅₀, γ, and k_e0 of a 60-kg subject were 8.36, 58.0, 650 L, 1.26, 0.917, 0.669 L/min, 92.1, 18.7, 2.21 μg/mL, 2.89, and 0.138 /min, respectively. In the Choi model and Eleveld model, pooled biases (95% CI) of the propofol concentration were 7.78 ( 3.09-12.49) and 16.70 (9.46-23.93) and pooled inaccuracies were 22.84 (18.87-26.81) and 24.85 (18.07-31.63), respectively. The Choi PK model was less biased than the Eleveld PK model in Korean elderly subjects (age range: 65.0-79.0 yr; weight range: 45.0-75.3 kg). Our results suggest that the Choi PK model, particularly, is applicable to target-controlled infusion in non-obese Korean elderly subjects.

Predictive performance of pharmacokinetic models for target concentration-controlled infusion of cefoxitin as a prophylactic antibiotic in patients with colorectal surgery

We aimed to evaluate the predictive performance of previously constructed free (C_free) and total (C_total) cefoxitin pharmacokinetic models and the possibility of administering cefoxitin via the target‐controlled infusion (TCI) method in clinical practice. Two external validation studies (N = 31 for C_free model, N = 30 for C_total model) were conducted sequentially. Cefoxitin (2 g) was dissolved in 50 mL of normal saline to give a concentration of 40 mg mL⁻¹. Before skin incision, cefoxitin was infused with a TCI syringe pump. Target concentrations of free concentration and total concentration were set to 25 and 80 μg mL⁻¹, respectively, which were administered throughout the surgery. Three arterial blood samples were collected to measure the total and free plasma concentrations of cefoxitin at 30, 60 and 120 min, after the start of cefoxitin administration. The predictive performance was evaluated using four parameters: inaccuracy, divergence, bias and wobble. The pooled median (95% confidence interval) biases and inaccuracies were − 45.9 (−47.3 to −44.5) and 45.9 (44.5 to 47.3) for C_free model (Choi_F model), and − 16.6 (−18.4 to −14.8) and 18.5 (16.7 to 20.2) for C_total model (Choi_T_old model), respectively. The predictive performance of the newly constructed model (Choi_T_new model), developed by adding the total concentration data measured in the external validation, was better than that of the Choi_T_old model. Models constructed with total concentration data were suitable for clinical use. Administering cefoxitin using the TCI method in patients maintained the free concentration above the minimal inhibitory concentration (MIC) breakpoints of the major pathogens causing surgical site infection throughout the operation period.

Obesity and anesthetic pharmacology: simulation of target-controlled infusion models of propofol and remifentanil

The prevalence of obesity is increasing, resulting in an increase in the number of surgeries performed to treat obesity and diseases induced by obesity. The associated comorbidities as well as the pharmacokinetic and pharmacodynamic changes that occur in obese patients make it difficult to control the appropriate dose of anesthetic agents. Factors that affect pharmacokinetic changes include the increase in adipose tissue, lean body weight, extracellular fluid, and cardiac output associated with obesity. These physiological and body compositional changes cause changes in the pharmacokinetic and pharmacodynamic parameters. The increased central volume of distribution and alterations in the clearance of drugs affect the plasma concentration of propofol and remifentanil in the obese population. Additionally, obesity can affect pharmacodynamic properties, such as the 50% of maximal effective concentration and the effect-site equilibration rate constant (ke0). Conducting a simulation of target-controlled infusions based on pharmacokinetic and pharmacodynamic models that include patients that are obese can help clinicians better understand the pharmacokinetic and pharmacodynamic changes of anesthetic drugs associated with this population.

Breath-by-breath measurement of intraoperative propofol by unidirectional anisole-assisted photoionization ion mobility spectrometry via real-time correction of humidity

Humidity as a major issue affects the quantitative performance of ion mobility spectrometry (IMS) in field applications. According to the kinetic equations of ion-molecular reaction, the intensity ratio of the product ion peak (PIP) over the reactant ion peak (RIP) is proposed as a quantitative factor to correct real-time humidity variation. By coupling this method with a unidirectional anisole-assisted photoionization IMS, direct breath-by-breath measurement of intraoperative propofol was achieved for the first time, which provided more clinical information for studying the anesthetics pharmacokinetics. Although the signal intensities of the RIP and the propofol PIP both declined along with the increase of humidity, the intensity ratio of Propofol/(RIP + Propofol) kept almost constant in a wide relative humidity range of 0%-98%, enabling direct quantitation of exhaled propofol with varying humidity. Furthermore, interfering ion peaks resulted from the high concentration humidity and anesthetics in single exhalation were eliminated during the balanced anesthesia as the exhaled sample was diluted by the unidirectional gas flow scheme. As a demonstration, breath-by-breath variation profiles of propofol were obtained via monitoring end-tidal propofol concentration of intraoperative anesthetized patients (n = 7). The analyses were quantitative, corrected for humidity in real-time, without measuring the humidity content of each breath sample during operation, which show potential for the quantitative analysis of other high humidity samples.

Do epoch lengths of hypnotic depth indicators affect estimated of blood-brain equilibration rate constants of propofol?

This study aimed to investigate the effect of epoch length of hypnotic depth indicators on the blood-brain equilibration rate constant (k_e0) estimates of propofol. Propofol was administered by zero-order infusion (1.5, 3.0, 6, and 12 mg·kg^-1·h^-1) for one hour in 63 healthy volunteers. The k_e0 of propofol was estimated using an effect-compartment model linking pharmacokinetics and pharmacodynamics, in which response variables were electroencephalographic approximate entropy (ApEn) or bispectral index (BIS) (n = 32 each for propofol infusion rates of 6 and 12 mg·kg^-1·h^-1). Epoch lengths of ApEn were 2, 10, 30, and 60 seconds (s). The correlations between plasma propofol concentrations (Cp) and BIS and ApEn 2, 10, 30, and 60 s were determined, as was the Ce associated with 50% probability of unconsciousness (Ce_50,LOC). The pharmacokinetics of propofol were well described by a three-compartment model. The correlation coefficient between Cp and ApEn 2, 10, 30, and 60 s were -0.64, -0.54, -0.39, and -0.26, respectively, whereas correlation coefficient between Cp and BIS was -0.74. The blood-brain equilibration half-life based on the k_e0 estimates for ApEn at 2, 10, 30, 60 s and BIS were 4.31, 3.96, 5.78. 6.54, 5.09 min, respectively, whereas the Ce_50,LOC for ApEn at 2, 10, 30, 60 s and BIS were 1.55, 1.47, 1.28, 1.04, and 1.55 μg·ml^-1, respectively. Since k_e0, which determines the onset of drug action, varies according to the epoch length, it is necessary to consider the epoch length together when estimating k_e0.

Prospective clinical validation of the Eleveld propofol pharmacokinetic-pharmacodynamic model in general anaesthesia

BACKGROUND

Target-controlled infusion (TCI) systems incorporating pharmacokinetic (PK) or PK-pharmacodynamic (PK-PD) models can be used to facilitate drug administration. Existing models were developed using data from select populations, the use of which is, strictly speaking, limited to these populations. Recently a propofol PK-PD model was developed for a broad population range. The aim of the study was to prospectively validate this model in children, adults, older subjects, and obese adults undergoing general anaesthesia.

METHODS

The 25 subjects included in each of four groups were stratified by age and weight. Subjects received propofol through TCI with the Eleveld model, titrated to a bispectral index (BIS) of 40-60. Arterial blood samples were collected at 5, 10, 20, 30, 40, and 60 min after the start of propofol infusion, and every 30 min thereafter, to a maximum of 10 samples. BIS was recorded continuously. Predictive performance was assessed using the Varvel criteria.

RESULTS

For PK, the Eleveld model showed a bias < ±20% in children, adults, and obese adults, but a greater bias (-27%) in older subjects. Precision was <30% in all groups. For PD, the bias and wobble were <5 BIS units and the precision was close to 10 BIS units in all groups. Anaesthetists were able to achieve intraoperative BIS values of 40-60 using effect-site target concentrations about 85-140% of the age-adjusted Ce₅₀.

CONCLUSIONS

The Eleveld propofol PK-PD model showed predictive precision <30% for arterial plasma concentrations and BIS predictions with a low (population) bias when used in TCI in clinical anaesthesia practice.

Accuracy assessment of a PION TCI pump based on international standards

Background

Inaccuracies associated with target-controlled infusion (TCI) delivery systems are attributable to both software and hardware issues, as well as pharmacokinetic variability. However, little is known about the inaccuracy of the syringe pump operating in TCI mode. This study aimed to evaluate the accuracy of the TCI pump based on international standards.

Methods

A test apparatus for accuracy evaluation of a syringe pump (PION TCI^®, Bionet Co. Ltd.) was designed to apply the gravimetric method. Pump accuracy was evaluated in terms of deviation defined by the following equation: infusion rate deviation (%) = (Rate_mea - Rate_est ) / Rate_est × 100, where Rate_mea is the infusion rate (ml/h) as measured by the gravimetric system, and Rate_est is the infusion rate (ml/h) as estimated by the pump. An infusion rate representing TCI mode was determined from previous clinical trial data which evaluated the predictive performance of the pharmacokinetic model. The PION TCI pump used in that clinical trial was used to evaluate accuracy of the syringe pump. The distribution of infusion rates obtained from the clinical trial was calculated, and the median value of the distribution was determined as the representative value.

Results

The representative infusion rate representing TCI mode was 31 ml/h, at which the infusion rate deviation was 4.5 ± 1.6%.

Conclusions

The inaccuracy of the syringe pump contributing to TCI system inaccuracy is insignificant.