Predictive ability of propofol effect-site concentrations during fast and slow infusion rates

Schnider and Eleveld Models for Propofol Target-Controlled Infusion Anesthesia: A Clinical Comparison

BACKGROUND

Various pharmacokinetic/pharmacodynamic (PK/PD) models have been developed to accurately dose propofol administration during total intravenous anesthesia with target-controlled infusion (TIVA-TCI). We aim to clinically compare the performance of the Schnider model and the new and general-purpose Eleveld PK/PD model during TIVA-TCI.

METHODS

We conducted a prospective observational study at a single center, enrolling 78 female patients, including 37 adults (aged < 65 years) and 41 elderly patients (aged ≥ 65 years). These patients underwent breast surgery with propofol-remifentanil TIVA-TCI guided by the bispectral index (BIS) for depth of anesthesia monitoring (target value 40-60) and the surgical plethysmographic index (SPI) for antinociception monitoring (target value 20-50) without neuromuscular blockade. The concentration at the effect site of propofol (CeP) at loss of responsiveness (LoR) during anesthesia maintenance (MA) and at return of responsiveness (RoR), the duration of surgery and anesthesia (min), the time to RoR (min), the propofol total dose (mg), the deepening of anesthesia events (DAEs), burst suppression events (BSEs), light anesthesia events (LAEs) and unwanted spontaneous responsiveness events (USREs) were considered to compare the two PK/PD models.

RESULTS

Patients undergoing BIS-SPI-guided TIVA-TCI with the Eleveld PK/PD model showed a lower CeP at LoR (1.7 (1.36-2.25) vs. 3.60 (3.00-4.18) μg/mL, p < 0.001), higher CePMA (2.80 (2.55-3.40) vs. 2.30 (1.80-2.50) μg/mL, p < 0.001) and at RoR (1.48 (1.08-1.80) vs. 0.64 (0.55-0.81) μg/mL, p < 0.001) than with the Schnider PK/PD model. Anesthetic hysteresis was observed only in the Schnider PK/PD model group (p < 0.001). DAEs (69.2% vs. 30.8%, p = 0.001) and BSEs (28.2% vs. 5.1%, p = 0.013) were more frequent with the Eleveld PK/PD model than with the Schnider PK/PD model in the general patient population. DAEs (63.2% vs. 27.3%, p = 0.030) and BSEs (31.6% vs. 4.5%, p = 0.036) were more frequent with the Eleveld PK/PD model than with the Schnider PK/PD model in the elderly.

CONCLUSIONS

The Schnider and Eleveld PK/PD models impact CePs differently. A greater incidence of DAEs and BSEs in the elderly suggests more attention is necessary in this group of patients undergoing BIS-SPI-guided TIVA-TCI with the Eleveld PK/PD than with the Schnider model.

Influence of rate of administration on the mechanism behind propofol induced loss of consciousness

BACKGROUND

Propofol effect-site time course models included in TCI systems have been under discussion. We hypothesized that the rate of administration is a major contributor affecting the construction of a useful effect-site model: yielding different plasmatic concentrations, loss of consciousness may occur by different mechanisms more complex than the pharmacological effect-site.

METHODOLOGY

ASA III patients were randomized in two groups: rapid induction (RI) received TCI of propofol effect-site (CeCALC) 5.4 μg/mL (modified Marsh model), and slow induction (SI) propofol infusion of 10 mg/kg/hour. A neurologist, blinded to induction method, performed neurological assessments using the FOUR score until the loss of consciousness (LOC). At LOC, the presence of brain stem reflexes, EEG index (PSI) and infusion time/mass of drug were registered. Fisher's exact test was used to describe differences between brain stem reflexes and respiration components of the FOUR score and CeCALC for 4 propofo models at LOC time.

RESULTS

16 patients divided in two groups were included. All patient in SI had brainstem reflexes free at LOC. In the RI, all patients had brain stem reflexes abolished and 1 patient had B and R of 4 points in the FOUR score (brain stem reflexes unaffected; P < .001). CeCALC at LOC time were contradictory at LOC in both groups and using 4 different Pk/Pd models.

CONCLUSIONS

Depending of the infusion rate, propofol CeCALC at LOC calculated by different Pk/Pd models could be the source of confuse data to be used to guide the state of general anesthesia.

Influence of rate of administration on the mechanism behind propofol induced loss of consciousness

BACKGROUND

Propofol effect-site time course models included in TCI systems have been under discussion. We hypothesized that the rate of administration is a major contributor affecting the construction of a useful effect-site model: yielding different plasmatic concentrations, loss of consciousness may occur by different mechanisms more complex than the pharmacological effect-site.

METHODOLOGY

ASA III patients were randomized in two groups: rapid induction (RI) received TCI of propofol effect-site (CeCALC) 5.4 μg/mL (modified Marsh model), and slow induction (SI) propofol infusion of 10 mg/kg/hour. A neurologist, blinded to induction method, performed neurological assessments using the FOUR score until the loss of consciousness (LOC). At LOC, the presence of brain stem reflexes, EEG index (PSI) and infusion time/mass of drug were registered. Fisher's exact test was used to describe differences between brain stem reflexes and respiration components of the FOUR score and CeCALC for 4 propofo models at LOC time.

RESULTS

16 patients divided in two groups were included. All patient in SI had brainstem reflexes free at LOC. In the RI, all patients had brain stem reflexes abolished and 1 patient had B and R of 4 points in the FOUR score (brain stem reflexes unaffected; P < .001). CeCALC at LOC time were contradictory at LOC in both groups and using 4 different Pk/Pd models.

CONCLUSIONS

Depending of the infusion rate, propofol CeCALC at LOC calculated by different Pk/Pd models could be the source of confuse data to be used to guide the state of general anesthesia.

Critical view of the effect site modelling of propofol

Target controlled infusion (TCI) of Propofol has been the subject of discussion during its 20 years of use, including the validity of the models that represent the course of the effect, such as: Are the different EEG indexes representative of the effect? Is the reactivity of the EEG index used to build models comparable to each other? What is the real reacting time of each monitor? Is the ke0 influenced by the infusion speed? Is the ke0 or the time to peak effect affected by age? How valid are the current Emax models? Are the induction and wakening simple mirror phenomenon as they are represented in the E max models? This review discusses issues related to the complexity and difficulty in obtaining a representation of the effect, and the lack of agreed definitions to be able to construct representative models of the temporary installation of the effect of Propofol for its use in TCI.

Do we have today a reliable method to detect the moment of loss of consciousness during induction of general anaesthesia?

This review aims to give an overview of the current state of monitoring depth of anaesthesia and detecting the moment of loss of consciousness, from the first clinical signs involved in anaesthesia to the latest technologies used in this area. Such techniques are extremely important for the development of automatic systems for anaesthesia control, including preventing intraoperative awareness episodes and overdoses. A search in the databases Pubmed and IEEE Xplore was performed using terms such anaesthetic monitoring, depth of anaesthesia, loss of consciousness, as well as anaesthesia indexes, namely BIS. Despite the several methods capable of monitoring the hypnotic state of anaesthesia, there is still no methodology to accurate detect the moment of loss of consciousness during induction of general anaesthesia.

Comparison of the Cortínez and the Schnider models with a targeted effect-site TCI of 3 mcg/ml in biophase in healthy volunteers

OBJECTIVE

To compare the Cortínez and Schnider models in effect-site TCI mode (3 mcg/ml) in healthy volunteers.

METHODS

Ten healthy volunteers were prospectively studied on 2 occasions. Propofol was administered with the Cortínez or the Schnider models, as randomly assigned. Times and predicted concentrations at the time of loss and recovery of consciousness (LOC and ROC), mass of drug administered, BIS, and haemodynamic variables were compared. Statistical analysis was with paired Wilcoxon test. A P<.05 was considered significant.

RESULTS

The propofol bolo was higher (1.4 [1.3-1.6] versus 0.9 [0.7-1.3] mg/kg, P=.005) and the LOC occurred earlier (1.33 [0.67-6.83] versus 3.87 [1.66-11.08] minutes, P=.02) with the Cortínez model compared to the Schnider model. With the Cortínez model, LOC occurred at an effect site concentrations of 2.6 (1.65-3.0) mcg/ml. With the Schnider model, LOC occurred at 3.87 min (1.66-11.8) after reaching the target of 3 mcg/ml. (P=.001). BIS values, infusion rates, and haemodynamic variables were similar between models after 20minutes of infusion (P>.5). Recovery (ROC) was longer with the Cortínez model (11.6 [8.1-16.2] vs. 8.5 [4.7-15.5] min, P=.003).

CONCLUSIONS

The Cortínez model is a good alternative to the Schnider model for use in effect-site TCI mode in normal weight subjects. With the target used in this study (3 mcg/ml), the slower Ke0 incorporated into the Cortínez model better discriminated the LOC time.

[Reevaluation of the time course of the effect of propofol described with the Schnider pharmacokinetic model]

BACKGROUND

The first order plasma-effect-site equilibration rate constant (k(e0)) links the pharmacokinetics (PK) and pharmacodynamics (PD) of a given drug. This constant, calculated for each specific PK drug model, allowed us to predict the course of the effect in a target controlled infusion (TCI). The PK-PD model of propofol, published by Schnider et al., calculated a k(e0) value of 0.456min(-1) and a corresponding time to peak effect (t peak) of 1.6min. The aim of this study was to reevaluate the k(e0) value for the predicted Schnider model of propofol, with data from a complete effect curve obtained by monitoring the bispectral index (BIS).

METHODS

The study included 35 healthy adult patients (18-90 years) scheduled for elective surgery with standard monitoring and using the BIS XP(®) (Aspect), and who received a propofol infusion to reach a plasma target of 12 μg/ml in 4min. The infusion was then stopped, obtaining a complete effect curve when the patient woke up. The Anestfusor™ (University of Chile) software was used to control the infusion pumps, calculate the plasma concentration plotted by Schnider PK model, and to store the BIS data every second. Loss (LOC) and recovery (ROC) of consciousness was assessed and recorded. Using a traditional parametric method using the "k(e0) Objective function" of the PK-PD tools for Excel, the individual and population k(e0) was calculated. Predictive Smith tests (Pk) and Student t test were used for statistical analysis. A P<.05 indicated significance.

RESULTS

The evaluation included 21 male and 14 female patients (18 to 90 years). We obtained 1,001 (±182) EEG data and the corresponding calculated plasma concentration for each case. The population k(e0) obtained was 0.144min(-1) (SD±0.048), very different from the original model (P<.001). This value corresponds with a t peak of 2.45min. The predictive performance (Pk) for the new model was 0.9 (SD±0.03), but only 0.78 (SD±0.06) for the original (P<.001). With a baseline BIS of 95.8 (SD±2.34), the BIS at LOC was 77.48 (SD±9.6) and 74.65(SD±6.3) at ROC (P=.027). The calculated Ce in the original model at LOC and ROC were 5.9 (SD±1.35)/1.08 μg/ml (SD±0.32) (P<.001), respectively, and 2.3 (SD±0.63)/2.0 μg/ml (SD±0.65) (NS) for the new model. The values between LOC/ROC were significantly different between the 2 models (P<.001). No differences in k(e0) value were found between males and females, but in the new model the k(e0) was affected by age as a covariable (0.26-[age×0.0022]) (P<.05).

CONCLUSIONS

The dynamic relationship between propofol plasma concentrations predicted by Schnider's pharmacokinetic model and its hypnotic effect measured with BIS was better characterized with a smaller k(e0) value (slower t½k(e0)) than that present in the original model, with an age effect also not described before.