An Allometric Model of Remifentanil Pharmacokinetics and Pharmacodynamics

Practicalities of Total Intravenous Anesthesia and Target-controlled Infusion in Children

Propofol administered in conjunction with an opioid such as remifentanil is used to provide total intravenous anesthesia for children. Drugs can be given as infusion controlled manually by the physician or as automated target-controlled infusion that targets plasma or effect site. Smart pumps programmed with pharmacokinetic parameter estimates administer drugs to a preset plasma concentration. A linking rate constant parameter (keo) allows estimation of effect site concentration. There are two parameter sets, named after the first author describing them, that are commonly used in pediatric target-controlled infusion for propofol (Absalom and Kataria) and one for remifentanil (Minto). Propofol validation studies suggest that these parameter estimates are satisfactory for the majority of children. Recommended target concentrations for both propofol and remifentanil depend on the type of surgery, the degree of surgical stimulation, the use of local anesthetic blocks, and the ventilatory status of the patient. The use of processed electroencephalographic monitoring is helpful in pediatric total intravenous anesthesia and target-controlled infusion anesthesia, particularly in the presence of neuromuscular blockade.

Opioid Sensitivity in Children with and without Obstructive Sleep Apnea

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Opioids are a mainstay of perioperative analgesia. Opioid use in children with obstructive sleep apnea is challenging because of assumptions for increased opioid sensitivity and assumed risk for opioid-induced respiratory depression compared to children without obstructive sleep apnea. These assumptions have not been rigorously tested. This investigation tested the hypothesis that children with obstructive sleep apnea have an increased pharmacodynamic sensitivity to the miotic and respiratory depressant effects of the prototypic μ-opioid agonist remifentanil.

Methods

Children (8 to 14 yr) with or without obstructive sleep apnea were administered a 15-min, fixed-rate remifentanil infusion (0.05, 0.1, or 0.15 μg · kg-1 · min-1). Each dose group had five patients with and five without obstructive sleep apnea. Plasma remifentanil concentrations were measured by tandem liquid chromatography mass spectrometry. Remifentanil effects were measured via miosis, respiratory rate, and end-expired carbon dioxide. Remifentanil pharmacodynamics (miosis vs. plasma concentration) were compared in children with or without obstructive sleep apnea.

Results

Remifentanil administration resulted in miosis in both non-obstructive sleep apnea and obstructive sleep apnea patients. No differences in the relationship between remifentanil concentration and miosis were seen between the two groups at any of the doses administered. The administered dose of remifentanil did not affect respiratory rate or end-expired carbon dioxide in either group.

Conclusions

No differences in the remifentanil concentration–miosis relation were seen in children with or without obstructive sleep apnea. The dose and duration of remifentanil administered did not alter ventilatory parameters in either group.

Effects of propofol combined with remifentanil anesthesia on the NO, endothelin and inflammatory cytokines in the plasma of patients with liver cirrhosis during the perioperative period

The effects of propofol combined with remifentanil on the nitric oxide (NO), endothelin (ET-1) and inflammatory cytokines in the plasma of patients with liver cirrhosis were investigated. A retrospective analysis of 68 patients with liver cirrhosis who underwent hepatectomy in the Hunan Provincial People's Hospital from March 2016 to July 2018 was made. According to different anesthesia methods, 30 patients anesthetized with propofol were enrolled into Group A. The other 38 patients anesthetized with propofol combined with remifentanil were enrolled into Group B, and the operation time, amount of bleeding during operation and postoperative awake time of the two groups were recorded. At three separate time-points T₁ (30 min before the anesthesia), T₂ (after the portal triad clamping), T₃ (3 days after the operation), aspartate transaminase (AST) and alanine transaminase (ALT) levels in the plasma were measured by rate method, and the levels of NO, ET-1, interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) in plasma were detected by enzyme-linked immunosorbent assay (ELISA). The plasma NO levels at the T₂ time-point were significantly lower than those at the T₁ and T₃ time-points (P<0.05); at the T₂ time-point, the concentrations of AST, ALT, ET-1, IL-6 and TNF-α in the plasma in Group A were significantly higher than those of Group B (P<0.05), while the levels of plasma NO in Group A were the opposite (P<0.05). The anesthesia of propofol combined with remifentanil could contribute to the balance of NO/ET-1 and the inhibition of inflammatory factors during the hepatectomy operation in patients with liver cirrhosis, and help to protect the liver function of patients, reducing the incidence of liver ischemia-reperfusion injury in patients.

Application of remifentanil combined with propofol in the diagnosis of colon cancer with awakening painless digestive endoscopy

Anesthetic effect of remifentanil combined with propofol in awakening painless endoscopy was analyzed. Retrospective analysis of 120 cases of colon cancer were treated in Dongying People's Hospital from June 2015 to December 2017. All of them were treated by awakening painless digestive endoscopy, divided into 60 cases in observation group (combined with remifentanil and propofol anesthesia), and 60 cases in control group (combined intravenous anesthesia of finanib and propofol). The data were respectively recorded at time-points of oxygen inhalation, intubation for 10 min, awakening time, waking time, and the time-points for each represented as the time-points of T1, T2, T3, T4, T5 and recorded the diastolic blood pressure (DBP), respiratory rate (RR) and heart rate (HR), and compared the awakening effect and the occurrence of adverse reaction. There was no significant difference in the DBP index between the two groups at time-point T1 (P>0.05). The time-points of T2, T3, T4 and T5 were significantly different from the observation group (P<0.05). There was no significant difference in RR index between the two groups and between the same groups (P>0.05). Compared with the control group, the awakening time and consciousness recovering of the observation group is lower (P<0.05). The incidence of adverse reactions after awakening operation between the two groups was statistically significant (P<0.05). The local pain rate in the observation group after the awakening operation was lower than the control group. The combined use of trace remifentanil and small dose propofol in the awakening painless digestive endoscopy can make the patients with colon cancer more stable when they are in the awakening state, so as to improve the safety of awakening painless digestive endoscopy. It is worth promoting in clinical practice.

Clinical Pharmacokinetics and Pharmacodynamics of Propofol

Propofol is an intravenous hypnotic drug that is used for induction and maintenance of sedation and general anaesthesia. It exerts its effects through potentiation of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) at the GABAA receptor, and has gained widespread use due to its favourable drug effect profile. The main adverse effects are disturbances in cardiopulmonary physiology. Due to its narrow therapeutic margin, propofol should only be administered by practitioners trained and experienced in providing general anaesthesia. Many pharmacokinetic (PK) and pharmacodynamic (PD) models for propofol exist. Some are used to inform drug dosing guidelines, and some are also implemented in so-called target-controlled infusion devices, to calculate the infusion rates required for user-defined target plasma or effect-site concentrations. Most of the models were designed for use in a specific and well-defined patient category. However, models applicable in a more general population have recently been developed and published. The most recent example is the general purpose propofol model developed by Eleveld and colleagues. Retrospective predictive performance evaluations show that this model performs as well as, or even better than, PK models developed for specific populations, such as adults, children or the obese; however, prospective evaluation of the model is still required. Propofol undergoes extensive PK and PD interactions with both other hypnotic drugs and opioids. PD interactions are the most clinically significant, and, with other hypnotics, tend to be additive, whereas interactions with opioids tend to be highly synergistic. Response surface modelling provides a tool to gain understanding and explore these complex interactions. Visual displays illustrating the effect of these interactions in real time can aid clinicians in optimal drug dosing while minimizing adverse effects. In this review, we provide an overview of the PK and PD of propofol in order to refresh readers' knowledge of its clinical applications, while discussing the main avenues of research where significant recent advances have been made.

Scaling beta-lactam antimicrobial pharmacokinetics from early life to old age

AIMS

Beta-lactam dose optimization in critical care is a current priority. We aimed to review the pharmacokinetics (PK) of three commonly used beta-lactams (amoxicillin ± clavulanate, piperacillin-tazobactam and meropenem) to compare PK parameters reported in critically and noncritically ill neonates, children and adults, and to investigate whether allometric and maturation scaling principles could be applied to describe changes in PK parameters through life.

METHODS

A systematic review of PK studies of the three drugs was undertaken using MEDLINE and EMBASE. PK parameters and summary statistics were extracted and scaled using allometric principles to 70 kg individual for comparison. Pooled data were used to model clearance maturation and decline using a sigmoidal (Hill) function.

RESULTS

A total of 130 papers were identified. Age ranged from 29 weeks to 82 years and weight from 0.9-200 kg. PK parameters from critically ill populations were reported with wider confidence intervals than those in healthy volunteers, indicating greater PK variability in critical illness. The standard allometric size and sigmoidal maturation model adequately described increasing clearance in neonates, and a sigmoidal model was also used to describe decline in older age. Adult weight-adjusted clearance was achieved at approximately 2 years postmenstrual age. Changes in volume of distribution were well described by the standard allometric model, although amoxicillin data suggested a relatively higher volume of distribution in neonates.

CONCLUSIONS

Critical illness is associated with greater PK variability than in healthy volunteers. The maturation models presented will be useful for optimizing beta-lactam dosing, although a prospective, age-inclusive study is warranted for external validation.

Advances in pharmacokinetic modeling: target controlled infusions in the obese

PURPOSE OF REVIEW

The use of conventional pharmacokinetic parameters sets 'models' derived from nonobese patients has proven inadequate to administer intravenous anesthetics in the obese population and is commonly associated with higher than anticipated plasma propofol concentrations when used with target (plasma or effect site) controlled infusion pumps. In this review we will describe recent modeling strategies to characterize the disposition of intravenous anesthetics in the obese patient and will show clinically relevant aspects of new model's performance in the obese population.

RECENT FINDINGS

Because clearance of a drug increases in a nonlinear manner with weight, nonlinear relationships better scale infusion rates between lean and obese individuals. Allometric concepts have been successfully used to describe size-related nonlinear changes in clearances. Other nonlinear scaling options include the use of descriptors such as body surface area, lean body weight, fat-free mass, and normal fat mass. Newer pharmacokinetic models, determined from obese patient data, have been developed for propofol and remifentanil using allometric concepts and comprehensive size descriptors.

SUMMARY

Pharmacokinetic models to perform target-controlled infusion in the obese population should incorporate descriptors that reflect with greater precision the influence of body composition in volumes and clearances of each drug. It is our hope that commercially available pumps will soon incorporate these new models to improve the performance of this technique in the obese population.

Pharmacokinetic-pharmacodynamic model for propofol for broad application in anaesthesia and sedation

BACKGROUND

Pharmacokinetic (PK) and pharmacodynamic (PD) models are used in target-controlled-infusion (TCI) systems to determine the optimal drug administration to achieve a desired target concentration in a central or effect-site compartment. Our aim was to develop a PK-PD model for propofol that can predict the bispectral index (BIS) for a broad population, suitable for TCI applications.

METHODS

Propofol PK data were obtained from 30 previously published studies, five of which also contained BIS observations. A PK-PD model was developed using NONMEM. Weight, age, post-menstrual age (PMA), height, sex, BMI, and presence/absence of concomitant anaesthetic drugs were explored as covariates. The predictive performance was measured across young children, children, adults, elderly, and high-BMI individuals, and in simulated TCI applications.

RESULTS

Overall, 15 433 propofol concentration and 28 639 BIS observations from 1033 individuals (672 males and 361 females) were analysed. The age range was from 27 weeks PMA to 88 yr, and the weight range was 0.68-160 kg. The final model uses age, PMA, weight, height, sex, and presence/absence of concomitant anaesthetic drugs as covariates. A 35-yr-old, 170 cm, 70 kg male (without concomitant anaesthetic drugs) has a V₁, V₂, V₃, CL, Q₂, Q₃, and k_e0 of 6.28, 25.5, 273 litres, 1.79, 1.75, 1.11 litres min^-1, and 0.146 min^-1, respectively. The propofol TCI administration using the model matches well with recommendations for all age groups considered for both anaesthesia and sedation.

CONCLUSIONS

We developed a PK-PD model to predict the propofol concentrations and BIS for broad, diverse population. This should be useful for TCI in anaesthesia and sedation.

What is the best size predictor for dose in the obese child?

Lean body mass is commonly proposed for anesthesia maintenance drug dosing calculations. However, total body mass used with allometric scaling has been shown to be better for propofol in obese adults and children. Fat-free mass has also been used instead of lean body mass. Fat-free mass is essentially the same as lean body mass but excludes a small percentage of mass of lipids in cell membranes, CNS, and bone marrow. Normal fat mass is a size descriptor that partitions total body mass into fat-free mass and fat mass calculated from total body mass minus fat-free mass. The relative influence of fat mass compared with fat-free mass is described by the fraction of fat mass that makes fat equivalent to fat-free mass in terms of allometric size. This fraction (Ffat) will differ for each drug and each parameter affected by body size (eg, clearance and volume of distribution). This fraction is based on the concept of theory-based allometric size. The normal fat mass based on allometric theory and partition of body mass into fat and fat-free components provides a principle-based approach explaining size and body composition effects on pharmacokinetics of all drugs in children and in adults.