Computerized continuous infusion of intravenous anesthetic drugs during pediatric cardiac surgery

Pharmacokinetics of Fentanyl and Its Derivatives in Children: A Comprehensive Review

Fentanyl and its derivatives sufentanil, alfentanil, and remifentanil are potent opioids. A comprehensive review of the use of fentanyl and its derivatives in the pediatric population was performed using the National Library of Medicine PubMed. Studies were included if they contained original pharmacokinetic parameters or models using established routes of administration in patients younger than 18 years of age. Of 372 retrieved articles, 44 eligible pharmacokinetic studies contained data of 821 patients younger than 18 years of age, including more than 46 preterm infants, 64 full-term neonates, 115 infants/toddlers, 188 children, and 28 adolescents. Underlying diagnoses included congenital heart and pulmonary disease and abdominal disorders. Routes of drug administration were intravenous, epidural, oral-transmucosal, intranasal, and transdermal. Despite extensive use in daily clinical practice, few studies have been performed. Preterm and term infants have lower clearance and protein binding. Pharmacokinetics was not altered by chronic renal or hepatic disease. Analyses of the pooled individual patients' data revealed that clearance maturation relating to body weight could be best described by the Hill function for sufentanil (R ² = 0.71, B _max 876 mL/min, K ₅₀ 16.3 kg) and alfentanil (R ² = 0.70, B _{max (fixed)} 420 mL/min, K ₅₀ 28 kg). The allometric exponent for estimation of clearance of sufentanil was 0.99 and 0.75 for alfentanil clearance. Maturation of remifentanil clearance was described by linear regression to bodyweight (R ² = 0.69). The allometric exponent for estimation of remifentanil clearance was 0.76. For fentanyl, linear regression showed only a weak correlation between clearance and bodyweight in preterm and term neonates (R ² = 0.22) owing to a lack of data in older age groups. A large heterogeneity regarding study design, clinical setting, drug administration, laboratory assays, and pharmacokinetic estimation was observed between studies introducing bias into the analyses performed in this review. A limitation of this review is that pharmacokinetic data, based on different modes of administration, dosing schemes, and parameter estimation methods, were combined.

Quality control and stability of ketamine, remifentanil, and sufentanil syringes in a pediatric operating theater

BACKGROUND

Transforming a drug from its commercial form into a ready-to-use drug is common practice, especially in pediatrics. However, the risk of compounding error is real and data on drug stability in practice are not always available.

AIMS

The aim of this study was to assess, in real conditions, both the error rate and stability of three drugs: ketamine, remifentanil, and sufentanil.

METHODS

A new rapid and easy-to-use high-performance liquid chromatography method with a diode array detector has been developed and validated to quantify these drugs and detect their degradation products. Over a 1-month period, 151 syringes were collected in the postanesthesia care unit. Seventy-three were stock solution syringes containing a 10-fold dilution of commercial drugs and 78 were serial dilution syringes made from successive dilutions of stock solutions. A comparison between real and expected concentrations as well as the detection of possible degradation products was carried out on these samples.

RESULTS

All stock solution syringes had good chemical stability throughout the working day. A 4-µg/mL remifentanil serial dilution syringe, however, had to be discarded as a degradation peak was detected. Overall, 15.3% (95% CI, 9.5-21.1%) of syringes had a drug concentration outside the ±10% acceptability range, that is, 11.0% (95% CI, 3.7-18.2%) and 19.5% (95% CI, 10.6%-28.4%) of stock and diluted syringes respectively, with drug amounts ranging from -25.3% to 22.0%. The highest error rates were observed with sufentanil syringes: 20% and 28% for stock solution and serial dilution, respectively.

CONCLUSION

The study shows that stock solution syringes prepared in advance are chemically stable throughout the day, unlike certain serial dilution syringes, indicating that the latter should be prepared just before administration to ensure chemical stability. Our results show that the error rate for serial dilution syringes is twice that of stock solution. Different safety measures are under discussion and have to be further studied.

Correction to: Pharmacokinetics of Fentanyl and Its Derivatives in Children: A Comprehensive Review

Fentanyl and its derivatives sufentanil, alfentanil, and remifentanil are potent opioids. A comprehensive review of the use of fentanyl and its derivatives in the pediatric population was performed using the National Library of Medicine PubMed. Studies were included if they contained original pharmacokinetic parameters or models using established routes of administration in patients younger than 18 years of age. Of 372 retrieved articles, 44 eligible pharmacokinetic studies contained data of 821 patients younger than 18 years of age, including more than 46 preterm infants, 64 full-term neonates, 115 infants/toddlers, 188 children, and 28 adolescents. Underlying diagnoses included congenital heart and pulmonary disease and abdominal disorders. Routes of drug administration were intravenous, epidural, oral-transmucosal, intranasal, and transdermal. Despite extensive use in daily clinical practice, few studies have been performed. Preterm and term infants have lower clearance and protein binding. Pharmacokinetics was not altered by chronic renal or hepatic disease. Analyses of the pooled individual patients' data revealed that clearance maturation relating to body weight could be best described by the Hill function for sufentanil (R ² = 0.71, B _max 876 mL/min, K ₅₀ 16.3 kg) and alfentanil (R ² = 0.70, B _{max (fixed)} 420 mL/min, K ₅₀ 28 kg). The allometric exponent for estimation of clearance of sufentanil was 0.99 and 0.75 for alfentanil clearance. Maturation of remifentanil clearance was described by linear regression to bodyweight (R ² = 0.69). The allometric exponent for estimation of remifentanil clearance was 0.76. For fentanyl, linear regression showed only a weak correlation between clearance and bodyweight in preterm and term neonates (R ² = 0.22) owing to a lack of data in older age groups. A large heterogeneity regarding study design, clinical setting, drug administration, laboratory assays, and pharmacokinetic estimation was observed between studies introducing bias into the analyses performed in this review. A limitation of this review is that pharmacokinetic data, based on different modes of administration, dosing schemes, and parameter estimation methods, were combined.

The impact of extracorporeal life support and hypothermia on drug disposition in critically ill infants and children

Extracorporeal membrane oxygenation (ECMO) support is an established lifesaving therapy for potentially reversible respiratory or cardiac failure. In 10% of all pediatric patients receiving ECMO, ECMO therapy is initiated during or after cardiopulmonary resuscitation. Therapeutic hypothermia is frequently used in children after cardiac arrest, despite the lack of randomized controlled trials that show its efficacy. Hypothermia is frequently used in children and neonates during cardiopulmonary bypass (CPB). By combining data from pharmacokinetic studies in children on ECMO and CPB and during hypothermia, this review elucidates the possible effects of hypothermia during ECMO on drug disposition.

Less isoflurane is required after than before cardiopulmonary bypass to maintain a constant bispectral index value

OBJECTIVE

To test whether patients require less volatile anesthetic after cardiopulmonary bypass (CPB).

DESIGN

Prospective, observational clinical study.

SETTING

Cardiovascular operating rooms of a large teaching hospital.

PARTICIPANTS

Twenty adult patients undergoing surgery with CPB.

INTERVENTIONS

Subjects received a computer-controlled fentanyl infusion designed to maintain effect site concentrations of 3 ng/mL, combined with a variable amount of isoflurane.

MEASUREMENTS AND MAIN RESULTS

The end-tidal isoflurane concentration associated with a target bispectral index of 55 was recorded during skin preparation, after sternotomy, during rewarming, and after separation from CPB. Adjusted, geometric mean (95% confidence intervals), end-tidal isoflurane concentrations associated with a bispectral index of 55 were 0.46% (0.38% to 0.58%) during skin preparation, 0.47% (0.39% to 0.58%) after sternotomy, 0.35% (0.29% to 0.42%) during rewarming, and 0.36% (0.31% to 0.43%) after separation from CPB. The last 2 concentrations (recorded near the end and after CPB) were significantly (p < 0.05) less than the first 2 concentrations (recorded before CPB).

CONCLUSION

Because the level of surgical stimulation was relatively constant and minimal at the times of the measurements, these results are consistent with a reduced need for isoflurane after compared with before CPB.

Pharmacokinetic model-driven infusion of sufentanil and midazolam during cardiac surgery: assessment of the prospective predictive accuracy and the quality of anesthesia

OBJECTIVE

To evaluate the prospective predictive accuracy and the quality of anesthesia of pharmacokinetic model-driven infusion of sufentanil and midazolam designed to establish and maintain a plasma level of drug during cardiac surgery.

DESIGN

Prospective analysis.

SETTING

Operating room at a university hospital.

PARTICIPANTS

Twenty adult patients younger than 75 years old scheduled for valvular or coronary artery bypass graft surgery.

INTERVENTIONS

Patients were anesthetized using a variable predicted concentration of sufentanil (1 to 10 ng/mL) combined with a stable predicted concentration of midazolam (100 ng/mL).

MEASUREMENTS AND MAIN RESULTS

For each patient, arterial samples were taken before (6 samples), during (2 samples), and after (2 samples) cardiopulmonary bypass (CPB). Plasma sufentanil and midazolam concentrations were measured by specific radioimmunoassay and high-performance liquid chromatography techniques. Predicted sufentanil and midazolam concentrations were derived using the data sets of Gepts et al and Maitre et al. The predictive performance, the percentage prediction error (PE), and the absolute percentage error were calculated for each sample. The bias, inaccuracy, and dispersion were assessed by determining the median of the individual medians of the prediction errors (MDPE), the median of the individual median of the absolute prediction errors (MDAPE), and the 10th and 90th percentiles of PE. For midazolam, the inaccuracy was low (MDAPE < 21%), but CPB was associated with a dilution of the measured concentration associated with a negative bias. For sufentanil, the inaccuracy was also low before CPB (MDAPE = 18%) but increased during and after CPB (MDAPE > 40%). During the whole procedure, the hemodynamic control necessitated only a few interventions.

CONCLUSIONS

Pharmacokinetic model-driven infusion of sufentanil and midazolam using the pharmacokinetic sets of Gepts et al and Maitre et al is a safe and accurate anesthetic technique before CPB in adult patients undergoing cardiac surgery when high sufentanil (1 to 10 ng/mL) and low midazolam (100 ng/mL) predicted plasma concentrations are targeted.

[Target-controlled intravenous anesthesia]

Target-controlled infusion (TCI) is a new delivery system for i.v. anaesthetic agents with which the anaesthetist targets a plasma drug concentration to achieve a predetermined effect. With this system, the tedious task of calculating the amount of administered drug required to achieve the target concentration is left in charge of a microprocessor which commands the infusion device. TCI has long been used only by a few research teams, but this year a much wider field opens to this delivery system through marketing of Diprifusor, a TCI system specifically designed for administration of propofol in everyday practice. This article describes the rationale for administering i.v. agents through TCI delivery systems, the pharmacokinetic basis of TCI, the regulations and a broad overview of clinical applications, both recent and yet to come.

Aging increases pharmacodynamic sensitivity to the hypnotic effects of midazolam

The effect of aging on the pharmacodynamics of midazolam was investigated in a double-blinded study involving 39 consenting patients ranging in age from 39 to 77 yr. Midazolam was infused intravenously (i.v.) using a pharmacokinetic model-driven drug infusion device to achieve a plasma midazolam concentration that was held constant for the 10-min duration of the study. Blood samples were obtained from the radial artery at 5 and 10 min for subsequent measurement of the plasma midazolam concentrations. With the 10-min sample, the patients were also assessed for the presence or absence of responsiveness to verbal command. To ensure that the pharmacodynamic end-point was assessed under the condition of a relative steady-state effect-site midazolam concentration, only those patients (n = 33) in whom the plasma midazolam concentration at 10 min was within 30% of the measured concentration at 5 min were included in the subsequent data analyses. Logistic regression was used to fit the verbal command response/no response data to a mathematical model that included patient age and the plasma midazolam concentration measured at 10 min. Cp50, the steady-state plasma midazolam concentration at which 50% of patients would be expected not to respond to a specific stimulus (e.g., verbal command), was calculated as a function of age from the parameterized logistic model. The midazolam Cp50 for response to verbal command decreased significantly (P = 0.034) with increasing patient age, demonstrating that aging increases pharmacodynamic sensitivity to the hypnotic effects of midazolam independent of pharmacokinetic factors.

Predictive accuracy of midazolam in adult patients scheduled for coronary surgery

STUDY OBJECTIVE

To evaluate the predictive accuracy of midazolam during cardiac anesthesia so as to orient the selection of the most appropriate pharmacokinetic model for use in a computer-assisted continuous-infusion system.

DESIGN

Retrospective analysis.

SETTING

Operating room at a university hospital.

PATIENTS

66 consecutive middle-aged and elderly coronary patients scheduled for coronary artery bypass graft (CABG) surgery.

INTERVENTIONS

Patients were anesthetized using a variable-rate infusion of alfentanil combined with midazolam in an attempt to achieve and maintain target concentrations of 100 ng/ml or 500 ng/ml.

MEASUREMENTS AND MAIN RESULTS

A total of 323 arterial blood samples were taken, and serum midazolam concentrations were measured by high-performance liquid chromatography. Predicted midazolam concentrations were calculated using 3 selected data sets. Their bias, inaccuracy, and dispersion were assessed by determining the median performance error, the median absolute performance error (MDAPE), and the 10th and 90th percentiles. Two of the selected data sets of midazolam, with a clearance lower than 5 ml/kg/min, were very accurate (MDAPE less than 20%) in predicting low or high prebypass concentrations of midazolam in adult patients with good left ventricular function.

CONCLUSIONS

Two of the 3 pharmacokinetic data sets of midazolam studied may be selected when using a computer-assisted infusion system in adult coronary patients.

Midazolam and fentanyl continuous infusion anesthesia for cardiac surgery: a comparison of computer-assisted versus manual infusion systems

Continuous infusion of intravenous anesthetics can be achieved either by a manually controlled infusion (MCI) pump, or by a computer-assisted continuous infusion (CACI) pharmacokinetic model-driven infusion system. Randomized double-blind comparisons of the two infusion systems for general anesthesia were performed in 24 patients undergoing coronary artery bypass grafting. Patients were allocated to receive continuous infusions of midazolam and fentanyl by either a MCI device or CACI. Midazolam and fentanyl infusions were independently titrated to maintain hemodynamic stability, defined as mean arterial pressure (MAP) and heart rate (HR) within 20% of baseline values. As directed by the study design, comparable hemodynamic control was achieved in both groups. Mean plasma fentanyl concentrations measured at specific timepoints were similar between groups. The plasma midazolam level for induction was 196 +/- 139 ng/mL in the CACI group and 300 +/- 128 ng/mL in the MCI group, and the fentanyl level was similar in both groups, 6.7 +/- 1.9 ng/mL in CACI and 6.3 +/- 4.6 ng/mL in the MCI group. The drug levels were lower (P < or = .05) for midazolam during maintenance of anesthesia and similar for fentanyl during the maintenance of anesthesia. In the MCI group, the average duration of anesthesia was 246.5 +/- 35.0 minutes, with a mean total fentanyl dose of 30.27 +/- 11.14 micrograms/kg. In the CACI group, the average duration of anesthesia was 230.8 +/- 44.1 minutes, with a mean total fentanyl dose of 34.61 +/- 5.40 micrograms/kg (P > 0.05 for comparisons between groups for duration of anesthesia and total fentanyl dose).(ABSTRACT TRUNCATED AT 250 WORDS)

[Computer-assisted intravenous anesthesia: value, method and use]

Total intravenous anaesthesia (TIVA) is becoming increasingly popular among anaesthetists. It has several advantages, namely each component of the anaesthetic protocol can be independently controlled, and the operating room remains unpolluted with nitrous oxide or volatile anaesthetic agents. TIVA aims to maintain a constant blood concentration of each anaesthetic agent. This means that infusion rates need to be repeatedly altered. A computer calculates theoretical blood concentrations of agent according to a pharmacokinetic model, and drives an infusion device. Only a few programmes have been developed by research teams. No commercial device is available as yet. However, there are several syringe pumps and volumetric pumps which are accurate enough for use in TIVA and which may be controlled by computer. Clinical studies have shown the benefits of TIVA: greater haemodynamic stability, decreased drug consumption, more rapid recovery, and a lesser need for postoperative ventilatory support. The most appropriate agents are propofol and etomidate as hypnotics, alfentanil and sufentanil for opioids, vecuronium and atracurium as muscle relaxants. Etomidate is not recommended for prolonged infusions, because of the risk of adrenocortical suppression. TIVA seems to be attractive for neurosurgery, thoracic surgery, day case surgery, endoscopic procedures, and anaesthesia in remote locations. Unfortunately, it is an expensive technique. Moreover, there is considerable interpatient variability of the drug concentration required for a same clinical effect. Two methods are proposed to decrease this variability: population pharmacokinetic models and Bayesian forecasting. Closed loop systems are still research tools. It is concluded that computer-driven anaesthesia is the equivalent to the vaporizer for volatile agents. However, further clinical studies are needed to determine whether the advantages of this technique outweigh its disadvantages.

Development of a real-time algorithm for predicting sufentanil plasma levels during cardiopulmonary-bypass surgery using a systems approach

During cardiopulmonary-bypass (CB) procedures, anesthesiologists have traditionally based the administration of narcotics on general dosage recommendations and past experience. Initial doses are usually based on body weight and supplemental amounts are given in anticipation of, or in response to, the effects of surgical stimuli. There has been considerable recent interest in using the population pharmacokinetics of narcotics to optimize the attainment and maintenance of drug plasma concentrations at analgesic target levels which will blunt the hemodynamic responses to noxious stimuli. Moreover, the undershooting or overshooting of the target can be reduced by application of these principles making drug administration more effective and safer. The present study concerns the development of a model for the computer-guided administration of sufentanil throughout surgical procedures involving CB; there is a paucity of studies which have attempted to model the pharmacokinetics of drugs during CB because of the lack of information on the effects of bypass conditions on the pharmacokinetic parameters. We have attempted to approach the effects of hypothermia on sufentanil clearance by applying a continuous temperature correction to the ultimate elimination rate constant (the terminal eigenvalue). This correction is based primarily on the anticipated effects of temperature on the enzyme-catalyzed reactions which are essential for the elimination of drug from the body. An algorithm for the application of the model is also presented.

Anesthesia for Cardiac Surgery in Newborns with Congenital Heart Disease

The repair of complex congenital heart disease in the neonatal period is an evolving practice that began a decade ago and has resulted in improved survival and reduced long term morbidity. Surgical techniques and anesthetic, management hare evolved to care for these uniquely challenging infants. Optimal anesthetic care requires a sound basis in normal and pathologic neonatal and dedopmental physiology, cardiac and pediatric anesthesia, and cardiopulmonary bypass management. Typical concerns in neonatal congenital heart patients include a reduced myocardial compliance and poorly

developed calcium transport systems. This limits stroke volume and increases calcium requirements in the post-bypass patient. Additionally, abnormal blood flow patterns are common and may be adversely affected by inotropes, inspired oxygen concentration, carbon dioxide tension, and mechanical ventilation. The impact of cardiopulmonary bypass on the neonate is extreme. Infants are commonly cooled to 15°C, their blood volume is hemodiluted by 3-4 fold, and periods of total circulatory arrest for up to 60 min are routine. The physiologic impact of these biologic extremes on organ function must be anticipated. Bypass management must assure optimal distribution of pump flow to minimize any impediment to organ protection. High dose narcotics combined with a benzodiazepine have become an integral part of anesthetic management for the neonate with congenital heart disease. Combined intravenous drug therapy provides a complete anesthetic (sedation amnesia and analgesia), hemodgnamic stability, reduced stress hormone release, and control of pulmonary vascular reactivity. Thii reduces the need for multiple inotropic therapy aimed at decreasing pulmonary vascular resistance and improving myocardial contractility. This manuscript provides an in-depth understanding of the anesthetic management of the neonate with complex congenital heart disease. The overall goal of anesthetic management is to deliver a patient to the ICU with stable hemodynamics and a clear management plan based on intraoperative observations and interventions.