Pharmacokinetics of cisatracurium in patients receiving nitrous oxide/opioid/barbiturate anesthesia

BACKGROUND

Cisatracurium, one of the ten isomers in atracurium, is a nondepolarizing muscle relaxant with an intermediate duration of action. It is more potent and less likely to release histamine than atracurium. As one of the isomers composing atracurium, it presumably undergoes Hofmann elimination. This study was conducted to describe the pharmacokinetics of cisatracurium and its metabolites and to determine the dose proportionality of cisatracurium after administration of 2 or 4 times the ED(95).

METHODS

Twenty ASA physical status 1 or 2 patients undergoing elective surgery under nitrous oxide/opioid/barbiturate anesthesia were studied. Patients received a single rapid intravenous bolus does of 0.1 or 0.2 mg x kg-1 (2 or 4 times the ED(95), respectively) cisatracurium. All patients were allowed to recover spontaneously to a train-of-four ratio > or = 0.70 after cisatracurium-induced neuromuscular block. Plasma was extracted, acidified, and stored frozen before analysis for cisatracurium, laudanosine, the monoquaternary acid, and the monoquaternary alcohol metabolite.

RESULTS

The clearances (5.28 +/- 1.23 vs. 4.66 +/- 0.67 ml x min(-1) x kg(-1) and terminal elimination half-lives (22.4 +/- 2.7 vs. 25.5 +/- 4.1 min) were not statistically different between patients receiving 0.1 mg x kg(-1) and 0.2 mg x kg(-1), respectively. Maximum concentration values for laudanosine averaged 38 +/- 21 and 103 +/- 34 ng x ml(-1) for patients receiving the 0.1 and 0.2 mg x kg(-1) doses, respectively. Maximum concentration values for monoquaternary alcohol averaged 101 +/- 27 and 253 +/- 51 ng x ml(-1), respectively. Monoquaternary acid was not quantified in any plasma sample.

CONCLUSIONS

Cisatracurium undergoes Hofmann elimination to form laudanosine. The pharmacokinetics of cisatracurium are independent of dose after single intravenous doses of 0.1 and 0.2 mg x kg(-1).

Estimation of the hepatic extraction ratio of indocyanine green in swine

1. The hepatic extraction ratio of Indocyanine Green was measured directly by trans-hepatic catheterization in 14 outbred swine (eight well-fed, six malnourished). A specific two-compartment pharmacokinetic model was fitted to the arterial Indocyanine Green concentration-time data and used to estimate the hepatic extraction ratio of Indocyanine Green. 2. The specific two-compartment pharmacokinetic model was modified to represent more accurately the physiological process of Indocyanine Green removal. Simulations were performed using this new model to estimate the hepatic extraction ratio of Indocyanine Green in the swine. 3. Similarly to previous findings, our data showed that the original model consistently overestimated the hepatic extraction ratio of Indocyanine Green (i.e. the model estimate was compared with the true, directly measured value). 4. The comparison of the modified model and the original model clearly indicates the reason for the overprediction of the hepatic extraction ratio of Indocyanine Green by the latter. The simulations using the new model indicate that the percentage of binding of Indocyanine Green to its transport protein (glutathione S-transferase) for removal in the bile will affect the estimation of the hepatic extraction ratio of Indocyanine Green. Thus, the amount of Indocyanine Green available for removal is less than that assumed by the original model.

Population pharmacokinetics: development of a medical intensive care unit-specific gentamicin dosing nomogram

OBJECTIVE

This study was designed to develop a population-specific dosing nomogram for gentamicin in medical intensive care unit (MICU) patients using the population pharmacokinetic program nonparametric expectation maximization (NPEM).

DESIGN

Observational clinical gentamicin dosing data were collected, entered into the USC*PACK database program PASTRX, and downloaded into the population pharmacokinetic program NPEM. NPEM generated population pharmacokinetic parameter values that were used to develop a gentamicin dosing nomogram. The nomogram was tested in the next 15 patients admitted to MICU to determine accuracy. Doses given per the MICU and the Hull-Sarubbi nomograms were compared with doses based on actual patient-specific pharmacokinetic parameter values. Reliability coefficients (intraclass correlation coefficients) were calculated to assess the agreement between observations.

SETTING

Data were gathered from patients receiving gentamicin therapy in the MICU, Presbyterian University Hospital, Pittsburgh.

PATIENTS

Baseline population pharmacokinetic parameter values were determined in 36 MICU patients receiving gentamicin therapy. Patients with renal failure receiving hemodialysis or another mechanical method of blood clearance or fluid removal were excluded. The population parameter values in the form of a dosing nomogram were then used prospectively to dose gentamicin in 15 patients.

RESULTS

NPEM generated population parameter values similar to those previously published using the Sawchuk-Zaske method in ICU patients. The mean volume of distribution generated using NPEM was 0.34 +/- 0.12 L/kg. The relationship between creatinine clearance (Clcr) and elimination rate constant (Ke) was: Ke = 0.00218 x Clcr + 0.007. The nomogram-derived doses correlated with doses determined by using actual patient-specific pharmacokinetic values (p < 0.05). The Hull-Sarubbi derived doses, however, did not correlate with patient-specific doses (p > 0.05). Only one patient had a peak concentration < 6 mg/L. Two of 15 patients had trough concentrations prior to the first maintenance dose > 2 mg/L.

CONCLUSIONS

The use of NPEM to generate population-specific pharmacokinetic parameter values has been previously described. Application of population-specific dosing nomograms can improve initial dosing regimens such that conventional therapeutic concentrations can be achieved early in therapy. This nomogram, however, does not preclude follow-up patient-specific pharmacokinetic analysis.

Population pharmacokinetics of gentamicin. Use of the nonparametric expectation maximisation (NPEM) algorithm

A new nonparametric expectation maximisation (NPEM) algorithm for the estimation of population pharmacokinetic parameter values was evaluated. The algorithm, in the form of a personal computer program, was used to compute population pharmacokinetic parameter densities of gentamicin in a group of 9 patients with indicators of malnutrition. The 1-compartment parameter values for clearance (CL), volume of distribution (Vd) and elimination rate constant (k) were compared with values generated using a standard 2-stage (STS) approach. NPEM was used with a full data set (72 gentamicin concentrations) and a sparse data set (only peak and trough concentrations for each patient; 18 in total). There were no differences in parameter value estimations between the STS and NPEM with all the data (p greater than 0.05) or with the sparse data (p greater than 0.05). Mean parameter value estimates from the STS and NPEM (with sparse data) were used as a priori data sets in the USC*PACK gentamicin Bayesian program to predict concentrations in 8 subsequent patients with similar indicators of malnutrition. There were no differences in predicted gentamicin concentrations between STS (3.75 +/- 2.06 mg/L) and NPEM (3.75 +/- 2.17 mg/L). NPEM was able to generate population pharmacokinetic parameter values for gentamicin in a defined population of patients using sparse routine clinical data. It was also shown to function with only a single data point per patient.

Effect of nutritional status on organic anion clearance by the swine liver

Hepatic dysfunction follows a wide range of insults. Impaired excretion of organic dyes such as bilirubin often occurs before other obvious clinical defects in metabolic processes. Indocyanine green (ICG) is excreted through pathways similar to those of bilirubin. To determine the effectiveness of ICG as a marker of hepatic dysfunction related to clinical malnutrition, pigs received 5 mg/kg ICG with simultaneous sampling from the hepatic vein, pulmonary artery, and aorta over 3 hours. Group I remained well nourished, group II was fasted to a weight loss equal to 20% of initial body weight, and group III was fasted to a 20% weight loss and then refed until the animals regained their initial weight. Both systemic and intrinsic hepatic clearance were depressed significantly with fasting but returned above baseline after refeeding. No significant difference appeared between systemic and intrinsic hepatic clearance. Extraction ratios were low in all groups. In outbred swine, ICG clearance reflects the function of hepatic organic anion excretion in vivo, and venous sampling reflects intrinsic hepatic clearance. The impairment of the carrier-mediated transport system is reversible with refeeding.

Therapeutic drug monitoring. Practical considerations for primary care physicians

Primary care physicians who understand the complexities of drug dosing may be able to provide their patients with more effective pharmacologic therapy. Standard or empirical methods of dosing are appropriate for most agents. The evolution of pharmacokinetics and clinically useful drug assays has led to pharmacokinetic dosing, a more sophisticated and exact method of dosing certain agents.