Pharmacodynamic Analysis of Morphine Time-to-Remedication Events in Infants and Young Children After Congenital Heart Surgery.

OBJECTIVE: The aim of this study was to characterize the relationship between morphine plasma concentration and repeated time to postoperative remedication events in children undergoing cardiac surgery. METHODS: Data from our previously published study of morphine pharmacokinetics were utilized in this pharmacodynamic study. A population survival analysis based on hazard functions was undertaken in NONMEM(®). RESULTS: Hazard was best described by a Gompertz function changing in steps over time. Concentration and age were the only predictors of the hazard function. Concentration producing 50 % reduction in hazard was 19.6 (bootstrap 95 % confidence interval 5.90-49.5 ng/ml). The hazard ratio for a 1-year-old child to a 1-month-old child was 1.91 (1.35-2.86). Sensitivity to morphine decreased with age and leveled off after 1-year of life. Morphine sulfate doses >0.1 mg/kg did not noticeably increase tolerable pain durations. CONCLUSION: Time to remedication is a clinically useful endpoint for assessing opioid-induced analgesia. Sensitivity to morphine treatment is age-dependent. Morphine sulfate doses of 0.1-0.2 mg/kg are adequate for the management of postoperative pain in children. Our findings may help avoid unnecessary large morphine doses in children.

Pharmacokinetics of Morphine and Its Metabolites in Infants and Young Children After Congenital Heart Surgery.

The objective of this study was to characterize morphine glucuronidation in infants and children following cardiac surgery for possible treatment individualization in this population. Twenty children aged 3 days to 6 years, admitted to the cardiovascular intensive care unit after congenital heart surgery, received an intravenous (IV) loading dose of morphine (0.15 mg/kg) followed by subsequent intermittent IV bolus doses based on a validated pain scale. Plasma samples were collected over 6 h after the loading dose and randomly after follow-up doses to measure morphine and its major metabolite concentrations. A population pharmacokinetic model was developed with the non-linear mixed effects software NONMEM. Parent disposition was adequately described by a linear two-compartment model. Effect of growth (size and maturation) on morphine parameters was accounted for by allometric body weight-based models. An intermediate compartment with Emax model best characterized glucuronide concentrations. Glomerular filtration rate was identified as a significant predictor of glucuronide formation time delay and maximum concentrations. Clearance of morphine in children with congenital heart disease is comparable to that reported in children without cardiac abnormalities of similar age. Children 1-6 months of age need higher morphine doses per kilogram to achieve an area under concentration-time curve comparable to that in older children. Pediatric patients with renal failure receiving morphine therapy are at increased risk of developing opioid toxicity due to accumulation of morphine metabolites.

Pharmacokinetics of prophylactic cefazolin in parturients undergoing cesarean delivery.

The objectives of this work were (i) to characterize the pharmacokinetics of cefazolin in pregnant women undergoing elective cesarean delivery and in their neonates; (ii) to assess cefazolin transplacental transmission; (iii) to evaluate the dosing and timing of preoperative, prophylactic administration of cefazolin to pregnant women; and (iv) to investigate the impact of maternal dosing on therapeutic duration and exposure in newborns. Twenty women received 1 g of cefazolin preoperatively. Plasma concentrations of total cefazolin were analyzed from maternal blood samples taken before, during, and after delivery; umbilical cord blood samples obtained at delivery; and neonatal blood samples collected 24 h after birth. The distribution volume of cefazolin was 9.44 liters. [corrected] The values for pre- and postdelivery clearance were 7.18 and 4.12 liters/h, respectively. Computer simulations revealed that the probability of maintaining free cefazolin concentrations in plasma above 8 mg/liter during scheduled caesarean surgery was <50% in the cord blood when cefazolin was administered in doses of <2 g or when it was administered <1 h before delivery. Therapeutic concentrations of cefazolin persisted in neonates >5 h after birth. Cefazolin clearance increases during pregnancy, and larger doses are recommended for surgical prophylaxis in pregnant women to obtain the same antibacterial effect as in nonpregnant patients. Cefazolin has a longer half-life in neonates than in adults. Maternal administration of up to 2 g of cefazolin is effective and produces exposure within clinically approved limits in neonates.