The relationship between age and morphine infusion rate in children

AIM: We performed a retrospective audit of intravenous morphine infusion administered to children in an effort to characterize the relationship between dose and age. METHODS: A retrospective audit of morphine infusions was reviewed for a 24-months period and included all children who received continuous intravenous nurse-controlled morphine infusions and patient-controlled analgesia; a population undergoing acute and elective surgical procedures, as well as medical and oncological treatments. The relationship between age and infusion rate was investigated using nonlinear mixed effects models. RESULTS: There were 886 children whose data were acceptable for review. Morphine dose increased with age from 9.97 (CV 28%) μg · kg(-1) per h in neonates. The Hill equation with an exponential of 1.5 best described these changes. Morphine rate reached 90% of its mean final rate of 22.5 (CV 167%) μg · kg(-1) per h, observed in teenagers, at approximately 5 years of age. There was considerable uncertainty of this age-morphine rate profile, and the maturation half-life of this profile was 20 months of age (CV 632%). An increase in dosing variability was observed with increasing age. CONCLUSIONS: Morphine infusions at steady-state did not mirror clearance maturation in children nursed in our hospital. We suggest that initial infusion rates in children are started at 10 μg · kg(-1) per h in neonates, 15 μg · kg(-1) per h in toddlers and 25 μg · kg(-1) per h in children above the age of 5 years. The large variability associated with infusion rates means that subsequent infusion rates will depend on feedback from pain scores, adjuvant medications and adverse effects.