The relationship between age and morphine infusion rate in children

We performed a retrospective audit of intravenous morphine infusion administered to children in an effort to characterize the relationship between dose and age.

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.

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.

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.