129 Morphine and Morphine-6-Glucuronide Serum Concentrations and their Correlation with Pain Responses

Abstract

The aim of the study was to assess morphine and morphine-6-glucuronide (M6G) serum concentrations in preterm neonates requiring mechanical ventilation after continuous morphine infusion during first days of life. We also examined correlation between morphine, its metabolite and pain scores. 14 preterm neonates were randomly assigned. Neonates (mean birth weight 1196g (780 to 1760g), mean gestational age 29 weeks (26 to 33weeks) received morphine for 1 to 5 days (100mcg/kg over 30 minutes followed by 20mcg/kg/h). Blood samples (3–4 samples) were obtained from neonates at 0, 30 minutes, 6 and 24 hours after the start of morphine infusion (at a median postnatal age 4 hours) and 24 hours after discontinuation of infusion. The pain responses were assessed using two scales- Premature Infant Pain Profile (PIPP) and Comfort Scale (CS) at 6 and 24 hours. M6G was found in all samples, even after 30 minutes of morphine infusion. In most samples M6G concentration exceeded morphine concentration. Mean (SD) morphine clearance was 5.5(3.0)ml/min/kg. There was a significant negative correlation between morphine plasma concentration and Comfort Scale scores at 6 hour of the study (r=-0.72, p=0.003). There was no correlation between M6G and pain scores. Clearance value was similar to values published by other authors. Finding correlation only between morphine concentration and CS at 6th hour of the study confirms sophisticated problem of pain assessment.