<qa>  What is the study population intubated and mechanically ventilated?The purpose of this study is to determine the absorption and bioavailability of codeine in relation to postnatal PNA and postconceptional PCA age; determine the parent drug codeine, its active metabolites, their formation rates and their ratios in relation with PCA and PNA; and identify relevant genetic polymorphisms of opioid metabolism in the study population and their potential relationship to the biodisposition and pharmacodynamic effects of codeine. The study population is intubated and mechanically ventilated infants equal to or greater than 26 weeks gestational age at birth and less than 4 weeks postnatal age. 