Babies who were recruited had blood samples collected on days 1, 2, 3 and 7 for measurement of Nt pro-BNP. Each baby also had an echocardiogram performed between the fifth and seventh day of life. Decisions about treatment of the duct were made by attending physicians independent of the study. Physicians, investigators and echocardiographers were blinded to knowledge of the Nt pro-BNP concentration. Nt pro-BNP was also measured before and after treatment in all babies who had a PDA treated and echocardiography performed to confirm closure.