This was an observational study conducted at the BC Women’s and Children’s Health Centre, a quaternary referral NICU, between January 2013 and December 2014 (retrospective) and from January 2015 to June 2016 (prospective). Population-based data of the total number of very low birth weight (VLBW) infants born at <34 weeks of gestation, including inborn and outborn, were obtained from the local neonatal database. All VLBW infants without any major congenital malformations or cardiac anomalies, except for PDA or patent foramen ovale, who were admitted to our NICU were enrolled in the study (Figure 1). A retrospective chart review was performed for all infants who underwent PDA ligation during Epoch I (January 2013 to December 2014), whereas prospective data were collected during Epoch II (January 2015 to June 2016). A dedicated comprehensive TNE program was introduced to preoperatively assess the PDA’s hemodynamic significance and to triage patients for ligation starting in January 2015. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the University of British Columbia Children and Women’s Research Ethics Board (No. H15-01343) and individual consent for this study was waived as data were already being collected for standard clinical care purposes (secondary use of data).