Echocardiographic examinations (Vivid S6, GE Healthcare, Chicago, USA) were performed daily after birth by a pediatric cardiologist or a neonatologist. Doppler samples of PDA flow were obtained using two-dimensional echocardiographic imaging. The velocity–time integral (VTI) of each of the left-to-right and right-to-left flows was calculated. We defined the LR ratio as the percentage of VTI of the left-to-right flow of PDA against the overall VTI consisting of left-to-right and right-to-left flows (Fig. 1). The LR ratio of > 50% was considered as “left-to-right dominant.” When the PDA had already been closed, the LR ratio was set at 100% for the purpose of convenience.

Quantification of patent ductus arteriosus (PDA) flow. The LR ratio is defined as the percentage of velocity–time integral (VTI) of the left-to-right (LR) flow of PDA against the overall VTI containing the LR and right-to-left (RL) flows on echocardiography

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