The maximal velocity across the PDA is denoted as V (PDA) max. This method has been validated by invasive measurements in cardiac catheterization ( 34 ). The flow and velocity across the PDA are dependent on the pressure between the aorta and the main pulmonary artery. Right to left shunting across the PDA indicates SPAP higher than the aortic pressure (Figure ​ (Figure6). 6 ). Left to right shunting across the PDA indicates lower SPAP compared to the aortic pressure. Bidirectional shunting across the PDA is a common finding in newborns until the pulmonary vascular resistance (PVR) has decreased (Figure ​ (Figure6 6 ).