In a newborn with a patent ductus arteriosus that remains open after birth, which shunt direction becomes more prominent as pulmonary vascular resistance falls?

• A. Right-to-left
• B. Bidirectional
• C. No shunt
• D. Left-to-right

Answer: (D)

As newborn circulation stabilizes, the key factor determining flow through a patent ductus arteriosus is the pressure gradient between the systemic (aorta) and pulmonary (artery) circuits. Immediately after birth, pulmonary vascular resistance is high, so the gradient can be small or even favor flow toward the pulmonary artery. As PVR falls, the aortic pressure remains higher than the now lower pulmonary artery pressure, driving blood from the left side of the heart into the pulmonary circulation via the PDA. This creates a left-to-right shunt, increasing pulmonary blood flow and potentially leading to signs of pulmonary overcirculation if the PDA is large. The pattern would be right-to-left only if pulmonary pressures stayed elevated or surpassed systemic pressures, and a bidirectional or no-shunt pattern would occur under different hemodynamic circumstances. Thus, left-to-right shunting becomes more prominent as pulmonary vascular resistance falls.