Final answer:
Babies with PDA are non-cyanotic because of a left-to-right shunt which increases pulmonary blood flow allowing for proper oxygenation of blood.
Step-by-step explanation:
Babies with a patent ductus arteriosus (PDA) are non-cyanotic primarily due to a left-to-right shunt. This occurs when blood flows from the higher pressure aorta into the lower pressure pulmonary artery, which leads to increased pulmonary blood flow rather than decreased flow. This is the opposite of a right-to-left shunt, which would cause poorly oxygenated blood to bypass the lungs and enter systemic circulation, resulting in cyanosis. In PDA, the shunt allows more blood to reach the lungs where it can become oxygenated, helping maintain a normal oxygen saturation in the blood and preventing cyanosis.