What indicates a patent ductus arteriosus with a right to left shunt based on PaO2 measurements?

A patent ductus arteriosus (PDA) that results in a right-to-left shunt typically leads to a lower post-ductal partial pressure of oxygen (PaO2) compared to the pre-ductal values. This phenomenon occurs because the right-to-left shunt allows deoxygenated blood from the pulmonary circulation to bypass the lungs and mix with the oxygenated blood returning to systemic circulation.

When there is a significant right-to-left shunt through a PDA, the blood that is directly delivered to the systemic circulation lacks sufficient oxygen due to its mixing with poorly oxygenated blood from the right heart. Consequently, if we measure oxygen levels at different anatomical sites (like the pre-ductal site, usually the right arm, and the post-ductal site, generally from the lower body), we would observe that the post-ductal PaO2 would be lower than the pre-ductal PaO2.

In this context, if the pre-ductal PaO2 is 15 torr higher than that of the post-ductal PaO2, it specifically indicates that a significant amount of deoxygenated blood is entering the systemic circulation as a result of the right-to-left shunt. Thus, the difference