What setting indicates a patent ductus arteriosus with a right to left shunt based on pre and post-ductal blood gas studies?

A patent ductus arteriosus (PDA) with a right-to-left shunt can significantly affect oxygenation as the shunt allows deoxygenated blood to move from the right heart to the systemic circulation, resulting in lower oxygen levels in the blood that is measured post-ductally (after passing through the ductus arteriosus) compared to the pre-ductal blood (before passing through the ductus).

In the scenario where the pre-ductal PaO2 is 15 torr higher than the post-ductal PaO2, this indicates a significant right-to-left shunt, as it demonstrates that the pre-ductal blood (which has not mixed with deoxygenated blood from the right side of the heart) is receiving a greater amount of oxygen compared to the post-ductal blood. This difference reflects the mixing of oxygenated and deoxygenated blood resulting from the shunt, as evidenced by the lower levels of oxygen in the post-ductal measurements.

In essence, the substantial decrease in post-ductal PaO2 compared to pre-ductal PaO2 explicitly points to the pathology of the right-to-left shunt associated with the PDA, which is the key factor in interpreting these