What does the failure of cyanosis and hypoxia to improve with 100% oxygen suggest in an infant?

The failure of cyanosis and hypoxia to improve with 100% oxygen in an infant strongly suggests a congenital heart defect. In cases of congenital heart defects, especially those that result in significant shunting of blood (like a right-to-left shunt), oxygen therapy may not alleviate hypoxemia. This is because the circulatory system is bypassing the lungs, either due to structural anomalies such as tetralogy of Fallot or transposition of the great arteries.

In contrast, conditions like respiratory distress syndrome, pneumonia, or cystic fibrosis typically involve oxygenation issues that can be improved with supplemental oxygen. In these cases, oxygen therapy enhances gas exchange in the lungs; thus, the infant's hypoxic state would improve as the lungs function to oxygenate the blood effectively. The persistence of hypoxia despite high-flow oxygen delivery is a hallmark of congenital heart problems, indicating that the issue lies within the circulatory system rather than purely in the lungs.