What is a key diagnostic criterion for persistent pulmonary hypertension of the newborn (PPHN)?

A key diagnostic criterion for persistent pulmonary hypertension of the newborn (PPHN) is the gradient of greater than 10% between preductal and postductal arterial blood gas measurements. This gradient indicates a significant right-to-left shunting of blood, which is characteristic of PPHN. In a healthy newborn, there should be little to no difference between the oxygen saturation levels measured preductally (in the right arm) and postductally (in the lower extremities) due to normal pulmonary vascular resistance and proper oxygenation.

In PPHN, the abnormal persistence of high pulmonary vascular resistance leads to a right-to-left shunt through the ductus arteriosus or foramen ovale, resulting in decreased oxygen saturation in the lower parts of the body compared to the upper body. This marked difference reflects the inadequate oxygenation and is therefore a vital diagnostic marker for PPHN.

The other options do not serve as direct diagnostic indicators for PPHN. For instance, low oxygen saturation in the mother pertains more to maternal conditions rather than the newborn's pulmonary status. The presence of structural heart disease can be a contributing factor to pulmonary hypertension but is not a standalone criterion for diagnosing PPHN itself, which is specifically about the pulmonary