In the congenital heart screen, pulse oximetry compares oxygen saturation between which locations?

• A. Left hand and left foot
• B. Right hand and right foot (pre- and post-ductal)
• C. Right hand and left foot
• D. Umbilical vessels

Answer: (B)

Pulse oximetry screening for congenital heart disease hinges on comparing oxygen saturation before and after the ductus arteriosus has any influence on circulation. The right hand is used as the pre-ductal site because its arterial blood comes from vessels that branch from the aorta before the ductus arteriosus connects to the descending aorta. The post-ductal site is usually a leg, such as the foot, because that blood has circulated after potential mixing through the ductus. In a healthy newborn, these two readings are similar; a meaningful difference or an unexpectedly low saturation suggests a ductal-dependent or cyanotic heart defect and requires further evaluation. Using umbilical vessels isn’t relevant to this postnatal pre/post comparison, and other site pairings don’t consistently reflect pre- versus post-ductal blood.