Pulse oximetry screening is a simple, non-invasive procedure performed on newborns to check for Critical Congenital Heart Disease (CCHD). The test measures oxygen saturation (“sats” or SpO2) using sensors taped to the baby’s skin. This routine check identifies life-threatening heart defects that might otherwise go unnoticed before discharge. CCHD affects two to three out of every 1,000 live births and often requires intervention within the first year of life. Taking two separate saturation measurements leverages differences in blood circulation characteristic of certain cardiac defects.
The Role of the Ductus Arteriosus in Neonatal Circulation
The use of two separate measurements relates directly to the newborn’s circulatory transition from fetal life to independent breathing. Before birth, the lungs are not used for oxygen exchange, and the fetal circulatory system employs the Ductus Arteriosus (DA). The DA acts as a shunt, connecting the pulmonary artery directly to the aorta, allowing blood to bypass the inactive lungs and flow into the systemic circulation.
When a baby takes their first breaths, the lungs inflate, pulmonary vascular resistance drops, and blood flow is redirected to the lungs for oxygenation. This rapid change in pressure signals the Ductus Arteriosus to begin closing. Functional closure of the DA typically occurs within the first 12 to 24 hours of life in healthy infants.
In newborns with certain types of CCHD, the DA may remain open (Patent Ductus Arteriosus), or it may be structurally necessary to sustain life. If a heart defect causes higher blood pressure on the right side, deoxygenated blood can be forced through the open DA into the aorta. This phenomenon, called right-to-left shunting, means blood with lower oxygen content enters the systemic circulation after a specific branching point. Detecting this mixing is the reason for taking both pre-ductal and post-ductal measurements.
Identifying Pre-Ductal and Post-Ductal Measurement Sites
To detect right-to-left shunting, the screening requires one measurement before the Ductus Arteriosus (DA) insertion and one after it. The pre-ductal measurement is the oxygen saturation in the blood supply that has not yet mixed with deoxygenated blood from the DA shunt. This reading is taken on the Right Upper Extremity, specifically the right hand. The artery supplying the right arm is the first major vessel to branch off the aorta, originating proximal to the DA connection.
The post-ductal measurement is taken from a location where the blood supply would be affected by right-to-left shunting through a patent DA. This reading is taken on the Lower Extremity, specifically on either foot. The arteries supplying the lower body branch off the aorta distal to the DA insertion point, meaning they receive the mixed, potentially lower-oxygen blood. Comparing the saturation values from the right hand and the foot allows clinicians to identify a significant difference suggesting a problem with blood flow or oxygenation.
Interpreting Results and Screening Criteria
The comparison of pre-ductal (right hand) and post-ductal (foot) oxygen saturation values determines the screening outcome. Current guidelines define three results: a Pass, a Fail (indicating immediate further evaluation), or a Re-screen (requiring a repeat test). A newborn receives a Pass if the SpO2 is 95% or greater in both the right hand and the foot. The difference between the two measurements must also be small, typically 3 percentage points or less.
The screening results are considered Borderline or require a Re-screen if the saturation is between 90% and 94% in both limbs, or if the difference between the hand and foot is greater than 3 percentage points. In such cases, the test is repeated, usually after one hour, allowing the newborn’s circulation time to stabilize. A large saturation difference, even if both values are above 95%, suggests right-to-left shunting that warrants careful monitoring.
An immediate Fail result occurs if the oxygen saturation is less than 90% in either the right hand or the foot on any single reading. A fail also occurs if the saturation remains in the borderline range (90% to 94%) or the difference remains high after up to three sequential screenings. Any failure necessitates an immediate referral for a comprehensive medical evaluation, typically including an echocardiogram to rule out CCHD.