Pulse oximetry screening, often called Critical Congenital Heart Disease (CCHD) screening, is a standard, non-invasive method used globally to assess newborn health. This simple test is performed using a light-emitting sensor placed on the baby’s skin to measure the amount of oxygen in the blood, expressed as a percentage of saturation. The goal of this screening is to identify babies with specific, serious heart defects before they show any outward symptoms, allowing for timely medical intervention. The screening is typically conducted when the newborn is at least 24 hours old, which allows the circulatory system to complete its initial transition from fetal life.
What Pre- and Post-Ductal Measurements Represent
The screening involves taking two simultaneous measurements on different parts of the baby’s body to compare oxygen levels before and after a specific circulatory connection. The “pre-ductal” measurement is taken on the right hand, specifically the wrist or palm. This site receives blood flow from the aorta before it encounters the ductus arteriosus, reflecting the highest level of oxygenated blood supplied to the upper body, including the brain. The “post-ductal” measurement is taken on either of the baby’s feet, representing circulation after the ductus arteriosus connects to the aorta. This dual-site measurement is performed because the ductus arteriosus, a temporary blood vessel, can potentially mix poorly oxygenated blood into the lower body’s circulation.
The Transition of Newborn Circulation
A baby’s circulation undergoes dramatic changes immediately after birth as the lungs take over the function of gas exchange from the placenta. In the womb, the ductus arteriosus serves a necessary purpose by acting as a shunt, diverting most of the blood away from the fetus’s non-functioning lungs directly into the aorta. This allows highly oxygenated blood to preferentially supply the upper body and brain.
Once the umbilical cord is clamped and the baby takes its first breath, the change in pressure and the sudden increase in blood oxygen levels trigger the functional closure of the ductus arteriosus. This process typically begins within hours of birth and is usually complete within the first few days of life for healthy infants. If the ductus arteriosus fails to close as expected, remaining “patent,” it can allow deoxygenated blood to flow from the pulmonary artery into the descending aorta.
This abnormal flow is known as a right-to-left shunt, causing poorly oxygenated blood to mix with the systemic circulation supplying the lower body. Because the pre-ductal site (right hand) receives blood before this mixing point, and the post-ductal site (foot) receives blood after it, a persistent, open ductus arteriosus can lead to a measurable difference in oxygen saturation between the two sites. This difference signals a failure in the normal circulatory transition or the presence of a congenital heart defect.
Interpreting the Saturation Difference
In a healthy newborn, the pre-ductal and post-ductal oxygen saturation readings should be nearly the same, indicating that all parts of the body are receiving well-oxygenated blood. Both measurements should also be high, typically 95% or greater, with an absolute difference of no more than 3 percentage points between the two sites.
The possibility of a problem is raised when the readings are either low overall or when there is a significant discrepancy between the two sites. A screen is considered a failure if the oxygen saturation is below 90% in either the right hand or the foot. Readings that fall into a borderline range (90% to 94% in both extremities) also suggest a need for further evaluation, as does a saturation difference greater than 3% between the hand and the foot.
If a difference exists, the post-ductal (foot) reading will be lower than the pre-ductal (right hand) reading. This pattern, where the lower body has a lower oxygen saturation, is a key indicator of a right-to-left shunt through a patent ductus arteriosus. The deoxygenated blood enters the circulation that supplies the lower half of the body, leading to the measured drop in the post-ductal oxygen saturation level.
Follow-Up Care After an Abnormal Screening
An abnormal pulse oximetry result requires immediate attention from the healthcare team. If the initial screen is borderline or failed, the newborn is often re-screened after a short period, typically one hour, to allow more time for the transitional circulation to stabilize. The medical team will notify the pediatrician to assess the baby and determine the next steps based on the severity of the reading and the infant’s overall clinical appearance.
An abnormal screen does not automatically confirm a severe heart defect. Other conditions, such as respiratory issues or infection, can also cause low oxygen saturation readings. However, if the repeat screening remains abnormal, the definitive next step is usually an echocardiogram. This specialized ultrasound provides a detailed view of the heart’s structure and function, which is necessary to confirm or rule out a congenital heart defect.