Preductal oxygen refers to a measurement of blood oxygen saturation taken at a specific location on a newborn’s body, defined as being “before the ductus arteriosus.” In newborn medicine, this reading is a standard component of screening protocols used to assess how well the infant’s circulatory system is adapting after birth. It provides a snapshot of the highest available oxygen level in the circulation, serving as a baseline for comparison.
Anatomy: Understanding the Preductal Position
The preductal location is defined by its relationship to the Ductus Arteriosus (DA), a temporary blood vessel present during gestation. The DA is a remnant of the fetal circulatory system, connecting the pulmonary artery directly to the descending aorta, allowing blood to bypass the lungs. After birth, the DA typically begins to close within 12 to 24 hours as the baby starts breathing.
The artery supplying the right upper extremity—the right hand—branches off the aorta before the DA connects to the descending aorta. This anatomical arrangement means the blood flowing to the right hand has not yet mixed with any deoxygenated blood that might be shunting through a still-open DA. Therefore, a measurement taken on the right hand reflects the most highly oxygenated blood available, protecting the upper body and head.
The Importance of Preductal Oxygen Screening
Preductal oxygen saturation is routinely measured using non-invasive pulse oximetry screening. This involves placing a sensor on the newborn’s right hand (preductal) and a second sensor on one of the feet (post-ductal). This side-by-side comparison forms the basis of the screening protocol.
The rationale is that if the DA remains open, poorly oxygenated blood shunted into the aorta will lower the saturation in the post-ductal site (feet). The preductal site (right hand) remains protected from this mixing because its arterial supply originates upstream of the DA connection. The test is typically performed at approximately 24 hours of life to maximize accuracy. A passing result requires both saturation values to be high, typically 95% or greater, and the difference between the two readings must be minimal, generally three percent or less.
What Oxygen Discrepancies Reveal
A significant difference between the preductal and post-ductal readings indicates a potential abnormality in the newborn’s circulatory transition. A foot reading notably lower than the right hand suggests a right-to-left shunt. This occurs when deoxygenated blood from the right side of the heart bypasses the lungs and enters the systemic circulation through an open DA.
This mixing immediately lowers the oxygen concentration in the aorta after the DA connection, reducing the saturation measured on the foot while the right hand reading remains higher. A difference greater than three percent is considered abnormal and warrants immediate investigation. These discrepancies are a marker for Critical Congenital Heart Disease (CCHD), which requires timely intervention. CCHD involves defects where pulmonary circulation is restricted or heart chambers are poorly formed, forcing blood to shunt away from the lungs. The screening identifies these life-threatening issues before symptoms become severe, allowing for prompt treatment.