The term “preductal” in newborns refers to a specific anatomical location relative to a temporary blood vessel present during fetal development. Medical professionals use “preductal” measurements, particularly of oxygen levels, to assess a newborn’s circulatory adaptation to life outside the womb, helping monitor infant health and detect certain conditions early after birth.
Understanding the Ductus Arteriosus and Preductal Location
The “ductus arteriosus” is a blood vessel crucial for fetal circulation. Before birth, a fetus receives oxygen and nutrients from the placenta, not its lungs, which are filled with amniotic fluid and not yet functional. The ductus arteriosus acts as a shunt, connecting the pulmonary artery directly to the aorta. This allows most blood to bypass the inactive fetal lungs and go straight to the rest of the body.
After birth, as a baby takes its first breaths, the lungs inflate and begin to function. This causes increased blood flow to the lungs and a drop in pulmonary vascular resistance. Simultaneously, the placenta’s removal from circulation leads to a decrease in chemicals, called prostaglandins, that helped keep the ductus arteriosus open. These changes cause the ductus arteriosus to constrict and close, often within the first 12 to 24 hours of life in healthy full-term newborns. Permanent anatomical closure is generally completed within two to three weeks.
The term “preductal” refers to the part of the circulatory system that receives blood before it passes through the ductus arteriosus. This includes the blood supply to the head, neck, and the right arm. The right hand is considered the preductal site for measurements, as the brachiocephalic artery branches off the aorta before the ductus arteriosus. In contrast, “postductal” refers to areas receiving blood after it has passed through or bypassed the ductus arteriosus, such as the lower extremities.
The Significance of Preductal Oxygen Saturation
Measuring preductal oxygen saturation is a routine practice in newborn care, offering insights into a baby’s circulatory transition. Oxygen saturation, or SpO2, indicates the percentage of hemoglobin in the blood carrying oxygen. In a healthy newborn, as the ductus arteriosus closes and the lungs take over oxygenation, oxygen levels throughout the body should become uniform.
A significant difference between preductal and postductal oxygen saturation can indicate blood shunting through a patent (open) ductus arteriosus or other circulatory issues. For instance, if deoxygenated blood from the pulmonary artery mixes with oxygenated blood in the aorta through an open ductus arteriosus, oxygen levels in the lower body (postductal) may be lower than in the right arm (preductal). A difference of 3% or more between the preductal (right hand) and postductal (foot) oxygen saturation readings is significant and warrants further investigation.
Normal preductal oxygen saturation levels rise steadily after birth. For example, at one minute of age, the median preductal SpO2 is 66%, increasing to 89% by five minutes and reaching 95% by eight minutes of age in healthy infants. This differential measurement is a simple, non-invasive screening tool that helps identify newborns who may have underlying heart or lung problems.
Conditions Indicated by Preductal Measurements
Preductal oxygen saturation measurements, especially when compared with postductal readings, are a screening tool for conditions in newborns. They are particularly useful for screening Critical Congenital Heart Defects (CCHD). CCHDs are serious heart conditions requiring medical intervention, often surgery, in the first few weeks or months of life. Early detection through screening can lead to timely treatment and reduce severe complications or mortality.
One condition often identified is a Patent Ductus Arteriosus (PDA) that remains open after birth. While the ductus arteriosus is normal in the fetus, its failure to close can cause extra blood to flow to the lungs, straining the heart, potentially leading to lung congestion or pulmonary hypertension.
Another condition indicated by preductal measurements is Coarctation of the Aorta (CoA). This involves a narrowing of the aorta, often near where the ductus arteriosus connects. If the narrowing is located before the ductus arteriosus (preductal coarctation) and the ductus remains open, blood flow to the lower body can be impaired, resulting in lower oxygen saturations in the feet compared to the right hand. This differential saturation, along with other clinical signs like weak femoral pulses or unequal blood pressure between arms and legs, prompts further diagnostic steps like an echocardiogram.
How Preductal Saturation is Measured
Preductal oxygen saturation is measured using a non-invasive device called a pulse oximeter. This device uses a small, soft sensor wrapped around a baby’s right hand or wrist. The sensor contains light-emitting diodes that send red and infrared light through the skin to a photodetector. The amount of light absorbed is then analyzed to determine the oxygen saturation level.
To ensure an accurate reading, the sensor should be placed on the thin part of the hand, away from the thumb, with the light emitter and detector directly opposite each other. The baby should be quiet and still, as excessive movement can interfere with the reading. The measurement takes a few minutes until a stable reading is obtained. This simple, painless test is part of routine newborn screening for heart conditions, often performed around 24 hours after birth.