Newborn Cord Blood Gases
For optimal interpretation, a section of umbilical cord should be clamped on each end to isolate it from the placenta, and samples drawn soon after birth. Use a separate 1mL heparinized syringe for each vessel, and as soon as the blood is drawn, place the specimen on ice and send it to respiratory therapy for analysis.
The umbilical artery is the smaller vessel and should be drawn first, being careful to insert the needle superficially and not poke through the artery and into the vein.
The umbilical vein is the larger vessel and should be drawn after the artery. It is important to sample both arterial and venous blood, especially if the infant is depressed at birth.
Blood from the umbilical artery represents the state of the fetus at the time of delivery, because it is blood returning from the fetus. Umbilical venous blood provides the placental status.
In the case of cord compression, the placenta is still functioning normally, so the venous gas would still be normal. However, the arterial gas would reflect a lower pH and increased PCO2 because the fetus would be retaining CO2 and not receiving enough oxygen when the cord is compressed.
Many studies have been done on healthy term infants to determine a normal reference range for newborn blood gases. However, the exact values accepted as normal vary in literature and in various facilities.
One study example of umbilical cord blood gas values following uncomplicated term vaginal deliveries are shown on the chart. The umbilical artery values were: pH, 7.28 +/- 0.05; PCO2 49.2 +/- 8.4mmHg; PO2, 18.0 +/- 6.2mmHg; and bicarbonate, 22.3 +/- 2.5mEq/L. The umbilical venous values were: pH, 7.35 +/- 0.05; PCO2, 38.2 +/- 5.6mmHg; PO2, 29.2 +/- 5.9mmHg; and bicarbonate, 20.4 +/- 4.1mEq/L.
A pH of <7.1 may indicate birth asphyxia/hypoxia severe enough to cause neurological deficits. In a sick newborn, acidosis is seen more often than alkalosis. Take note that infants with abnormal blood gas results may be acceptable for that infant, depending on the gestational age of the infant or the disease process. Remember to look not only at the lab results, but also Apgar scores, and the child’s general condition.
Disturbances of acid-base balance signal the body to attempt to return the pH back to a normal level. For example, if CO2 levels are increased, the body attempts to correct the problem by excreting more of it. The neonate is usually unable to correct an acid-base imbalance because of its immaturity, such as immature lungs. For more detailed explanation of how to interpret blood gas disturbances, see our video on “Acid base balance and Blood gas interpretation.’
In neonates, you may often see respiratory acidosis, due to hypoventilation, ventilation-to-perfusion mismatching, or cardiac disease. The neonate may have decreased lung tissue, apnea, meconium aspiration, or persistent pulmonary hypertension of the newborn.
Metabolic acidosis may also occur due to increased acid formation from inborn errors of metabolism or hyperalimentation. It is also seen with diarrhea and renal tubular acidosis from a loss of bases.
Respiratory alkalosis can occur with hyperventilation or the CNS response to hypoxia or maternal heroin addiction. Metabolic alkalosis occurs with a gain of bases or loss of acids, which can occur with gastric suctioning.
Umbilical cord blood gas analysis is recommended in all high-risk deliveries and some institutions are collecting samples from all deliveries. The best results are obtained when both umbilical artery and venous samples are taken soon after birth from a segment of cord that has been clamped in two places to isolate it from the placenta.
An infant with a low cord pH who is vigorous at birth and has no other compromise is not necessarily at an increased risk for adverse outcomes. However, an infant with a pH <7.1 at birth, who is also not vigorous, is at a high risk for adverse outcome. In combination with other clinical information, the umbilical blood gas results provide important information about the past, present, and possibly the future condition of the infant.
Thank you for watching this video tutorial on newborn cord blood gases. Be sure to check out our other videos!