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Cord Blood Gases

Evaluation of the status of a newborn is best accomplished by assessing multiple criteria. Because the Apgar scoring system is considered subjective, it is not always accurate in the prediction of asphyxia. Umbilical cord acid base balance is a more effective means of identifying hypoxia and acidemia.

Umbilical venous cord blood gas values are similar to the maternal intervillous oxygen and acid-base status because oxygen and carbon dioxide can equilibrate between these two compartments. The umbilical artery contains blood returning from the fetus to the placenta and provides the most useful information regarding the acid-base status of the fetus. Umbilical venous blood has higher pH, pO2, and base excess than umbilical arterial blood, but lower pCO2.

There is a lack of consensus concerning when umbilical cord blood gases should be performed. Some physicians order them on all deliveries. One study suggested that arterial blood should be sampled from an umbilical artery in the following situations:

  • All premature newborns
  • Presence of meconium stained amniotic fluid
  • Operative vaginal or abdominal deliveries for non-reassuring fetal heart rate pattern
  • Term newborns who are depressed at birth or have a 5 minute Apgar score below 7.

Umbilical cord samples should be collected before delivery of the placenta by double clamping of a section of the umbilical cord. A minimum of 1.5 mL of blood collected into heparinized syringes is required for each venous and arterial cord sample. Syringes should be immediately placed in ice for transportation to the laboratory.

Published reference ranges for cord blood gases are:

Parameter

Arterial

Venous

pH

7.14 – 7.42

7.22 – 7.44

pO2

3 – 40

12 – 43

pCO2

34 - 78

30 – 63

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