Umbilical cord blood acid-base state: what is normal?

Umbilical cord blood gases and acid-base data from vigorous neonates were examined to determine normal values and ranges.

The University of California, San Francisco, Perinatal Data Base was used to retrieve information from deliveries between 1977 and 1993. Newborns with 5-minute Apgar scores > or = 7 were selected because it is generally accepted that a vigorous newborn has not had substantial intrapartum asphyxia lasting until delivery.

Full blood gas and obstetric data were available for 16,060 newborns. Of these, 15,073 (94%) had a 5-minute Apgar score > or = 7. The median umbilical artery values, with 2.5th percentile values in parenthesis, were pH 7.26 (7.10), PCO2 52 mm Hg (74), base excess -4 mEq x L(-1) (-11), and PO2 177 mm Hg (6). Although the distributions were skewed, the mean +/- 2 SDs were similar to these values. Data for these babies were further analyzed by method of delivery, gestational age, presentation, and presence of thick meconium. Although the means were significantly different in all groups, the differences between groups were relatively small.

A wide range of acid-base values were found in babies with normal Apgar scores, defining the "physiologic acidemia" of the normal vigorous newborn.