The umbilical cord arterial acid-base status reflects the status of newborn infants more objectively than the Apgar score. Delayed sampling may be carried out because of the need for resuscitative efforts, an inadequate primary sample, or a lack of early indications for immediate sampling. This prospective study was an attempt to determine the accuracy of delayed arterial blood gas sampling from 2 sites, the umbilical cord and the placental surface. Paired cord and placental samples were taken after double clamping the cord at baseline and after 30, 60, and 90 minutes. One-third each of the 90 placentas sampled were normal vaginal deliveries, high-risk deliveries, and elective cesarean section deliveries.
At birth, the mean cord pH of 7.207 was significantly lower than the placental value of 7.240. Cord blood pH values declined significantly over time, by 0.112 at 90 minutes after birth. The placental arterial pH decreased at twice the rate of the cord blood pH over 90 minutes. At baseline, the mean cord base excess of −7.0 mmol/liter was significantly lower than the placental value of −6.3 mmol/liter. The cord base excess declined steadily, by 9.0 mmol/liter at 90 minutes. Despite different baseline levels of pH and base excess, the decrease in both these parameters was similar for normal vaginal deliveries, high-risk deliveries, and cesarean section deliveries. Values for base excess were significantly lower when chorioamnionitis was confirmed histologically, but pH and CO2 were unaffected. Base excess values decreased over time at similar rates whether or not chorioamnionitis was present.
These findings demonstrate that, when arterial blood sampling is delayed for any reason, apparent deterioration of blood gas parameters over time may not reflect the status of the newborn infant. The investigators recommend that arterial blood gas samples be taken from the umbilical cord as soon as possible after delivery. When this is not possible, however, delayed sampling does have a role in estimating arterial pH and base excess values at the time of birth.