Placental compared with umbilical cord blood to assess fetal blood gas and acid-base status.

OBJECTIVE: To estimate the extent to which placental cord blood sampled from the umbilical cord at its insertion into the placenta and after delivery of the placenta, is in agreement with umbilical cord blood sampled from a clamped segment of the umbilical cord after delivery of the infant, for the assessment of fetal blood gas, acid-base status, and hemoglobin levels at birth.

METHODS: Forty-eight patients were studied with arterial and venous blood sampling from the umbilical cord and from the placental cord insertion, with subsequent measurement of blood gases, pH, base excess, O2 saturation, and hemoglobin. The relationships of corresponding measurements from the placental and umbilical vein and from the placental and umbilical artery were analyzed using regression analysis, paired analysis of grouped means, and by estimating limits of agreement.

RESULTS: The relationships between placental and umbilical cord blood measurements were described using a linear mathematical model, and although respective measurements were all significantly related (P < .01), this was strongest for both venous and arterial base excess and hemoglobin measurements (r values 0.91 to 0.99) and variably weaker for venous and arterial Po2 (and thereby O2 saturation measurements [r values 0.36 to 0.89]) and arterial Pco2 (and thereby pH measurements [r values 0.66 to 0.73]). Whereas base excess and hemoglobin measurements for both the venous and arterial placental and umbilical cord bloods were close in value over the range of values studied, Po2 and thereby O2 saturation values were variably lower in the placental vein compared with the umbilical vein, while Pco2 values were variably lower and thereby pH values conversely higher in the placental artery compared with the umbilical artery. Limits of agreement as a measure of the difference between paired placental and umbilical cord blood measurements were such that only those for base excess and hemoglobin were likely narrow enough to be acceptable for clinical purposes.

CONCLUSION: Placental cord blood provides for a close estimate of fetal base excess and hemoglobin status at birth, but with more error for Po2 and thereby O2 saturation and Pco2 and thereby pH due to continued blood gas exchange within and across the placenta after cord clamping.