Objectives To address the practical problems of routine umbilical cord blood sampling, to determine the ranges for pH, pCO2 and base deficit and to examine the relationships of these parameters between cord vessels.
Design An observational study of umbilical cord artery and vein blood gas results.
Setting A large district general hospital in the UK.
Subject One thousand nine hundred and forty-two cord results from 2013 consecutive pregnancies of 34 weeks or more gestation, monitored by fetal scalp electrode during labour.
Results Only 1448 (74.6%) of the 1942 supposedly paired samples had validated pH and pCO2data both from an artery and the vein; 54 (2.8%) had only one blood sample available, 90 (4.6%) had an error in the pH or pCO2 of one vessel and in 350 (18%) pairs the differences between vessels indicated that they were not sampled from artery and vein as intended. Only 60% of the cases with an arterial pH less than 7.05 had evidence of a metabolic acidosis (base deficit in the extracellular fluid 10 mmol/1 or more). Of all the cases, 2.5% had a venous-arterial pH difference greater than 0.22 units.
Conclusions Both artery and vein cord samples must be taken and the results screened to ensure separate vessels have been sampled. Interpretation of the results requires the examination of pCO2 and base deficit of the extracellular fluid from each vessel as well as the pH. Confusion about the value of cord gas measurements may be due to the use of erroneous data and inadequate definitions of acidosis which do not differentiate between respiratory and metabolic components.