Umbilical cord blood gases: probability of arterial or venous source in acidemia

Umbilical cord blood gases (UBG) may be a critical element in the assessment of a depressed newborn infant but in some cases the arterial or venous UBG source is uncertain making clinical and/or medical-legal interpretation difficult. Objective: to estimate the probability of an arterial (ProbAS) or venous (ProbVS) UBG source depending on blood gas parameters in acidemic cases.

A total of 56,703 pairs of concomitant arterial and venous (CAV) UBG results assayed over an 8.8-year period were analyzed. Specimen pairs with preanalytical issues, duplicate source, or physiologically out-of-range or uninterpretable results were excluded. The 3,579 CAV-UBGs with an arterial and venous pH 6.70 to 7.25 were analyzed. Generalized additive model (gam)-based binomial logistic regressions were used to determine the ProbAS and ProbVS according to the blood gas parameters.

The relative differences between arterial and venous medians were: pO₂ ‒47%, pCO₂ 22%, pH -11%, and BD 4%. Below a median of 2.4 kPa, the lower the pO₂, the higher the ProbAS. Above this value, the higher the pO₂, the lower the ProbAS. An Excel worksheet is provided to calculate ProbAS and ProbVS from the regression model for different combinations of pH, pCO₂, and pO₂ values. Considering ProbAS and ProbVS above a cutoff 0.8, the model correctly identified the source in 56% of cases while 41% were indeterminant and 3% were erroneous.

The probability of an arterial or venous source of an umbilical blood gas can be estimated based on the pH, pCO₂, and pO₂ in most acidemic specimens.