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

Objectives 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. Methods 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. Results The relative differences between arterial and venous medians were: pO(2) -47%, pCO(2) 22%, pH -11%, and BD 4%. Below a median of 2.4 kPa, the lower the pO(2), the higher the ProbAS. Above this value, the higher the pO(2), the lower the ProbAS. An Excel worksheet is provided to calculate ProbAS and ProbVS from the regression model for different combinations of pH, pCO(2), and pO(2) 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. Conclusions The probability of an arterial or venous source of an umbilical blood gas can be estimated based on the pH, pCO(2), and pO(2) in most acidemic specimens.