Can venous cord gas values predict fetal acidemia?

BACKGROUND: Umbilical cord arterial blood gas values are used to diagnose fetal acidemia; however, arterial cord blood specimens are frequently not available.

OBJECTIVE: We sought to assess whether umbilical cord venous blood gas values can be used to reliably predict fetal acidemia.

STUDY DESIGN: This is an observational study of women with a singleton gestation at a single tertiary care hospital who delivered from September 2010 through August 2015 and had both umbilical cord arterial and venous blood gas samples measured. Fetal acidemia was defined in 2 ways: (1) umbilical cord arterial pH <7.0, and (2) umbilical cord arterial base deficit ≥12 mEq/L. Receiver operating characteristic curves for fetal acidemia were generated using umbilical cord venous blood gas values and the areas under the curve were calculated. Umbilical cord venous blood gas cutoffs associated with <1%, <5%, <10%, and <50% probability of acidemia were calculated.

RESULTS: Of the 36,325 women who met inclusion criteria, 563 (1.5%) had an umbilical cord arterial pH <7.0 and 1535 (4.2%) had an umbilical cord arterial base deficit ≥12 mEq/L. Umbilical cord venous pH was highly predictive of umbilical cord arterial pH <7.0 (area under the curve, 0.955; 95% confidence interval, 0.946-0.965). Similarly, umbilical cord venous base deficit was highly predictive of umbilical cord arterial base deficit ≥12 mEq/L (area under the curve, 0.967; 95% confidence interval, 0.963-0.971). While the combination of venous pH and base deficit was statistically significantly more predictive of umbilical cord arterial pH <7.0 (area under the curve, 0.961; 95% confidence interval, 0.952-0.969; P < .001), this difference has negligible clinical meaning. Similarly, the combination of venous pH and base deficit (area under the curve, 0.967; 95% confidence interval, 0.962-0.971) was no more predictive than venous base deficit alone in the prediction of umbilical cord arterial base deficit ≥12 mEq (P = .622). The likelihood of an arterial cord venous pH <7.0 was <1%, <5%, <10%, and <50% with cord venous pH of 7.22, 7.16, 7.14, and 7.07, respectively. The likelihood of an arterial cord base deficit ≥12 mEq/L was <1%, <5%, <10%, and <50% with cord venous base deficit of 7.0, 8.5, 9.2, and 11.3, respectively.

CONCLUSION: Umbilical cord venous pH and base deficit are each highly predictive of fetal acidemia, and can be used to evaluate the likelihood of fetal acidemia in the absence of umbilical cord arterial blood gas values.