Umbilical cord gases in relation to the neonatal condition: the EveREst plot

To validate umbilical arterial and venous pH and base deficit (pH(UA), pH(UV), BD(UA), and BD(UV), respectively), as well as venous-arterial pH differences, as measures of perinatal condition at birth (in relation to emergency intervention, resuscitation, facial mask, low Apgar, seizures, other cerebral problems or death); to investigate whether BD(UA) or pH(UA) better measures perinatal risk.

A novel method (Event Rate Estimate (EveREst) plots) was used to analyze cord blood gases and perinatal outcomes of 34,510 term singleton deliveries: cord blood gas values were grouped into exclusive quantiles (containing equal proportions of cases); the quantiles were plotted against per cent rates (event rates) for perinatal outcomes; the event rates for the different blood gases were compared using the χ² test for difference of proportions.

Low pH(UA) predicts poor perinatal outcome better than or comparably to high BD(UA): pH(UA) is significantly better than BD(UA) for predicting low Apgar, resuscitation and facial mask (p<0.001, p<0.05, and p<0.001, respectively). For seizures and other cerebral problems, low pH(UA) is better than high BD(UA) but the difference is not statistically significant. For death, both measures perform equally well. Interventions for the specific reason of "fetal distress" increased as pH(UA) decreased but only where electronic fetal monitoring was used. In acidemic neonates (pH(UA) ≤ 7.05, n=1752), significantly more cord prolapses and placenta abruptions were associated with large and small venous-arterial pH difference (pH(VAD)) respectively (p<0.01).

EveREst plots display clearly the diagnostic value of cord gases. They allow for the easy identification of background rates and increases above background, thresholds of interest, and comparison of the blood gas measures. Overall pH(UA) is the best umbilical blood measure of perinatal outcome. BD(UA) is comparable or inferior. Extremes of pH(VAD) (large or small) identify higher proportions of specific poor outcomes in acidemic neonates.