Is it time to end the use of base deficit for fetal well-being assessment?

Authors have expressed reservations regarding the use of base deficit measured in umbilical artery blood samples to assess fetal well-being during the course of labor and to predict neonatal neurologic morbidity. Despite its integration into clinical practice for more than 50 years, obstetricians and maternal-fetal medicine specialists may not realize that this marker has significant limitations in accurately identifying neonatal metabolic acidosis as a proxy for fetal well-being. In brief, there are 2 large families of base deficit, namely whole blood and extracellular fluid. Both rely on equations that use normal adult acid-base characteristics (pH 7.40 and partial CO2 pressure of 40 mm Hg) that overlook the specificity of the normal in utero acid-base status of pH 7.27 and partial CO2 pressure of 54 mm Hg. In addition, it ignores the unique characteristic of the in utero fetal response to acute hypoxia. The dependence on placental circulation for CO2 elimination may lead to extremely high values (up to 130 to 150 mm Hg) during hypoxic events, a phenomenon that is absent in adults with acute metabolic acidosis who can hyperventilate. The dispute over if to include a correction for high partial CO2 pressure in the bicarbonate estimation, as presented in the Great Trans-Atlantic Debates, remains unresolved. The key constants computed for adult acid-base physiology in the current base deficit algorithms, without accounting for the impact of high partial CO2 pressure or other fetal characteristics of buffering capacity (eg, differences in body water content composition, plasma protein, and hemoglobin attributes), may lead to an overestimation of metabolic acidosis, especially in newborns who are experiencing hypercarbia during the early stages of the hypoxic response. These unrecognized limitations impact the base deficit results and may mislead clinicians on fetal well-being assessments when discussing the management of fetal heart rate monitoring and neonatal outcomes. Based on our arguments, we believe that it is prudent to consider an alternative to base deficit for drawing conclusions regarding fetal well-being during the course of birth management. We propose a marker specifically related to the newborn acid-base physiology--the neonatal eucapnic pH correction. This marker can be added to arterial cord blood gas analysis, and we have described how to interpret it as a marker of neonatal metabolic acidosis.

Umbilical artery eucapnic pH to assess fetal well-being.

BACKGROUND: Umbilical artery gas results help obstetricians assess fetal well-being during labor and guide screening decisions on eligibility for therapeutic hypothermia (ie, whole-body or head cooling). The accuracy of results, especially for the base deficit on arterial cord gas analysis, in predicting brain injury is questioned. A novel biomarker specifically calculated for fetal acid-base physiology and response to asphyxia-neonatal eucapnic pH as a marker of neonatal metabolic acidosis-has the potential to be an accurate predictor of hypoxic-ischemic encephalopathy. OBJECTIVE: We aimed to compare false-negative rates of hypoxic-ischemic encephalopathy for umbilical artery pH, base deficit, and neonatal eucapnic pH in assessing fetal acid-base balance as a marker of fetal well-being and predicting acute brain injury. STUDY DESIGN: This is a retrospective single-center cohort study of newborns ≥ 35 weeks of gestation diagnosed with hypoxic-ischemic encephalopathy. We compared false-negative rates for any grade of hypoxic-ischemic encephalopathy using unilateral paired chi-square statistical analysis based on cutoff values for umbilical artery pH ≤7.00, base deficit ≥16 mmol/L, base deficit ≥12 mmol/L and neonatal eucapnic pH ≤7.14. We performed an analysis of variance between umbilical artery pH, base deficit, and neonatal eucapnic pH for each hypoxic-ischemic encephalopathy grade. RESULTS: We included 113 newborns. False-negative rate for hypoxic-ischemic encephalopathy was significantly higher for base deficit <16 mmol/L (n=78/113; 69.0%) than <12 mmol/L (n=46/113; 40.7%), pH >7.00 (n=41/113; 36.3%), or neonatal eucpanic pH >7.14 (n=35/113; 31.0%) (P<.0001). All true-positive cases were identified using only umbilical artery pH and neonatal eucapnic pH. Base deficit ≥16 or ≥12 mmol/L did not add any value in identifying newborns with hypoxic-ischemic encephalopathy when using umbilical artery pH and neonatal eucapnic pH. No association emerged between any marker and hypoxic-ischemic encephalopathy severity grading. CONCLUSION: Our findings support the accuracy of neonatal eucapnic pH to assess fetal well-being during labor and to improve predictive performance for acute brain injury. Neonatal eucpanic pH, in addition to umbilical artery pH, may be a viable alternative in identifying newborns at risk for hypoxic-ischemic encephalopathy.

Eucapnic pH coupled with arterial cord pH improves hypoxic-ischemic encephalopathy prediction.

OBJECTIVE: To consider the classical use of "pH < 7.0 and/or a base deficiency ≥12 mmol/L" as markers of the risk of neonatal hypoxic-ischemic encephalopathy (HIE), recalling various criticisms of the use of these markers in favor of that of neonatal eucapnic pH, which appears to be a better marker of this risk. METHODS: Fifty-five cases of acidemia with pH < 7.00 were collected from a cohort from the Nice University Hospital with eight cases of HIE. We compared the receiver operating characteristics curves established from the positive likelihood ratio (+LR) for each case of: umbilical cord artery pH (pHa), neonatal eucapnic pH (pH euc-n) in isolation (not matched to pHa), and matched pHa to its own pH euc-n. RESULTS: The areas under the curve (AUC) are identical for pHa and pH euc-n, but AUC for the matched pair pHa-pH euc-n appears superior but non-significant because of the small number in our cohort. However, using the bootstrap method, the partial AUC for a sensitivity greater than 75% indicates the significant superiority (P < 0.01) of the matched pair pHa-pH euc-n approach. CONCLUSION: The originality of this study lies in the use of two methodologic approaches: (1) standardized partial analysis of the AUCs of the pHa curve and that of pHa matched to its own pH euc-n, and (2) bootstrap statistical technique, that allowed us to conclude (P < 0.01) that the combined use of pH measured at the cord coupled with its eucapnic correction is better for diagnosing metabolic acidosis and best predicting the risk of HIE.