[Measured quantities in perinatal medicine--the base excess].

Background: There are convincing data from neonatal studies that the base excess (BE, mmol/L) measured in capillary or cord blood offers special diagnostic and prognostic power in the new-born compromised by hypoxia. For computation of BE the hemoglobin concentration (Hb, %), the oxygen saturation (%) and the distribution of the fetal fluid compartments are necessary. Until now these three factors, which differ in the fetus when compared with adults, have not been taken into account using automatic blood gas equipment. This study therefore attempts to analyse and verify which BE will offer the best fit with the outcome conditions in the newborn.

Methods: Using a cohort of 7701 singletons delivered at term by the vaginal route in whom a blood gas analysis in cord blood together with Hb measurements were available and plausible, three BE values were computed: the ordinary BE (actual BE, according to Radiometer determinations), the BE corrected to the real oxygen saturation (BE,oxy) and the BE in the extracellular fluid compartment of the fetus (BE,ecff) using the algorithms of O. Siggaard-Andersen. Moreover, three different oxygen saturation curves for fetal Hb (HbF) were applied, namely those of Zander et al., Hellegers and Schruefer and Ruiz et al. (Severinghaus). The prognostic potential of the different BE values was analysed using a correlation with the APGAR-score after 1 min, the sum of the APGAR indices after 1 and 5 min, and additionally with the fetal heart frequency (FHF) of the last 30 min ante-partum quantified by a score electronically in a group of 342 fetuses.

Results: The median of the actual BE in umbilical artery blood amounted to -4.9 mmol/L. Correction to the real oxygen saturation of the blood sample leads to a median of -7.7 mmol/L. Using three different oxygen saturation curves and thus taking into account HbF reveals little influence on the numerical values of BE. However, application of the true distribution of fetal fluid compartments in the evaluation of BE (BE,ecff) leads to a significant reduction of the numerical value of BE (median = -3.2 mmol/L). Computational correction of BE using the real oxygen saturation (%) in each fetus leads to a important and highly significant increase of the correlation (r and rho) with the APGAR indices. The correlation with the FHF score also increased but failed to reach significance.

Conclusions: Our data show that, in perinatal medicine, a computational correction of BE according to the real oxygen saturation of the blood sample is mandatory. An algorithm for HbF should be used. The BE in the extracellular fluid compartment does not fit to reality. The determination of the Hb concentration in the umbilical blood of each fetus is essential since its variability is higher than expected. In legal controversies the computational origin of BE should be documented and carefully regarded.