Base Excess in Arterial Cord Blood: A Calculated Parameter
Yes, base excess (BE) reported on an arterial umbilical cord blood gas is a calculated parameter, not a directly measured value. It is computed using measured values of pH, pCO₂, hemoglobin concentration, and oxygen saturation through the Henderson-Hasselbalch equation 1.
How Base Excess is Calculated
Base excess represents the metabolic component of acid-base status and indicates the contribution of non-volatile substances to blood acid-base balance. The calculation requires:
- Measured pH (directly measured)
- Measured pCO₂ (directly measured)
- Hemoglobin concentration (directly measured)
- Oxygen saturation (calculated or measured)
The BE value is then computed using established algorithms, with hemoglobin concentration and oxyhemoglobin saturation modulating the buffer capacity of blood in the calculation 1.
Critical Considerations for Umbilical Cord Blood
The calculation becomes particularly complex in umbilical artery blood due to the normally low fetal oxygen saturation. In term infants delivered spontaneously, the median oxygen saturation in umbilical artery blood is only approximately 27.7% (due to median pO₂ of ~18 mmHg) 2. This creates a significant computational challenge.
Correction for Oxygen Saturation
Computational correction of BE to the actual oxygen saturation is mandatory in perinatal medicine 2, 3. When BE is corrected to 100% oxygen saturation based on the actual (calculated) oxygen saturation:
- Median uncorrected BE in umbilical artery: -4.6 to -4.9 mmol/L
- Median corrected BE (BEoxy): -7.4 to -7.6 mmol/L
- The correction uniformly leads to lower BE values (median difference: 2.7 mmol/L) 2
Corrected BE values show significantly higher correlations with clinically important outcomes including Apgar scores and pCO₂ 2, 3.
Factors Influencing the Calculation
The calculated BE value in umbilical cord blood is influenced by:
- Gestational age - BE values increase with advancing gestational age 4
- Choice of fluid compartment - BE can be calculated for blood (actual BE) or extracellular fluid (BE,ecf), yielding different numerical values 4, 5
- Algorithm used - Different calculation algorithms produce different results 3, 4
- Hemoglobin concentration - Essential to measure in each fetus due to higher-than-expected variability 3
Clinical Implications
For clinical interpretation, the base excess of blood (not extracellular fluid) provides better correlation with neonatal outcomes 4. When evaluating metabolic acidosis in cord blood:
- A BE ≤ -12 mmol/L (or base deficit ≥12 mmol/L) is one of the four essential criteria for determining an acute intrapartum hypoxic event sufficient to cause cerebral palsy 6
- Cord pH <7.00 combined with base deficit >12 mmol/L indicates significant metabolic acidosis 7
Common Pitfall
The most critical error is failing to account for the actual oxygen saturation when interpreting BE values in umbilical cord blood. Blood gas analyzers may report uncorrected values that underestimate the true metabolic acidosis by approximately 2.7 mmol/L 2. Always verify whether your laboratory reports corrected or uncorrected values, and document which calculation method was used, particularly in medicolegal contexts 3.