The assessment of fetal well-being during labor and delivery increasingly relies on umbilical cord arterial blood gas analysis, particularly the measurement of pH, to objectively evaluate an infant’s acid-base status. While traditionally used to identify hypoxic events, recent research highlights potential inaccuracies in interpreting these values, especially when delayed cord clamping is practiced.
A normal birth is characterized as a beneficial stress response, triggering a surge in catecholamines designed to protect against intrauterine hypoxia and facilitate the transition to extrauterine life. However, when oxygen supply becomes critically low, hypoxemia – a deficit of oxygen in the blood – can progress to hypoxia, a tissue-level oxygen deficiency, potentially leading to organ damage, a condition known as asphyxia. A decrease in pH, reflecting an increase in hydrogen ion concentration, alongside an elevated base deficit, serves as an indicator of fetal oxygen deficiency.
The base deficit, a measure intended to reflect the metabolic component of a low pH and differentiate it from respiratory acidosis caused by high carbon dioxide concentrations, is not a directly measured value. Instead, it is calculated by blood gas analyzers using pH, partial pressure of carbon dioxide, and hemoglobin levels. Variations in calculation equations between different analyzer brands can lead to significant discrepancies in base deficit values, potentially influencing the diagnosis of metabolic acidosis, typically defined as a pH below 7.00 combined with a base deficit of 12.0 mmol/L or greater in umbilical cord arterial blood.
Researchers have noted that the ambition to enhance neonatal hemoglobin and iron stores through delayed cord clamping can inadvertently result in falsely acidic blood gas and lactate values if blood sampling is also delayed. This is because delaying sampling doesn’t account for the metabolic changes occurring after birth. Blood gases and lactate levels in cord blood naturally change as pregnancy progresses, trending towards a mixed metabolic and respiratory acidemia due to increased fetal metabolism and carbon dioxide production.
The distinction between base deficit calculated in blood versus extracellular fluid is also crucial. Extracellular fluid calculations are advocated for fetal blood due to the common occurrence of high carbon dioxide levels in newborns, which can significantly impact pH values and be misinterpreted as respiratory acidosis. Clinicians must be aware of these potential pitfalls in collecting, handling, and interpreting umbilical cord blood gas values, as highlighted by recent educational efforts aimed at improving understanding of the process.
