From the Research
The average difference in cord blood pH between venous and arterial blood is typically less than 0.02 pH units, with venous blood generally having a higher pH than arterial blood, as indicated by a recent study 1. This difference occurs because arterial cord blood reflects fetal status more accurately, while venous blood primarily reflects placental function. During normal labor and delivery, arterial cord blood pH usually ranges from 7.18 to 7.38, while venous cord blood pH ranges from 7.25 to 7.45. This pH difference exists because venous blood has given up carbon dioxide to the maternal circulation through the placenta, making it less acidic than arterial blood which contains the metabolic byproducts from the fetus. Understanding this normal difference is clinically important when interpreting cord blood gases to assess fetal well-being, as a reversal of this pattern (where arterial pH is higher than venous) may indicate acute cord compression events. When collecting cord blood samples for analysis, it's essential to properly identify and label arterial and venous samples to ensure accurate interpretation of results, as noted in various studies 2, 3, 4, 5. Key factors to consider include the correlation between lactate levels in arterial and venous cord blood, as well as between lactate levels and pH and base excess in cord arterial blood, which have been explored in research 1, 5. In clinical practice, the most recent and highest quality study 1 should guide the interpretation of cord blood pH differences to prioritize morbidity, mortality, and quality of life outcomes. Some studies have provided reference values for cord blood gases and lactate, which can help in assessing fetal well-being 3, 1. However, it is crucial to consider the most recent evidence when making clinical decisions, as older studies may not reflect current best practices or understanding of fetal physiology 2, 4, 5. Ultimately, accurate interpretation of cord blood pH requires consideration of the venoarterial pH gradient, which is commonly used to differentiate arterial from venous samples, as discussed in the latest research 1.