Pitfalls in Interpreting Umbilical Cord Blood Gases and Lactate at Birth
Sammanfattning: Acid-base status in umbilical cord blood is an objective measure of the fetus’ exposure to and ability to handle hypoxia. The objective of this thesis was to clarify some of the methodological pitfalls in interpreting umbilical cord blood gases and lactate values at birth. Study I pinpoints the methodological confounding in calculating base deficit (BD) with algorithms used in different brands of blood gas analyzers and reports the consequences for diagnosing metabolic acidosis (MA) at birth. Neonatal MA rates cannot be compared between maternity units or between scientific articles where different fetal compartments (blood or extracellular fluid) and different algorithms for calculating BD have been used. Study II addresses the issue of possible diagnostic discrepancies when acid-base parameter value decimals are rounded off. A drift of a dichotomy parameter value cut-off due to decimal rounding will result in a shift in distribution of negative and positive cases in a population sample. The findings warrant a discussion on standardization of round-off rule and the number of decimals for a specific analyte result. Study III demonstrates that delayed cord blood sampling with intact pulsations affects umbilical acid-base values and hematological parameters in both vaginal and cesarean deliveries. The changes were more marked after vaginal delivery. A change towards acidemia and lactemia can be explained by the hidden acidosis phenomenon, i.e. a surge into the central circulation of peripherally trapped acid metabolites when the newborn starts to breathe. Study IV shows that clinical characteristics have a significant influence on the distribution of veno-arterial and arterio-venous gradients (Δ values) in umbilical cord blood. Validation criteria based on fixed ΔpH and ΔpCO2 values may then exclude correct samples of clinical outliers. Lactate cannot be used for validation of umbilical cord blood samples. A negative ΔpO2 value indicates delayed cord blood sampling or mix-up of samples and is the only certain validation criterion.
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