Hidden acidosis: an explanation of acid–base and lactate changes occurring in umbilical cord blood after delayed sampling

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Abstract
Objective: To explore the ‘hidden acidosis’ phenomenon, in which there is a washout of acid metabolites from peripheral tissues in both vaginal and abdominal deliveries, by investigating temporal umbilical cord blood acid–base and lactate changes after delayed blood sampling.Design: Prospective comparative study.Setting: University hospital.Sample: Umbilical cord blood from 124 newborns.Methods: Arterial and venous cord blood was sampled immediately after birth (T0), and at 45 seconds (T45), from unclamped cords with intact pulsations taken from 66 neonates born vaginally and 58 neonates born via planned caesarean section at 36–42 weeks of gestation. Non‐parametric tests were used for statistical comparisons, with P < 0.05 considered significant.Main outcome measures: Temporal changes (T0–T45) in umbilical cord blood pH, the partial pressure of CO2 () and O2 (), and in the concentrations of lactate, haematocrit (Hct), and haemoglobin (Hb).Results: In both groups all arterial parameters, except for in the group delivered by caesarean section, changed significantly (pH decreased and the other variables increased). There were corresponding changes in venous acid–base parameters. When temporal arterial changes were compared between the two groups, the decrease in pH and increase in were more pronounced in the group delivered vaginally. Neonates born vaginally had significantly lower pH and higher lactate, Hct, and Hb concentrations at T0 and T45 in both the artery and the vein. At T45, arterial and levels in the group delivered vaginally were also significantly higher.Conclusions: Delayed umbilical cord sampling affected the acid–base balance and haematological parameters after both vaginal and caesarean deliveries, although the effect was more marked in the group delivered vaginally. The hidden acidosis phenomenon explains this change towards acidaemia and lactaemia. Arterial haemoconcentration was not the explanation of the acid–base drift.

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