The neonatal metabolic acidosis rate varied significantly with blood gas analyzer brand and fetal fluid compartment for calculation of BD.

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 (T 0 ), and at 45 seconds (T 45 ), 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 (T 0 -T 45 ) in umbilical cord blood pH, the partial pressure of CO 2 (P CO 2 ) and O 2 (P O 2 ), and in the concentrations of lactate, haematocrit (Hct), and haemoglobin (Hb).Results In both groups all arterial parameters, except for P CO 2 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 P CO 2 were more pronounced in the group delivered vaginally. Neonates born vaginally had significantly lower pH and higher lactate, Hct, and Hb concentrations at T 0 and T 45 in both the artery and the vein. At T 45 , arterial P CO 2 and P O 2 levels in the group delivered vaginally were also significantly higher.Conclusions Delayed umbilical cord sampling affected the acidbase 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.

Due to pH and BD case value decimal round-offs, a diagnostic discrepancy of acidotic pH values occurred in 8%, and of metabolic acidosis diagnosis in 10.7% of cases. A drift of a dichotomy parameter value cut-off due to decimal round-offs will result in a shift in distribution of positive and negative cases in a population sample.