• P Mokarami, N Wiberg, and P Olofsson.
• Institution of Clinical Sciences, Department of Obstetrics and Gynaecology, Skåne University Hospital, Lund University, Malmö, Sweden. parisa.mokarami@med.lu.se
• BJOG. 2013 Jul 1; 120 (8): 996-1002.

ObjectiveTo 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.DesignProspective comparative study.SettingUniversity hospital.SampleUmbilical cord blood from 124 newborns.MethodsArterial 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 MeasuresTemporal changes (T₀-T₄₅) in umbilical cord blood pH, the partial pressure of CO₂ (PCO₂) and O₂ (PO₂), and in the concentrations of lactate, haematocrit (Hct), and haemoglobin (Hb).ResultsIn both groups all arterial parameters, except for PCO₂ 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 PCO₂ 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 PCO₂ and PO₂ levels in the group delivered vaginally were also significantly higher.ConclusionsDelayed 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.© 2013 The Authors BJOG An International Journal of Obstetrics and Gynaecology © 2013 RCOG.

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