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.

Introduction Traditional validation of umbilical cord blood samples with positive veno‐arterial ΔpH and arterio‐venous ΔpCO2 values confirms the source of samples, whereas negative Δvalues represent mix‐up of samples. To investigate whether this is true, the distributions of V‐A ΔpO2 and A‐V Δlactate were also explored and related to clinical characteristics. In addition, different cord blood sampling techniques were evaluated. Material and methods Register study with cord blood acid‐base and clinical data from 27 233 newborns. Clinical characteristics were related to positive, zero and negative Δvalues. Blood samplings from unclamped and double‐clamped cords were compared. A two‐sided P < 0.05 was considered significant. Results ΔpH and ΔpCO2 values distributed into positive, around zero, and negative sub‐populations, with significant differences in pH and clinical characteristics between sub‐populations. No such sub‐populations were distinguished for ΔpO2 and Δlactate. The 2.5th and 5th ΔpH percentiles were 0.013 and 0.022, respectively, and for ΔpCO2 0.30 and 0.53 kPa. Applying 5th percentile criteria resulted in 3.5% of “approved” cases showing a ΔpO2 ≤ 0. Puncture and sampling of the unclamped cord resulted in significantly better sample quality. Conclusions Unphysiological negative ΔpO2 values occurred despite correct validation with traditional criteria. Δlactate cannot be used for validation because both positive and negative values are physiological. Positive/around zero/negative ΔpH and ΔpCO2 sub‐populations were associated with significant differences in pH and clinical characteristics, indicating that defective sampling and sample handling are not the sole explanations for negative Δvalues. Prompt puncture and sampling of the unclamped cord resulted in best sample quality.

In both SGA and AGA fetuses, glucose was mobilized during acidemia; lactate was higher in SGA at both a normal and low pH. Considering SGA being a proxy for fetal growth restriction, and acidemia a proxy for hypoxia, it is concluded that growth-restricted fetuses have an intact ability to develop lacticemia during hypoxia.