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

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 (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.

The effect of delayed umbilical cord clamping on cord blood gas analysis in vaginal and caesarean‐delivered term newborns without fetal distress: a prospective observational study

“…Arterial and venous cord blood was sampled immediately after birth from the unclamped umbilical cord (T0), as previously described . At 3 minutes (T3), the cord was triple clamped: the first clamp at 5 cm from the newborn, the second clamp at 1–2 cm from the first, and the third clamp at 10–20 cm from the second.…”

“…A sample size of 84 neonates in each group was calculated to demonstrate a 0.04 reduction in pH between T0 and T3, with a two‐sided alpha level of 0.05 and 90% power. Based on previous work, we increased the sample size of VDs to 97 and the sample size of CDs to 124.…”

“…Previous studies on parameter variations of CBG after DCC are scant and refer mainly to VDs. They showed a trend towards mixed acidosis when sampling is deferred, as a result of acid metabolites accumulating during labour in peripheral fetal tissue (hidden acidosis), and released after birth, and with the increase in partial pressure of carbon dioxide (pCO 2 ) before effective neonatal ventilation is established …”

Arterio‐venous blood gas Δvalues for validation of umbilical cord blood samples at birth are not only biased by sample mix ups but also affected by clinical factors

“…When sampling is delayed and the cord blood flow continues, the arterial blood values change toward acidemia, hypercapnia and lacticemia . This phenomenon, called hidden acidosis, might garble the Δvalues.…”

“…When sampling is delayed and the cord blood flow continues, the arterial blood values change toward acidemia, hypercapnia and lacticemia . This phenomenon, called hidden acidosis, might garble the Δvalues. When the newborn's respiration has started, the arterial pO 2 increases until the cord pulsations ebb away, whereas venous pO 2 peaks at 45 seconds and thereafter decline .…”