Acidosis at birth – significance for very premature, low-birthweight infants

Umbilical-cord blood gas analysis provides objective evidence of the metabolic status of neonates at the time of delivery. Perinatal metabolic acidosis is indicative of hypoxia (sometimes the result of asphyxia during birth) and associated risk of permanent brain damage. 

A recently published study sought to assess the incidence and significance of perinatal acidosis for that very small subset of newborns that are born very prematurely (<28 weeks gestation) and have extremely low birthweight (<1000 g). 

Around a third of these most vulnerable newborns do not survive and for those who do survive, around a third suffer neurodevelopmental impairment, manifest as one or more of a range of permanent life-disabling conditions that include cerebral palsy, blindness, deafness, cognitive deficit, etc. 

The principal question addressed by the study is: can the presence of acidosis at birth be used to help predict outcome for these highly vulnerable babies? The study population comprised 3979 babies born at hospitals across the US between 2002 and 2007 with birthweight in the range of 400-1000 g. Mean (± SD) gestational age for the cohort was 26 (± 2.0) weeks. 

For each of these very premature babies the results of cord blood gas analysis were retrieved. Additionally, clinical data relating to each pregnancy/birth, as well as eventual outcome for the baby were recovered. For the purposes of the study acidosis was defined as either cord-blood pH less than 7.0 or cord-blood base excess less than –12 mmol/L. The primary study outcome measure was death/neurodevelopmental impairment (NDI).

Of the 3976 study subjects, 249 (6.3 %) suffered acidosis at birth and 2124 (53 %) had the primary outcome (i.e. either died or survived with NDI). Statistical analysis revealed that acidosis is significantly associated with death or NDI; 66 % of acidosis-affected patients had the primary outcome compared with just 52.5 % of those without acidosis. 

The calculated odds ratio (OR) for relationship between acidosis (defined as less than 7.0) and either death or NDI was 2.5, and for the relationship between acidosis (defined as base excess less than –12 mmol/L) and either death of NDI, the calculated OR was 1.5. 

Despite this, further statistical analysis found that inclusion of pH and base excess in a multivariable model that included other established predictors (gestational age, birthweight, 5-minutes Apgar score, etc.) did not improve the prediction of outcome.

In summary, the authors of this, the largest ever study to address the issue, conclude that although perinatal acidosis increases the likelihood of a poor outcome for these extremely low-birthweight infants it is a relatively rare occurrence (affecting only 6 % of infants in their study) and measurement of pH (or base excess) at birth is not useful in predicting outcome; other factors such as gestational age, birthweight and Apgar score are more useful in this regard.