Expanding the role for umbilical cord blood gas analysis – a clinical study

Umbilical cord blood gas analysis is a well-validated tool for evaluation of the oxygenation and metabolic status of babies at the time of birth. Current expert guidelines advise that the finding of pH ≤7.00 and base deficit ≥12 mmol/L in blood sampled from the umbilical artery within minutes of birth confirms a diagnosis of neonatal metabolic acidemia. The finding is associated with high risk of tissue hypoxia and consequent adverse neurological outcome (e.g. hypoxic-ischemic encephalopathy, HIE) as well as other adverse events.

Although practice does vary, umbilical cord blood gas analysis is usually reserved for newborns who are judged on the basis of fetal-maternal monitoring prior to birth and other factors such as degree of prematurity, to be at high risk of asphyxia, tissue hypoxia and their potentially devastating consequences.

All newborns are assessed at 1 minute and 5 minutes after birth using the Apgar scoring system that depends on rapid evaluation of five parameters:

• Activity (muscle tone)
• Pulse (heart rate)
• Grimace (response to stimulation)
• Appearance (skin colour pink/grey)
• Respiratory activity

Each of these components is given a score of 0,1 or 2, so overall Apgar score can range from 0 to 10. In general, umbilical cord blood gas analysis would not be performed on babies with a reassuring 5 minutes Apgar score (that is in the range 7-10).

The principal author of this highlighted study works at an Italian hospital where it is routine practice to perform umbilical arterial blood gas analysis on all newborns, not just those at high risk. This policy enabled this retrospective cohort study, which was designed to test the notion that umbilical cord arterial blood analysis can be used to predict development of neonatal respiratory distress syndrome in full-term babies with reassuring Apgar scores. A secondary aim was to determine if the analysis is helpful in predicting resuscitation in the delivery room and admission to neonatal intensive care among these apparently healthy neonates.

The studied cohort comprised 352 full-term neonates with body weight appropriate for gestational age; 63.7 % were born by vaginal delivery, 24.7 % by instrumental delivery, 8.7 % by emergency cesarean operation and 2.8 % by elective cesarean operation. All had reassuring Apgar scores (7-10) at both 1 minute and 5 minutes after birth. And, in line with local policy, all 352 had umbilical cord arterial blood sampled within 15 minutes of delivery for measurement of pH, base excess and lactate.

Despite reassuring Apgar score and other positive indicators – full-term and expected gestational weight – 16 (4.7 %) of these neonates developed respiratory distress syndrome, 28 (8 %) were admitted to neonatal intensive care and 18 (5.1 %) required delivery-room resuscitation.

Binary logistic regression analysis showed the blood gas analyzer parameter with greatest ability to predict adverse events (respiratory distress syndrome, resuscitation and admission to intensive care) was pH. A statistically significant association was found between pH and respiratory distress syndrome. Receiver operating curve (ROC) analysis determined that the optimum pH cut-off point to predict respiratory distress syndrome is 7.12 with a sensitivity and specificity of 68 % and 63 %, respectively.

The authors conclude that umbilical cord arterial blood pH <7.12, even in the presence of reassuring Apgar score, represents a risk factor for respiratory distress syndrome. In this sense, they have demonstrated a value for the use of umbilical cord blood gas analysis in the assessment of all neonates, not just those at high risk of adverse outcomes.