The logic that sits behind the assumptions for how intrapartum CTG monitoring works go a little bit like this:
- Oxygen levels can fall during labour
- Low oxygen levels cause damage to the fetus in a predictable fashion
- Low oxygen levels also cause changes to the fetal heart rate in a predictable fashion
- Detecting changes in the fetal heart rate associated with low oxygen levels allows clinicians to achieve birth earlier
- Damage to the fetus is therefore minimised
Today I want to look at the second item of logic – that low oxygen levels are predictably associated with damage to the fetus.
Johnson et al. (2021) have published an analysis of the relationship between umbilical artery pH levels and base excess with one- and five-minute Apgar scores this week. Why measure pH and base excess? As oxygen levels fall in cells in the body, metabolic processes switch over to one that produces excess acid. pH and base excess fall. It is easier to measure pH and base excess and they reflect longer term oxygenation, whereas oxygen levels themselves can fluctuate from moment to moment.
The research team were based in Texas, USA, and analysed outcomes from the births of 29,787 babies. They found that the correlation between either pH or base excess and Apgars at either one or five minutes was weak. The only neonatal death occurred in a baby with normal pH and base excess. A higher caesarean section rate was found when the baby was born with a pH of under 7 (53.6% vs 35.5% for babies born with a pH of 7 or more, p<0.01).
The authors commented:
These results document the presence of wide biological variability in fetal tolerance of the hypoxic stress of labor and suggest that such tolerance is poorly reflected by arterial blood gas analysis, and hence also by the recognition of fetal heart rate patterns designed to predict such values. Given this poor correlation between acidemia and short-term neonatal outcomes (Apgar scores) and considering the remarkable recuperative capacities of the newborn, the well documented poor correlation between any fetal heart rate pattern or Apgar score and long-term neurologic outcome should not be surprising.p. 4-5
The finding of only a weak association between low oxygen levels (manifesting as low pH) and worse outcomes for babies is not new and has been repeatedly noted by researchers over an extended period of time (for example Beeby et al., 1994; Leinonen et al., 2019; Lissauer & Steer, 1987). What we have been slow to take into consideration is that this knowledge requires a different response from health care providers if we are to improve outcomes for the baby and for women (by avoiding caesarean sections that do not lead to improved outcomes for the baby). Simply trying harder to do the same thing – for example by focusing on education regarding CTG interpretation – is destined to fail.
As Johnson and colleagues point out in their paper:
First, the significant contribution of electronic fetal heart rate monitoring to the current cesarean delivery rate combined with the failure of such monitoring to significantly reduce the rate of neonatal hypoxic ischemic encephalopathy or subsequent cerebral palsy has less to do with inadequate pattern interpretation by clinicians and much more to do with a wide biological variability in the ability of the human fetus to tolerate the hypoxic stress of labor, leading to a highly variable and unpredictable individual threshold for injury. Second, further refinement of techniques of fetal heart rate interpretation, designed to more accurately predict fetal pH, or the use of alternative approaches to the evaluation of fetal acidemia (such as lactate levels) do not affect this fundamental weakness in the use of electronic fetal heart rate monitoring and are unlikely to improve the clinical usefulness of this tool.p. 6
Purchasing more technology and training, while well intentioned, is not going to achieve the desired outcomes if the foundational principles on which our approaches to fetal monitoring are based are fundamentally flawed. To borrow a term from Catherine Williams (@BerksMaternity) – this is “safety theatre”. It is designed to look like something is being done, while doing nothing.
Beeby, P. J., Elliott, E. J., Henderson-Smart, D. J., & Rieger, I. D. (1994, Sep). Predictive value of umbilical artery pH in preterm infants. Archives of Disease in Childhood, 71(2), F93-96. http://eutils.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&id=7979485&retmode=ref&cmd=prlinks
Johnson, G. J., Salmanian, B., Denning, S. G., Belfort, M. A., Sundgren, N. C., & Clark, S. L. (2021, Aug 5). Relationship Between Umbilical Cord Gas Values and Neonatal Outcomes: Implications for Electronic Fetal Heart Rate Monitoring. Obstetrics & Gynecology, in press. https://doi.org/10.1097/AOG.0000000000004515
Leinonen, E., Gissler, M., Haataja, L., Andersson, S., Rahkonen, P., Rahkonen, L., & Metsäranta, M. (2019, Apr 07). Umbilical artery pH and base excess at birth are poor predictors of neurodevelopmental morbidity in early childhood. Acta Paediatrica, 108(10), 1801-1810. https://doi.org/10.1111/apa.14812
Lissauer, T. J., & Steer, P. J. (1987). The Relation Between the Need for Intubation at Birth, Abnormal Cardiotocograms in Labour and Cord Artery Blood Gas and pH Values. Obstetric Anesthesia Digest, 7(2), 57. https://doi.org/10.1097/00132582-198707000-00019