When you have been using a test for years and years, you assume someone way back at the beginning did some pretty decent research to determine what numbers are “normal” and a sign of good health, and what numbers are “abnormal” and a sign of poor health. But is that always the case?
Why do we test cord blood acid levels?
The testing of fetal and neonatal blood acid levels has played an important role in the fetal monitoring story – even before CTGs started to be used. Back in 1965 Saling, a German obstetrician was testing umbilical artery blood pH levels (a test of how acid the blood is) (Boenisch & Saling, 1974). The theory goes like this: when oxygen levels fall low, the fetus switches over to anaerobic metabolism to produce energy needed to stay alive and well, anaerobic metabolism produces lactic acid, so pH falls. (Of course the physiology behind this is actually way more complicated, but in the world of fetal monitoring, folks seem not to let facts get in the way of a good theory.)
Knowing the pH is normal is meant to provide reassurance that over the bit of time before the test was done, fetal oxygen levels have been alright. Knowing it is less than normal suggests that oxygen levels were low. If this is before birth, that would usually prompt advice for caesarean section or instrumental birth. After birth, it suggests closer attention should be paid looking out for potential complications from low oxygen levels, and treatments like therapeutic cooling might be used if any signs of this are seen.
What do CTGs have to do with this?
CTG monitoring has been suggested as a way to prevent high acid levels in the umbilical cord blood just after birth. Only two randomised controlled trials have look at this (the 1985 Dublin trial by MacDonald et al, and the plagued by a major breach of randomisation Athens trial Vintizileos, et al., 1993). Neither trial found less cases of acidosis when CTGs were used. The Dublin trial used a pH cut off of under 7.2, and the Athens trial used 7.1 as their limit of normal. Back in 1974, Boenisch and Saling used 7.3. There’s a hint here that evidence for a definition of a normal pH might be lacking.
So what is a normal pH?
A team of researchers based in the USA decided it was time to find this out (Albrecht, et al., 2023). Their October 2023 paper explains the approach they used. Their data came from 45,457 births (only singletons, no twins or more) in a hospital where it was routine to collect cord blood samples at every birth. They didn’t seem to exclude births that resulted in a poor outcome – which strikes me a odd if the goal is to set a normal range.
They found arterial cord pH levels as follows:
- After elective (not in labour) caesarean section – 7.28 with 95% of the results falling between 7.16 and 7.35
- After caesarean section in labour – 7.27 with 95% of the results falling between 7.10 and 7.34
- After vaginal birth (they don’t say whether this includes instrumental birth or not – it seems like it does from the numbers of births) – 7.27 with 95% of the results falling between 7.14 and 7.36.
They recommend a cutoff point of 7.1 regardless of the mode of birth, saying this “represents an evidence-based lower limit of normal and should replace the historic, arbitrarily chosen value of 7.2” (p. 2). This seems straightforward enough but it means hitting a major rest button on definitions for legal cases where a pH of between 7.1 and 7.2 would previously been seen as evidence of a problem.
There were some other interesting findings. There was very little difference in pH levels of infants born by caesarean section for prolonged labour (7.28 with a range of 7.15 to 7.34) and those born by elective caesarean section. This undermines the longstanding belief that pH falls over the duration of labour. The “your baby is going to get tired” arguments seems not to hold water after all.
They looked to see what effect anaesthesia had on the results. Mean pH was lower with general anaesthesia than for regional (epidural or spinal) – at 7.24 for elective caesarean and 7.21 for in labour caesarean. Vaginal births with regional anaesthesia had a lower pH than those without with a pH of 7.25. These all remain firmly in the range considered normal even by previous standards but hint at physiological changes not fully compensated for.
I’ll leave the final word to the authors of the paper:
Baseline fetal arterial pH levels are significantly lower than those seen in the newborn period; this difference must be considered when using umbilical cord pH to assess the impact of intrapartum events on the fetus. Uncomplicated labor, even when prolonged, does not generally lead to significant cumulative hypoxic stress to the human fetus.
p. 6
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References
Albrecht, K. D., Denning, S., Hosek, K., Burnett, B. A., Sangi-Haghpeykar, H., Belfort, M. A., & Clark, S. L. (2023, Aug 19). Umbilical cord gas analysis: clinical implications of a comprehensive, contemporary determination of normal ranges. Am J Obstet Gynecol MFM, 5(10), 101134. https://doi.org/10.1016/j.ajogmf.2023.101134
Boenisch, H., & Saling, E. (1974). A combined clinical-biochemical scoring of the newborn. Results of the past four years. Journal of Perinatal Medicine, 2(2), 122-129. https://doi.org/10.1515/jpme.1974.2.2.122
MacDonald, D., Grant, A., Sheridan-Pereira, M., Boylan, P. C., & Chalmers, I. (1985, Jul 01). The Dublin randomized controlled trial of intrapartum fetal heart rate monitoring. American Journal of Obstetrics & Gynecology, 152(5), 524-539. https://doi.org/0002-9378
Vintzileos, A. M., Antsaklis, A., Varvarigos, I., Papas, C., Sofatzis, I., & Montgomery, J. T. (1993, Jun 01). A randomized trial of intrapartum electronic fetal heart rate monitoring versus intermittent auscultation. Obstetrics & Gynecology, 81(6), 899-907. https://www.ncbi.nlm.nih.gov/pubmed/8497353
Categories: CTG, EFM, New research
Tags: cord blood, pH, Physiology
Birth Small Talk 
Thanks. Even the taking of the blood test might be relevant if it interferes with the outcome (you know I mean cord clamping!). Do ANY of the studies take blood from a continuing (rather than amputated) circulation and look at changes in the pH with time over transition?
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None that I am aware of, no.
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So, we don’t have a proper description of ‘normal’ uninterupted human neonatal behaviours and pH – all outcome studies based pH and/or Apgar scores and/or NN admissions are flawed – from the outset – as these are proxies taken from interrupted transition? Doesn’t this make the whole of obstetric and neonatal science rather shaky?
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It was always thus…
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