The use of fetal blood sampling for pH testing (a way to measure acidity) was pioneered by German obstetrician Erich Saling in the 1960s. Saling’s focus on acidosis predated the widespread introduction of intrapartum CTG monitoring but was motivated by the same underlying aim: to find a way to identify which fetuses will benefit from having labour ended sooner through surgery. CTG monitoring overtook the use of fetal blood sampling in labour, which eventually found a role as a secondary test used when the CTG was considered to be normal.
The equipment required to calculate pH is typically bulky, making “point of care” testing difficult. I have memories of being a junior registrar waiting for a midwife to run to the intensive care unit elsewhere in the hospital to run the test, quietly hoping that the precious sample would not be dropped, or the machine would not refuse to run! Lactate, or lactic acid, is one of the acids generated by metabolic acidosis which contributes to the fall in pH seen with lower oxygen levels. Small portable lactate testing devices began to be used instead of the bulky and expensive pH equipment and also had the advantage that much smaller volumes of blood are needed (Wang et al., 2017).
Concerns have been raised about the ability of pH testing to accurately reflect the risk of poorer outcomes (Al Wattar et al., 2019; Leinonen et al., 2019). There is much less research on the relationship between lactate levels and outcomes. It has been assumed that the two tests produce results which are interchangeable, so that a normal pH level is considered equivalent to a normal lactate level, but research suggests that this is not true. Prouhèze et al. (2020) tested both pH and lactate in 480 fetal blood samples collected during labour. Of 388 normal scalp pH samples, 24% had an abnormal lactate. On the basis of these findings, using the lactate level rather than pH for decision making would push the surgical birth rate up unnecessarily.
It has been recently pointed out that lactate meters are not all alike (Holzmann et al., 2021). A lactate level of 4.8 mmol/L or above is widely used as an indicator that surgical birth is warranted, but this level was established on a particular brand of monitor (the Lactate Pro 1), no longer commercially available. Two independent research groups have sought to calculate a cut off for the newer Lactate Pro 2, with one settling on 7.3 mmol/L and the other 7.1 mmol/L. Again, using the newer meters but the older reference range would push up the surgical birth rate.
Given the research, we once again need to take great care when introducing new technology or updating that technology in our birth environments. Like the CTG, fetal lactate testing has been under assessed, with no clear evidence to establish the impact of lactate testing on perinatal outcomes nor on surgical birth rates. It does seem possible that switching from pH to lactate, and then updating to newer lactate monitors without adjusting cut off levels contributes to the maintenance of high caesarean section rates.
Al Wattar, et al. (2019). Evaluating the value of intrapartum fetal scalp blood sampling to predict adverse neonatal outcomes: A UK multicentre observational study. European Journal of Obstetrics, Gynecology, and Reproductive Biology, 240, 62-67. https://doi.org/10.1016/j.ejogrb.2019.06.012
Holzmann, M., Nordstrom, L., & Steer, P. (2021). Inconsistency between lactate meters in the assessment of fetal metabolic acidemia. Acta Obstetrica Gynecologica Scandanavica, 100(5), 815-817. https://doi.org/10.1111/aogs.14140
Leinonen, E., Gissler, M., Haataja, L., Andersson, S., Rahkonen, P., Rahkonen, L., & Metsäranta, M. (2019). Umbilical artery pH and base excess at birth are poor predictors of neurodevelopmental morbidity in early childhood. Acta Paediatrica, 108, 1801-1810. https://doi.org/10.1111/apa.14812
Prouhèze, A., Girault, A., Barrois, M., Lepercq, J., Goffinet, F., & Le Ray, C. (2020). Fetal scalp blood sampling: Do pH and lactates provide the same information? Journal of Gynecology Obstetrics and Human Reproduction, 50(4), 101964. https://doi.org/10.1016/j.jogoh.2020.101964
Wang, M., Chua, S. C., Bouhadir, L., Treadwell, E. L., Gibbs, E., & McGee, T. M. (2017). Point-of-care measurement of fetal blood lactate – Time to trust a new device. Australian and New Zealand Journal of Obstetrics and Gynaecology, 355, i6405-6407. https://doi.org/10.1111/ajo.12671