The goal of fetal heart rate monitoring is to identify the fetus who has low oxygen levels before irreversible damage has been done, in order to prevent such damage. Cardiotocograph (CTG) monitoring relies on clinicians being able to recognise particular patterns of the fetal heart rate which may occur more often when low oxygen levels are present. There is increasing suspicion that our understanding of the physiological links between low oxygen levels and various heart rate patterns isn’t as well established as we like to think it is (Lear et al, 2018).
When you step back and look at what we are doing with CTG monitoring – it doesn’t really make a lot of sense. If you felt short of breath and presented to an emergency department with this, you would expect that someone would check your oxygen level as part of your assessment. If you left having only had your heart rate checked, you’d have grounds to complain about the quality of your care. Yet that is essentially all we offer in maternity care – a surrogate measure only and not particularly accurate guesstimate of what is going on with the oxygen levels of the fetus.
Directly measuring tissue oxygen levels in the fetus can, and has been, done. The sensors required to measure oxygen saturation directly from the fetus are no longer in commercial production but have been assessed in research. Christine East and colleagues summarised the randomised controlled trial evidence in a Cochrane review in 2014. Seven trials examined outcomes from 8013 women. Using fetal oximetry in addition to the CTG, rather than the CTG alone, did not change the caesarean section rate. There was no difference in outcomes for the fetus but the trials were underpowered to show such a difference if it were present. The trials all examined fetal oximetry in addition to, rather than instead of, CTG monitoring.
The sensors used in this research were placed onto the fetal cheek during labour, requiring that the membranes had either opened spontaneously or artificially and a vaginal examination be performed to place the device through the woman’s partially open cervix. This placed limits on how useful the monitoring device was. Advances in technology are beginning to make it possible to monitor fetal oxygen levels using sensors placed externally on the woman’s abdomen instead. Information about one such system can be found here.
We have a long (possibly very long…) way to go before trans-abdominal fetal oximetry turns up on a birth suite near you. I fervently hope that before it does, we subject it to rigorous research that includes a head to head comparison with intermittent auscultation rather than considering oximetry only as an addition to CTG monitoring. It is also important that we ask women about their experiences with the new technology and that we examine the long term outcome of hours of exposure to near infra-red light in utero. Who knows whether this might end up being a better approach than either CTG monitoring or intermittent auscultation. It does intuitively make sense that if you want to know about oxygen levels, then actually measuring them might be a pretty good starting point.
East, C. E., Begg, L., Colditz, P. B., & Lau, R. (2014). Fetal pulse oximetry for fetal assessment in labour. Cochrane Database of Systematic Reviews, 10, CD004075. https://doi.org/10.1002/14651858.CD004075.pub4
Lear, C. A., Wassink, G., Westgate, J., Nijhuis, J. G., Ugwumadu, A., Galinsky, R., Bennet, L., & Gunn, A. J. (2018, Dec). The peripheral chemoreflex: indefatigable guardian of fetal physiological adaptation to labour. The Journal of Physiology, 596(23), 5611-5623. https://doi.org/10.1113/JP274937