Arrhythmias – is this a missing link in the fetal monitoring story?

There’s a very strong narrative around the way that fetal heart rate monitoring in labour is meant to prevent poor outcomes. But what if that story isn’t quite true? Recent physiology research suggests that the mechanism of fetal harm might not be quite what we have been taught.

What did they do?

A team of researchers in Finland (Lantto et al., 2025) studied fetal sheep, as they wanted to know whether acidosis might provoke arrhythmias in the fetal heart . Under anaesthesia, sensors were placed on fetal sheep that permitted the researchers to record the fetal ECG (electrical activity from the heart), EEG (electrical activity from the brain), and blood gases. A device was placed around the umbilical cord that allowed the researchers to intermittently block blood flow through the cord.

Once the sheep recovered from the surgery several days later, a baseline fetal ECG recording was made. The umbilical cord was then occluded for one minute and released for two minutes, and this was repeated until the fetal blood pH fell below 7.0 and / or the base excess level fell to less than -16 mol/L. Once this point was reached, the sheep were anaesthetised again and a caesarean section was performed, while the monitoring continued. The umbilical cord was kept intact until after ventilation was established.

What happened?

Data from eight fetal sheep were available for analysis. All developed decelerations in the heart rate during occlusion followed by a rise in the heart rate over baseline after occlusion was released. Three fetuses developed acidaemia after the fifth cord occlusion, and all had atrioventricular condition abnormalities in the fetal ECG. All three of these fetuses required resuscitation after birth, with one being defibrillated for ventricular fibrillation.

The remaining five fetuses developed acidaemia after 10 – 17 cord occlusions but showed no evidence of abnormal atrioventricular conduction. One of these went on to develop ventricular fibrillation and died in utero before caesarean section was able to be performed. The remaining four had normal heart rate patterns at birth.

What does this mean for CTG monitoring in humans?

The software built into CTG monitoring systems works hard to build a heart rate recording that looks like what it has been trained a heart rate recording should look like. To achieve this, it uses a process called signal averaging to reduce the “noise” from the original input. But if that “noise” is an irregular heart rate, it becomes invisible on the CTG output.

This research alerts us to the possibility that there could be serious abnormalities in heart rhythm (as distinct from heart rate patterns) in response to low oxygen levels. CTG monitoring is likely to be missing these and this could be part of the explanation why mortality rates are not being improved by using CTGs. Handheld Dopplers that count heart rates use the same technology and are likely to have the same problem detecting the abnormal rhythms. And that’s a worry. Using a direct auscultation method (like a Pinard) is likely to detect abnormal heart rhythms, as long as the listener is specifically paying attention and alert to the possibility of an irregular heart rate.

Final thoughts

This is very preliminary research, and it needs to be tested again in larger studies. There are a few things about this study that make me feel cautious about immediately changing fetal monitoring technology design. Compared to other similar experiments using fetal sheep with repeated cord occlusions, this population of sheep developed acidosis very quickly, and the mortality rate was higher than I have seen in other studies. It makes me wonder whether part of what this team found related to the prior health of the animals they used, or their experience at performing the fetal surgery, or at managing the experimentation after surgery.

Nonetheless, it raises a new and interesting question that we should seek further answers for.

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References

Lantto, J., Johnson, J., Huhta, H., Haapsamo, M., Kiviranta, P., Rasanen, K., Voipio, H. M., Sonesson, S. E., Voipio, J., Rasanen, J., & Acharya, G. (2025, Jun). Atrioventricular conduction abnormalities are associated with poor outcome following intermittent umbilical cord occlusions in fetal sheep. Acta Obstetric et Gynecologica Scandinavica, 104(6), 1207-1217. https://doi.org/10.1111/aogs.15109