ST segment analysis and the caesarean section rate

One of the downsides to CTG use in labour is the high false positive rate. That is to say, the high number of fetuses who have an abnormal heart rate pattern but who have normal oxygen levels. This high false positive rate drives higher rates of caesarean section and instrumental birth for fetuses who cannot possibly benefit from these procedures as they were never at risk of harm in the first place. Standard approaches to interpretation of the CTG pattern have not been able to tell the difference between a fetus who really would benefit from earlier birth and those who won’t. One approach developed to attempt to solve this issue is STAN or analysis of the ST segment of the fetal ECG pattern.

What is STAN?

Computer analysis of the ST segment of the fetal ECG (that’s the electrical signal generated by the heart) was developed in the early 1990s (Westgate et al., 1992). The theory was that, as is the case in adults, low oxygen levels in the tissues of the fetal heart would show up as changes in the shape of the T wave and the height of the ST segment above the baseline (Rosen et al., 1976). The presence of these changes is believed to provide evidence of low oxygen levels in a critical tissue in the body, and therefore is considered a more accurate marker of fetal oxygen status than abnormal heart rate patterns.

The ECG changes are tricky to see so computer systems have been developed to analyse the ST segment of the fetal ECG by calculating the T/QRS ratio. This requires the use of a sensor that can record the fetal ECG – like a fetal spiral electrode. In recent years, externally worn noninvasive fetal ECG sensors have also started coming to the market (more about that here and here). For STAN monitoring to work, a period of around 20 mins of monitoring are required to establish a baseline and for this to be reliable it must be done while the fetal heart rate pattern is normal (Steer & Hvidman, 2014). It’s therefore not something you can turn on only once the fetal heart rate pattern is of concern.

Does STAN work?

The most recent systematic review of trials about STAN was the 2021 paper by Al Wattar and colleagues. They found no improvements in perinatal outcomes and no impact on the caesarean section rate when STAN was used.

Since their review, Blix and colleagues (2022) have published a population study examining the impact of introducing STAN in Sweden. They found no difference in perinatal outcomes, other than a rise in the rate of Apgar scores of less than seven at five minutes of age. There were no changes in the rate of caesarean section or instrumental births that could be attributed to STAN.

Victor et al. (2023), in Denmark recently reported their randomised controlled trial, where they compared CTG use with STAN against CTG and fetal blood sampling when the CTG was abnormal. They found no differences in the incidence of metabolic acidosis at birth between the two approaches. STAN use was associated with a higher rate of instrumental birth, with no difference in the caesarean section rate.

START

Kuah and team (2023) are the most recent group to add data about STAN use. Their STAN Australian randomised controlled trial (called START) has just been published. STAN is not widely used in Australia, with this study believed to be the only application of the technology here.

The study compared CTG with STAN with CTG use alone. Fetal blood sampling could be used in either arm of the study, but was used only once (in the CTG only group). The primary aim of the study was to examine differences in the use of unplanned caesarean section in labour, and the sample size required to achieve this was 1,818 women. 970 were ultimately recruited. This left the trial underpowered, but it nonetheless provided data that will be useful in future meta-analyses. The COVID pandemic hampered recruitment, and many women declined to participate as they were concerned about the use of a fetal spiral electrode or impaired mobility, as telemetry was not available when STAN was in use.

The START trial found no difference in the caesarean section rate or in the instrumental birth rate. The sample size was too small to permit statistical analysis of most of the less common perinatal outcomes. They did provide an analysis for admission to the nursery, which was not statistically significant between the two groups.

Where does that leave us?

To date, STAN technology has not demonstrated an ability to reduce the use of surgical birth by reducing the false positive rate of abnormal CTG patterns. This might be because the technology and the assumptions about how it works are flawed. It could also be that in every trial conducted to date, STAN has always been used in conjunction with CTG monitoring.

Looking at the guidelines about how to interpret the significance of the T/QRS ratio, the first step is always to interpret the CTG using a standardised approach. Then the T/QRS ratio is used to provide further refinement to support the decision to recommend fetal blood sampling or surgical birth. Maternity professionals, particularly those who are far more familiar with CTG interpretation than with STAN, and likely to trust the decisions they make on the basis of the CTG alone. When the fetal heart rate pattern is abnormal, but the T/QRS ratio is reassuring, they may still advise surgical birth. The studies therefore continue to be dominated by the high false positive rate inherent in CTG use.

To truly assess the potential of STAN, a trial making a comparison between STAN alone (without CTG data) and CTG use might be fruitful. We won’t make significant inroads into further improving perinatal outcomes until we find biological markers for fetal hypoxic damage that are better than relying on heart rate pattern analysis. STAN might be that thing, but we’ll never know if we don’t remove its attachment to CTG use when testing it.

References

Al Wattar, B. H., Honess, E., Bunnewell, S., Welton, N. J., Quenby, S., Khan, K. S., Zamora, J., & Thangaratinam, S. (2021). Effectiveness of intrapartum fetal surveillance to improve maternal and neonatal outcomes: a systematic review and network meta-analysis. Canadian Medical Association Journal, 193(14), E468-E477. https://doi.org/10.1503/cmaj.202538

Blix, E., Eskild, A., Skau, I., & Grytten, J. (2022, Jul). The impact of the introduction of intrapartum fetal ECG ST segment analysis. A population study. Acta Obstetricia et Gynecologica Scandinavica, 101(7), 809-818. https://doi.org/10.1111/aogs.14347

Kuah, S., Simpson, B., Salter, A., Matthews, G., Louise, J., Bednarz, J., Chandraharan, E., Symonds, I., McPhee, A., Mol, B. W., Turnbull, D., & Wilkinson, C. (2023, Jun 8). Comparing the effect of CTG+STan with CTG alone on emergency cesarean section rate: STan Australian Randomized controlled Trial (START). Ultrasound in Obstetrics & Gynecology, in press. https://doi.org/10.1002/uog.26279

Rosen, K. G., Hökegård, K. H., & Kjellmer, I. (1976). A study of the relationship between the electrocardiogram and hemodynamics in the fetal lamb during asphyxia. Acta physiologica Scandinavica, 98(3), 275-284. https://doi.org/10.1111/j.1748-1716.1976.tb10312.x

Steer, P. J., & Hvidman, L. E. (2014, Apr 15). Scientific and clinical evidence for the use of fetal ECG ST segment analysis (STAN). Acta Obstetricia et Gynecologica Scandinavica, 93(6), 533-538. https://doi.org/10.1111/aogs.12369

Victor, S. F., Bach, D. B. B., Hvelplund, A. C., Nickelsen, C., Lyndrup, J., Wilken-Jensen, C., Scharff, L. J., Weber, T., Secher, N. J., & Krebs, L. (2023, Jun). Cardiotocography combined with ST analysis versus cardiotocography combined with fetal blood sampling in deliveries with abnormal CTG: a randomized trial. Arch Gynecol Obstet, 307(6), 1771-1780. https://doi.org/10.1007/s00404-022-06649-3

Westgate, J., Harris, M., Curnow, J. S. H., & Greene, K. R. (1992). Randomised trial of cardiotocography alone or with ST waveform analysis for intrapartum monitoring. Lancet, 340(8813), 194-198. https://doi.org/10.1016/0140-6736(92)90465-f