Comparing continuous CTG use with intermittent CTG use

In some settings, women are offered the option of having intermittent CTG use rather than continuous CTG use. This might look something like having 20 to 30 minutes of CTG use every hour or two, with intermittent auscultation used during the time off the CTG.

A first look, this seems like it might be a way to reduce some of the downsides associated with CTG use (like the increased rate of caesarean section and restrictions to mobility), while still using CTG monitoring. This week’s post explores evidence from the one and only randomised controlled trial that has compared intermittent CTG use with continuous CTG use (Herbst & Ingemarsson, 1994).

How was the research done?

This research was conducted in Sweden in the years 1989 to 1991. Only women who were considered to be low risk were eligible to take part. Women were excluded if they were having a pre labour caesarean section, had a hypertensive disorder, diabetes, fetal growth restriction, reduced or meconium stained liquor, multiple pregnancy, breech presentation, abruption, an abnormal admission CTG, a known fetal abnormality, or were under 33 weeks pregnant.

After consenting to take part, women were randomly assigned to either continuous CTG use or intermittent CTG use. External monitors were used until the membranes ruptured, and then a fetal spiral electrode. Telemetry was available if women wanted to mobilise, but they don’t tell us how often this was used. In both arms of the study, fetal blood sampling was able to be used it there were concerns about the heart rate pattern.

Intermittent CTG use involved a 15 to 30 minute period of CTG monitoring every second hour during the first stage of labour, with intermittent auscultation every 15 to 30 minutes when the CTG was not in use. If oxytocin was started, CTG monitoring was used until 30 minutes after the last dose adjustment to the infusion. If an epidural was administered, CTG monitoring was used for the first hour and for 30 minutes after any bolus doses. If the CTG recording was considered “equivocal” or “ominous”, or meconium staining or bleeding were noted, it was left on. All women in the intermittent CTG monitoring group had continuous CTG monitoring in the second stage of labour.

It is important to note that the aim of the study was not to show whether there were better or worse outcomes for women or for babies. The study was designed to find out whether intermittent CTG use would mean that “ominous” heart rate patterns would be missed. They designed the size of the study with this in mind.

Were the women really low risk?

Table 3 in the paper provides information about the women who took part in the study. In total, 2,015 women were randomised to intermittent CTG use and 2029 to continuous CTG use. Oxytocin was used for induction or stimulation of labour for 48% of women. Three percent were preterm and seven percent post term (> 42 weeks). Epidurals were used by 18% of women.

I have written about this before in relation to other older studies about fetal heart rate monitoring – the goal posts have shifted when it comes to what is considered risky. The majority of the women in this study would now be considered to be high risk. (Keep that in mind for the next time someone tries to tell you that most of the research about CTG use was done in low risk populations – this is simply untrue!)

What did the study show?

There was no significant difference in the rate of “abnormal” fetal heart rate patterns (bradycardia, tachycardia, reduced variability, sinusoidal, late, prolonged or severe decelerations) between the two approaches to monitoring. There was a difference in the amount of time CTG monitoring was in use in the first stage and for the total duration of labour (but not for second stage, given the study design). So the lack of difference was not because the intermittent CTG use turned into continuous use in most cases.

Noninstrumental vaginal birth rates were high in both groups (92% in the intermittent group and 90% in the continuous group) with no significant difference between any of the modes of birth. Both the average duration of labour and the number of women labouring for more than 12 hours were the same in both groups. Outcomes for the babies were also no different – cord blood gases, Apgar scores, and admission to the nursery were measured. They didn’t report on deaths – presumably there were none – or on neonatal seizures.

Importantly – the authors noted that there was never a situation where intermittent auscultation was considered to be normal but when the CTG was commenced, a significantly abnormal heart rate pattern was found.

So what does this mean?

For women who decide to use CTG monitoring for labour, and for one reason or another want to take a break from it for an hour or two – this study supports the idea that this is a safe thing to do. The findings from this study don’t provide proof that spending more time using CTG monitoring leads to better outcomes for women or their babies. It is of course, important to be careful when applying research findings produced in a different time and place to a specific current situation, and that this is only one small study. It is also important to note that no research has ever compared intermittent auscultation with intermittent CTG use. We need more research!

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References

Herbst, A., & Ingemarsson, I. (1994, Aug 01). Intermittent versus continuous electronic monitoring in labour: a randomised study. British Journal of Obstetrics & Gynaecology, 101(8), 663-668. https://doi.org/10.1111/j.1471-0528.1994.tb13181.x