Hyperstimulation During an Induction

What it is

Hyperstimulation is the uterus contracting too often, or for too long, in response to a drug given to start or speed up labour. ACOG describes the same thing in plainer words: "when oxytocin is used, the uterus may be overstimulated. This may cause the uterus to contract too often."

The problem is not the contractions themselves. It is the gaps between them. The placenta refills with blood between contractions, and that is when your baby is resupplied with oxygen. Take the gaps away and the resupply goes with them. ACOG: "Too many contractions may lead to changes in the fetal heart rate."

The threshold

NICE's guideline on fetal monitoring in labour puts a number on it: "If 5 or more contractions per 10 minutes are present: perform a full risk assessment; take action to reduce contraction frequency ...; explain to the woman what is happening, and ensure that she has adequate pain relief."

Five in ten is the line at which the team is asked to act rather than watch. It is one of the reasons the frequency of your contractions is documented half-hourly in established labour, and continuously if you are on a monitor.

What causes it

The methods used to induce labour, in NICE's own ordering of risk.

NICE asks teams to explain, before an induction starts, that "both dinoprostone and misoprostol can cause hyperstimulation" — dinoprostone being the prostaglandin given as a pessary, gel or tablet, and misoprostol the oral tablet. It also asks teams to explain that "when using pharmacological methods of induction, uterine activity and fetal condition must be monitored regularly".

An oxytocin infusion, used to start or strengthen contractions once the waters are broken, can do the same thing. ACOG names it as the main example.

Mechanical methods are different. NICE: "mechanical methods are less likely to cause hyperstimulation than pharmacological methods." The HSE says it more directly about balloon induction: "This method avoids any risk of too many contractions", and notes it may be recommended if you have had one previous caesarean, or if other methods have not opened the cervix enough.

What it feels like

Contractions that arrive before the last one has finished, with little or no recovery in between; pain that does not let up; and a sense that something is different from the labour you were braced for. NICE's own transfer criteria include "pain reported by the woman that differs from the pain normally associated with contractions", which is a recognition that this is something a woman notices before a machine does.

Say it. That sentence is on the guideline's list because it is diagnostic information, not a complaint.

What is done about it

The response is to remove the cause, and the options depend on which method was used. NICE: "if hyperstimulation does occur, the induction treatment will be stopped by giving no further medication, or by removal of vaginally administered products when possible."

Removability is not equal across products, and NICE flags the difference in advance: "there are differences in the ease with which different vaginal products can be removed (for example, dinoprostone controlled-release vaginal delivery systems can be more easily removed than gel or vaginal tablets)."

That is why the pessary has a string. The HSE describes the Propess pessary as looking like a small tampon, left in for 24 hours, with "a short string attached to the pessary to remove it easily". A gel cannot be taken back out, which is worth knowing when you are choosing between them.

If an oxytocin infusion is running, NICE's fetal monitoring guideline asks teams to reduce contraction frequency "by reducing or stopping oxytocin if it is being used", and to consider offering a tocolytic drug — a medicine that relaxes the uterus. ACOG describes the same sequence: "If there are problems with the fetal heart rate, oxytocin may be reduced or stopped. Other treatments may be needed to steady the fetal heart rate."

NICE adds one caution about the oral tablet: hyperstimulation "caused by misoprostol may be more difficult to reverse", because a swallowed tablet cannot be retrieved.

What happens alongside

Position change, because maternal position affects uterine blood flow and cord compression. Intravenous fluids if your blood pressure is low, particularly after an epidural top-up. And a review of the whole picture rather than the trace alone.

Two things NICE says not to do are worth knowing, because both were once routine. Facial oxygen should not be offered as part of conservative measures "because it may harm the baby", although it can be given if you need it yourself or before an anaesthetic. And intravenous fluids should not be offered to treat fetal heart rate abnormalities unless you are hypotensive or have signs of sepsis.

How likely is it?

NICE publishes hyperstimulation rates for different induction agents in an appendix to its guideline for clinicians, and asks teams to discuss them with women choosing between methods. We are not going to reproduce a single headline figure here, because the rate depends entirely on which drug, which preparation and which regimen is used, and a number stripped of that context would mislead you.

The useful version of the question for your own appointment is: which method are you offering me, and what is the chance of hyperstimulation with that one compared with the alternatives?

Where it fits in the wider picture

Hyperstimulation is one of the reasons an induction is a hospital procedure with monitoring attached, and one of the reasons NICE asks that the risks and benefits of each method be discussed rather than the method simply being chosen for you.

The NHS's description of induction notes that hormones are usually offered first "unless there's a medical reason you cannot take hormones", and that induction can take a while to work. NICE's intrapartum guideline lists an obstetric emergency, and changes in the baby's heart rate, among the findings that trigger a change in where and how you are cared for.

What you can ask, and decline

Before an induction starts: which method, how removable is it, and what happens if I contract too often? Can I have a mechanical method instead, and would that suit my situation?

During one: how many contractions am I having in ten minutes? Is the pessary coming out? Is the drip being turned down? Can I have more pain relief while you do it?

NICE is explicit that women can have an amniotomy and "choose whether or not to have an oxytocin infusion, or can delay starting this", while being told that this may mean labour takes longer and there may be an increased risk of neonatal infection. The infusion is an offer with a rate that can be turned down as well as up, and asking for it to be slowed is a normal request rather than an obstruction.