Cardiotocography (CTG) is used to measure the fetal heart rate and the contractions of the uterus. It is also known as electronic fetal monitoring. It is a useful way of monitoring the condition of the fetus and the activity of labour.
CTG can help guide decision making and delivery. However, it should not be used in isolation for decision making, and it is essential to take into account the overall clinical picture.
Two transducers are placed on the abdomen to get the CTG readout:
- One above the fetal heart to monitor the fetal heartbeat
- One near the fundus of the uterus to monitor the uterine contractions
The transducer above the fetal heart monitors the heartbeat using Doppler ultrasound. The transducer above the fundus uses ultrasound to assess the tension in the uterine wall, indicating uterine contraction.
Indications for Continuous CTG Monitoring
The indications for continuous CTG monitoring in labour include:
- Maternal tachycardia (> 120)
- Significant meconium
- Pre-eclampsia (particularly blood pressure > 160 / 110)
- Fresh antepartum haemorrhage
- Delay in labour
- Use of oxytocin
- Disproportionate maternal pain
There are five key features to look for on a CTG:
- Contractions – the number of uterine contractions per 10 minutes
- Baseline rate – the baseline fetal heart rate
- Variability – how the fetal heart rate varies up and down around the baseline
- Accelerations – periods where the fetal heart rate spikes
- Decelerations – periods where the fetal heart rate drops
Contractions are used to gauge the activity of labour. Too few contractions indicate labour is not progressing. Too many contractions can mean uterine hyperstimulation, which can lead to fetal compromise. It is also important to interpret the fetal heart rate in the context of the uterine contractions.
Accelerations are generally a good sign that the fetus is healthy, particularly when occurring alongside contractions of the uterus.
Baseline Rate and Variability
Baseline rate and variability can be described as reassuring, non-reassuring and abnormal (adapted from NICE guidelines 2017):
110 – 160
100 – 109 or 161 – 180
Below 100 or above 180
5 – 25
Less than 5 for 30 – 50 minutes or
More than 25 for 15 – 25 minutes
Less than 5 for over 50 minutes or
More than 25 for over 25 minutes
Decelerations are a more concerning finding. The fetal heart rate drops in response to hypoxia. The fetal heart rate is slowing to conserve oxygen for the vital organs. There are four types of decelerations to be aware of:
- Early decelerations
- Late decelerations
- Variable decelerations
- Prolonged decelerations
Early decelerations are gradual dips and recoveries in heart rate that correspond with uterine contractions. The lowest point of the declaration corresponds to the peak of the contraction. Early decelerations are normal and not considered pathological. They are caused by the uterus compressing the head the fetus, stimulating the vagus nerve of the fetus, slowing the heart rate.
Late decelerations are gradual falls in heart rate that starts after the uterine contraction has already begun. There is a delay between the uterine contraction and the deceleration. The lowest point of the declaration occurs after the peak of the contraction. Late decelerations are caused by hypoxia in the fetus, and are a more concerning finding. They may be caused by excessive uterine contractions, maternal hypotension or maternal hypoxia.
Variable decelerations are abrupt decelerations that may be unrelated to uterine contractions. There is a fall of more than 15 bpm from the baseline. The lowest point of the declaration occurs within 30 seconds, and the deceleration lasts less than 2 minutes in total. Variable decelerations often indicate intermittent compression of the umbilical cord, causing fetal hypoxia. Brief accelerations before and after the deceleration are known as shoulders, and are a reassuring sign that the fetus is coping.
Prolonged decelerations last between 2 and 10 minutes with a drop of more than 15 bpm from baseline. This often indicates compression of the umbilical cord, causing fetal hypoxia. These are abnormal and concerning.
The NICE guidelines (2017) have criteria for describing findings of decelerations as reassuring, non-reassuring and abnormal. It is worth remembering that the CTG is reassuring when there are no decelerations, early decelerations or less than 90 minutes of variable decelerations with no concerning features.
Regular variable decelerations and late decelerations are classed as non-reassuring or abnormal, depending on the features. Prolonged decelerations are always abnormal.
Management Based on the CTG
The NICE guidelines (2017) recommend categorising the CTG based on three features of the CTG described above:
- Baseline rate
The four categories for CTG are:
- Suspicious: a single non-reassuring feature
- Pathological: two non-reassuring features or a single abnormal feature
- Need for urgent intervention: acute bradycardia or prolonged deceleration of more than 3 minutes
The outcome of the CTG will guide management, such as:
- Escalating to a senior midwife and obstetrician
- Further assessment for possible causes, such as uterine hyperstimulation, maternal hypotension and cord prolapse
- Conservative interventions such as repositioning the mother or giving IV fluids for hypotension
- Fetal scalp stimulation (an acceleration in response to stimulation is a reassuring sign)
- Fetal scalp blood sampling to test for fetal acidosis
- Delivery of the baby (e.g. instrumental delivery or emergency caesarean section)
There is a “rule of 3’s” for fetal bradycardia when they are prolonged:
- 3 minutes – call for help
- 6 minutes – move to theatre
- 9 minutes – prepare for delivery
- 12 minutes – deliver the baby (by 15 minutes)
A sinusoidal CTG is a rare pattern to be aware of, as it can indicate severe fetal compromise. It gives a pattern similar to a sine wave, with smooth regular waves up and down that have an amplitude of 5 – 15 bpm. It is usually associated with severe fetal anaemia, for example, caused by vasa praevia with fetal haemorrhage.
DR C BRaVADO
DR C BRaVADO is a mnemonic often taught to assess the features of a CTG in a structured way. It involves assessing in order:
- DR – Define Risk (define the risk based on the individual woman and pregnancy before assessing the CTG)
- C – Contractions
- BRa – Baseline Rate
- V – Variability
- A – Accelerations
- D – Decelerations
- O – Overall impression (given an overall impression of the CTG and clinical picture)
If you are asked to assess a CTG in your exams, use the DR C BRaVADO structure to describe each feature in turn. Give an overall impression of the CTG as being normal (all features are reassuring), suspicious, pathological, or need for urgent intervention, as described in the NICE guidelines (2017).
Last updated September 2020