What is cardiotocography?
Cardiotocography (CTG) is used during pregnancy to monitor both the foetal heart and the contractions of the uterus. It is usually only used in the 3rd trimester. It’s purpose is to monitor foetal well-being and allow early detection of foetal distress. An abnormal CTG indicates the need for more invasive investigations and may lead to the need for emergency caesarian section.
How it works
The device used in cardiotocography is known as a cardiotocograph.
It involves the placement of 2 transducers onto the abdomen of a pregnant women.
One transducer records the foetal heart rate using ultrasound.
The other transducer monitors the contractions of the uterus.
It does this by measuring the tension of the maternal abdominal wall.
This provides an indirect indication of intrauterine pressure.
The CTG is then assessed by the midwife and obstetric medical team.
How to read a CTG
To interpret a CTG you need a structured method of assessing its various characteristics.
The most popular structure can be remembered using the acronym DR C BRAVADO
DR – Define Risk
C – Contractions
BRa – Baseline Rate
V – Variability
A – Accelerations
D – Decelerations
O – Overall impression
You first need to assess if this pregnancy is high or low risk.
This is important as it gives more context to the CTG reading – e.g. If the pregnancy is high risk, your threshold for intervening may be lowered.
Reasons a pregnancy may be considered high risk are shown below¹
Maternal medical illness
Previous cesarean section
Intrauterine growth restriction
Premature rupture of membranes
Oxytocin induction/augmentation of labour
Other risk factors
Absence of prenatal care
Record the number of contractions present in a 10 minute period – e.g. 3 in 10
Each big square is equal to 1 minute, so look at how many contractions occurred within 10 squares.
Individual contractions are seen as peaks on the part of the CTG monitoring uterine activity.
You should assess contractions for the following:
- Duration – how long do the contractions last?
- Intensity – how strong are the contractions? (assessed using palpation)
In this example there are 2-3 contractions in a 10 minute period – e.g. 3 in 10
Baseline rate of the foetal heart
The baseline rate is the average heart rate of the foetus within a 10 minute window.
Look at the CTG and assess what the average heart rate has been over the last 10 minutes.
Ignore any accelerations or decelerations.
A normal foetal heart rate is between 110-150 bpm¹.
Foetal tachycardia is defined as a baseline heart rate greater than 160 bpm.
It can be caused by:¹
- Foetal hypoxia
- Chorioamnionitis – if maternal fever also present
- Foetal or maternal anaemia
- Foetal tachyarrhythmia
Foetal bradycardia is defined as a baseline heart rate less than 120 bpm.
Mild bradycardia of between 100-120 bpm is common in the following situations:
- Postdate gestation
- Occiput posterior or transverse presentations
Severe prolonged bradycardia (< 80 bpm for > 3 minutes) indicates severe hypoxia.
Causes of prolonged severe bradycardia are:¹
- Prolonged cord compression
- Cord prolapse
- Epidural & spinal anaesthesia
- Maternal seizures
- Rapid foetal descent
If the cause cannot be identified and corrected, immediate delivery is recommended.
Baseline variability refers to the variation of foetal heart rate from one beat to the next.
Variability occurs as a result of the interaction between the nervous system, chemoreceptors, baroreceptors and cardiac responsiveness.
Therefore it is a good indicator of how healthy the foetus is at that particular moment in time.
This is because a healthy foetus will constantly be adapting its heart rate to respond to changes in its environment.
Normal variability is between 10-25 bpm³
To calculate variability you look at how much the peaks and troughs of the heart rate deviate from the baseline rate (in bpm)
Variability can be categorised as: 4
- Reassuring – ≥ 5 bpm
- Non-reassuring – < 5bpm for between 40-90 minutes
- Abnormal – < 5bpm for >90 minutes
Reduced variability can be caused by: ³
- Foetal sleeping – this should last no longer than 40 minutes – most common cause
- Foetal acidosis (due to hypoxia) – more likely if late decelerations are also present
- Foetal tachycardia
- Drugs – opiates / benzodiazepines / methyldopa / magnesium sulphate
- Prematurity – variability is reduced at earlier gestation (<28 weeks)
- Congenital heart abnormalities
Accelerations are an abrupt increase in baseline heart rate of >15 bpm for >15 seconds.
The presence of accelerations is reassuring.
Antenatally there should be at least 2 accelerations every 15 minutes¹.
Accelerations occurring alongside uterine contractions is a sign of a healthy foetus.
However the absence of accelerations with an otherwise normal CTG is of uncertain significance.
Decelerations are an abrupt decrease in baseline heart rate of >15 bpm for >15 seconds.
There are a number of different types of decelerations, each with varying significance.
Early decelerations start when uterine contraction begins and recover when uterine contraction stops.
This is due to increased foetal intracranial pressure causing increased vagal tone.
It therefore quickly resolves once the uterine contraction ends and intracranial pressure reduces.
This type of deceleration is therefore considered to be physiological and not pathological³.
Variable decelerations are observed as a rapid fall in baseline rate with a variable recovery phase.
They are variable in their duration and may not have any relationship to uterine contractions.
They are most often seen during labour and in patients’ with reduced amniotic fluid volume.
Variable decelerations are usually caused by umbilical cord compression¹:
- The umbilical vein is often occluded first causing an acceleration in response.
- Then the umbilical artery is occluded causing a subsequent rapid deceleration.
- When pressure on the cord is reduced another acceleration occurs and then the baseline rate returns.
- Accelerations before and after a variable deceleration are known as the “shoulders of deceleration“.
- Their presence indicates the foetus is not yet hypoxic and is adapting to the reduced blood flow.
Variable decelerations can sometimes resolve if the mother changes position.
The presence of persistent variable decelerations indicates the need for close monitoring.
Variable decelerations without the shoulders is more worrying as it suggests the foetus is hypoxic.
Late decelerations begin at the peak of uterine contraction and recover after the contraction ends.
This type of deceleration indicates there is insufficient blood flow through the uterus and placenta.
As a result blood flow to the foetus is significantly reduced causing foetal hypoxia and acidosis.
Reduced utero-placental blood flow can be caused by: ¹
- Maternal hypotension
- Uterine hyperstimulation
The presence of late decelerations is taken seriously and foetal blood sampling for pH is indicated.
If foetal blood pH is acidotic it indicates significant foetal hypoxia and the need for emergency C-section.
A deceleration that last more than 2 minutes.
If it lasts between 2-3 minutes it is classed as non-reassuring.
If it lasts longer than 3 minutes it is immediately classed as abnormal.
Action must be taken quickly – e.g. foetal blood sampling / emergency C-section
This type of pattern is rare, however if present it is very serious.
It is associated with high rates of foetal morbidity & mortality¹.
It is described as:
- A smooth, regular, wave-like pattern
- Frequency of around 2-5 cycles a minute
- Stable baseline rate around 120-160 bpm
- No beat to beat variability
A sinusoidal pattern indicates:
- Severe foetal hypoxia
- Severe foetal anaemia
- Foetal/maternal haemorrhage
Immediate C-section is indicated for this kind of pattern.
Outcome is usually poor.
Once you have assessed all aspects of the CTG you need to give your overall impression.
The overall impression can be described as either: 4
The overall impression is determined by how many of the CTG features were either reassuring, non-reassuring or abnormal. The NICE guideline below demonstrates how to decide which category a CTG falls into.4
3. Clinical obstetrics & gynaecology. 2nd Edition. 2009. B.Magowan, Philip Owen, James Drife
4. Nice guidelines http://www.nice.org.uk/nicemedia/live/11837/36273/36273.pdf