What is cardiotocography?
Cardiotocography (CTG) is used during pregnancy to monitor both the fetal heart and contractions of the uterus. It is most commonly used in the third trimester. Its purpose is to monitor fetal well-being and allow early detection of fetal distress. An abnormal CTG indicates the need for more invasive investigations and potentially emergency caesarian section.
How it works
The device used in cardiotocography is known as a cardiotocograph.
It involves the placement of two transducers onto the abdomen of a pregnant woman.
One transducer records the fetal 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 the pregnancy is high or low risk.
This is important as it gives more context to the CTG reading (If the pregnancy is high risk, the threshold for intervention will likely be lower.)
Some reasons a pregnancy may be considered high risk are shown below ¹
Maternal medical illness
- Gestational diabetes
- Multiple gestation
- Postdate gestation
- Previous cesarean section
- Intrauterine growth restriction
- Premature rupture of membranes
- Congenital malformations
- Oxytocin induction/augmentation of labour
Other risk factors
- Absence of prenatal care
- Drug abuse
Next, you need to record the number of contractions present in a 10 minute period.
Each big square on the example CTG chart below is equal to 1 minute, so look at how many contractions occurred within 10 big 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 (this is often referred to as “3 in 10”)
Baseline rate of the fetal heart
The baseline rate is the average heart rate of the fetus 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 fetal heart rate is between 110-150 bpm¹.
Fetal tachycardia is defined as a baseline heart rate greater than 160 bpm.
It can be caused by: ¹
- Fetal hypoxia
- Chorioamnionitis – if maternal fever also present
- Fetal or maternal anaemia
- Fetal tachyarrhythmia
Fetal bradycardia is defined as a baseline heart rate less than 100 bpm for 3 minutes or more.
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 and spinal anaesthesia
- Maternal seizures
- Rapid fetal descent
If the cause cannot be identified and corrected, immediate delivery is recommended.
Baseline variability refers to the variation of fetal 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.
It is, therefore, a good indicator of how healthy a fetus is at that particular moment in time, as a healthy fetus will constantly be adapting its heart rate in response to changes in its environment.
Normal variability is between 5-25 bpm. 4
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 into the following categories: 4
- Reassuring – 5 – 25bpm
- less than 5bpm for between 30-50 minutes
- more than 25bpm for 15-25 minutes
- less than 5bpm for more than 50 minutes
- more than 25bpm for more than 25 minutes
Reduced variability can be caused by any of the following: ³
- Fetal sleeping – this should last no longer than 40 minutes (most common cause)
- Fetal acidosis (due to hypoxia) – more likely if late decelerations are also present
- Fetal 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 the baseline fetal heart rate of greater than 15 bpm for greater than 15 seconds. ¹
The presence of accelerations is reassuring.
Accelerations occurring alongside uterine contractions is a sign of a healthy fetus.
The absence of accelerations with an otherwise normal CTG is of uncertain significance.
Decelerations are an abrupt decrease in the baseline fetal heart rate of greater than 15 bpm for greater than 15 seconds.
There are a number of different types of decelerations, each with varying significance.
Early decelerations start when the uterine contraction begins and recover when uterine contraction stops.
This is due to increased fetal 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 pathological. ³
Variable decelerations are observed as a rapid fall in baseline fetal heart 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 fetus 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 are more worrying as it suggests the fetus is becoming hypoxic.
Late decelerations begin at the peak of the uterine contraction and recover after the contraction ends.
This type of deceleration indicates there is insufficient blood flow to the uterus and placenta.
As a result, blood flow to the fetus is significantly reduced causing fetal hypoxia and acidosis.
Reduced uteroplacental blood flow can occur due to: ¹
- Maternal hypotension
- Uterine hyperstimulation
The presence of late decelerations is concerning and fetal blood sampling for pH is indicated.
If fetal blood pH is acidotic it indicates significant fetal hypoxia and the need for emergency C-section.
A prolonged deceleration is defined as a deceleration that lasts 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. fetal blood sampling/emergency C-section
This type of pattern is rare, however, if present it is very concerning.
It is associated with high rates of fetal morbidity and mortality. ¹
A sinusoidal CTG patter has the following characteristics:
- A smooth, regular, wave-like pattern
- Frequency of around 2-5 cycles a minute
- Stable baseline rate around 120-160bpm
- No beat to beat variability
A sinusoidal pattern usually indicates one or more of the following:
- Severe fetal hypoxia
- Severe fetal anaemia
- Fetal/maternal haemorrhage
Immediate C-section is indicated for this kind of pattern.
The 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 and gynaecology. 2nd Edition. 2009. B.Magowan, Philip Owen, James Drife
4. Intrapartum care: NICE guideline CG190 (February 2017) [LINK]