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What is cardiotocography?

Cardiotocography (CTG) is used during pregnancy to monitor 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 CTG 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


DRDefine Risk
CContractions

 

BRaBaseline Rate

VVariability

A – Accelerations

DDecelerations

O Overall impression


Define risk

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 (e.g. if the pregnancy is high risk, the threshold for intervention will be lower.)

Some reasons a pregnancy may be considered high risk are shown below ¹

Maternal medical illness

  • Gestational diabetes
  • Hypertension
  • Asthma

Obstetric complications

  • Multiple gestation
  • Post-date gestation
  • Previous cesarean section
  • Intrauterine growth restriction
  • Premature rupture of membranes
  • Congenital malformations
  • Oxytocin induction/augmentation of labour
  • Pre-eclampsia

Other risk factors

  • Absence of prenatal care
  • Smoking
  • Drug abuse

Contractions

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 one 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)

Uterine contractions (CTG)

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In this example, there are 2 contractions in a 10 minute period (this is often referred to as “2 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 bpm. ¹

 

CTG – Baseline Heart Rate

 

Fetal tachycardia

Fetal tachycardia is defined as a baseline heart rate greater than 160 bpm.

Causes include:

  • Fetal hypoxia
  • Chorioamnionitis – if maternal fever also present
  • Hyperthyroidism
  • Fetal or maternal anaemia
  • Fetal tachyarrhythmia

 

Fetal bradycardia

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 (less than 80 bpm for more than 3 minutes) indicates severe hypoxia.

Causes of prolonged severe bradycardia include:

  • 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.


Variability

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. 3

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 follows: 3

  • Reassuring –  5 – 25bpm
  • Non-reassuring:
    • less than 5bpm for between 30-50 minutes
    • more than 25bpm for 15-25 minutes
  • Abnormal:
    • less than 5bpm for more than 50 minutes
    • more than 25bpm for more than 25 minutes
    • sinusoidal

 

CTG – Variability

..

Reduced variability can be caused by any of the following: ²

  • 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

 

CTG – Reduced Variability

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Accelerations

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.

CTG – Accelerations


Decelerations

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 deceleration

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

 

CTG – Early Decelerations

Variable deceleration

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 compression: ¹

  • 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.

 

CTG – Variable Decelerations

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Late deceleration

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
  • Pre-eclampsia
  • 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.

 

CTG – Late Decelerations

Prolonged deceleration

A prolonged deceleration is defined as a deceleration that lasts more than 3 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

 

CTG – Prolonged Deceleration

.

Sinusoidal pattern

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 pattern 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.

Sinusoidal pattern


Overall impression

Once you have assessed all aspects of the CTG you need to give your overall impression.

The overall impression can be described as either: 3

  • Reassuring
  • Suspicious
  • Abnormal

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.3

 

Reassuring

Baseline heart rate

  • 110 to 160 bpm

 

Baseline variability

  • 5 to 25 bpm

 

Decelerations

  • None or early
  • Variable decelerations with no concerning characteristics* for less than 90 minutes

Non-reassuring

Baseline heart rate

Either of the below would be classed as non-reassuring:

  • 100 to 109 bpm
  • 161 to 180 bpm

 

Baseline variability

Either of the below would be classed as non-reassuring:

  • Less than 5 for 30 to 50 minutes
  • More than 25 for 15 to 25 minutes

 

Decelerations

Any of the below would be classed as non-reassuring:

  • Variable decelerations with no concerning characteristics* for 90 minutes or more
  • Variable decelerations with any concerning characteristics* in up to 50% of contractions for 30 minutes or more
  • Variable decelerations with any concerning characteristics* in over 50% of contractions for less than 30 minutes
  • Late decelerations in over 50% of contractions for less than 30 minutes, with no maternal or fetal clinical risk factors such as vaginal bleeding or significant meconium

 

Abnormal

Baseline heart rate

Either of the below would be classed as abnormal:

  • Below 100bpm
  • Above 180 bpm

 

Baseline variability

Any of the below would be classed as abnormal:

  • Less than 5 for more than 50 minutes
  • More than 25 for more than 25 minutes
  • Sinusoidal

 

Decelerations

Any of the below would be classed as abnormal:

  • Variable decelerations with any concerning characteristics* in over 50% of contractions for 30 minutes (or less if any maternal or fetal clinical risk factors [see above])
  • Late decelerations for 30 minutes (or less if any maternal or fetal clinical risk factors)
  • Acute bradycardia, or a single prolonged deceleration lasting 3 minutes or more

 

Regard the following as concerning characteristics of variable decelerations: lasting more than 60 seconds; reduced baseline variability within the deceleration; failure to return to baseline; biphasic (W) shape; no shouldering.


References

1. AMIR SWEHA, M.D. Interpretation of the Electronic Fetal Heart Rate During Labor. Am Fam Physician. 1999 May 1;59(9):2487-2500. [LINK]

2. Clinical obstetrics and gynaecology. 2nd Edition. 2009. B.Magowan, Philip Owen, James Drife

3. Intrapartum care: NICE guideline CG190 (February 2017) [LINK]


 

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