Download - Understanding Cardiotocography – “CTGs”
Understanding Cardiotocography – “CTGs”
A Normal Antenatal CTG
Features of a CTG
• Baseline• Short term variability• Accelerations• Decelerations• Response to stimuli
• Contractions• Fetal movements• Other
Baseline Fetal Heart Rate
• 110 to 150 bpm at term• Faster in early pregnancy• Below 100 = baseline bradycardia• Below 80 = severe bradycardia• Tachycardia common with maternal fever• Tachycardia with reduced STV = early
hypoxia
Accelerations
• Must be >15 bpm and >15 sec above baseline• Should be >2 per 15 min period• Always reassuring when present• May not occur when fetus is “sleeping”• Should occur in response to fetal movements or
fetal stimulation• Non reactive periods usually do not exceed 45 min
• (>90 min and no accelerations is worrying)
Short Term Variability(or Beat to Beat Variability with a Scalp Clip)
• Should be >5 bpm• The most important feature of any CTG• Is a reflection of competing acceleratory and
decelerating CNS influences on the fetal heart• And therefore represents the best measure of CNS
oxygenation• Will be affected by drugs• Will be reduced in the pre term fetus
Decelerations• Early: mirrors the contraction
• Typically occurs as the head enters the pelvis and is compressed, i.e. it is a vagal response
• Late: Follows every contraction and exhibits a slow return to baseline
• Is quite rare but is the response of a hypoxic myocardium
• Variable: Show no relationship to contractions• Mild • Moderate• Severe
• In practice many “decels” or “dips” are MIXED
An Abnormal Antenatal CTG
An Abnormal Antenatal CTG cont’d
Abnormal CTG Features
• Reduced STV• No accelerations• Decelerations after
most contractions with a slow return to baseline
In Practice a CTG is best regarded as a screening tool:
• High negative predictive value• >98% of fetuses with a normal CTG will be OK
• Poor positive predictive value• Up to 50% of fetuses with an abnormal CTG will be
hypoxic and acidotic but 50% will be OK
• Therefore the CTG should always be interpreted in its clinical context
• And backed by fetal blood sampling PRN
The RCOG Classification of CTGs
• Normal = all 4 features are reassuring
• Suspicious = One non reassuring feature
• Pathological = Two or more non reassuring features or a abnormal pattern
Non Reassuring Features of a CTG
• Baseline <110>100 or >160<180• STV <5 for >40 min but <90 min• Early decelerations• Variable decelerations• A single prolonged deceleration up to 3 min
A CTG is abnormal when:
• Baseline is <100 or >180 bpm• STV is <5 for >90 min• Late decelerations are repeated• Atypical variable decelerations occur• Two prolonged decelerations for >3 min
occur• Sinusoidal pattern >10 min