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