Cardiotocography

Cardiotocography records the foetal heart rate and uterine activity simultaneously. The heart rate is detected by a Doppler ultrasound transducer on the maternal abdomen, or by a foetal scalp electrode in labour. Uterine contractions are recorded by an external tocodynamometer or an internal pressure catheter.

Abnormal patterns indicate possible foetal compromise but have low specificity, so interpretation depends on the clinical context. A normal trace is strongly reassuring.

Antenatal recording is used from the late second or third trimester in high risk pregnancy. Continuous intrapartum recording is used when labour is high risk, and intermittent auscultation is used in low risk labour.

Foetal behavioural states. Foetal sleep cycles last 20 to 40 minutes and occasionally up to 90 minutes. They lower variability and abolish accelerations, which can mimic compromise.

Indications. Reduced foetal movements, foetal growth restriction, hypertensive disorders, diabetes, post-term pregnancy, antepartum haemorrhage, multiple pregnancy, and high risk labour (induction or augmentation with oxytocin, meconium, abnormal auscultation).

Preparation and technique. The mother is positioned semi-recumbent or lateral, avoiding supine hypotension. Transducers are repositioned until the signal is clear. A trace of 20 to 40 minutes is recorded antenatally. Intrapartum monitoring is continuous.

Contraindications and precautions. Foetal scalp electrodes are avoided in maternal HIV, hepatitis B and C infection, and active genital herpes, and in foetal bleeding disorders.

Interpretation.

• Baseline rate is the mean level over 10 minutes, excluding accelerations and decelerations. Normal is 110 to 160 beats/min. Tachycardia is over 160 beats/min and bradycardia is under 110 beats/min.
• Baseline variability of 5 to 25 beats/min is reassuring. Under 5 beats/min for over 40 minutes in an antenatal trace, or over 30 minutes in labour, is non-reassuring.
• Accelerations at 32 weeks or later are rises of at least 15 beats/min lasting at least 15 seconds. Before 32 weeks the thresholds are 10 beats/min for 10 seconds.
• Early decelerations mirror contractions. Late decelerations begin after the peak of the contraction and suggest uteroplacental insufficiency. Variable decelerations vary in timing, shape and depth and suggest cord compression.
• A sinusoidal pattern is a regular oscillation of 3 to 5 cycles/min lasting at least 20 minutes, associated with severe foetal anaemia or hypoxia.
• Uterine tachysystole is over 5 contractions in 10 minutes, averaged over 30 minutes.

Classification. Traces are usually classified as normal (all features reassuring), suspicious (one non-reassuring feature) or pathological (two or more non-reassuring features, or any abnormal feature).

Causes of an abnormal trace. Placental insufficiency, cord compression, uterine hyperstimulation, maternal fever or hypotension, foetal anaemia, foetal infection, foetal arrhythmia, foetal sleep, and maternal drugs.

Critical values and findings needing immediate action.

• A sinusoidal pattern, persistent bradycardia, or a prolonged deceleration over 3 minutes requires immediate review and preparation for urgent delivery.
• Late decelerations with reduced variability require immediate correction of reversible factors and escalation.

Limitations and artefacts. Maternal heart rate can be recorded in place of the foetal rate, particularly in the second stage. Signal loss, foetal movement and transducer displacement cause artefacts. Opioids and magnesium sulphate reduce variability.

Follow up tests. Foetal blood sampling for pH and lactate, ultrasound with amniotic fluid assessment, umbilical artery Doppler, biophysical profile, and umbilical cord blood gas analysis at birth.

Special groups. Preterm fetuses have lower baseline variability and fewer accelerations. Pre-existing foetal arrhythmia requires specialist foetal echocardiography.