How do I interpret a cardiotocograph (CTG)?

How to Interpret Cardiotocography (CTG)

Use a physiological interpretation approach that recognizes specific patterns of fetal hypoxic stress and distinguishes compensatory responses from decompensation, rather than relying solely on arbitrary feature-based classification systems. This method, supported by the 2024 International Expert Consensus 1, represents a paradigm shift from traditional "Normal/Suspicious/Pathological" categorization to understanding the underlying fetal pathophysiology.

Core Principles of CTG Interpretation

The Physiological Framework

The key to modern CTG interpretation is recognizing that labor creates gradually evolving hypoxic stress through:

• Repeated constriction of maternal spiral arterioles during contractions
• Umbilical cord compression as labor progresses

Most fetuses mount adequate compensatory responses to maintain aerobic metabolism, but those with pre-existing compromise (chronic uteroplacental insufficiency, chorioamnionitis, chronic anemia) may rapidly decompensate 2.

Sequential Pattern Recognition

Interpret CTG changes in this specific order 3:

1. Look for decelerations first (variable or late)
2. Assess baseline heart rate changes following decelerations
3. Evaluate baseline variability as the critical determinant of compensation status

Specific Features to Assess

Baseline Fetal Heart Rate

• Normal: 110-160 bpm
• Tachycardia developing AFTER repetitive decelerations indicates compensatory response to hypoxic stress 3

Baseline Variability (Most Critical Feature)

This is your primary indicator of fetal compensation status:

• Normal variability (5-25 bpm) = Fetal compensation intact - Even with ongoing decelerations and tachycardia, if variability remains normal, perinatal outcomes are favorable 3
• Abnormal variability (either increased >25 bpm or reduced <5 bpm) = Decompensation - Significantly increases risk of:
  • 5-minute Apgar ≤7: 29.6% vs 0.9% with normal variability
  • Umbilical artery pH <7.0: 29.5% vs 0%
  • NICU admission: 27.3% vs 3.7% 3

Decelerations

• Spontaneous repetitive unprovoked decelerations warrant delivery consideration 4
• The pattern matters less than the fetal response (baseline rate and variability changes)

Computerized CTG (cCTG) Short-Term Variation

For growth-restricted fetuses specifically:

• STV <3.5 ms at 32-34 weeks indicates need for delivery
• STV <4.5 ms at ≥34 weeks indicates need for delivery 4

Clinical Application Algorithm

For Fetal Growth Restriction (FGR)

Without absent/reversed end-diastolic velocity (AEDV/REDV):

• Weekly CTG testing after viability 5

With AEDV/REDV:

• CTG at least 1-2 times daily 5
• Consider hospitalization 5

Delivery indications based on CTG:

• Spontaneous repetitive unprovoked decelerations 4
• Abnormal cCTG short-term variation (thresholds above) 4

For General Intrapartum Monitoring

Progression of hypoxic stress pattern 3:

1. Repetitive decelerations alone → Continue monitoring
2. Decelerations + tachycardia + normal variability → Fetus compensating, continue careful observation
3. Decelerations + tachycardia + abnormal variability → Decompensation, urgent intervention needed

Critical Pitfalls to Avoid

Don't Rely on Traditional Classification Alone

Traditional "Normal/Suspicious/Pathological" systems have poor diagnostic precision 6:

• Sensitivity ranges from 39-92% across different guidelines
• Positive predictive values only 15-24%
• These systems either miss acidemic fetuses (low sensitivity) or trigger unnecessary interventions (low specificity) 6

Don't Ignore the Sequence

A common error is focusing on individual features rather than their temporal relationship 2. Tachycardia appearing BEFORE decelerations has different implications than tachycardia developing AFTER repetitive decelerations.

Don't Overinterpret Isolated Findings

• Tachycardia alone without preceding decelerations may indicate maternal fever, medications, or fetal infection rather than hypoxia
• Single deceleration without baseline changes doesn't indicate decompensation

Additional Considerations

Emerging Features 1

The 2024 consensus now includes recognition of:

• ZigZag pattern - indicates autonomic instability from hypoxic stress
• Features of chorioamnionitis - inflammatory patterns
• Relative uteroplacental insufficiency of labor (RUPI-L)

Limitations of CTG

• Normal CTG has excellent negative predictive value (95-99%) but poor positive predictive value 6
• CTG should be integrated with other assessments (Doppler, biophysical profile, clinical context)
• For FGR specifically, CTG is a late marker - Doppler changes precede CTG abnormalities 5

The physiological approach improves both sensitivity for detecting true compromise AND specificity for avoiding unnecessary interventions, with emerging evidence showing reduced rates of both hypoxic-ischemic encephalopathy and emergency cesarean sections 2.