Cardiotocography (CTG) Interpretation

1. Introduction

Cardiotocography (CTG) is a standard tool for intrapartum and antepartum fetal surveillance, primarily used to assess fetal well-being by continuously monitoring fetal heart rate (FHR) and uterine contractions. It aims to detect fetal hypoxia early, allowing timely interventions to reduce perinatal morbidity and mortality.

2. Components of CTG

Component	Definition
Baseline FHR	Average FHR over a 10-minute segment (excluding accelerations & decelerations)
Baseline Variability	Fluctuations in FHR baseline, reflecting autonomic nervous system activity
Accelerations	Transient increase in FHR indicating fetal responsiveness
Decelerations	Transient decrease in FHR; may indicate hypoxia or other stressors

3. NICE (2021) CTG Feature Classification

Feature	Reassuring	Non-Reassuring	Abnormal
Baseline FHR	110–160	100–109 or 161–180	<100 or >180, sinusoidal
Variability	5–25 bpm	<5 for 30–50 min or >25 for 15–25 min	<5 >50 min or >25 >25 min
Accelerations	Present	Absence not significant	Absence with abnormal features
Decelerations	None/early	Variable <90s	Late, prolonged >3 min, atypical

4. Overall CTG Classification

Category	Interpretation Criteria
Normal	All features reassuring
Suspicious	One non-reassuring feature with others reassuring
Pathological	One or more abnormal OR ≥2 non-reassuring features

5. Summary Table: CTG Cheat Sheet

Feature	Normal	Suspicious	Pathological
Baseline FHR	110–160	100–109 or 161–180	<100 or >180
Variability	5–25	<5 for 30–50 mins	<5 >50 mins or >25 >25 mins
Accelerations	Present	Absent	Absent with abnormal features
Decelerations	None/early	Variable	Late, prolonged, atypical

6. Types of Hypoxia & Corresponding CTG Patterns

Type of Hypoxia	CTG Pattern	Clinical Implication
Acute	Bradycardia or prolonged deceleration >3 min	Immediate evaluation; consider delivery
Subacute	Progressive decelerations, rising baseline, loss of variability	Significant compromise likely
Gradually evolving	Tachycardia, late decelerations, shallow variability	Monitor/prepare for delivery
Chronic	Reduced variability, shallow decelerations, persistent tachycardia	Indicates long-standing compromise

7. Stepwise Interpretation: DR C BRAVADO Mnemonic

D: Determine uterine contraction frequency
R: Rate – Assess baseline FHR
C: Contractions – Frequency, duration
B: Baseline FHR assessment
R: Reassuring features?
A: Accelerations present?
V: Variability – Normal (5–25 bpm)?
A: Atypical features?
D: Decelerations – Type & significance
O: Overall impression – Normal, Suspicious, Pathological

8. CTG Pattern Examples

Baseline Heart rate& Unstable Baseline

Normal Variability

Saltatory Pattern

Reduced Variability

Sinusoidal Pattern

Pseudo-sinusoidal

Acceleration

Early, Variable & Late Decelerations

Prolonged Deceleration

Physiology of Hypoxia in Labour

During labour, the fetus employs various adaptive mechanisms in response to hypoxia, similar to the physiological response to exercise. Intrapartum hypoxia generally follows one of four pathways:

Acute, Subacute, Gradually Progressive, and Chronic.

Each has distinct CTG features and management approaches.

1. Acute Hypoxia

Presents as a prolonged deceleration lasting >5 minutes or >3 minutes if accompanied by reduced variability. (FIGO 2015)
• Causes:
– Cord prolapse
– Placental abruption
– Uterine rupture
– Maternal hypotension (e.g., supine hypotension, epidural)
– Uterine hyperstimulation (oxytocin, prostaglandins)
• Fetal pH drops at 0.01/min during deceleration (Gull et al. 1996)• Management: The 3-Minute Rule
0–3 min: Call emergency team
3–6 min: Diagnose cause; correct iatrogenic factors or prepare for immediate delivery
6–9 min: Monitor for recovery (variability, FHR rise)
9–12 min: Proceed with delivery (AVD/CS) if no recovery observed

Important:
• If deceleration is preceded by reduced variability/loss of cycling, expedite delivery immediately (Williams & Galerneau 2002).
• If normal variability is present before the deceleration, 90% recover in 6 min and 95% in 9 min if no acute event. (Physiological-CTG.com)

2. Subacute Hypoxia

CTG shows fetus in frequent decelerations, commonly caused by uterine hyperstimulation (Albertson et al. 2016).
• Fetal pH drops at ~0.01 per 2–3 minutes.• Management:
1. Stop/reduce uterotonics
2. Avoid supine positioning
3. Start IV fluids
4. Administer tocolytics if needed
5. Expedite delivery if no recovery

• If in second stage, stop pushing to allow fetal recovery. Reassess in 10 min; if no improvement, expedite delivery.

3. Gradually Evolving Hypoxia

Most common type of intrapartum hypoxia (Richardson et al. 1996).
• Mimics exercise physiology – stepwise progression:
1. Decelerations (hypoxic stress)
2. Loss of accelerations and cycling
3. Deeper/wider decelerations
4. Rise in baseline (catecholamine-driven redistribution)
5. Reduced variability (vasoconstriction affecting brain)
6. Terminal heart failure (‘step-ladder’ FHR decline)Important:
• Stages 1–4 = Fetal compensation
• Stages 5–6 = Decompensation
• Stages 4 & 5 may be reversible if promptly managed
• Act at stage 4 to prevent organ damage by improving fetal oxygenation

4. Chronic Hypoxia

Presents as a baseline rate at the upper end of normal associated with reduced variability and blunted responses (infrequent accelerations and lack of cycling) and is frequently associated with shallow decelerations.
This represents a fetus with reduced reserve and increased susceptibility to hypoxic injury during labour.
Careful consideration should be given when planning interventions potentially increasing the risk of hypoxia, with low threshold for surgical intervention.

References

FIGO. Intrapartum fetal monitoring: Guidelines for practice. Int J Gynaecol Obstet. 2015.
2. NICE NG25. Intrapartum care for healthy women and babies. 2021 update.
3. RCOG Green-top Guideline No. 31. The Use of Electronic Fetal Monitoring. 2022.
4. Ayres-de-Campos D, et al. FIGO consensus guidelines. Int J Gynaecol Obstet. 2015.
5. Royal College of Obstetricians and Gynaecologists (RCOG) – Clinical guidelines.