What is Cardiotocography (CTG)?
Cardiotocography (CTG) is a monitoring technique used primarily in obstetrics to assess fetal well-being by recording the fetal heart rate (FHR) and uterine contractions simultaneously. It is widely employed in the antepartum period (before labor) and intrapartum period (during labor) to detect signs of fetal distress, allowing for timely interventions to prevent adverse outcomes.
.png "Cardiotocography (CTG) is a monitoring technique used primarily in obstetrics to assess fetal well-being by recording the fetal heart rate (FHR) and uterine contractions simultaneously. It is widely employed in the antepartum period (before labor) and intrapartum period (during labor) to detect signs of fetal distress, allowing for timely interventions to prevent adverse outcomes.")
Table of Contents
Principle of CTG
The core concept of CTG is that the fetal heart rate (FHR) is not a static number but a dynamic pattern regulated by the fetal central nervous system (CNS), particularly the balance between sympathetic (increases heart rate) and parasympathetic (decreases heart rate) activity. This interplay is oxygen-dependent — a well-oxygenated fetus has an intact CNS that can respond rapidly to stimuli, while an oxygen-deprived fetus shows characteristic changes in FHR.
1. CNS Control of Fetal Heart Rate
🔹 Sympathetic system: Stimulates the heart to beat faster (tachycardia), often during fetal movement or stress.
🔹 Parasympathetic system (via the vagus nerve): Slows the heart rate (bradycardia), particularly during rest or head compression.
🔹 Healthy variability in baseline FHR comes from the constant fine-tuning between these two systems.
🔹 Brainstem reflexes (baroreceptor and chemoreceptor reflexes) allow the fetus to adapt to transient changes in oxygen and blood flow.
2. Role of Oxygen Supply
🔹 Adequate oxygen ensures that the myocardium (heart muscle) and CNS function optimally, allowing normal FHR patterns and variability.
🔹 Hypoxia (low oxygen) affects the CNS:
✔ Initially, it may stimulate the sympathetic system, causing transient tachycardia.
✔ Prolonged hypoxia leads to CNS depression, reduced variability, and abnormal decelerations.
🔹 Acidosis (build-up of acidic byproducts from anaerobic metabolism) occurs if hypoxia persists, further blunting CNS responsiveness and causing serious FHR abnormalities.
3. Physiologic Basis of CTG Patterns
🔹 Baseline FHR: Reflects resting CNS tone.
🔹 Variability: Shows the ability of the CNS to react to internal/external stimuli — healthy fetuses have 5–25 bpm variability.
🔹 Accelerations: Represent a reactive and well-oxygenated fetus responding to movements or external stimuli.
🔹 Decelerations: Often occur as a reflex to stress (e.g., cord compression, head compression, or placental insufficiency).
The timing and shape of these decelerations give clues about whether the cause is benign or worrisome.
4. Why Hypoxia Changes the FHR
When oxygen delivery to the fetus is reduced (due to placental issues, cord problems, or maternal factors), the fetal body tries to compensate:
🔹 Short-term compensation – Increase in heart rate and cardiac output.
🔹 Prolonged stress – Redistribution of blood flow to vital organs (brain, heart, adrenals) and away from peripheral tissues.
🔹 If stress continues – CNS becomes hypoxic → variability decreases → pathological decelerations develop → risk of metabolic acidosis rises.
The principle of CTG rests on the neurocardiac reflex arc between fetal oxygenation, CNS function, and heart rate control.
🔹 Normal oxygen + intact CNS = healthy variability and accelerations.
🔹 Low oxygen or acidosis = loss of variability, abnormal decelerations, and potential bradycardia.
This is why CTG is such a powerful indirect monitor of fetal well-being — it doesn’t measure oxygen directly, but it picks up the CNS and cardiac signs that change when oxygenation is compromised.
Equipment and Setup of CTG
A standard CTG machine has:
1. Ultrasound transducer – detects fetal heart rate using Doppler ultrasound.
2. Tocodynamometer (TOCO) – measures uterine contractions by detecting changes in abdominal wall tension.
3. Paper or digital display – prints or shows real-time tracings of both FHR and contractions.
Two types of monitoring are possible:
🔹 External CTG: Uses surface transducers placed on the maternal abdomen.
🔹 Internal CTG: Involves a fetal scalp electrode for heart rate and an intrauterine pressure catheter (IUPC) for contractions, usually reserved for high-risk situations during labor.
Types of CTG Use
🔹 Antenatal (Non-Stress Test, NST) – Used to assess fetal health before labor, especially in high-risk pregnancies (e.g., maternal hypertension, diabetes, post-term pregnancy).
🔹 Intrapartum CTG – Continuous monitoring during labor to detect early signs of fetal compromise.
Cardiotocography (CTG) Procedure
A cardiotocography (CTG) is a non-invasive test that monitors the fetal heart rate and uterine contractions during pregnancy. It is usually performed in the third trimester, but can also be done earlier if necessary.
Here's how the CTG is performed:
1. The mother will lie down on an examination table with her abdomen exposed.
2. Two sensors will be attached to her abdomen: one to measure the fetal heart rate, and the other to measure uterine contractions. The sensors are attached with elastic belts, similar to those used during an ultrasound.
3. The fetal heart rate sensor uses ultrasound technology to detect the baby's heart rate. The sensor is placed over the baby's back or chest, depending on the baby's position.
4. The uterine contraction sensor uses a pressure-sensitive pad to detect contractions. The sensor is placed over the uterus, typically near the top of the mother's uterus.
5. The sensors are connected to a machine that records the fetal heart rate and uterine contractions. The machine produces a graph called a "trace," which shows the changes in fetal heart rate and uterine contractions over time.
6. The CTG is usually performed for about 20-30 minutes, although it can be done for longer if necessary.
During the CTG, the mother may be asked to press a button on the machine when she feels the baby move so that any changes in fetal heart rate can be correlated with the baby's movements. The CTG is typically performed in the hospital or doctor's office and can be done as an outpatient procedure.
Overall, the CTG is a safe and non-invasive test that provides important information about the fetal heart rate and uterine contractions during pregnancy.
What Parameters are Measured by Cardiotocography (CTG)?
The CTG (Cardiotocogram) measures two main parameters: the fetal heart rate (FHR) and uterine contractions.
.png "The CTG (Cardiotocogram) measures two main parameters: the fetal heart rate (FHR) and uterine contractions.")
1. Fetal Heart Rate (FHR)
The upper line on the CTG trace records the fetal heart rate, typically measured in beats per minute (bpm). This is detected using an ultrasound transducer placed on the mother's abdomen.
2. Uterine Activity
The lower line on the trace records uterine contractions. This is measured by a tocodynamometer, a pressure sensor placed on the mother's abdomen. It shows the frequency and duration of contractions but not their actual strength.
Parameters Assessed in CTG
The interpretation of a CTG trace is a skilled process that involves analyzing several key features to determine the fetal condition.
1. Baseline Fetal Heart Rate (FHR)
This is the average heart rate of the fetus between contractions. A normal baseline FHR is typically between 110 and 160 bpm.
🔹 Tachycardia: A baseline FHR greater than 160 bpm. This can be caused by maternal fever, infection, or fetal hypoxia.
🔹 Bradycardia: A baseline FHR less than 110 bpm. This can be a sign of fetal distress, especially if it's accompanied by other concerning features.
2. Baseline Variability
This refers to the normal, irregular fluctuations in the FHR. It's a key indicator of a healthy fetal nervous system and reflects the balance between the sympathetic and parasympathetic nervous systems.
🔹 Normal Variability: Fluctuations of 6-25 bpm. This is a reassuring sign of fetal well-being.
🔹 Reduced Variability: Fluctuations of less than 5 bpm. This can be a sign of fetal sleep, but if it persists, it may indicate fetal compromise.
🔹 Absent Variability: No detectable fluctuations. This is a highly concerning sign and may indicate severe fetal hypoxia or acidosis.
🔹 Saltatory Variability: Marked variability with fluctuations greater than 25 bpm. This can be a sign of acute hypoxia.
🔹 Sinusoidal Pattern: A smooth, wave-like pattern with regular oscillations. This is a rare but highly pathological finding that can be associated with severe fetal anemia or hypoxia and requires immediate attention.
3. Accelerations
These are temporary increases in the FHR, at least 15 bpm above the baseline and lasting for at least 15 seconds. The presence of accelerations is a reassuring sign of a healthy fetus with adequate oxygenation.
4. Decelerations
These are temporary decreases in the FHR. Their timing, shape, and relationship to uterine contractions are crucial for interpretation. There are three main types:
🔹 Early Decelerations: These are gradual, symmetrical drops in FHR that "mirror" the uterine contractions. They begin and end with the contraction and are considered benign, often caused by fetal head compression.
🔹 Variable Decelerations: These are abrupt, sharp drops in FHR that can occur at any time, with or without a contraction. They have a "V," "W," or "U" shape and are typically caused by umbilical cord compression. If they are frequent, deep, or prolonged, they can be a sign of fetal compromise.
🔹 Late Decelerations: These are gradual drops in FHR that begin after the peak of the contraction and end well after the contraction has finished. They are a serious sign of uteroplacental insufficiency (inadequate blood flow to the placenta) and can indicate fetal hypoxia.
5. Uterine Activity
The frequency, duration, and intensity of uterine contractions are assessed. Excessive uterine activity (tachysystole) can compromise fetal oxygenation.
🔹 Excessive contractions (>5 in 10 min, called tachysystole) can reduce fetal oxygenation.
How is Cardiotocography (CTG) Interpreted?
Cardiotocography (CTG) interpretation is a critical skill in obstetrics, allowing clinicians to assess fetal well-being by analyzing the fetal heart rate (FHR) patterns in relation to uterine contractions. Accurate interpretation guides decisions on whether to continue monitoring, perform additional testing, or intervene to prevent fetal compromise. CTG interpretation involves evaluating multiple parameters that reflect the fetus’s autonomic nervous system response and oxygenation status.
Please note: This article is for informational purposes only. To learn how to interpret CTG tracing, consult your Educator, Medical Guidelines, or online tools recommended by your Educators. Interpreting CTG tracings requires both theoretical knowledge and practical skills.(alert-passed)
Key Components of CTG Interpretation
CTG interpretation centers on five main parameters:
1. Baseline Fetal Heart Rate (FHR)
2. Baseline Variability
3. Accelerations
4. Decelerations
5. Uterine Activity
Reactive vs. Non-Reactive CTG
The terms reactive and non-reactive primarily apply to antepartum non-stress tests (NST), a form of CTG performed before labor to evaluate fetal well-being. A reactive Cardiotocography (CTG) is a normal, reassuring finding that indicates a healthy fetus. A non-reactive CTG is an abnormal or suspicious finding that suggests a fetus may be at risk of hypoxia and requires further assessment.
A. Reactive CTG
A reactive CTG is characterized by:
🔹 Normal Baseline Fetal Heart Rate (FHR): The average heart rate is between 110 and 160 beats per minute (bpm).
🔹 Moderate Variability: The FHR fluctuates by 6 to 25 bpm, which shows a healthy nervous system in the fetus.
🔹 Presence of Accelerations: There are at least two FHR accelerations (increases of at least 15 bpm for at least 15 seconds) within a 20-minute period. The presence of accelerations is a key indicator of fetal well-being and adequate oxygenation.
A reactive CTG is a sign that the fetus is well-oxygenated and coping well with the intrauterine environment. It typically does not require any immediate intervention, and monitoring can continue as planned.
B. Non-Reactive CTG
A non-reactive CTG is defined by the absence of accelerations over a 20-minute period. This lack of accelerations can be a sign of fetal distress, though it doesn't always mean the baby is in immediate danger.
The reasons for a non-reactive CTG can include:
🔹 Fetal Sleep Cycle: A fetus often sleeps for 20-40 minutes at a time, during which its heart rate variability and accelerations can be reduced. If the CTG is non-reactive but the baby is likely just sleeping, the mother might be encouraged to have a snack or a drink, or a vibroacoustic stimulator might be used to wake the baby up.
🔹 Maternal Medication: Certain medications, like sedatives or pain relievers given to the mother, can temporarily affect fetal heart rate patterns.
🔹 Fetal Hypoxia: This is the most concerning cause. A non-reactive CTG, especially when combined with other abnormal features like reduced variability or decelerations, can be a sign of fetal oxygen deprivation.
When a CTG is non-reactive, a clinician will usually extend the monitoring period to 40-60 minutes to see if the baby wakes up. If the CTG remains non-reactive after this time, or if other concerning features are present, further evaluation is needed. This might involve additional testing, such as a biophysical profile, or, in more urgent cases, preparations for delivery.
Classification of CTG Tracings: Normal, Suspicious, and Pathological
International guidelines, such as those from FIGO and NICE, classify CTG traces into three broad categories based on the combination of baseline rate, variability, accelerations, and decelerations.
1. Normal CTG
A normal CTG has all the following features:
✔ Baseline FHR between 110–160 bpm.
✔ Baseline variability between 5–25 bpm.
✔ Presence of accelerations.
✔ Absence of decelerations or presence of early decelerations only.
✔ Normal uterine activity.
Normal CTG indicates fetal well-being, and routine monitoring can continue.
2. Suspicious (Indeterminate) CTG
A suspicious CTG displays one or two features outside normal limits but without clear evidence of fetal compromise. Examples include:
✔ Baseline tachycardia or bradycardia slightly outside normal range.
✔ Reduced variability (3–5 bpm) lasting less than 40 minutes.
✔ Occasional variable decelerations without repetitive late decelerations.
✔ Mild uterine tachysystole.
Suspicious CTG warrants closer observation and further evaluation, including possible additional fetal assessment methods.
3. Pathological CTG
A pathological CTG shows signs strongly suggestive of fetal hypoxia or acidemia, including:
✔ Baseline FHR <100 bpm or >170 bpm for more than 10 minutes.
✔ Absent baseline variability (<5 bpm) for over 40 minutes.
✔ Repetitive late decelerations or prolonged decelerations (>3 minutes).
✔ Severe variable decelerations with slow return to baseline.
✔ Sinusoidal pattern (a smooth, sine wave-like baseline) indicating severe fetal anemia or hypoxia.
Pathological CTG requires urgent intervention, which may include intrauterine resuscitation measures or expedited delivery.
Additional Considerations in CTG Interpretation
✅ Contextual factors: Maternal fever, medications, fetal sleep cycles, and gestational age influence CTG patterns and must be considered during interpretation.
✅ Clinical correlation: CTG is a screening tool, not diagnostic by itself. Abnormal CTG should prompt correlation with clinical findings and possibly additional tests like fetal scalp blood sampling or biophysical profiling.
✅ Limitations: False positives are common, potentially leading to unnecessary interventions; hence, interpretation requires experience and should be integrated with the overall clinical picture.
Patterns are categorized as normal, suspicious, or pathological to guide clinical management. Reactive CTG indicates fetal well-being, while non-reactive tracings require further evaluation. Correct interpretation, combined with clinical judgment, is essential to optimize fetal outcomes during pregnancy and labor.(alert-passed)
Limitations of CTG
While the CTG is a valuable tool for monitoring fetal well-being, it is not a perfect test. Some CTGs may be falsely non-reactive, leading to unnecessary interventions or cesarean sections. In addition, some CTGs may be falsely reactive, leading to unnecessary monitoring or interventions.
Summary
Cardiotocography remains a cornerstone of fetal surveillance, offering continuous, non-invasive monitoring of the fetal heart rate in relation to uterine activity. When interpreted within the clinical context, CTG provides valuable insight into fetal well-being, enabling obstetric teams to make informed decisions and reduce adverse perinatal outcomes. However, reliance on CTG alone without correlating with maternal and fetal factors can lead to unnecessary interventions, making training and experience in its interpretation essential.
