Fetal Cardiothoracic Ratio (CTR) — Ultrasound Overview: The fetal cardiothoracic ratio (CTR) is an important sonographic index used to evaluate the relative size of the fetal heart in comparison to the thoracic cavity. It provides an indirect measure of cardiac enlargement, chamber dilatation, or mass effect within the fetal chest. Unlike the postnatal CTR measured on chest radiographs, the fetal CTR is obtained through real-time ultrasound in specific cardiac views.
Method of Measurement: The fetal CTR can be assessed using two main approaches: Transverse (4-chamber) view: Cardiac transverse diameter (outermost heart borders) divided by thoracic transverse diameter (inner ribs or thoracic wall at the same level).
Section structure
1. CA (Cardiac Area)
2. CD (Cardiac Diameter)
3. TD (Thoracic Diameter)
4. AP (Anteroposterior axis)
Area method: Cardiac area (traced around the heart) divided by thoracic area (traced inside thoracic cavity) on the standard four-chamber view.
Both diameter-based and area-based methods are valid, but area measurement is considered more accurate in many centers.
Reference Values: In normal fetuses, the CTR (diameter-based) is usually 0.45–0.50 (45–50%). The area-based CTR typically falls within a similar range, with minor gestational variation. Importantly, the CTR remains relatively constant throughout gestation. Values exceeding 0.50 suggest cardiomegaly and require further evaluation. Some sources allow a slightly wider normal range (0.35–0.55) depending on technique and gestational age.
Clinical Significance: An increased fetal CTR indicates cardiomegaly, which may be due to structural heart defects (e.g., hypoplastic left heart, Ebstein’s anomaly), myocardial disease, high-output states (anemia, arteriovenous malformations, twin-to-twin transfusion syndrome), or pericardial effusion. A reduced CTR may suggest pulmonary hypoplasia or technical error. Persistent abnormal CTR is associated with adverse perinatal outcomes and warrants detailed fetal echocardiography.
Technical Considerations: Accurate measurement requires a true axial four-chamber view, avoidance of oblique sections, and inclusion of the entire thoracic cavity. Care should be taken not to include pericardial effusion or lung masses within the cardiac contour. The measurement should be repeated if fetal position or maternal habitus degrades image quality.
Limitations: Fetal CTR is a screening tool, not a diagnostic measure of cardiac function. It cannot differentiate between chamber-specific enlargement and extracardiac causes of apparent heart size increase. Interpretation must always be integrated with fetal echocardiography, Doppler studies, and maternal/fetal clinical findings.
Summary: The fetal CTR is a simple, reproducible ultrasound parameter that remains fairly constant across gestation. A CTR > 0.50 suggests cardiomegaly and should prompt detailed cardiac evaluation and search for systemic causes. Its value lies in early recognition of abnormal heart size, guiding timely referral for fetal echocardiography and perinatal management.
Fetal Cardiothoracic Ratio (CTR) and Associated Anomalies: The fetal CTR is an important screening parameter for detecting abnormal heart size. A CTR > 0.50 (50%) generally indicates cardiomegaly, while a CTR < 0.35–0.40 may point to reduced cardiac size or thoracic expansion anomalies. Abnormal CTR values should always prompt detailed fetal echocardiography and systemic evaluation. The anomalies associated with CTR alterations include:
1. Increased CTR (Cardiomegaly):
- Structural congenital heart disease — Hypoplastic left heart syndrome, Ebstein’s anomaly, tricuspid dysplasia, ventricular aneurysm.
- Cardiomyopathies — Dilated cardiomyopathy, endocardial fibroelastosis.
- High-output cardiac states — Severe fetal anemia (alloimmunization, parvovirus infection), arteriovenous malformations (chorioangioma, vein of Galen malformation), twin-to-twin transfusion syndrome (recipient twin).
- Obstructive lesions — Critical aortic stenosis, coarctation, pulmonary atresia with dilated right heart.
- Pericardial effusion / hydrops fetalis — Can mimic or contribute to cardiomegaly.
- Chromosomal syndromes — Certain aneuploidies (e.g., trisomy 21, 13, 18) with cardiac involvement.
2. Decreased CTR (Relatively small heart):
- Pulmonary hypoplasia — Due to oligohydramnios, congenital diaphragmatic hernia, skeletal dysplasias, thoracic malformations.
- Thoracic space-occupying lesions — Congenital pulmonary airway malformation (CPAM), bronchopulmonary sequestration, large pleural effusions displacing the heart.
- Technical factors — Oblique planes, improper imaging, or compression of thorax.
3. Clinical Importance: Abnormal CTR is not a diagnosis in itself but a warning sign. Persistent CTR > 0.50 is strongly associated with adverse perinatal outcomes and requires:
- Detailed fetal echocardiography (chamber anatomy, function, Doppler).
- Assessment for hydrops fetalis, anemia, or systemic causes.
- Genetic evaluation if associated with structural anomalies.
- Close surveillance and delivery planning in a tertiary care setting.
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