Cardiothoracic Ratio Formula and Cutoff Values for Cardiomegaly
The cardiothoracic ratio (CTR) is calculated by dividing the maximum transverse cardiac diameter by the maximum internal thoracic diameter, with cardiomegaly defined as CTR >0.5 on posteroanterior (PA) films and >0.55 on anteroposterior (AP) films. 1
Measurement Technique
To calculate the CTR, measure the maximum transverse cardiac diameter by identifying the widest point of the cardiac silhouette from the right heart border to the left heart border. 1 Then measure the maximum internal thoracic diameter at the level of the diaphragm, from inner rib to inner rib. 1
The formula is:
CTR = Maximum Cardiac Diameter ÷ Maximum Thoracic Diameter
Diagnostic Cutoff Values
Posteroanterior (PA) View
- CTR >0.5 indicates cardiomegaly 1, 2, 3, 4
- This is the standard threshold used in clinical practice 4
Anteroposterior (AP) View
- CTR >0.55 indicates cardiomegaly 1, 2, 3
- The higher threshold accounts for cardiac magnification that occurs with AP projection 5
For portable AP chest radiographs, a correction formula can be applied when a previous PA film is available: CD(Chest PA)/CD(Chest AP) ratio = {0.00099 × (radiation distance [cm])} + 0.79. 5 This allows calculation of a "corrected" CTR that approximates the PA measurement. 5
Critical Measurement Considerations
The CTR must be measured on a properly positioned PA chest radiograph to be reliable; measurements on AP films without correction are unreliable due to cardiac magnification. 4 The CTR should express the relationship between heart size and transverse chest dimension specifically on PA radiographs. 4
Age and gender affect normal CTR values, so the same cutoff may not be appropriate for all populations. 6 Mean CTR values differ between males (46.6% ± 3.9) and females (47.8% ± 4.8), and an increase in transverse cardiac diameter of just 1 cm can push CTR above 50% in most age groups except males aged 21-40 years. 6
Essential Clinical Pitfalls
Pericardial effusion can mimic cardiomegaly on chest X-ray without true cardiac chamber enlargement, making echocardiography mandatory for confirmation. 1, 2 An enlarged cardiac silhouette does not distinguish between true myocardial hypertrophy, ventricular dilatation, or pericardial fluid. 1
A normal CTR does not exclude significant cardiac disease—up to 18-20% of patients with acute decompensated heart failure have normal chest X-rays. 3 Conversely, significant left ventricular dysfunction may be present without radiographic cardiomegaly. 3
Mandatory Next Steps After Identifying Cardiomegaly
When CTR exceeds the diagnostic threshold, immediately order transthoracic echocardiography to verify true cardiac enlargement, measure ejection fraction, assess chamber dimensions, and evaluate valvular structure. 1, 2 This is the essential confirmatory test. 1
Obtain a 12-lead ECG to identify rhythm disturbances, conduction abnormalities, chamber enlargement patterns, and evidence of prior myocardial infarction. 1, 2
Draw natriuretic peptides (BNP or NT-proBNP) for heart failure assessment, as these have reasonable negative predictive value for excluding heart failure. 1, 3
Additional Radiographic Findings to Assess
Beyond the CTR calculation, look for pulmonary vascular redistribution with prominent upper lobe vessels indicating elevated left ventricular filling pressures. 1, 3
Identify Kerley B lines from increased lymphatic pressures, which indicate interstitial edema. 1, 3
Note alveolar edema appearing as fluffy opacities or consolidations in severe fluid overload. 1, 3
Document pleural effusions, particularly bilateral effusions, which support heart failure diagnosis. 1, 3
On lateral view, assess for retrosternal fullness suggesting right ventricular enlargement. 1
Context-Specific Performance
In emergency department settings with acute presentations, chest X-ray demonstrating pulmonary edema has a positive likelihood ratio of 4.8 for confirming acute heart failure, making it more useful in acute rather than chronic presentations. 1, 3 Radiologists achieve 95% accuracy in identifying congestive heart failure on chest X-ray, compared to 85% for emergency medicine attendings and 78% for first-year residents. 3
In congenital heart disease patients, cardiomegaly correlates with significant right or left ventricular enlargement in the absence of pericardial effusion. 1 Throughout gestation in fetal echocardiography, a normal cardiothoracic ratio using area is 0.25 to 0.35 and <0.5 using circumference. 7