Cardiothoracic Ratio on AP Sitting Chest X-Ray
The upper limit of normal for the cardiothoracic ratio (CTR) on an anteroposterior (AP) chest radiograph in the sitting position is 55-60%, not the traditional 50% used for posteroanterior (PA) films.
Key Differences Between AP and PA Projections
The AP projection inherently magnifies the cardiac silhouette compared to PA radiographs due to fundamental geometric principles 1, 2, 3:
- AP chest radiographs demonstrate CTR values approximately 7.5% higher than axial CT measurements 1
- The heart diameter measures approximately 3 cm larger on AP compared to PA films 1
- The thoracic diameter is also affected, measuring approximately 2 cm larger on AP projections 1
Recommended CTR Cutoff Values for AP Films
Optimal Threshold: 55-60%
A CTR cutoff of 55% provides the best balance of sensitivity (61%) and specificity (66%) for detecting cardiac chamber enlargement on AP chest radiographs 1. The traditional 50% cutoff used for PA films performs poorly on AP projections:
- CTR ≥50% on AP films: 86% sensitivity but only 32% specificity (too many false positives) 1
- CTR ≥55% on AP films: 61% sensitivity and 66% specificity (balanced performance) 1
- CTR ≥60% on AP films: 34% sensitivity but 92% specificity (highly specific but misses many cases) 1
Historical Validation
Studies from 1987 established that an upper limit of 55% CTR and heart diameter of 165 mm in males (150 mm in females) provides useful discrimination between normal and abnormal heart size on AP erect chest radiographs 2.
Clinical Application Algorithm
When interpreting AP sitting chest radiographs 1, 2:
- CTR <45%: Likely normal heart size (high sensitivity for excluding cardiomegaly)
- CTR 45-55%: Indeterminate zone—avoid making clinical decisions based solely on this range; consider additional imaging or clinical correlation
- CTR >55%: Likely cardiac enlargement (reasonable balance of sensitivity and specificity)
- CTR >60%: Highly specific for true cardiac chamber enlargement (92% specificity)
Important Caveats
Technical Factors Affecting Measurement
The magnification effect of AP projection is greater for smaller hearts than larger hearts 3. Additionally:
- Radiation distance significantly impacts measurements: The correction formula is {0.00099 × radiation distance (cm)} + 0.79 4
- The position of the thoracic diameter (anterior vs. posterior) markedly affects CTR calculations 3
Limitations of CTR as a Diagnostic Tool
CTR correlates only weakly with true cardiac chamber volumes measured by cardiac MRI (the gold standard), with area under the curve values of only 0.6-0.7 5. This weak correlation exists because:
- Substantial overlap exists in cardiac MRI parameters between patients with normal and increased CTR 5
- CTR cannot reliably distinguish which specific cardiac chambers are enlarged 5
Role in Trauma Settings
AP chest radiographs are commonly used as first-line imaging in trauma centers, particularly for hemodynamically unstable patients 6. However, they have limited ability to detect direct cardiac injuries and primarily identify indirect findings such as:
- Enlarged cardiomediastinal silhouette
- Hemothorax or pneumopericardium
- Displaced rib fractures (especially ribs 3-9) 6
When cardiac injury is suspected based on clinical presentation and radiographic findings, further evaluation with transthoracic echocardiography or CT should be pursued 6.