Cardiothoracic Ratio Measurement and Clinical Significance

Definition and Measurement Standards

The American Heart Association defines an enlarged cardiothoracic ratio (CTR) as a cardiac transverse diameter exceeding 50% of the thoracic diameter on a posteroanterior (PA) chest radiograph, indicating cardiomegaly, though this measurement primarily reflects right ventricular enlargement rather than left ventricular function and has significant limitations in clinical practice 1, 2

The cardiac silhouette enlargement primarily reflects right ventricular volume changes rather than left ventricular function, since the right ventricle forms most of the cardiac border on radiographs 1, 2

The American College of Cardiology recommends echocardiography for ejection fraction, chamber dimensions, and valvular function assessment when cardiac enlargement is suspected 1, 2
The European Society of Cardiology recommends cardiac MRI for precise volumetric measurements, tissue characterization, and functional assessment when cardiac enlargement is suspected 3
The American Heart Association recommends BNP/NT-proBNP levels to assess heart failure severity, though these do not replace imaging for structural assessment 1, 2

Serial chest radiographs are not recommended for routine chronic heart failure management, as changes in pulmonary vascular congestion are too insensitive to detect meaningful fluid status changes, according to the American College of Cardiology 1, 2

The American College of Radiology emphasizes that the cardiothoracic ratio (CTR) should only be reliably measured and interpreted on posteroanterior (PA) radiographs, as attempts to use CTR in other projections face significant limitations 4
Bedside anteroposterior (AP) radiographs are commonly limited in quality due to motion artifacts, overlying devices and garments, and suboptimal patient positioning, which can affect CTR measurement and interpretation 4
The evaluation of the mediastinum is inherently limited with only an AP projection, and AP radiographs have lower diagnostic yield for thoracic pathology compared to CT, missing 80% of hemothorax cases and 50% of vertebral and rib fractures 4

AP chest radiographs remain valuable as first-line imaging in hemodynamically unstable trauma patients and critically ill patients who cannot stand for PA imaging, and can identify indirect findings suggesting cardiac injury 4
AP radiographs can identify indirect findings suggesting cardiac injury including hemothorax, widened mediastinum, enlarged cardiomediastinal silhouette, and abnormal cardiac contour, but clinicians must remember that an apparently enlarged cardiac silhouette on AP view may be artifactual rather than pathological 4