What does an enlarged CT (Cardiothoracic) ratio indicate?

What is an Enlarged Cardiothoracic (CT) Ratio?

An enlarged cardiothoracic ratio (CTR) is defined 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

Definition and Measurement Standards

• The CTR is calculated by dividing the maximal transverse cardiac diameter by the maximal internal thoracic diameter on a PA chest radiograph 2
• A CTR ≥0.50 (50%) is the traditional threshold for defining cardiomegaly 3, 2
• This measurement is only reliable on PA radiographs—anteroposterior (AP) projections magnify the cardiac silhouette and should not be used for CTR interpretation 4, 2
• Bedside AP radiographs are limited by motion artifacts, overlying devices, suboptimal positioning, and inherently lower diagnostic yield 4

Clinical Significance and Prognostic Value

• An enlarged CTR indicates cardiac chamber enlargement but does not directly correlate with cardiac function or ejection fraction 2, 5
• 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
• In elderly populations, cardiomegaly (CTR ≥0.50) is associated with increased mortality (9.1 vs 4.8 deaths per 100 person-years) and cardiovascular disease incidence independent of age, diabetes, and prior myocardial infarction 6
• Development of new cardiomegaly increases cardiovascular disease risk 1.8-fold compared to those maintaining normal CTR 6

Limitations and Pitfalls

The universal 50% cutoff has poor discriminatory power for true cardiac enlargement:

• CTR correlates only weakly with actual cardiac chamber volumes measured by cardiac MRI (the gold standard), with substantial overlap between normal and enlarged hearts 5
• CTR has only mild-to-moderate discriminatory ability (AUC 0.6-0.7) for detecting true cardiac enlargement 5
• Intermediate CTR values (45-55%) are neither sensitive nor specific and should not guide clinical decisions 5
• On CT imaging, the optimal CTR threshold for predicting left ventricular systolic dysfunction is actually 0.56, not 0.50 3

Practical Clinical Algorithm

For CTR interpretation, use this stratified approach:

• CTR <0.45: Likely normal cardiac size (sensitive but not specific) 5
• CTR 0.45-0.55: Indeterminate zone—avoid making clinical decisions based solely on this finding; pursue echocardiography or cardiac MRI for definitive assessment 5
• CTR >0.55: Likely true cardiac enlargement (specific but not sensitive); warrants further cardiac evaluation 5
• CTR ≥0.56 on CT: Suggests at least mild left ventricular systolic dysfunction 3
• CTR ≥0.60 on CT: Can exclude severe left ventricular systolic dysfunction with 98% negative predictive value 3

When CTR Assessment is Appropriate

• 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 1
• CTR remains valuable as first-line screening in hemodynamically unstable trauma patients and critically ill patients who cannot undergo PA imaging 4
• An enlarged cardiac silhouette on AP radiographs may be artifactual due to projection magnification rather than pathological 4

Alternative Assessments Required

When cardiac enlargement is suspected, definitive evaluation requires:

• Echocardiography for ejection fraction, chamber dimensions, and valvular function assessment 1
• Cardiac MRI for precise volumetric measurements, tissue characterization, and functional assessment 1, 5
• BNP/NT-proBNP levels to assess heart failure severity, though these do not replace imaging for structural assessment 1