CT Criteria for Right Heart Strain
Right heart strain on CT is diagnosed when the right ventricle to left ventricle diameter ratio (RV:LV) is ≥0.9, measured on axial images at the level of the tricuspid valve, or when interventricular septal bowing toward the left ventricle is present. 1, 2
Primary Diagnostic Criteria
RV:LV Diameter Ratio
- Measure the RV and LV diameters perpendicular to the interventricular septum on axial CT images at the level of the tricuspid valve 2, 3
- RV:LV ratio ≥0.9 indicates right heart strain with 88% sensitivity and 39% specificity compared to echocardiography 1, 2
- This measurement demonstrates excellent inter-observer reliability (ICC 0.95-0.96) even among non-radiologists 3
- RV diameter >45 mm alone increases the likelihood of clinically significant strain 4
Interventricular Septal Configuration
- Leftward bowing (flattening) of the interventricular septum indicates RV pressure overload 1, 2
- This finding reflects RV systolic pressure approaching or exceeding LV pressure 1
- Note that subjective assessment of septal bowing shows significant inter-observer variability among non-radiologists 3
Secondary CT Findings
Pulmonary Artery Measurements
- Main pulmonary artery (MPA) diameter ≥29 mm suggests pulmonary hypertension with 87% sensitivity and 89% specificity 1, 5, 6
- MPA to ascending aorta diameter ratio (MPA:Ao) >1.0 has 96% positive predictive value for pulmonary hypertension 1, 6
- MPA:Ao ratio measurement shows excellent inter-observer agreement (ICC 0.92-0.93) 3
- Segmental pulmonary artery to bronchus ratio >1:1 indicates elevated pulmonary vascular pressures 1, 5, 6
Venous Congestion Markers
- Contrast reflux into the inferior vena cava (IVC) and hepatic veins indicates elevated right atrial pressure 1, 7
- IVC diameter >20 mm suggests volume overload 7
- These subjective findings show higher inter-observer variability 3
Cardiac Chamber Changes
- Right atrial enlargement on CT is 100% predictive of right heart strain on subsequent echocardiography 7
- RV wall thickening ≥6 mm indicates chronic RV pressure overload 1
- Pericardial effusion may be present in severe cases 5
Prognostic Implications
Patients with RV:LV ratio ≥0.9 plus interventricular septal bowing have 30% risk of severe clinical deterioration within 5 days, compared to 4% in those without these findings 2. The combination of CT and echocardiographic evidence of strain identifies the highest-risk patients 2, 8.
Clinical Context for Interpretation
When CT Findings Are Sufficient
**Normal CT-RV/LV ratio (<0.9) adequately excludes critical RV strain in 97% of patients who lack all five high-risk features**: congestive heart failure, RV diameter >45 mm, age >60 years, central embolus location, and stage IV cancer 4. This represents approximately 37% of PE patients and eliminates the need for additional echocardiography 4.
When Additional Testing Is Needed
Proceed directly to echocardiography when CT shows RV:LV ≥0.9 with any of the following: central PE, elevated troponin, hemodynamic instability, or known cardiopulmonary disease 2, 8. Echocardiographic findings of RV dilation, TAPSE <16 mm, or McConnell's sign in these patients warrant consideration of advanced interventions including thrombolysis or catheter-directed therapy 8.
Common Pitfalls to Avoid
- Do not rely on MPA diameter alone to exclude pulmonary hypertension, as sensitivity varies with underlying lung disease and diameter <29 mm does not rule out disease 1, 5
- Measure RV:LV ratio at the correct anatomic level (tricuspid valve on axial images) to ensure reproducibility 2, 3
- Recognize that isolated interventricular septal bowing on CT is only 61% predictive of strain on echocardiography when other CT findings are absent 7
- Understand that CT is highly sensitive (88%) but only moderately specific (39%) for RV strain compared to the gold standard echocardiography 2
- Be aware that ground-glass opacities can occur in pulmonary arterial hypertension and do not always indicate pulmonary edema 5