The contrast-enhanced CT image of the chest shown above demonstrates filling defects (arrows) within the main pulmonary arteries. This finding is most consistent with a diagnosis of:
A. Pulmonary hypertension
B. Mediastinal lymphadenopathy
C. Aortic dissection
D. Saddle pulmonary embolism
Explanation
Image Findings
Axial contrast-enhanced CT image of the chest demonstrating the main pulmonary artery bifurcation.
A large, low-attenuation filling defect is visible within the lumen of the right main pulmonary artery (indicated by the right arrow).
A similar large, low-attenuation filling defect is also present within the lumen of the left main pulmonary artery (indicated by the left arrow).
These intraluminal defects appear to straddle the main pulmonary artery bifurcation.
The surrounding pulmonary arterial lumen is well-opacified by contrast, confirming the intraluminal nature of the defects.
Diagnosis
Key Point
The presence of large, low-attenuation filling defects straddling the main pulmonary artery bifurcation on a contrast-enhanced CT is pathognomonic for a saddle pulmonary embolism.
A pulmonary embolism (PE) occurs when a blood clot (thrombus), usually originating from the deep veins of the legs, travels to the pulmonary arteries and obstructs blood flow. A saddle embolus is a specific type of large PE that lodges at the bifurcation of the main pulmonary artery, extending into both the right and left main pulmonary arteries, as clearly demonstrated in this image. This type of embolus can cause significant hemodynamic compromise due to the extensive obstruction of pulmonary blood flow.
Differential Diagnosis
Table
Feature
Saddle Pulmonary Embolism
Pulmonary Hypertension
Aortic Dissection
Mediastinal Lymphadenopathy
Image Findings
Intraluminal filling defects in main pulmonary arteries
Dilated pulmonary arteries, RV hypertrophy, no intraluminal defects
Intimal flap in aorta, true/false lumens, not in pulmonary arteries
Enlarged lymph nodes external to vessels, potential extrinsic compression
Vessel Involved
Pulmonary arteries
Pulmonary arteries (dilatation)
Aorta
Lymphatic vessels/nodes (external to major vessels)
Pathology
Thrombus
Increased pulmonary vascular resistance
Tear in aortic intima
Inflammatory/malignant enlargement of lymph nodes
Clinical Context
Acute dyspnea, pleuritic chest pain, risk factors for DVT
Progressive dyspnea, fatigue, signs of right heart failure
Sudden severe chest/back pain, pulse deficits
Often asymptomatic, or symptoms related to underlying cause (infection, malignancy)
Clinical Relevance
Clinical Pearl
Saddle pulmonary emboli are particularly dangerous as they can cause acute right heart failure and cardiogenic shock due to massive obstruction of pulmonary blood flow. Patients typically present with sudden onset dyspnea, pleuritic chest pain, and hypoxemia. Risk factors include prolonged immobility, recent surgery, malignancy, oral contraceptive use, and inherited thrombophilias.
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CT Pulmonary Angiography (CTPA) is the gold standard for diagnosing pulmonary embolism. The classic finding is an intraluminal filling defect within a contrast-opacified pulmonary artery.
Key Point
The Wells' score and PERC rule are used for pre-test probability assessment of PE, guiding the decision for D-dimer testing or imaging.
Do not confuse motion artifact or beam hardening artifact with true filling defects. True emboli are typically well-defined and persist across multiple slices. Also, be careful not to misinterpret partial volume averaging of adjacent structures as intraluminal pathology.
