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:

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 | 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. ## High-Yield for NEET PG **High-Yield:** **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. ## Mnemonic **Mnemonic:** For risk factors of DVT/PE, think **VIRCHOW'S TRIAD**: * **V**enous stasis (immobility, heart failure) * **I**ntimal injury (surgery, trauma, catheter) * **R**eversible hypercoagulability (malignancy, OCPs, pregnancy) ## Common Traps **Warning:** 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. ## Reference [cite:Harrison's Principles of Internal Medicine, 20th Ed, Ch 278; Robbins Basic Pathology, 10th Ed, Ch 13]