B64-22 When the Clot Thickens: A Case of Mistaken Thrombo-Identity

Abstract Chronic thromboembolic pulmonary hypertension (CTEPH) and in situ thrombosis associated with pulmonary arterial hypertension (PAH) are distinct entities with overlapping clinical and radiologic features. We present a case of in situ thrombosis in a patient with PAH, initially suspected to have CTEPH, to highlight key diagnostic distinctions. A 57-year-old male with a 40-year history of methamphetamine and tobacco use was transferred to Banner University Medical Center for management of hypoxemia and right heart strain after being diagnosed with a massive saddle pulmonary embolism (PE). Initial catheter-directed thrombolysis at an outside hospital was unsuccessful, raising concern for chronic thromboembolic disease (CTEPH). Echocardiography showed severe right ventricular (RV) dilation and dysfunction. Despite partial success with interventional thrombectomy, extensive clot burden persisted.Right and left heart catheterizations revealed precapillary pulmonary hypertension with a pulmonary vascular resistance of 7.8 Wood units and a normal left ventricular end-diastolic pressure of 10 mmHg, suggesting pulmonary arterial hypertension rather than post-capillary disease. Further review of imaging demonstrated eccentric thrombus adherent to the vessel wall, forming an obtuse angle, without recanalization or organized thromboembolic remnants—findings more consistent with in situ thrombosis rather than CTEPH. The patient was treated with therapeutic anticoagulation, diuresis, inhaled epoprostenol, and tadalafil (Opsynvi), resulting in clinical improvement. He was discharged to a skilled nursing facility with plans for follow-up and transition to inhaled treprostinil (Tyvaso). CTEPH, a WHO Group 4 pulmonary hypertension, results from chronic, organized thromboembolic obstruction of the pulmonary arteries. In contrast, in situ thrombosis in PAH (Group 1) arises from endothelial dysfunction, shear stress, and hypercoagulability without prior embolic events. Radiographically, CTEPH is characterized by webs, bands, intimal irregularities, and mosaic perfusion patterns, while in situ thrombosis typically manifests as an eccentric lesion forming an obtuse angle with the vessel wall and lacking recanalization or organized thromboembolic remnants. Non-responsiveness to EKOS or thrombectomy should raise suspicion for chronic in situ clot formation.Ventilation-perfusion (V/Q) scans remain the most sensitive tool for detecting CTEPH, demonstrating multiple mismatched perfusion defects, whereas in situ thrombosis often lacks these abnormalities. Advanced imaging, such as dual-energy CT, can further differentiate the two by revealing characteristic perfusion and attenuation differences. Differentiating CTEPH from in situ thrombosis is crucial, as treatments differ: CTEPH requires surgical endarterectomy, while PAH-related thrombosis responds to vasodilator therapy. Careful integration of hemodynamic and imaging findings ensures accurate diagnosis and optimized management. This abstract is funded by: none