Catheter-directed thrombolysis for acute pulmonary thromboembolism (PTE) is associated with a reduction in mortality, although it also carries an increased risk of bleeding. This study aimed to visualize the extent of fibrinolysis within pulmonary artery thrombi using rare specimens obtained via non-obstructive general angioscopy (NOGA). Under direct visualization with NOGA, blood samples were collected from the site of pulmonary artery thrombi and analyzed histologically and immunohistochemically. Fibrinolysis was assessed using the fibrin network index (FNI). The FNI was significantly higher in the Monteplase-treated group than in the untreated group, clearly visualizing the enhanced fibrinolysis induced by Monteplase. In a case where Monteplase was administered in advance for the treatment of deep vein thrombosis (DVT), a marked increase in D-dimer levels was observed; however, the FNI remained low. This suggests that the thrombus may have partially dissolved at the DVT stage, limiting the efficacy of Monteplase by the time the embolus reached the pulmonary artery. The presence of neutrophil extracellular traps (NETs) in association with fibrin was confirmed by immunohistochemical analysis. The influence of NETs in the progression from DVT to PTE was suggested:
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