Catheter-guided alteplase has a growing role in the management of acute pulmonary embolism (PE). Following a 12-24 hour alteplase infusion (for bilateral or unilateral PE, respectively) patients are routinely managed with therapeutic unfractionated heparin (UFH) as a bridge to chronic warfarin therapy. It is our desire to study the effects of rivaroxaban vs. standard care following catheter-guided alteplase thrombolytic therapy (CDT) in patients with acute pulmonary embolism. Use of rivaroxaban may offer several important advantages compared to standard therapy in this setting. Among these is the potential for rivaroxaban to improve novel biomarkers of thrombosis including inhibition neutrophil extracellular traps (NETs), tissue factor-positive microparticles, and markers of inflammation. Neutrophil release of extracellular DNA may provide a scaffold upon which venous thrombosis propagates. NETs are associated with thrombus organization. Their dissolution may facilitate thrombolysis. Circulating DNA, a surrogate marker for NETs, is elevated 2-3 fold in patients with venous thromboembolism (VTE) and correlates strongly with plasma myeloperoxidase (MPO), an inflammatory marker of neutrophil and monocyte activation. The investigators have previously demonstrated that heparin can trigger MPO release from leukocytes. Thus, it is resonable to speculate that anti-Xa therapy may reduce inflammation, MPO, and NET levels in circulation. Further, the investigators have observed that catheter-directed thrombolysis may increase length of stay (time frame = 8 hours to 30 days following administration of study drugs) and the investigators would also propose treatment with rivaroxaban may balance this by eliminating a "bridging" period.