A 62-year-old woman presents with acute onset pleuritic chest pain and dyspnea. ECG shows sinus tachycardia and new right bundle branch block, with S1Q3T3 pattern. CT pulmonary angiogram reveals a large saddle pulmonary embolus with right ventricular dilation on CT. Blood pressure is 82/54 mm Hg. What is the most appropriate immediate treatment?