A 54 year old man has a 3 hour history of chest pain and dyspnea. He is 1 week post colectomy for colon cancer. On examination, temperature 37.5 C, BP 110/60 mmHg, HR 130/min, and RR 24/min. O2 sat is 88% on room air and 94% on oxygen, 4 L/min, cardiac examination shows tachycardia. Breath sounds are normal. Serum creatinine concentration is 3 mg/dl. CT pulmonary angiography shows left sided pulmonary embolism. Which of the following is the most appropriate immediate treatment?