Following pelvic gynecologic surgery, a 34-year-old woman becomes dyspneic, her peripheral aerial O2 saturation falls from 94% to 81%, and her measured PaO2 is 52 on a 100% non-rebreather mask. She is hemodynamically stable. A CT angiogram is consistent with a right lower lobe pulmonary embolus. Which of the following is the next step in her management?