A 65-year-old man presents with a 2-day history of pleuritic chest pain and breathlessness. He has a recent history of total hip replacement 3 weeks ago. Heart rate is 110 bpm, respiratory rate 26 breaths/min, oxygen saturation 92% on air, blood pressure 118/74 mmHg. His Wells score for pulmonary embolism is 6 (PE likely). What is the most appropriate next step in management?