A 70-year-old man presents to the acute medical unit with sudden-onset shortness of breath and pleuritic chest pain. His respiratory rate is 28/min, oxygen saturation 89% on air, blood pressure 110/70 mmHg and heart rate 120 bpm. He had a total hip replacement 2 weeks ago. A CTPA confirms a large pulmonary embolism without right ventricular strain. What is the most appropriate initial management?