A 45-year-old woman presents to the emergency department with acute onset pleuritic chest pain and dyspnoea. She recently had knee replacement surgery and has been relatively immobile. Heart rate is 115 beats/min, respiratory rate 28 breaths/min, blood pressure 110/70 mmHg, oxygen saturation 90% on room air. D-dimer is elevated. CT pulmonary angiography confirms a large pulmonary embolus without haemodynamic compromise. What is the most appropriate initial management?