A 58-year-old man presents to the emergency department with sudden onset pleuritic chest pain and shortness of breath. He has a recent history of lower-limb immobilisation following knee surgery. Pulse 118/min, respiratory rate 26/min, blood pressure 120/78 mmHg and oxygen saturation 91% on air. His Wells score for pulmonary embolism is high. CT pulmonary angiography will be delayed by several hours. What is the most appropriate immediate management?