A 64-year-old man develops sudden onset pleuritic chest pain and severe breathlessness while on a medical ward following hip surgery. He is pale and clammy with a BP of 78/45 mmHg, respiratory rate 32 breaths per minute and oxygen saturation 84% on 15 L oxygen. His jugular venous pressure is elevated. Massive pulmonary embolism is suspected. What is the most appropriate immediate management?