A 62-year-old man presents with sudden onset pleuritic chest pain and breathlessness. He is tachycardic at 128 beats per minute, hypotensive at 78/40 mmHg and hypoxic with saturations of 86% on high-flow oxygen. ECG shows sinus tachycardia and CXR is normal. Massive pulmonary embolism is suspected. What is the most appropriate immediate management?