A 65-year-old woman collapses with sudden onset pleuritic chest pain and breathlessness. She is hypotensive at 80/50 mmHg, tachycardic at 130 beats per minute and hypoxic with oxygen saturation of 86% on air. ECG shows sinus tachycardia. CT pulmonary angiography demonstrates a large saddle pulmonary embolus. What is the most appropriate immediate treatment?