A 65-year-old man on ICU develops acute severe dyspnoea and hypotension 7 days after a total knee replacement. CT pulmonary angiography confirms bilateral massive pulmonary embolism with RV strain. HR 130 bpm, BP 80/50 mmHg, SpO2 85% on 15 L/min O2. He is commenced on unfractionated heparin. Despite 1 litre of crystalloid and noradrenaline 0.2 mcg/kg/min, he remains hypotensive. What is the most appropriate next step?