A 72-year-old man on ICU is receiving therapeutic anticoagulation with IV unfractionated heparin for a submassive pulmonary embolism. He develops life-threatening gastrointestinal haemorrhage with haemoglobin dropping from 95 to 55 g/L over 2 hours. The heparin infusion is stopped. APTT ratio 30 minutes after stopping remains 4.2. What is the dose of protamine required?