1. Royal Brompton & Harefield NHS Foundation Trust, Adult Intensive Care Unit, Sydney Street, London, SW3 6NP, UK
2. Respiratory Medicine Research Programme, Centre for Infection and Immunity, Queen’s University, Belfast, BT12 6BN, Northern Ireland, UK
3. Regional Intensive Care Unit, Royal Victoria Hospital, Belfast, BT12 6BA, Northern Ireland, UK
4. Unit of Critical Care, National Heart and Lung Institute, Imperial College London, London, SW3 6LY, UK