1. Faculty of Public Health, London School of Hygiene and Tropical Medicine , Keppel Street, London, WC1E 7HT, United Kingdom
2. Department of Cardiothoracic Surgery, Golden Jubilee Hospital , Agamemnon St, Clydebank, G81 4DY, United Kingdom
3. Department of Cardiothoracic Surgery, Royal Infirmary of Edinburgh , 51 Little France Cres, Edinburgh, EH16 4SA, United Kingdom