[1] Imperial Coll London, Charing Cross Hosp, Fac Med, London W6 8RF, England
[2] Univ Oxford, Med Sci Div, Oxford OX1 3AZ, England
[3] Hammersmith Hosp, Imperial Coll NHS Trust, Dept Cardiothorac Surg, London W12 0HS, England
[4] Univ Thessaly, Dept Cardiothorac Surg, Biopolis, Larisa 41110, Greece
[5] Univ Thessaly, Dept Cardiothorac Surg, Unit Qual Improvement, Biopolis, Larisa 41110, Greece
[6] Imperial Coll London, St Marys Hosp, Dept Surg & Canc, London W2 1NY, England