[1] East & North Hertfordshire NHS Trust, Dept Cardiol, Stevenage, Herts, England
[2] Univ Cambridge, Dept Cardiovasc Med, Cambridge, England
[3] Imperial Coll London, Natl Heart & Lung Inst, Fac Med, London, England
[4] St Georges Univ Hosp NHS Trust, Dept Cardiol, London, England