[1] South Cent Ambulance Serv, Otterbourne SO21 2RU, Hants, England
[2] Univ Southampton, NIHR Southampton Resp Biomed Res Unit, Southampton, Hants, England
[3] Southampton Univ Hosp, Shackleton Dept Anaesthet, Southampton, Hants, England
[4] Southampton Univ Hosp, Dept Cardiol, Southampton, Hants, England