[1] Univ Cambridge, Addenbrookes Hosp, Dept Anaesthesia, Neurosci Crit Care Unit, Cambridge CB2 2QQ, England
[2] Univ Cambridge, Dept Anesthesia, Cambridge, England
[3] Addenbrookes Hosp, Cambridge, England
[4] Univ Calgary, Dept Crit Care Med, Calgary, AB T2N 1N4, Canada