[1] Vancouver Gen Hosp, Dept Anesthesia, Vancouver, BC V5Z 1M9, Canada
[2] Univ British Columbia, Dept Med, Vancouver, BC, Canada
[3] Univ British Columbia, Dept Surg, Div Neurosurg, Vancouver, BC V6T 1W5, Canada
[4] Univ British Columbia, Dept Anesthesia Pharmacol & Therapeut, Vancouver, BC V5Z 1M9, Canada
[5] Vancouver Gen Hosp, Program Crit Care Med, Vancouver, BC V5Z 1M9, Canada
[6] Vancouver Coastal Hlth Res Inst, Ctr Clin Epidemiol & Evaluat, Vancouver, BC, Canada