[1] Univ British Columbia, Dept Med, Fac Med, Expt Med Program, Vancouver, BC, Canada
[2] Univ British Columbia, Dept Med, Div Resp Med, Vancouver, BC, Canada
[3] Univ British Columbia, Dept Med, Div Resp, Inst HEART LUNG Hlth, Vancouver, BC, Canada
[4] Univ British Columbia, Ctr Clin Epidemiol & Evaluat, Vancouver, BC, Canada