The noninvasive ventilation outcomes (NIVO) score: prediction of in-hospital mortality in exacerbations of COPD requiring assisted ventilation by Hartley, T, Lane, ND, Steer, J, Elliott, MW, Sovani, MP, Curtis, HJ, Fuller, ER, Murphy, PB, Shrikrishna, D, Lewis, KE, Ward, NR, Turnbull, CD, Hart, N, Bourke, SC