Efficacy and tolerability study of Symbicort Turbuhaler(160/4.5μg/inhalation,2inhalations twice daily) added to Atrovent (20μg/inhalation, 2 inhalations 4 times daily)+theophylline SR(0.1g/tablet,1 tablet p.o. twice daily) compared with Atrovent+theophylline SR in severe COPD patients