In 8 COPD patients on prolonged MV we applied in random order two MV settings : i ) pressure support ( PS ) 20 cm H2O + positive end-expiratory pressure ( PEEP ) 0 cm H2O ( setting-1 ) and ii ) PS 15 cm H2O + PEEP 5 cm H2O ( setting-2 ) , each followed by a 30 min T-piece trial .