The trial was stopped if at least one of the following intolerance criteria was present: SaO2<90% or PaO2 <60 mm Hg with FIO2 >0.4; PaCO2 > 50 mm Hg or increased by > 8 mm Hg; arterial pH < 7.33 or decreased by 0.07 or more; breathing frequency > 35 breaths/min or increased by 50% for 5 min or longer; heart rate > 140 beats/min or a sustained increase or decrease of > 20%; mean blood pressure > 130 or < 70 mm Hg; or presence of agitation, diaphoresis, disorientation, or depressed mental status.