assist-controlled modes are often used after sedation and sometimes curarization for endotracheal intubation. Usually a fIO2 of 100 is set to allow an adequate oxygenation until an arterial blod gas analysis is obtained. A respiratory rate from 12 to 20 breaths per minute is often adequate in most cases, preferably being lower in obstructive patients and higher in restrictive ones. Tidal volume must be set between 5-7 mL/predicted body weight, the lower values being the most suited in severe cases with extremely low compliances and high resistances (6-8). To have the fatigued respiratory muscles rest in these patients, 24-48 hours of full ventilatory support may be needed,with sedation to suppress the patients'respiratory effort.In severe ARDS patients the (PaO2/FIO2 less than 120),a randomized controlled trial comparing cisatracurium placebo for 48 hours showed an improved adjiusted 90-day survival rate and increased ventilator free days in the cisatracurium group without a significant increase in muscleweakness. Short-term paralysis may facilitate patient-vantilator synchrony in the setting of lung protective ventilaton. Short-term paralysis would eliminate patient triggering and expiratory muscle activity. In combination, these effects may serve to limit regional overdistenson and cyclic alveolar collapse. Paralysis may also act to lower metabolism and overall ventilator demand