Ventilatory failure may be generated by a deterioration of the neuromuscular capacity or by increasing the loads of the respiratory system. The decrease of the capacity may be owed to a depression of the respiratory center (whether by sedation, alkalosis or encephalic lesion), neuropathies (polyneuropathy of the critical patient), muscular disorders (malnutrition, hyperinsuflation, electrolyte disorders and the use of muscle relaxants and corticoids) and chest wall abnormality (unstable chest, post-operative pain). The increase in the load may be due to the high ventilation requirements (sepsis, anxiety and pain), resistive loads (bronchospasm, secretions), elastic loads (auto PEEP, decrease in compliance) and related to the ventilator and the endotracheal tube (valves, small tubes) [6, 12] .