Mechanical ventilation is often used in the setting of acute exacerbations of obstructive lung diseases like asthma and chronic obstructive pulmonary disease (COPD). These disorders are characterized by increased airway resistance secondary to bronchospasm and airway inflammation, collapse, and remodeling, often leading to inefficient exhalation and expiratory flow limitation. Patients with obstructive lung disease and inefficient exhalation usually require a longer expiratory time to achieve full exhalation.