Comparison of pressure support ventilation and assist control ventilation in patients with acute respiratory failure

We compared the effects of pressure support ventilation (PSV) with those of assist control ventilation (ACV) on the breathing pattern, work of breathing and blood gas exchange in 8 patients with acute respiratory failure. During ACV, the tidal volume was set at 10 ml/kg, and the inspiratory flow was set at 50 to 70l/min. During PSV, the pressure support level selected was 27±5 cm H₂O to make the breathing pattern regular. Tidal volume was significantly higher (908±179 ml vs. 633±96 ml) during PSV than during ACV at a lower peak airway pressure. Respiratory frequency was lower (15±4 breaths/min vs. 24±5 breaths/min) during PSV than during ACV, associated with a lower duty cycle, which improved synchrony between the patient and the ventilator. The oxygen cost of breathing, and estimate based on the inspiratory work added by a ventilator and the oxygen consumption, did not change significantly. PaO₂ was significantly higher during PSV than during ACV. We conclude that PSV using high levels of pressure support can improve the breathing pattern and oxygenation and fully sustain the patient's ventilation while matching his inspiratory efforts.