Ventilator gas flow into the lungs is based on two different methods of changing the transrespiratory pressure (pressure at the airway opening minus pressure at the body surface [P awo - P bs ]). A ventilator can control pressure either at the mouth or around the body surface. A negative pressure ventilator generates a negative pressure at the body surface that is transmitted to the pleural space and then to the alveoli. As a result, a pressure gradient develops between the airway opening and the alveoli, and air flows into the lungs. The volume delivered depends on the pressure difference between the alveolus and the pleural space (transpulmonary pressure [P L = P A - P PL ]) and lung and chest wall compliance.
With positive pressure ventilators, gas flows into the lung because the ventilator establishes a pressure gradient generating a positive pressure at the airway opening.
Again, volume delivery depends on the pressure distending the alveoli (P L ) and lung compliance.