FIGURE 1.

In the spontaneously breathing patient, the difference between airway pressure and pleural pressure (transpulmonary pressure) determines air flow and lung volumes. Pleural pressure is estimated by measuring esophageal pressure (Pes) with an esophageal balloon in the distal third of the esophagus; airway pressure (Pa_w) and air flow are measured at the mouth with a pressure sensor and pneumotachometer. Transpulmonary pressure is the difference between P_Aw and P_Es. In the normal person, the transpulmonary pressure varies with inspiration and expiration; increasing transpulmonary pressure results in increased lung volumes and inspiratory air flow. The cursor shows the transition from inspiratory to expiration at end-inspiratory lung volume (see text).