Fig. 3.
Role of esophageal manometry in the assessment of transpulmonary pressure. (A) A supine model, as it could be seen in a chest computed tomography scan, is shown. The pleural space, in blue, is subdivided into three zones (nondependent space, in red; the midlung space, in green; and the dependent space, in yellow). The esophagus, with the esophageal balloon in place, is represented by the violet mark. From classic physiology, it is known that the pleural pressure in the dependent pleura is around 2 cm H2O higher than the one in the nondependent space so that a gradient exists (vertical black arrow). Theoretically, esophageal pressure (Pes) is directly exposed to the pleural space and reflects pleural pressure at its same gravitational level (dotted line), but the absolute value could be overestimated due to compression by the mediastinum (M) on the esophagus, intrinsic tone of the esophageal musculature, esophageal content, and intrinsic tone of the esophageal balloon. In conditions like adult respiratory distress syndrome, high superimposed pressure caused by inflammatory edema increases this gradient, so that Pes is likely to significantly overestimate nondependent pleural pressure. The entity of this gradient is unknown in morbid obesity. (B) An upright model is shown, during endotracheal intubation and positive pressure mechanical ventilation. Transpulmonary pressure is calculated as the difference between alveolar pressure and pleural pressure. A high abdominal load increases pleural pressure, thus resulting in lower transpulmonary pressure. Being exposed to the pleural space, esophageal manometry can be assumed to be a surrogate for pleural pressure, otherwise not measurable in a human being in the clinical setting.