Why would it be important to measure transpulmonary pressure?

Concepts as Ventilator Induced Lung Injury (VILI), lung stress and strain as triggers of VILI, transpulmonary pressure as the real distending force of the lung renewed the interest on esophageal pressure measurement as surrogate of pleural pressure. This measure, however, is quite controversial in the critically ill patients. The elastance and the position of the balloon, the tone of the esophageal wall, the weight of the heart and of the lung and the position of the patients may influence the relationship between esophageal and pleural pressure. It has been shown that pleural pressure varies along the vertical axis of the lung. In an experimental study on dogs pleural pressure has been measured by surface wafers and it was lower than esophageal pressure in the upper part on the lung, nearly equal in the central part and higher in the lower part. Recently some authors proposed and arbitrary correction of the absolute value of the esophageal pressure by 5 cmH2O to obtain positive transpulmonary pressure values. However, pleural pressure variations are similar to esophageal pressure variations at every lung level so we can say that it will be better to evaluate pressure variations rather than absolute values.