Fig. 1

Pressures of the respiratory system. The respiratory system includes the lung and the chest wall, and the airway pressure is related to both transpulmonary pressure (lung assessment, =alveolar pressure − pleural pressure) and transthoracic pressure (chest and abdomen assessment, =pleural pressure − atmospheric pressure), which differ in the obese patient compared to the non-obese patient. The relative portion of pressure due to transthoracic pressure is higher in the obese patient than in the non-obese patient (elevated pleural pressure, which can be estimated by esophageal pressure). The plateau pressure represents the pressure used to distend the chest wall plus lungs. In obese patients, elevated plateau pressure may be related to an elevated transthoracic pressure, rather than an increase in transpulmonary pressure with accompanying lung overdistension. Usual driving pressure, i.e., transthoraco-pulmonary driving pressure (plateau pressure − positive end-expiratory pressure [PEEP]), may not be appropriate to assess the severity of obese patients with acute respiratory distress syndrome (ARDS). To differentiate the chest wall pressure from the lung pressure, assessing transpulmonary pressure (plateau pressure – PEEP – (inspiratory esophageal pressure − expiratory esophageal pressure)) using esophageal pressure may be appropriate in obese ARDS patients. Insp inspiratory, exp expiratory, esoph esophageal