This statement is arguably the least controversial regarding manometry. The concept seems straightforward: At the resting condition (ie, at functional residual capacity), the pleural pressure is slightly subatmospheric, a function of the lung elastic recoil and the tendency of the chest wall to expand outward. Disruptions in capillary homeostasis, as described by Starling, result in an accumulation of pleural fluid and an increase in pleural pressure.3 Conversely, thoracentesis should lead to a measurable gradual decrease in pleural pressure back to a more physiologic level if the entirety of the fluid is drained. In some situations (thick visceral pleural peel, endobronchial obstruction, or decreased lung compliance), the lung may reexpand only partially or not at all and may result in different elastance curves, as described in Figure 1. It is hypothesized that continued drainage when the lung cannot reexpand further will drive an increase in transpulmonary pressures that could result in complications such as chest pain, pneumothorax ex vacuo, or, rarely, REPE (see the “Pleural Manometry Helps Prevent REPE” section).4-8 Hence, monitoring pleural pressures during thoracentesis should be the gold standard a priori for the diagnosis of unexpandable lung.