There are three distinct pleural elastance curves that one can appreciate during a thoracentesis (Fig 1).3,4 In the first curve (normal physiology), pleural pressure is positive, and as fluid is withdrawn, pleural pressure approaches −3 to −5 cm H2O at FRC. This elastance curve is typical of effusions due to hepatic hydrothorax or congestive heart failure. In the second curve, termed “lung entrapment,” the initial elastance is normal, but toward the terminal portion of the thoracentesis the pressure drops and the slope of the curve becomes steeper. This is due to failure of the lung to fully expand and can be seen in patients who have a visceral pleural peel, endobronchial obstruction, or any cause of increased elastic recoil pressures of the lung parenchyma, such as lymphangitic carcinomatosis. This curve is commonly encountered in patients with malignant pleural effusions, occurring in up to 32% of patients in the largest study of pleurodesis to date.5 Additionally, it has been shown that the success of pleurodesis in these patients is significantly lower than in patients with a fully expandable lung.6 Although nonexpandable lung can be identified by postthoracentesis imaging, there are several mechanisms of pneumothorax following thoracentesis,7 and manometry is a useful and easy way to identify lung entrapment as the cause. Furthermore, manometry has been used to safely guide large-volume thoracenteses.8,9 The benefits of removing as much fluid as possible include maximizing symptom relief, maximizing the usefulness of postprocedure imaging, and identifying the terminal nonexpandable lung prior to attempts at pleurodesis. If one only removes 1 of 3 L in the thorax and assumes the rest of the lung will expand at the time of pleurodesis, the patient may be exposed to a procedure that will not be successful. These patients with lung entrapment due to malignant pleural effusions can be effectively managed with tunneled pleural catheters,10 and in fact, the spontaneous pleurodesis rate for tunneled pleural catheters is likely higher in patients with full lung expansion as compared with patients with nonexpandable lung.11 Finally, the third curve describes another cause of nonexpandable lung, termed “trapped lung.” Patients with trapped lung have pleural effusions ex vacuo—the negative pleural pressure is the cause of the effusion. These effusions are transudates, and as patients are often asymptomatic, specific therapy aimed at the effusion is not typically required.