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INTRODUCTION — The direct measurement of pleural pressures in a pleural effusion, also known as pleural manometry, can be performed at the time of thoracentesis. Assessment of the initial pleural fluid pressure and the changes in pressure as fluid is removed (known as the pleural space elastance) is useful in the identification of pleural processes (eg, lung entrapment, trapped lung) that prevent the lung from re-expanding when pleural fluid is removed. In addition, pleural pressure measurements can be used to guide fluid removal during large volume thoracentesis.
The technique for direct measurement of pleural fluid pressure will be reviewed here. The techniques for diagnostic thoracentesis and large volume thoracentesis, the analysis of pleural fluid, and the evaluation and management of trapped lung and lung entrapment are discussed separately. (See "Diagnostic thoracentesis" and "Large volume thoracentesis" and "Diagnostic evaluation of a pleural effusion in adults: Initial testing" and "Diagnostic evaluation of pleural effusion in adults: Additional tests for undetermined etiology" and "Diagnosis and management of pleural causes of unexpandable lung".)
●Pleural elastance is the change in pleural pressure with removal of a given volume of pleural fluid. It is calculated by dividing the change in pleural pressure by the volume removed. A normal pleural elastance is estimated to be <14.5 cm H2O/L . (See 'Pleural elastance' below.)
●Lung entrapment refers to visceral pleural restriction caused by an active pleural process in which the lung expansion is abnormal due to visceral pleural restriction from active pleural disease, such as malignancy or infection. The pleural fluid analysis is consistent with an exudate. (See "Diagnosis and management of pleural causes of unexpandable lung".)
●Trapped lung is a consequence of a remote inflammatory condition that has left behind a collagenous or fibrous peel, which restricts the ability of the lung to expand. The pleural fluid analysis is consistent with a transudate or a protein-discordant exudate and has a mononuclear cell predominance. All causes of trapped lung begin as a form of lung entrapment. The one exception is malignancy, where the vast majority of patients die as a result of the metastatic process. While most cases of lung entrapment resolve with resolution of the inflammatory process, in others, the resolution is incomplete resulting in a trapped lung. Therefore, a trapped lung and lung entrapment represent a continuum of the same disease process. (See "Diagnosis and management of pleural causes of unexpandable lung".)
●Large volume thoracentesis refers to removal of more than 1L during a therapeutic thoracentesis. (See "Large volume thoracentesis".)
INDICATIONS — The main indications for pleural pressure monitoring during thoracentesis include:
●Identifying visceral pleural processes that prevent lung expansion when pleural fluid is removed (eg, lung entrapment and trapped lung). (See "Diagnosis and management of pleural causes of unexpandable lung", section on 'Summary and recommendations'.)
●Determining whether lung entrapment will prevent successful pleurodesis for a malignant effusion. (See "Diagnosis and management of pleural causes of unexpandable lung", section on 'Lung entrapment'.)
●Identification of trapped lung as a cause of an undiagnosed pleural effusion. (See "Diagnostic evaluation of pleural effusion in adults: Additional tests for undetermined etiology", section on 'Trapped lung'.)
●Guiding fluid removal during therapeutic (large volume) thoracentesis. (See "Large volume thoracentesis", section on 'Determining the volume of fluid to be removed'.)
Identification of lung entrapment in a patient with a malignant pleural effusion is important, as thoracentesis is less likely to relieve dyspnea and pleurodesis is rarely successful when lung entrapment is present . (See "Diagnosis and management of pleural causes of unexpandable lung", section on 'Treatment'.)
TECHNIQUE — Direct measurement of pleural pressure is performed during thoracentesis. As with diagnostic thoracentesis, the patient’s identity and planned procedure are confirmed; the side of the thorax to be accessed is confirmed by reviewing the chest radiograph. The procedure is explained in detail to the patient and informed consent obtained. The correct procedure for site preparation, local anesthesia, and placement of the thoracentesis catheter in the pleural space is discussed separately. We recommend that the patient be in an upright and sitting position. (See "Diagnostic thoracentesis", section on 'Technique' and "Thoracic ultrasound: Indications, advantages, and technique".)
Equipment — The equipment needed for measurement of pleural pressures during thoracentesis is listed in the table (table 1). We prefer a water column manometer. A water column manometer can be constructed from two lengths of sterile intravenous tubing that are prefilled with normal saline solution and purged of air. A 22G needle is interposed between the thoracentesis catheter and the pleural manometer and serves as a resistor, dampening the pressure oscillations (figure 1 and picture 1) . Without the needle resistor, the water column of a simple water manometer swings widely during respiration and is difficult to record . The manometer is attached to a stopcock on the pleural catheter to allow repeated measures of pleural pressure during fluid removal [2,4]. When using a water manometer, the zero pressure level on the manometer is set at the level that the catheter enters the chest wall.
Alternatively, an electronic hemodynamic transducer attached to an intensive care unit (ICU) monitor can be used in addition to or instead of a water manometer. When both systems are used, two stopcocks are needed to make the connections. An important difference between the water manometer and the electronic transducer is that the electronic system will not measure negative pressure. To compensate, the height of the pressure transducer relative to the zero reference level is lowered such that pleural pressures are recorded as positive numbers and then corrected by subtracting the number of cm that the manometer is below the point of needle insertion (actual zero) . In addition, electronic transducers measure pleural pressure in mmHg, so the transducer pressures need to be converted to cm H2O (1 mmHg = 1.36 cm H2O).
Commercially available digital manometers can be directly attached to the thoracentesis catheter for easy acquisition of pleural liquid pressure . A three-way stopcock should be used to allow closure of the drainage side when pleural pressures are measured. Disadvantages of this technique include low sampling time (sampling time is for only 3 seconds so if a patient is breathing less than 20 breaths per minute, the entire respiratory cycle is not sampled), the ability to identify quiet and stable breathing is problematic, and the operator has to estimate the mean pressure due to the wide pressure oscillations occurring (this is especially true toward the end of drainage).
Insertion site — In order to measure pleural pressures and elastance at the time of thoracentesis, the patient should be in an upright sitting position with his or her arms resting on a surface such as a bedside table. Thoracic ultrasound is employed to confirm the location of the pleural effusion and select a dependent portion that is above the level of the diaphragm through all phases of the respiratory cycle [6,7]. The exact puncture site should be immediately above the superior aspect of a rib to avoid the neurovascular bundle and, when possible, 8 to 10 cm lateral to the paraspinous muscle. (See "Diagnostic thoracentesis", section on 'Avoidance of intercostal arteries' and "Thoracic ultrasound: Indications, advantages, and technique".)
Without moving the patient, a wide area surrounding the puncture site is sterilized with 0.05 percent chlorhexidine or 10 percent povidone-iodine solution, and sterile drapes are placed around the site. Local anesthetic (eg, 1 or 2 percent lidocaine) is administered to the site as for diagnostic thoracentesis. (See "Diagnostic thoracentesis", section on 'Site preparation and local anesthesia'.)
A thoracentesis catheter with a 50 mL syringe attached via a stopcock is inserted, advanced toward the rib, and then "walked" over the superior edge of the rib. As the needle is advanced, aspiration is attempted by intermittently pulling back on the plunger of the syringe. When pleural fluid is returned, the catheter is paced through the needle into the pleural space and the needle withdrawn from the skin. A sample of 50 to 60 mL of pleural fluid is obtained as needed for diagnostic testing. (See "Diagnostic thoracentesis", section on 'Specimen preparation'.)
Pressure measurement — The initial mean pleural pressure is measured just after insertion of the thoracentesis catheter into the pleural space and withdrawal of an initial sample of pleural fluid. We typically record the mean pleural pressure during tidal volume breathing, assessed over four to five respiratory cycles . This method requires use of an electronic pressure system with the capability to record tracings for review after the procedure (figure 2). The over-damped water manometer with a 22G needle acting as resistor is a validated method for measuring mean pleural pressures (picture 1). However, with periods of coughing, this system is not reliable and the electronic signal should be used for analysis.
During subsequent fluid removal, the pleural pressure is measured after each aliquot (eg, 100 to 250 mL) of pleural fluid, according to the same procedure (assessing mean pleural pressure over four to five respiratory cycles), and recorded [8,9]. We recommend stopping fluid removal when the mean pleural pressure decreases to -20 cm H2O.
Pleural elastance — To calculate the pleural elastance, pleural pressure is measured and recorded after each aliquot (eg, 100 to 250 mL) of pleural fluid is removed , and the change in pleural pressure (in cm H2O) is divided by the amount of pleural fluid removed (in liters). As an example, if 500 mL (0.5 L) of pleural fluid is removed, and the pleural pressure decreases from -5 to -24 cm H2O, the change in pleural pressure is 19 cm H2O, giving a calculated pleural elastance of 19 cm H2O/0.5 L = 38 cm H2O/L. (See 'Definitions' above and "Large volume thoracentesis", section on 'Definitions and normal values'.)
Elastance may vary depending on whether the assessment is made at a point when a substantial amount of pleural fluid remains, or at a point when the remaining amount of pleural fluid is minimal . Often, the lung has some capacity to re-expand normally, so the elastance remains low during initial fluid removal. When the lung is no longer able to re-expand, the pleural elastance increases.
Interpretation of pleural pressures — Analysis of pleural pressures includes two key components: the initial pressure and the change in pressure as fluid is removed. Understanding of the results is aided by drawing a graph with pleural pressure readings plotted against the volume of pleural fluid withdrawn (figure 1 and picture 1). The change in pleural pressure as fluid is withdrawn correlates with the ability of the lung to reexpand . The pattern of pressure changes can differentiate free-flowing fluid, lung entrapment, and trapped lung:
●Free flowing pleural fluid and an expandable lung – With a free-flowing pleural effusion and an expandable lung, the initial pleural pressure is slightly positive (pleural pressure in the absence of fluid is slightly negative) and changes minimally as fluid is withdrawn. The pressure/volume curve is monophasic with a calculated pleural elastance equal to or less than 14.5 cm H2O/L. A terminal pressure deflection occurs in all patients with an expandable lung when minimal pleural fluid remains. Excessively negative pleural pressures occur around the thoracentesis catheter in the absence of pleural fluid due to local deformation forces, which are similar to what occurs in the lobar regions of the lung. Therefore, to validate the last pressure measurement, adequate pleural fluid must be present and documented either by lung ultrasound or subsequent drainage of at least 50 mL of pleural fluid.
●Lung entrapment – Lung entrapment is commonly associated with a nearly normal initial pressure, but the pressure decreases gradually as fluid is withdrawn, culminating in a steep decrease in pressure when minimal fluid remains in the pleural space. The terminal decrease in pleural pressure yields a terminal increase in elastance and a biphasic line when pleural pressure is graphed against pleural fluid removal. The typical pressure/volume curve is biphasic with a calculated pleural elastance equal to or less than 14.5 cm H2O/L during the initial stages of drainage; however, during the terminal stages of drainage, the calculated pleural elastance exceeds 14.5 cm H2O/L. On occasion the pressure/volume curve of lung entrapment may be monophasic with a pleural elastance exceeding 14.5 cm H2O/L. (See "Diagnosis and management of pleural causes of unexpandable lung", section on 'Diagnosis'.)
●Trapped lung – Trapped lung is usually associated with a negative initial pressure, although low positive values have been recorded. Initial positive values may be due to the needle entering the pleural space at a point below the actual top of the effusion in the chest [4,10]. With fluid removal, a rapid decrease in pleural pressure occurs, correlating with a pleural elastance greater than 14.5 cm H2O/L (often greater than 25 cm H2O/L). However, a biphasic pressure/volume curve may be seen in this setting. This would imply a dual mechanism for pleural fluid formation. An example would be a patient with active heart failure with an underlying trapped lung due prior cardiac surgery. (See "Diagnosis and management of pleural causes of unexpandable lung", section on 'Diagnosis'.)
SUMMARY AND RECOMMENDATIONS
●The direct measurement of pleural pressures in a pleural effusion, also known as pleural manometry, can be performed at the time of thoracentesis. (See 'Introduction' above.)
●The main indications for pleural manometry are to identify visceral pleural processes that prevent lung expansion, determine whether lung entrapment will prevent successful pleurodesis, identify trapped lung as a cause of an undiagnosed pleural effusion, and guide fluid removal during therapeutic thoracentesis. Identification of lung entrapment in a patient with a malignant pleural effusion is important, as thoracentesis is less likely to relieve dyspnea and pleurodesis is rarely successful in treating a malignant pleural effusion in the presence of lung entrapment. (See 'Definitions' above and 'Indications' above.)
●The equipment used to measure pleural fluid pressure is listed in the table (table 1). We prefer either a water manometer or an electronic hemodynamic transducer that is attached to an intensive care unit (ICU) monitor; sometimes the two systems are used together (picture 1 and figure 1 and table 1). Digital manometers are also available. (See 'Equipment' above.)
●The patient should be in a sitting position for the procedure. Under ultrasound guidance, the location of the pleural effusion is confirmed, and an insertion site into a dependent portion of the fluid identified. After appropriate sterile preparation, draping, and injection of local anesthetic, a catheter is inserted into the pleural fluid. (See 'Insertion site' above.)
●The pleural pressure is measured after withdrawal of an initial sample of 50 to 60 mL of pleural fluid for diagnostic testing. We typically assess the pleural pressure during tidal volume breathing over four to five respiratory cycles and record the mean pressure. (See 'Pressure measurement' above and "Diagnostic thoracentesis", section on 'Fluid removal'.)
●As aliquots of pleural fluid (eg, 100 to 250 mL) are withdrawn, serial measurements of pleural pressure are made. The pleural elastance is calculated by dividing the change in pleural pressure (in cm H2O) by the volume (in liters) of pleural fluid removed. (See 'Pleural elastance' above.)
●Lung entrapment and trapped lung have different characteristic patterns of pleural pressure and elastance (figure 1 and picture 1 and figure 2). (See 'Interpretation of pleural pressures' above and "Diagnosis and management of pleural causes of unexpandable lung".)
•Lung entrapment is associated with a nearly normal initial pleural pressure, but the pressure decreases gradually as fluid is withdrawn, culminating in a steep decrease in pressure when minimal fluid remains in the pleural space.
•Trapped lung is usually associated with a slightly negative initial pressure. With fluid removal, a rapid decrease in pleural pressure occurs, correlating with a pleural elastance greater than 14.5 cm H2O/L (often greater than 25 cm H2O/L).
ACKNOWLEDGMENT — The editorial staff at UpToDate would like to acknowledge Peter Doelken, MD, FCCP, who contributed to an earlier version of this topic review.
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