Measurement and interpretation of pleural pressure (manometry): Indications and technique

INTRODUCTION — 

The direct measurement of pleural pressures during thoracentesis is known as pleural manometry. The indications, technique, and interpretation of pleural pressures during pleural fluid manometry will be reviewed here. The techniques for diagnostic and large-volume thoracentesis are discussed separately. (See "Ultrasound-guided thoracentesis" and "Large volume (therapeutic) thoracentesis: Procedure and complications".)

INDICATIONS — 

Performing manometry is not necessarily routine for all patients who undergo thoracenteses and is primarily a research tool [1]. However, pleural manometry can be very helpful in the following situations [2]:

●To diagnose and guide pleural drainage during large-volume thoracentesis in patients with suspected nonexpendable lung. Pleural manometry is clinically helpful when nonexpandable lung is suspected since it can facilitate the identification of and guide volume removal during thoracentesis (eg, due to chronic atelectasis, endobronchial or visceral pleural restriction). Nonexpandable lung may be suspected in those with pleural thickening or loculations on chest imaging, symptoms of chest discomfort during previous thoracentesis, or poor lung expansion or air in the pleural space ("pneumothorax ex vacuo") after previous thoracentesis.

●The assessment of stability of air leak in selected patients with pneumothorax who have nonexpandable lung pathology. In such cases, pleural manometry, can classify the leak as pressure-dependent or -independent. This type of classification is clinically important as a pleural intervention is needed for pressure-independent leak and can be avoided in patients with pressure-dependent leak [2]. Pressure-dependent pneumothorax may be seen with lung-thoracic cavity size/shape mismatch, such as after large-volume thoracentesis in patients with pleural fibrosis [3], after rapid atelectasis from bronchoscopic lung volume reduction [4], and after partial lung resection surgery [5].

However, pleural manometry does not appear to be able to predict those at risk of re-expansion pulmonary edema and does not appear to be useful in guiding fluid removal in those with free-flowing effusions who do not have nonexpandable lung [1,6-8]. Further details regarding the diagnosis of nonexpandable lung and large-volume thoracentesis are provided separately. (See "Diagnosis and management of pleural causes of nonexpandable lung" and "Large volume (therapeutic) thoracentesis: Procedure and complications", section on 'Determining the volume of fluid to be removed' and "Diagnostic evaluation of the hemodynamically stable adult with a pleural effusion", section on 'Thoracentesis indications and contraindications'.)

TECHNIQUE — 

Pleural manometry is performed during thoracentesis. The procedure for thoracentesis is discussed separately. (See "Ultrasound-guided thoracentesis", section on 'Technique' and "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax".)

Equipment set-up — The equipment needed for measurement of pleural pressures during thoracentesis is listed in the table (table 1). The manometer is attached to the side port of the three-way stopcock near the pleural catheter. Pleural pressures can be measured using a hemodynamic transducer, a water column, or a digital manometer. A hemodynamic transducer attached to an intensive care unit (ICU) monitor is our preferred tool for measuring pleural pressure because we feel it is the most accurate, although systems have been inadequately compared [9]. Because there is no gold standard, the choice is often dictated by institutional availability and physician preference.

There are three ways to measure pleural pressure:

●Hemodynamic (electronic) transducer – An electronic hemodynamic transducer attached to an ICU monitor is our preferred tool for measuring pleural pressure because it may be more accurate and is widely available. Electronic transduction 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 typically measure negative pressure. To compensate, the height of the pressure transducer relative to the zero reference level (ie, at the point of catheter insertion) is lowered so 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) [10]. Although there is no ideal level at which the manometer is placed, we typically place it 20 to 25 cm below the point of needle insertion. In addition, electronic transducers often measure pleural pressure in mmHg, so the transducer pressures need to be converted to measure pressure in cm H₂O (automatically or manually; 1 mmHg = 1.36 cm H₂O). Using electronic hemodynamic transducer systems, both static (mean or end-expiratory pleural pressures) and dynamic measures (pressure swings) can be recorded [11].

●Water column manometer – A U-shaped undamped water manometer made from sterile intravenous tubing and syringe aspiration system can be used to measure pleural pressure. The tube is prefilled with sterile saline to purge the system of air. This system is part of most standard thoracentesis trays. A disadvantage of this technique is that the large respiratory swings seen due to increased ventilation in a dyspneic patient or towards the end of the pleural drainage, where there may be coughing, may hinder precise measurement of the pleural pressure [12]. To overcome this problem, a 22-gauge needle can be interposed between the thoracentesis catheter and the pleural manometer to serve as a resistor, dampening the pressure oscillations (figure 1 and picture 1). This needle provides signal dampening around the mean pleural pressure and, thereby, minimizes the oscillations of the fluid column during ventilation [12]. Alternatively, some experts use two extension tubes to increase the length of the U-shaped manometer. In either case, the manometer is attached to a stopcock on the pleural catheter to allow repeated measures of pleural pressure during fluid removal [8,10]. Consequently, mean pleural pressures may be directly read from the measuring scale. When using a water manometer, the zero pressure level on the manometer is set at the level that the catheter enters the chest wall (ie, reference level zero). Pleural pressure is measured in cm H₂O above the reference level.

●Digital manometer (DM) – Commercially available DMs (picture 2) can be directly attached to the thoracentesis catheter for easy acquisition of pleural pressure [13]. A three-way stopcock should be used to allow closure of the drainage side of the thoracentesis catheter when pleural pressures are measured. Disadvantages of this technique include low sampling time (sampling time is for only three seconds, so if a patient is breathing <20 breaths per minute, the entire respiratory cycle is not sampled). In addition, the ability to estimate the mean pressure is often not feasible due to the wide pressure oscillations possible during breathing (this is especially true toward the end of drainage); thus, most measure end-expiratory pressures.

●Other – One study has reported successful transduction of pleural pressures in 10 patients using an epidural catheter that allows continuous recording of both pleural pressure and elastance throughout the procedure [14]. This technique remains investigational.

Small case series have compared these tools for measuring pleural pressure. In a small study comparing the accuracy of digital, water, and hemodynamic transduction manometers, strong correlation was found between digital and electronic manometers (r = 0.96), but poor correlation was found between water and electronic manometers [9]. In contrast, another study showed that damped manometer measurements (ie, oscillations damped by interposing a resistor in series) is as accurate as an electronic manometer (r = 0.97) [12].

Positioning — We recommend that the patient be in an upright and sitting position with his or her arms resting on a surface such as a bedside table, although it can be done in any position including the supine or decubitus position as long as the transducer is leveled. Patient positions are discussed separately. (See "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax", section on 'Patient position'.)

Pleural measurements — The initial mean pleural pressure is measured just after insertion of the thoracentesis catheter into the pleural space and after withdrawal of an initial small diagnostic sample of pleural fluid. Details are provided separately. (See "Ultrasound-guided thoracentesis", section on 'Technique'.)

Pleural pressure — Pleural pressure is recorded at the beginning of the procedure as well as during fluid removal.

●Initial pleural pressure – We typically calculate the initial mean pleural pressure using several measurements assessed over four to five consecutive respiratory cycles during tidal breathing [15]. This method requires use of an electronic hemodynamic transducer system with the capability of recording tracings for review after the procedure (figure 2). The damped water manometer with a 22-gauge needle acting as resistor is also a validated method for measuring mean pleural pressures (picture 1) [12]. However, with periods of coughing, this system is not reliable and the electronic signal should be used for analysis. The initial pleural pressure can also be measured at end expiration; however calculation of pleural elastance may become challenging with increasing pleural swings at the end of the drainage [16]. No method is proven to be superior.

●Change in pleural pressure during thoracentesis – 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 to calculate a pleural elastance [15,17]. It is generally agreed that fluid should not be removed if the mean pleural pressure decreases to -20 cm H₂O or lower; although this number is arbitrary, data from animal studies suggest that further removal of fluid may increase the risk of re-expansion pulmonary edema [18-20]. (See "Noncardiogenic pulmonary edema".)

Pleural elastance — Pleural elastance is the change in pleural pressure with removal of a given volume of pleural fluid. It can be measured throughout but is often only done at the beginning and end of the procedure.

It is calculated by dividing the change in pleural pressure by the volume removed in liters. Pleural pressure is measured and recorded after each aliquot (eg, 100 to 250 mL) of pleural fluid is removed [15], and the change in pleural pressure (in cm H₂O) is divided by the cumulative 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.

Elastance may vary over the course of pleural fluid removal [12]. Initially, the normal lung has the capacity to re-expand readily, so the elastance is generally low; however, as more fluid is removed, the normal lung may be less able to re-expand and pleural elastance can increase. But, in general, a normal pleural elastance will be <14.5 cm H₂O/L (although this number is somewhat arbitrary) [3]. Measuring elastance facilitates fluid removal during thoracentesis since it is an indicator of lung expansion. For example, if the lung is trapped, elastance will be high ≥14.5 cm H₂O/L indicating that little or no lung expansion can be expected with additional fluid removal.

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 2). The pattern of pressure changes can differentiate expandable lung, lung entrapment, and trapped lung [18].

Distinguishing expandable versus nonexpendable lung

Expandable lung: Monophasic pressure-volume curve — With expandable lung (typically with a free-flowing pleural effusion), the initial pleural pressure (relative to the point of insertion of the catheter in relation to the level of the left atrium) is slightly positive (eg, 5 to 10 cm H₂O; pleural pressure in the absence of fluid is slightly negative) and changes minimally as fluid is withdrawn (figure 2). The pressure-volume curve is flat (monophasic) with a calculated pleural elastance ≤14.5 cm H₂O/L throughout the procedure. A terminal pressure downward deflection (approaches -3 to -5 cm H₂O) occurs in all patients with an expandable lung when minimal pleural fluid remains. This curve is typical of that seen in hepatic hydrothorax or congestive heart failure. Excessively negative pleural pressures occur around the thoracentesis catheter in the absence of pleural fluid due to local deformation forces when there is a minimal amount of fluid left. Therefore, when negative pressures are seen, it is important to demonstrate residual fluid at the end of the procedure, either by lung ultrasound or subsequent drainage of at least 50 mL of pleural fluid to ensure that local deformation forces are not resulting in the falsely negative excessive pressure recording [7,21].

Partially expandable lung (lung entrapment): Biphasic pressure-volume and pleural elastance curve — With lung entrapment, the lung cannot expand fully because of an active disease, such as malignancy or infection that restricts expansion of the lung and/or visceral pleura. Representing a continuum, if the process causing entrapment progresses and does not resolve, the lung may eventually become trapped, the details of which are discussed below. (See 'Nonexpandable lung (trapped lung): Monophasic pressure-volume curve with high pleural elastance' below.)

Lung entrapment is commonly associated with a slightly positive initial pleural pressure (eg, 0 to 5 cm H₂O), but the pressure decreases gradually as fluid is withdrawn, culminating in a terminal steeper decrease in pressure when minimal fluid remains in the pleural space (ie, biphasic curve) (figure 2). Similarly biphasic, the calculated pleural elastance is ≤14.5 cm H₂O/L (ie, normal) 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.

Occasionally, a biphasic pressure-volume curve similar to that seen with lung entrapment may be seen in patients with trapped lung who also have a second pleural process [22]. An example would be a patient with active heart failure with an underlying trapped lung due prior cardiac surgery.

This curve is typical of inflammatory visceral pleural peel, any cause of increased elastic recoil pressures of the parenchyma (eg, lymphangitic carcinomatosis), or bronchial obstruction. These effusions are often exudative. They frequently do not respond to pleurodesis and a tunneled catheter may be required. Additional details regarding lung entrapment are discussed separately. (See "Diagnosis and management of pleural causes of nonexpandable lung", section on 'Diagnosis'.)

Nonexpandable lung (trapped lung): Monophasic pressure-volume curve with high pleural elastance — Pleural effusion from a trapped lung (pleural effusion ex vacuo) is a consequence of a remote inflammatory condition that has left behind a collagenous or fibrous pleural peel, which restricts the ability of the lung to expand. It is believed that trapped lung may begin as a form of lung entrapment (see 'Partially expandable lung (lung entrapment): Biphasic pressure-volume and pleural elastance curve' above). While most cases of lung entrapment resolve with resolution of the inflammatory process, the resolution is incomplete in others, resulting in a trapped lung. Some cases arise with lung-thoracic cavity size/shape mismatch after thoracic surgery or rapid atelectasis after bronchoscopic lung volume reduction.

Trapped lung is usually associated with an initial negative pressure, although occasional low positive values (eg, <2 to 3 cm H₂O) have been recorded [10,18]. With fluid removal, there is a rapid and steep decrease in pleural pressure (figure 2), correlating with a pleural elastance >14.5 cm H₂O/L (often >25 cm H₂O/L) throughout the procedure.

These effusions may be transudative and either require decortication or no therapy. Additional diagnostic features of trapped lung are discussed separately. (See "Diagnosis and management of pleural causes of nonexpandable lung", section on 'Diagnosis'.)

It is generally thought that fluid should not be removed if the mean pleural pressure decreases to -20 cm H₂O or lower. However, this is based upon indirect data from reports in patients without trapped lung in whom fluid removal resulted in re-expansion pulmonary edema when pleural pressures fell below -20 cm H₂O [18-20,23]. (See "Noncardiogenic pulmonary edema".)

Most clinicians stop thoracentesis when chest pain develops based upon the assumption that pain is due to excessive negative pressures and minimal remaining fluid. However, one large randomized study did not show any reduced occurrence of pleural pressure-related chest discomfort in patients undergoing large-volume thoracentesis with pleural manometry guidance [1].

Pressure-dependent- versus -independent pneumothorax

Pressure-dependent pneumothorax and air leak — Pressure-dependent pneumothorax occurs during pleural drainage in patients with nonexpandable lung physiology [2,4,5]. This type of air leak is caused by a pressure gradient between the subpleural lung parenchyma and the pleural space. Pleural manometry shows end-expiratory plateauing of pleural pressure when the thoracostomy tube is clamped for several minutes.

Clinical criteria can also be used for the diagnosis of pressure-dependent pneumothorax and air leak. We diagnose pressure-dependent pneumothorax when all of the following are present:

●Presence of clinical conditions associated with the presence of pressure-dependent pneumothorax

●Absence of symptoms associated with a pneumothorax

●Chest imaging demonstrating a pneumothorax on the side of a procedure without contralateral mediastinal shift

●Documentation of radiographic stability

●Absence of a continuous air leak without the application of suction (if chest thoracostomy is in place)

All criteria need to be fulfilled to make the diagnosis of pressure-dependent pneumothorax and air leak.

A pressure-dependent pneumothorax does not require pleural intervention and is a stable pneumothorax. However, once diagnosed, it may avoid additional chest tube placement or prompt safe removal of a tube that has already been placed.

Pressure-independent pneumothorax and air leak — Pressure-independent pneumothorax is a progressive type of pneumothorax where air may leak into the pleural space irrespective of the pressure gradient between the subpleural lung parenchyma and the pleural space. Pleural manometry shows a continued rise in end-expiratory pleural pressure without a discernible plateau after clamping the thoracostomy tube.

A pressure-independent pneumothorax is expansile and often needs pleural interventions for drainage and/or pleurodesis when the air leak persists. (See "Alveolopleural fistula and prolonged air leak in adults".)

SUMMARY AND RECOMMENDATIONS

●Indications – Pleural manometry is the direct measurement of pleural pressures in a pleural effusion during thoracentesis. The main indication for pleural manometry is:

•To identify nonexpandable lung and to guide fluid removal during large-volume (>1 L) therapeutic thoracentesis in those with suspected nonexpandable lung

•To assess the stability of air leak in patients with pneumothorax by detecting pressure-dependent or -independent pneumothorax

●Equipment – The equipment used to measure pleural fluid pressure is listed in the table (table 1). We prefer a water manometer and/or an electronic hemodynamic transducer that is attached to an intensive care unit monitor (picture 1 and figure 1 and table 1). Digital manometers are also available (picture 2). (See 'Equipment set-up' above.)

●Procedure – The patient should be in a sitting position for the procedure with both arms resting on a surface such as a table, although other positions can be used for pleural manometry. Pleural pressure is measured just after insertion of the thoracentesis catheter into the pleural space and withdrawal of an initial sample of pleural fluid. The manometer is attached to the side port of the three-way stopcock of the catheter. (See "Ultrasound-guided thoracentesis", section on 'Technique' and 'Positioning' above.)

•The mean initial pleural pressure is assessed by taking several measurements over four to five consecutive respiratory cycles during tidal volume breathing. During subsequent fluid removal, the pleural pressure is measured and recorded after each aliquot (eg, 100 to 250 mL) of pleural fluid, according to the same procedure. Fluid should not be removed if the mean pleural pressure decreases to -20 cm H₂O or lower. (See 'Pleural pressure' above.)

•As aliquots of pleural fluid (eg, 100 to 250 mL) are withdrawn and 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. A measurement of ≥14.5 cm H₂O/L indicates a stiff lung, with the expectation of poor lung expansion with additional fluid removal. (See 'Pleural elastance' above.)

●Interpretation – Expandable lung, lung entrapment, and trapped lung have different characteristic patterns of pleural pressure and elastance (figure 2). (See 'Interpretation of pleural pressures' above and "Diagnosis and management of pleural causes of nonexpandable lung".)

•Expandable lung (normal) – With a pleural effusion and underlying expandable lung, the initial pleural pressure is slightly positive (eg, 0 to 10 cm H₂O) and changes minimally as fluid is withdrawn. The pressure-volume curve is flat (monophasic) with a calculated pleural elastance ≤14.5 cm H₂O/L throughout the procedure. (See 'Expandable lung: Monophasic pressure-volume curve' above.)

•Partially expandable lung (lung entrapment) – Partially expandable lung cannot expand because of an active inflammatory or malignant pleural process. It is associated with a slightly positive initial pleural pressure, but the pressure-volume curve is biphasic; pressure decreases gradually as fluid is withdrawn, culminating in a steep decrease in pressure when minimal fluid remains in the pleural space. Similarly, the pleural elastance may be biphasic with a calculated elastance ≤14.5 cm H₂O/L during the initial stages of drainage and an increase to >14.5 cm H₂O/L during the terminal stages of drainage. (See 'Partially expandable lung (lung entrapment): Biphasic pressure-volume and pleural elastance curve' above.)

•Nonexpandable lung (trapped lung) – Nonexpandable lung cannot expand because of a remote inflammatory condition that has left behind a collagenous or fibrous peel or atelectasis due to endobronchial tumor or mismatch between lung-thoracic cavity size after lung surgery. It is usually associated with a slightly negative initial pressure. With fluid removal, there is a rapid decrease in pleural pressure, correlating with a pleural elastance >14.5 cm H₂O/L (often >25 cm H₂O/L) throughout the procedure. (See 'Nonexpandable lung (trapped lung): Monophasic pressure-volume curve with high pleural elastance' above.)

•Pressure-dependent versus -independent pneumothorax – Pleural manometry can diagnose pressure-dependent or -independent pneumothorax and air leak in patients with nonexpandable pathology. When the air leak is pressure-dependent, pleural manometry shows end-expiratory plateauing of pleural pressure when the thoracostomy tube is clamped for several minutes; in contrast, a continued rise in end-expiratory pleural pressure without a discernible plateau after clamping is seen in patients with pressure-independent pneumothorax. This distinction is clinically important as a pleural intervention is needed for pressure-independent leak and can be avoided in patients with pressure-dependent leak. (See 'Pressure-dependent- versus -independent pneumothorax' above.)

ACKNOWLEDGMENT — 

The UpToDate editorial staff acknowledges Peter Doelken, MD, FCCP, who contributed to an earlier version of this topic review.