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INTRODUCTION — Portable ultrasound devices are used at the bedside to evaluate pleural abnormalities and to guide thoracentesis and related procedures, such as pleural drainage catheter placement and needle aspiration biopsy of pleural or subpleural lung masses. The goals are to improve accuracy and safety in the characterization of pleural disease and performance of pleural access procedures.
The indications, contraindications, advantages, disadvantages, and technique of ultrasound-guided thoracentesis and related procedures will be reviewed here. The diagnostic evaluation and imaging of a pleural effusion and the technique of diagnostic thoracentesis are discussed separately. (See "Diagnostic evaluation of a pleural effusion in adults: Initial testing" and "Diagnostic evaluation of pleural effusion in adults: Additional tests for undetermined etiology" and "Imaging of pleural effusions in adults" and "Diagnostic thoracentesis".)
ADVANTAGES — Thoracic ultrasound has several advantages over traditional radiographic imaging of the pleura, including absence of radiation, better portability, real-time imaging, and the ability to perform dynamic imaging.
Ultrasound is substantially better at determining the location of pleural fluid than bedside physical examination and, in experienced hands, is associated with a lower rate of complications during thoracentesis. In addition, ultrasound guidance increases the likelihood of a successful tap compared to using physical examination for guidance. As an example, in a study of 17 patients who had a failed thoracentesis without ultrasound imaging, thoracentesis with ultrasound was successful in 15 . (See "Diagnostic thoracentesis", section on 'Site selection'.)
Ultrasound examination of the pleura is more sensitive than a plain chest radiograph at detecting the presence of pleural fluid and differentiating pleural fluid from lung consolidation. Compared with computed tomography (CT), pleural ultrasound has a 95 percent sensitivity for detection of pleural disease in patients with a “white out” on plain chest radiograph, but is slightly less sensitive in detecting small amounts of fluid [2,3]. Compared with CT scanning, ultrasound may better differentiate pleural fluid from pleural thickening and pleural masses . Bedside thoracic ultrasound is also faster and less resource-demanding than transporting a critically-ill patient to the CT scanner .
Ultrasound guidance is associated with a reduced risk of pneumothorax during thoracentesis [6,7]. In an observational cohort study of insurance database claims for 61,261 thoracenteses, ultrasound guidance was associated with a reduced likelihood of pneumothorax (OR, 0.81; 95% CI, 0.74-0.90) .
DISADVANTAGES — Thoracic ultrasound is an operator dependent technology. Focused, supervised training is needed to ensure that the operator correctly interprets the sonographic findings [8-10]. Inadequate training may increase the risk of complications.
Ultrasound is not as good as CT imaging for evaluation of the underlying lung parenchyma in the setting of complex pleural and lung parenchymal disease. Ultrasound guidance is not as good as guidance by CT imaging for complicated interventional procedures, such as empyema drainage with a pigtail catheter or biopsy of pleural masses. (See "Imaging of pleural effusions in adults", section on 'Ultrasonography'.)
INDICATIONS — The indications for pleural ultrasound include:
●Bedside detection of pleural fluid when the plain chest radiograph shows a “white out”
●Bedside detection of a pneumothorax
●Guidance for diagnostic and therapeutic thoracentesis
●Guidance for placement of thoracostomy tubes
MACHINE REQUIREMENTS — A wide variety of portable ultrasound machines with two-dimensional scanning capability are used for pleural ultrasonography and associated procedures [4,11].
Transducers — A 3.5 to 5.0 MHz transducer with a convex sector design works well for pleural imaging in most instances . Cardiac transducers are particularly effective, as they are designed with a small footprint, allowing scanning between rib interspaces. An additional advantage of using a cardiac transducer is that it may be used for other ultrasound applications (eg, cardiac, lung, abdominal); this reduces equipment costs.
Once an abnormality has been identified, a 7.5 to 10 MHz linear transducer can be used if needed to obtain more detailed images [9,11].
Image storage — Many portable ultrasound machines have digital image storage and transfer capability that meets the requirements for durable image documentation. Alternatively, the machine may be equipped with a printer if the clinician prefers a hard copy of the study to place directly in the patient chart.
Doppler — Doppler is generally not required for portable thoracic ultrasound-guided procedures, although it is occasionally used by ultrasonographers to differentiate a small pleural effusion from pleural thickening and to identify blood vessels that might be in the path of a needle during a procedure .
Preprocedure steps — The patient’s identity and planned procedure are confirmed. The results of other evaluations related to the patient’s problem are reviewed by the ultrasonographer. In particular, the chest radiograph is reviewed before the procedure to confirm the side of the pleural abnormality and the expected location of any masses or loculated accumulations of fluid.
Informed consent for the procedure is obtained, and clotting studies are confirmed to be adequate, if a thoracentesis or other pleural access procedure is planned.
Acoustic gel is placed on the patient’s skin in the area of interest to provide an airless interface, as ultrasound waves do not pass through air well. The gel also permits the ultrasound probe to slide gently over the skin.
Machine setup — The ultrasound machine is positioned so the screen is easily visible from the operator’s working position. Ambient lighting is reduced to maximize screen contrast . Typically, the thorax is scanned using gray-scale, real-time ultrasound . Dynamic noise filters are not used as they can obscure visualization of lung sliding. (See 'Lung sliding' below.)
The depth setting is adjusted such that the structures of the hemithorax deep to the chest wall are imaged. Gain is adjusted to maximize the contrast between different tissues .
Certain conventions are important to maintain orientation of thoracic anatomy and to standardize image acquisition. Ultrasound probes have a groove on one side that corresponds to a screen orientation marker, which appears as a white or blue dot on the screen. By convention, the dot is typically at the top or on the upper left of the screen (movie 1) .
The transducer is held perpendicular to the skin surface with the transducer marker (groove) pointed cephalad and the scanning plane directed between adjacent ribs. Structures near the skin surface appear close to the dot and deeper structures appear lower on the screen. A depth guide is usually located along the right side of the screen, to enable estimation of the depth of a structure relative to the chest wall.
Patient position — Pleural fluid is obedient to the law of gravity, so pleural fluid collects in the dependent portion of the thorax (unless loculated). The usual position for ultrasound examination of a pleural effusion is for the patient to sit with arms extended and resting on a firm surface that is just below the level of the shoulders. Raising the patient’s ipsilateral arm up to or above their head widens the intercostal spaces and facilitates scanning .
When the patient is sitting, the entire back is accessible for ultrasonographic examination, so free-flowing pleural fluid is readily identified in the dependent lower thorax (image 1). When scanning for a pneumothorax, the patient is usually positioned supine with the head of the bed elevated slightly.
The situation is more difficult in the critically-ill patient who is in a supine position and attached to multiple support devices . Patients with severe respiratory and/or hemodynamic failure may be intolerant of changes in body position. We typically position critically-ill patients supine with the ipsilateral arm held across the chest towards the opposite side. If the effusion is large, it may be identified in the mid axillary line. Smaller effusions may require that the transducer be moved medially such that it is pressed into the mattress and angled upwards to visualize the effusion. This is helpful for identifying the presence of the effusion, but impractical as a means to guide thoracentesis as the transducer position cannot be duplicated by the needle/syringe assembly.
Several alternative positions may be considered in order to permit access for ultrasound guided thoracentesis. These positions include slight elevation of the head of the bed with the patient rotated towards the lateral decubitus position, a flat bed with the patient in the lateral decubitus position and the side of the expected pleural effusion up, and the supine position with the patient lying such that the side with the effusion is at the edge of the bed (picture 1).
Scanning strategy — The transducer is oriented to scan between the ribs, as ribs block transmission of ultrasound. This orientation yields an image where the adjacent rib shadows appear on either side of the image on the screen. By moving the transducer longitudinally from one interspace to another, multiple interspaces may be examined in a short time. A methodical scanning strategy allows a comprehensive analysis of the target effusion or other pleural pathology (picture 2). After a general examination of the thorax using multiple scan lines, the examiner may concentrate on an area of particular interest in order to correctly identify nearby landmarks and determine the best site for device insertion, if needed (movie 1).
ULTRASOUND TERMINOLOGY — Terminology for thoracic ultrasound provides a description of a number of sonographic artifacts caused by air-tissue interfaces. The presence or absence of these artifacts can be used to aid in the diagnosis of pleural disease.
A, B, and E lines
A lines are horizontal lines (roughly parallel to the chest wall) that are brightly echogenic and located between the rib shadows when the probe is positioned longitudinally [14,15].
B lines arise at the border between aerated and compressed lung and are described as multiple ray-like, or comet-tail, vertical lines extending from the pleural line to the lower edge of the screen without fading (image 2) . B lines move synchronously with the lung during respiration and tend to erase A lines (movie 2).
E lines are vertical lines extending from the areas of subcutaneous emphysema deep into the chest . These sonographic artifacts can be confused with B lines and thus contribute to misidentification of a pneumothorax. The possibility of E lines should be suspected when subcutaneous emphysema can be palpated on the chest wall and when the vertical lines start at a level external to the ribs.
Lung sliding — The sonographic effect of lung sliding (also known as lung gliding or the lung sliding sign) is created by movement of the lung relative to the chest wall during respiration . The sonographic appearance is that of a thin, bright line moving horizontally along the pleural line with a wave-like pattern located above (towards the chest wall) and a granular pattern below. Lung sliding is an indirect sign indicating adherence of the visceral pleura to the parietal pleura. When air separates the two pleural layers as in a pneumothorax, the movement disappears (movie 3 and movie 2).
Following a pleural access procedure, the disappearance of lung sliding in an area where it was previously identified is a strong indicator of a postprocedure pneumothorax . (See 'Identification of pneumothorax' below.)
Lung pulse — The lung pulse is a vertical movement of the pleural line synchronous to the cardiac rhythm that is more commonly seen on the left hemithorax than the right. The lung pulse is caused by transmission of heart beats through consolidated, motionless lung. Intrapleural air prevents transmission of either horizontal or vertical movements to the parietal pleura. Visualization of a lung pulse excludes a pneumothorax. However, the lung pulse is only of significance if actually present; absence of a lung pulse is NOT diagnostic of a pneumothorax.
Lung point — A lung point is defined as a location where the lung adheres to the parietal pleura in a patient with a pneumothorax [16,18]. When scanning a patient with a suspected pneumothorax, the probe is moved laterally along the rib interspaces from the area without lung sliding or B lines to search for an area where lung sliding is seen intermittently (movie 4). The presence of lung sliding in one area (ie, the lung point) and not in another is a strong indicator of a pneumothorax. However, a lung point is not always present, such as when a large pneumothorax causes such extensive lung collapse that the lung does not abut the pleura .
ANATOMIC LANDMARKS AND ULTRASOUND APPEARANCE — Knowledge of the normal sonographic appearance of the chest wall, pleura, and adjacent structures guides accurate diagnosis of pleural pathology and is essential to safe needle insertion into the pleural space.
Chest wall and pleura — The normal thoracic ultrasound has certain characteristic features [9,20,21]. The intercostal muscles appear as hypoechoic, linear shadows of soft tissue density, containing echogenic fascial planes. The ribs appear as repeating curvilinear structures with a deeper, hypoechoic, posterior acoustic shadow that can be mistaken for pleural fluid . The parietal and visceral pleura normally appear as a single, bright echoic line no wider than 2 mm. A high resolution ultrasound probe (7.5 to 10 MHz) is sometimes needed to differentiate parietal and visceral portions of the pleura . The change in acoustic impedance at the pleura-lung interface results in a series of echogenic parallel lines equidistant from each other just deep to the pleural line.
Diaphragm and subdiaphragmatic recesses — The diaphragm must be positively identified before any pleural procedure to ensure that the needle insertion site and trajectory will remain above the diaphragm [4,9]. Subdiaphragmatic insertion of a needle or catheter can be catastrophic and potentially fatal, if the liver or spleen is lacerated.
The diaphragm typically appears as an echogenic line approximately 1 mm thick; downward (caudad) movement of the diaphragm should be seen with inspiration (image 1) . As a general rule, when the patient is sitting, the diaphragm is located caudad to the 9th rib. (See "Diagnostic thoracentesis", section on 'Site selection'.)
In addition to identifying the location of diaphragm, the location of the splenorenal and hepatorenal recesses should be confirmed, as the curvilinear sonographic appearance of the splenorenal and hepatorenal recesses is similar to that of the diaphragm (image 3). The splenorenal and hepatorenal recesses are identified by finding the liver or spleen craniad and the kidney caudad to the respective recess.
Lung — To avoid injury to the lung during an ultrasound guided thoracentesis, the sonographer must be able to identify the appearance of air-filled, fluid-filled, or atelectatic lung. The air-filled lung has an ultrasound appearance of bright echoes. When the lung is compressed by a surrounding pleural effusion, it appears hyperechoic or tissue dense; and, in large effusions, may appear to float in the effusion (image 2). Smaller effusions cause less compressive atelectasis.
At the border between aerated and compressed lung, multiple ray-like air artifacts termed “B” lines may be seen (image 2). Compressed lung frequently exhibits a characteristic flapping movement and moves with the respiratory cycle (movie 2).
The lung may move closer to the proposed needle path during respiratory cycling and possibly obscure the ultrasound field. Safe site selection requires that the pleural effusion space be clear throughout the respiratory cycle and that there is sufficient space between the chest wall and the visceral pleura to avoid inadvertent puncture of the lung. There is no definite rule as to the minimum allowable depth of pleural fluid at the needle insertion site. Greater than 10 mm is a reasonable estimate of a safe distance . By changing transducer position and angle, the examiner selects the site that yields the greatest distance between the chest wall and the surface of the lung.
Heart — Positive identification of the heart is important when performing needle insertion in the left anterolateral thoracic region. The heart may be surprisingly lateral in position, particularly in supine patients or when the patient has cardiomegaly or ipsilateral mediastinal shift. Ultrasonography allows site selection that is well away from the heart. The sonographic appearance of the heart depends on the axis of viewing (figure 1 and movie 5) and is discussed separately. (See "Transthoracic echocardiography: Normal cardiac anatomy and tomographic views".)
Pleural loculation may cause the target effusion to be in an unusual intrathoracic position. Anteromedial loculations require the examiner to identify and avoid adjacent mediastinal structures. For parasternal needle insertion, the internal mammary artery should be looked for using color Doppler.
IDENTIFICATION OF PLEURAL FLUID — The plain chest radiograph should be reviewed before the ultrasound guided procedure to confirm the expected site, size, and likelihood of loculations of the pleural effusion. By convention, the echogenicity of thoracic structures is determined relative to the liver.
Characteristic features — Pleural fluid usually appears as an anechoic (black), or hypoechoic compared to the liver, area surrounded by typical anatomic boundaries. Three ultrasonographic criteria must be satisfied to ensure the presence of a pleural effusion (movie 6) :
●The finding of an echo free space (appears black and without stippling) within the thoracic cavity
●The finding of typical anatomic boundaries that surround the effusion: the inside of the chest wall, the diaphragm, and the surface of the lung
●The presence of dynamic characteristics that are typical of pleural fluid, such as diaphragmatic movement, lung movement, movement of echogenic material within the fluid (septations, cellular debris, fronds), and changes in the shape of the pleural effusion with respiratory cycling.
Initially, the examiner should require that all of the above findings be met for identification of a pleural effusion. However, certain types of pleural effusions have variable or increased echogenicity, which can be accurately identified with advanced skill and experience. (See 'Atypical appearances of pleural fluid' below.)
Unlike intraabdominal fluid, a pleural effusion is not deformable with force application to the transducer on account of the rigidity of thoracic cage.
Atypical appearances of pleural fluid — With experience, the examiner will recognize that some pleural effusions have an atypical appearance, which may be due to obscuration of the typical findings or may reflect the characteristic presentation of the underlying process. As examples:
●Massive obesity and chest wall edema may degrade the image quality such that typical ultrasound features are not discernible.
●Complex loculated effusions may be hyperechoic and be located in a nondependent part of the thorax (movie 6). Hemothorax and empyema fluid may be isoechoic with the liver and have no dynamic changes with respiration.
●The presence of pleural or diaphragmatic thickening or nodularity, or an echogenic swirling pattern is suggestive of a malignant pleural effusion [9,24,25].
●The presence of air and fluid together (ie, hemopneumothorax) may present a complex sonographic picture.
When the examiner is uncertain, a more experienced ultrasonographer should review the findings or an alternative imaging modality should be used.
IDENTIFICATION OF PLEURAL MALIGNANT DISEASE — Morphologic characteristics used for identification of malignant pleural disease on chest computed tomography (CT) have been adapted for use in ultrasound examination of the pleura [9,24,26]. (See "Overview of the risk factors, pathology, and clinical manifestations of lung cancer", section on 'Pleural involvement' and "Management of malignant pleural effusions".)
The morphologic characteristics of pleural malignancy include:
●Diaphragmatic and parietal pleural nodule or nodules
●Pleural thickening >1 cm
In a study of 52 patients with suspected malignant pleural effusion, thoracic ultrasound correctly identified 26/33 malignant effusions and 19/19 benign effusions by using these criteria . However, ultrasound findings alone are not sufficient to make a diagnosis of pleural malignancy.
IDENTIFICATION OF PNEUMOTHORAX — Portable ultrasound is used to detect a pneumothorax is several situations, such as after a pleural access procedure, in the evaluation of a patient with chest trauma in the emergency department, and following chest tube placement to assess resolution of a pneumothorax [16,27-33]. (See "Imaging of pneumothorax" and "Primary spontaneous pneumothorax in adults" and "Secondary spontaneous pneumothorax in adults".)
As pleural air is usually located in the least dependent area, the patient is scanned in the supine position. A 3.5 to 5 MHz transducer probe is directed to the third or fourth intercoastal space between the parasternal and midclavicular lines. The first step is to identify the contiguous ribs on either side of the field. Next, the pleural line is identified between and under the ribs. For confirmation, the pleural line should be at the same depth on both sides of the chest. The four key features of a pneumothorax are the following (movie 7) :
●Absence of lung sliding which is a horizontal movement of the lung relative to the pleural line. (See 'Lung sliding' above.)
It is advisable to document the presence and location of lung sliding or B-lines prior to any procedure associated with a risk of pneumothorax as the disappearance of lung sliding or B-lines after the procedure confirms the interval development of a pneumothorax. Two other signs, absence of lung pulse and identification of the lung point, have been described as consistent with a pneumothorax, but these signs are generally not used as they are less accurate than the findings of absence of lung sliding and absence of B lines [16,18]. (See 'Lung pulse' above and 'Lung point' above.)
In a patient with COPD, the ultrasound appearance of emphysematous areas can mimic a pneumothorax . A chest computed tomography exam may be needed to accurately identify a pneumothorax in a patient with emphysematous bullae. In addition, subcutaneous emphysema can lead to sonographic artifacts, such as E lines that are created by the subcutaneous air and mimic B lines.
Performing multiple views during lung ultrasonography does not appear to increase the diagnostic sensitivity for clinically-significant pneumothorax with one study of 260 trauma patients reporting similar sensitivity (93 percent) and specificity (98 percent each) when single and four-view protocols were compared .
SUMMARY AND RECOMMENDATIONS
●Portable ultrasound is a useful technique for evaluation of the pleural abnormalities at the bedside. Pleural ultrasound improves the accuracy of the physical examination and enables a better understanding of pleural pathology than the plain chest radiograph alone. (See 'Advantages' above.)
●Ultrasound guidance for thoracentesis has several advantages over traditional radiographic imaging of the pleura, including absence of radiation, better portability, real-time imaging, and the ability to perform dynamic imaging. (See 'Advantages' above.)
●Use of a portable ultrasound machine requires additional, focused training to ensure that the operator correctly interprets the sonographic findings. (See 'Disadvantages' above.)
●The indications for pleural ultrasound include bedside detection of pleural fluid or a pneumothorax, guidance for diagnostic and therapeutic thoracentesis, and guidance for placement of thoracostomy tubes. (See 'Indications' above.)
●Terminology for thoracic ultrasound provides a description of a number of sonographic artifacts caused by air-tissue interfaces, such as A, B, and E lines, lung sliding, lung pulse, and lung point (image 2 and movie 2 and movie 4). The presence or absence of these artifacts is used in the diagnosis of pleural diseases (image 2). (See 'Ultrasound terminology' above.)
●Knowledge of the normal sonographic appearance of the chest wall, pleura, and adjacent structures guides accurate diagnosis of pleural pathology and is essential to safe needle insertion into the pleural space. (See 'Anatomic landmarks and ultrasound appearance' above.)
●Three ultrasonographic criteria must be satisfied to ensure the presence of a pleural effusion: (See 'Identification of pleural fluid' above.)
•The presence of dynamic characteristics that are typical of pleural fluid, such as diaphragmatic movement, lung movement, movement of echogenic material within the fluid (septations, cellular debris, fronds), and changes in the shape of the pleural effusion with respiratory cycling (image 4).
●Pleural fluid may have an atypical appearance in certain situations, such as when a thick chest wall degrades the ultrasound image, when the fluid is septated (eg, complex parapneumonic effusion or empyema), when malignancy causes a nodular or thickened pleura, or when air and fluid are both present (eg, hemopneumothorax) (image 4 and movie 6). (See 'Atypical appearances of pleural fluid' above and 'Identification of pleural malignant disease' above.)
●The ultrasound characteristics that suggest the presence of pleural malignancy include: diaphragmatic and parietal pleural nodule or nodules, pleural thickening >1 cm, and hepatic metastasis. (See 'Identification of pleural malignant disease' above.)
●Ultrasound signs of a pneumothorax include absence of lung sliding, absence of “B” lines, absence of a lung pulse, and absence of a lung point (movie 7 and movie 3). (See 'Identification of pneumothorax' above and 'Ultrasound terminology' above.)
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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