Chest ultrasound can provide quick, useful information about the lung and pleural space. It is an adjunct to chest auscultation and radiography to evaluate respiratory failure or suspected thoracic disease and guide thoracentesis. While ultrasound machines cannot image normal lung tissue, they can detect the lung surface and determine its smoothness and relative degree of aeration.
To follow convention and avoid overlooking subtle or focal abnormalities, consider the following technical recommendations when performing chest ultrasound.
Image 9.1 Diagram of four chest zones, including the upper anterior (1), basal anterior (2), upper lateral (3), and basal lateral (4). The anterior axillary line separates the anterior and lateral regions.
Normal intercostal views reveal two or more ribs with hyperechoic surfaces and underlying shadows. The pleura should be visible deep to the intercostal space, about 0.5 cm below the rib surfaces. The lung parenchyma, which is normally almost all air, should be just below the pleural surface. This air reflects nearly all of the ultrasound waves, forming a hyperechoic line, the pleural line, that's brightest when the probe is perpendicular to the lung surface. Tiny irregularities move horizontally along the pleural line as the visceral pleura and lung tissue slide along the surface of the parietal pleura and chest wall. This movement is called lung sliding and indicates that lung tissue abuts the chest wall at that spot and moves back and forth with respiration. It is called a lung pulse if it only jiggles due to adjacent cardiac motion.
Image 9.2 Normal lung ultrasound views with a high-frequency linear transducer (A) and a curvilinear transducer (B). Both reveal the pleural line exhibiting lung sliding between the rib shadows. A-lines are visible, though more so in the image obtained with the curvilinear transducer (B).
Strong ultrasound reflections off the pleural line can reverberate off the transducer and pass through the superficial tissue again before being sensed by the machine. The machine displays these delayed reverberation signals as equidistant horizontal lines below the pleural line. Termed A-lines, these artifacts suggest that below the pleural line is mostly air, which is consistent with normal or hyperinflated lung tissue or free air in the pleural space (pneumothorax).
Ultrasound waves enter lung tissue as aeration decreases (relatively more tissue or fluid). Once inside, ultrasound waves, especially ones of optimal frequency (size), can bounce around instead of attenuating while a consistent amount reflects back to the transducer continuously. These constant echoes generate vertical ringed-down artifacts termed B-lines or B-line artifacts (BLAs).3 B-lines are vertical artifacts that 1) start at the pleural line, 2) fall straight to the bottom of the image without fading, and 3) move back and forth with lung sliding. Normal lung tissue may occasionally exhibit one or two B-lines. Finding three or more B-lines suggests increased fluid or tissue at that location. This is termed interstitial syndrome because a thickened interstitium is often the cause. However, this can also occur with incomplete atelectasis and when fluid and debris partially fill alveoli.
Image 9.3 Histology of normal lung tissue (A) compared to tissue from a lung partially collapsed due to atelectasis (B). Normal lung tissue is mostly air (A), so reflects and scatters almost all ultrasound, creating strong reflections that generate A-lines artifacts. However, ultrasound entering thickened interstitium (B) or fluid-filled alveoli can continuously return some sound to the transducer, creating vertical lung artifacts known as B-lines.
Focal pneumonia or pneumonitis, atelectasis, pulmonary contusion, infarction, neoplasms, and pleural disease can cause focal or asymmetric B-lines. However, diffuse B-lines throughout the chest suggest pulmonary edema (from various causes), acute respiratory distress syndrome (ARDS), interstitial lung disease, or diffuse pneumonia or pneumonitis such as miliary tuberculosis. Consider examining the pleural line closely with a higher frequency transducer because pulmonary edema is usually associated with a smooth pleural line and normal lung sliding. In contrast, patients with ARDS, interstitial lung diseases, or pneumonia often have an irregular pleural line and diminished lung sliding. Lung surface irregularities can generate similar but distinct vertical artifacts, termed commit-tail artifacts (CTAs), that fade at increasing depths. Short commit tail artifacts are common in normal lung parenchyma, but larger ones can signify lung surface abnormalities. It’s also important not to confuse B-lines with gaseous stomach content and subcutaneous air, which create similar appearing ringed-down artifacts.
Image 9.4 Lung ultrasound views with a high-frequency linear transducer (A) and a curvilinear transducer (B). Both views reveal multiple B-lines, which start at the pleural line and emanate to the bottom of the image.
Although some ultrasound machines' post-processing can remove artifacts (decreasing sensitivity), a thorough exam to detect loss of aeration has a good negative predictive value for conditions like cardiopulmonary edema. However, these vertical artifacts lack specificity, so a specific diagnosis must incorporate other clinical data from the history, physical exam, laboratory studies, previous imaging, and other PoCUS findings.
When distinguishing acute heart failure from other causes of respiratory failure like pneumonia or COPD, diffuse sonographic B-lines as a sign of pulmonary edema is more sensitive than pulmonary edema on plain chest radiography (88% vs. 73%). Still, the specificities are equivalent at about 90%.4 This translates to positive/negative likelihood ratios of 8.8/0.13 for sonographic B-lines and 7.3/0.3 for edema on plain chest radiography. This disparity in favor of ultrasound is despite the fact that these studies were biased toward CXR sensitivity because they more often incorporated the CXR findings into the reference standard and performed the ultrasound exam hours after the CXR.4
Consolidation occurs when alveoli and small airways completely collapse or fill with fluid or debris. Without air in the tissue, ultrasound can penetrate and build an image. Consolidations may be limited to small subpleural foci surrounded by aerated lung. These appear as hypoechoic wedge or round-shaped irregularities on the lung surface. Multiple conditions are associated with these subpleural consolidations, including pneumonia, compressive or obstructive atelectasis, pulmonary infarction from pulmonary embolism, neoplasm, and contusion. Larger consolidations are usually associated with either pneumonia or atelectasis, both of which may have fluid or air-filled airways (hypo or hyperechoic spots or branching linear structures).
Image 9.5 Lung ultrasound views of subpleural consolidations. One is 1-2cm wide (A), and the other is >2cm wide (B).
A brief exam may fail to detect consolidations. However, a thorough exam can pick up small consolidations that are undetectable on plain chest radiography.5 Avoid confusing lung consolidations with the liver or splenic tissue, which reside below the diaphragm. The specificity of finding consolidations is also context-specific, so always incorporate other clinical data from the history, physical exam, laboratory studies, previous imaging, and other PoCUS findings before making a diagnosis.
When there is at least moderate risk of pulmonary embolism (Wells score >4), finding two or more round or wedge-shaped subpleural consolidations around 1-2cm wide has a positive likelihood ratio for PE that's >14.6,7 These findings 1) are not a substitute for CT angiography to diagnose or exclude PE, 2) do not apply to patients with concurrent lung pathology like COVID-19 pneumonia,8 and 3) require a thorough exam including scans of the posterior basal lung fields. Still, finding subpleural consolidations may be helpful in settings where CT is unavailable, another cause is less likely, and there is uncertainty in deciding whether to start or continue anticoagulation.
Image 9.6 A chest x-ray shows a dense right lower lung opacity that obscures the right hemidiaphragm (A). Right basal lateral view (B) reveals dense lobar consolidation above the diaphragm with hyperechoic air bronchograms and small pleural effusion consistent with right lower lobe pneumonia.
Image 9.7 The chest x-ray in this previously healthy adult (A) shows a complete whiteout of the right hemithorax with tracheal shift to the right and rib crowding on the right. This is consistent with a total collapse of the right lung. A right basal lateral chest view from this patient (B) reveals dense lobar consolidation above the diaphragm with no air bronchograms or pleural effusion consistent with atelectasis.
In patients with possible pneumonia, PoCUS assessment for consolidations had high sensitivity and specificity in multiple studies where skilled users scanned the whole chest (12 areas) for up to 13 minutes.9,10 Therefore, while not replacing radiography, chest ultrasound can help diagnose pneumonia sooner and improve diagnostic certainty when radiography is unavailable or one wants to avoid radiation.
Various factors, usually in combination, can cause fluid to accumulate in the pleural space. These include pulmonary venous congestion (high left atrial pressure), low serum protein levels (malnutrition, liver or kidney disease), and decreased relative pressure (lung collapse). When these factors predominate, the fluid is usually a transudate, with lower protein and lactate dehydrogenase (LDH) concentrations. Conversely, exudative fluid with a higher protein, LDH, or cholesterol usually arises from lymphatic dysfunction or increased tissue permeability associated with trauma, infection, malignancy, or other cause of inflammation. Fluid may also directly leak into the pleural space via bleeding (hemothorax), lymphatic leaks (chylothorax), esophageal rupture, feeding tube or central venous catheter penetration, or inflow of ascitic fluid, urine, or CSF.
PoCUS can detect pleural fluid with high accuracy.11 Most effusions accumulate in dependent areas and are best detected when the patient is upright. In supine patients, a sensitive method is to use a low-frequency probe to visualize the space above and below the most dependent part of the diaphragm, usually using the liver or spleen as an acoustic window. Aerated lung tissue casts a curtain that blocks any view of deeper tissues like the vertebrae. This curtain sign moves inferiorly with inspiration when pleural fluid is absent. However, a pleural effusion is present if the vertebrae are visible above the diaphragm. Views that clearly show both the diaphragm and the vertebrae will avoid the problem of scanning too anteriorly and missing fluid in the posterior pleural space. Typically, fluid in the pleural space is hypoechoic or anechoic, above the diaphragm, and surrounds lung tissue that's either aerated (appears bright) or consolidated (appears isoechoic). Identifying movable echodensities and fluid movement in color-doppler mode can confirm the pleural space has free-flowing fluid.
Image 9.8 A right basal lateral view showing the curtain sign indicates no pleural effusion (A). A similar view with a pleural effusion (B) provides a window to see the vertebrae above the diaphragm. To rule out pleural effusion using the curtain sign, find a lateral view like these that visualizes the vertebrae in the far field just below the diaphragm.
A therapeutic (large volume) thoracentesis is indicated if an effusion is symptomatic. While a diagnostic thoracentesis is indicated whenever a pleural effusion is new, large enough to perform the procedure safely, and it is not certain whether the cause is simply low serum protein or venous congestion. Don't confuse echoes in pleural fluid with artifacts. Actual echoes in pleural fluid will float and swirl around as opposed to artifacts, which may appear stationary or only move with the transducer. PoCUS can identify adjacent pneumonia, echogenic fluid, debris, septations/loculations, pleural nodules, or pleural thickening. Always consider performing a thoracentesis with fluid analysis in these cases because a transudative pleural effusion is unlikely.
Purulent pleural fluid (empyema) is usually more echogenic, nonhomogeneous, and surrounded by a capsular lining. If empyema is suspected, consider obtaining a CT scan to help differentiate empyema from a peripheral lung abscess. In addition to urgent thoracentesis and antimicrobial therapy, an empyema may necessitate surgery or at least an indwelling catheter for ongoing drainage.
Image 9.9 Basal lateral chest views reveal different types of pleural effusions: (A) anechoic pleural fluid without debris in an example of a large pleural effusion, (B) fluid with septations in an example of a complex parapneumonic effusion, (C) echogenic fluid in an example of an empyema, and (D) fluid with septations and debris in an example of a large hemothorax.
Use ultrasound to guide thoracentesis. PoCUS can rule out a pre-procedure pneumothorax or lack of lung sliding and determine the site, trajectory, and maximal depth of needle insertion. Ultrasound guidance has been repeatedly shown to decrease the risk of sub-diaphragmatic insertion and pneumothorax.12,13 First, locate the diaphragm using a sub-diaphragmatic landmark like the spleen or liver and mark the level equal to the superior-most point of the diaphragm during end-expiration. Then, find a fluid pocket above the diaphragm's high point with sufficient fluid depth regardless of slight variations in needle trajectory. Mark this spot well and note the maximum depth before hitting deeper tissue. Also, note any debris or loculations at this location that may impair drainage.
Image 9.10 An ideal location for thoracentesis is just superior to the diaphragm's high point with more than sufficient fluid depth. Avoid injuring the intercostal neurovascular bundle (arrow) by visualizing it with a linear transducer before performing a thoracentesis (A).
Passing the needle just above the lower rib can prevent injury to the intercostal neurovascular bundle. However, the location of the intercostal neurovascular bundle can vary. So, consider identifying it before the procedure using color Doppler with a high-frequency linear transducer. This is especially important when inserting the needle more posteriorly (closer to the spine). Consider an alternative location whenever the artery is near the middle of the rib space. If inserting a guidewire for the Seldinger technique, use ultrasound to confirm the guidewire is in the pleural space before inserting the dilator. When the procedure is complete, rescan to look for residual pleural fluid and whether a pneumothorax is present. Depending on the local standard of care, it may still be necessary to obtain a post-procedure chest radiograph.14
Free air can quickly fill the pleural space (pneumothorax) whenever the lung leaks air out or the chest wall leaks in air. Besides being a consequence of acute trauma or invasive procedures to the chest, a pneumothorax can occur spontaneously, usually due to pre-existing lung disease and exposure to changes in pressure from ventilators, high altitude, or underwater diving. The lung collapses as the volume of air in the pleural space expands. Lung collapse impairs ventilation and usually causes ipsilateral chest pain and shortness of breath. More air may accumulate with each effort to breathe. If this occurs, pressure can build in the pleural space and create a tension pneumothorax, which is life-threatening because it impairs venous return (right heart filling) and lead to obstructive shock.
Image 9.11 Normal lung sliding is confirmed with a linear transducer (A) as the pleural line moves back and forth horizontally during breathing. A pneumothorax may be present if the pleural line does not appear to move with respiration (B).
Ultrasound can show when the lung surface is just below the chest wall because it reveals lung sliding or a lung pulse when the lung moves back and forth with respiration or adjacent cardiac motion. In a pneumothorax, free air separates at least some lung tissue from the chest wall, erasing visible lung sliding at that location. Usually, this first occurs in the least dependent area, such as the anterior or superior-most regions, when the patient is supine or upright. If PoCUS reveals lung sliding, a lung pulse, lung consolidation, or pleural fluid in the upper or superior lung fields, then a pneumothorax is unlikely (high sensitivity).15,16 However, confined pleural air may be located elsewhere. False negatives can also occur if significant intercostal muscle movement is mistaken for lung sliding or subcutaneous air is mistaken for B-lines.
If PoCUS cannot identify lung sliding, a pneumothorax may be present, but it's also possible that the sliding is just hard to see or that part of the lung is ventilating poorly or adherent to the chest wall. To differentiate pneumothorax from other causes of no lung sliding, 1) use a high-frequency linear probe and decrease the depth to provide the best view of the pleural line, 2) look for and treat causes of poor ventilation, and 3) slide the probe laterally or inferiorly looking for the transition point where lung sliding becomes visible. This last finding, called the lung point, is where both sliding (from the partially deflated lung) and no sliding occur on different parts of the pleural line within a single view. There may be no lung point if the pneumothorax is large enough. However, other signs of pneumothorax may be present, including ipsilateral enlarged hemithorax, diminished breath sounds, hyper-resonance to percussion, decreased vocal fremitus, and subcutaneous emphysema with crackly sounds and sensations in the subcutaneous tissue.
Image 9.12 An example of the lung point where lung sliding is present and absent in the same view. When the pleural line is superficial, as in this case, consider using a high-frequency linear transducer for better image resolution instead of simply reducing the depth with a curvilinear transducer, as occurred in this example.
Image 9.13 This patient was brought to the hospital after severe blunt trauma. A quick physical exam revealed respiratory distress with obvious flail chest and local subcutaneous swelling and crepitus. PoCUS showed what looked like an irregular pleural line with B-lines above lung consolidation and pleural fluid. This subcutaneous air (*) can obscure the pleural line while mimicking B-lines and lung sliding.
In unstable patients where pneumothorax is suspected, finding what looks like lung sliding or B-lines should be examined carefully as significant intercostal muscle movement or subcutaneous air can mimic lung sliding and obscure a real pneumothorax, but finding no lung sliding and especially a lung point should prompt treatment with thoracostomy. Obtain a chest radiograph if able, especially if other parts of the history or exam are conflicting or uncertain. PoCUS guidance can also help ensure the thoracostomy is performed above the diaphragm and away from the heart.