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Updated:
1 October, 2024
Here, ultrasound of the lungs and thorax is described, including descriptions of pleural sliding, A-lines, B-lines, and pleural effusion.
Ultrasound of the Lungs and Thorax
Ultrasound has long been used to diagnose pleural effusion, but in intensive care, in the hands of anesthesiologists, it is a relatively new, clinically useful, and easily accessible tool for examining diseases and injuries in both the lungs and thorax.
Lung Ultrasound Examination (LUS) allows us to visualize pathological findings such as pleural effusion, pneumonia, lung consolidation, etc., in a simple and effective manner. Several different ultrasound machines are available on the market in various sizes and qualities, with rapid advancements in medical technology. There is currently no consensus on the best probe (ultrasound transducer) for LUS, but when specifically examining the pleura to, for example, detect or rule out pneumothorax, a linear, high-frequency probe is recommended, as it provides high resolution but does not penetrate deeply since the pleura is a superficial structure.
Appropriate lines for lung ultrasound examination
In a normal, fully air-filled lung, the only structure typically seen with ultrasound is the pleural line; the lung parenchyma is not visible. Air in the lungs scatters ultrasound waves, and normally, no echoes are reflected from the deeper parenchyma.
You can find a video link showing LUS here.
Lung Sliding
In healthy tissue, the parietal pleura and visceral pleura lie against each other and glide smoothly over one another during breathing. In a lung ultrasound examination (LUS), this produces the typical image of so-called “lung sliding” (see video).
You can find a video link showing lung sliding and pneumothorax here.
A-lines
An artifact that can appear when the lung is air-filled is the so-called “A-line“. A hyperechoic repetition artifact (reverberation artifact) appears at regular intervals, deep and parallel to the pleural line (see image). A-lines occur because some of the ultrasound waves bounce back and forth between the probe and the pleura, with some bouncing multiple times. The latter takes longer, and the ultrasound machine interprets this as an echo from a greater depth.
You can find a video link here. A-lines are shown around 5:30 minutes into the clip.
B-lines
The presence of A-lines indicates that the area under the probe is air-filled, which is a normal finding. When the air content in the lung decreases and the density increases, for example, with an increased amount of fluid content, transudate, or blood, this will result in an increased number of echogenic points. This appearance is called a “B-line“.
You can find a video link here. B-lines are shown around 8:13 minutes into the clip.
A B-line, by definition, is a hyperechoic reverberation artifact. The B-line starts at the pleural line and extends like a beam of light across the entire screen without fading. It moves synchronously with the pleural line[1]. The exact cause of the B-line is still not definitively established. During LUS, a distinction is made between a so-called “diffuse interstitial syndrome” (DIS) with B-lines in several zones, which indicates a global lung process such as pulmonary edema, ARDS, or lung fibrosis, while focal B-lines suggest a localized process such as pneumonia, atelectasis, or lung contusion. The number of B-lines has been shown to correlate with the degree of interstitial fluid (Extravascular Lung Water – EVLW) in both spontaneously breathing patients and those with controlled ventilation[2]. More than three B-lines in the same intercostal space indicates a pathological finding. More B-lines suggest more fluid, and in the case of fulminant pulmonary edema, the B-lines will merge into one large, spotlight-like artifact. To label the LUS finding as a diffuse interstitial syndrome, a pathological number of B-lines in two or more bilateral zones are required. Scientific studies show a correlation between the number of B-lines and biological markers for heart failure, such as BNP and wedge pressure.
B-lines appear and disappear in real-time[3], making lung ultrasound a tool for dynamic monitoring and continuous evaluation of changes in lung pathology. The finding of B-lines can be used, for example, in acute dyspnea to differentiate between a COPD exacerbation and increased interstitial fluid, as in heart failure. Scientific studies have shown that LUS can be used alone, but the combination with pro-BNP enhances its predictive value. The number of B-lines can be used as an independent prognostic indicator for adverse outcomes and 16-month mortality and rehospitalization in patients with decompensated heart failure[4].
Pneumothorax
Since pneumothorax involves the separation of the visceral pleura from the parietal pleura due to air leakage, LUS is well-suited for diagnosing and ruling out pneumothorax. Studies show that LUS has better sensitivity and the same high specificity as chest X-rays in diagnosing pneumothorax. As pneumothorax can be life-threatening, the high utility of LUS and the short time a targeted examination takes add to its value. Since air accumulates at the highest point, pneumothorax should be searched for at the highest point of the chest. In a supine patient, this means anteriorly, but it varies depending on body position. It should be noted that certain patient groups, such as those with emphysema, may have localized bullous emphysematous areas. Similarly, subcutaneous emphysema can complicate or even make the examination impossible.
There are several causes of absent lung sliding even when pneumothorax is not present, such as inflammation, previous pleurodesis, mainstem intubation, or apnea. In these cases, visualizing the so-called “lung pulse” can help. The lung pulse involves the transmission of heartbeats, causing the pleura to pulse in sync with the heartbeat (video).
The presence of lung sliding or lung pulse argues against pneumothorax[1]. To further strengthen the diagnosis, some users add an examination in “M-mode” (motion mode) during LUS. A moving lung normally produces the so-called “seashore sign” (wave patterns), where a clear difference can be seen between immobile subcutaneous tissue and the moving lung parenchyma (see image). With a pathological lung that does not move or is displaced as in pneumothorax, the M-mode pattern will look different, with no distinction seen between tissue above or below the pleura (see image). The strength of LUS is that it can rule out pneumothorax and allow further clinical diagnostics.
Since “lung sliding” occurs when the pleural layers glide against each other, this ultrasound finding rules out pneumothorax in the area being examined. A B-line, which by definition arises from the visceral pleura, also rules out pneumothorax. To definitively diagnose pneumothorax with LUS, one must locate the so-called “lung point.” This is the point that demarcates the pneumothorax from the area of the lung where the pleural layers are still gliding against each other (video). This finding is diagnostic for pneumothorax. By locating the lung point, one can also quantify the extent of the pneumothorax. LUS allows for the monitoring of a pneumothorax that does not require emergency drainage, which helps avoid unnecessary drain insertion. To diagnose pneumothorax, the following findings should be present:
- Lung point
- Absence of sliding
- Absence of B-lines
- Absence of lung pulse
You can find a video link showing lung sliding and pneumothorax here.
Lung Consolidation
In some cases, LUS can help distinguish between inflammation and atelectasis of the lung tissue (resorption vs compression). When a disease process causes consolidation of lung tissue with decreased air content and increased fluid content, it can be detected with LUS. For detection, the process must be close to the pleura. The causes can be, for example, pneumonia, atelectasis, pulmonary embolism, primary or metastatic cancer, or lung contusion. The appearance of consolidation in LUS aids in differential diagnosis. The consolidation may consist of a subpleural hypoechoic dark area, but with increasing consolidation, the lung will take on a more tissue-like appearance similar to that of the liver, known as hepatization (image).
An inflammatory process (such as bacterial pneumonia) can cause fluid leakage, which appears in LUS as a focal interstitial syndrome with B-lines at the site of the inflammation. Inflammation also affects the pleura, giving it a thickened and irregular appearance, compared to pulmonary edema, where the pleural line is thin and sharp (image). An inflammatory process can also reduce or eliminate “lung sliding.” A consolidation can contain air, known as an air bronchogram. In LUS, this appears as punctiform or linear hyperechoic (white) areas within the consolidation. Dynamic and static air bronchograms are distinguished, with the former showing gas bubbles moving in the bronchi (video + image). This rules out atelectasis and further supports a pneumonia diagnosis[5].
Pleural Effusion
Using ultrasound, pleural effusion can be visualized relatively easily. The fluid is typically seen as an anechoic (dark) space between the parietal and visceral pleura.[5] Compression atelectasis of the lung bases is often also observed. Although the echogenicity of the fluid may suggest its content, pleural puncture and analysis are required for exact diagnosis.[6]
The puncture is preferably performed under ultrasound guidance. In M-mode, one can observe the so-called “Sinusoid sign,” which is a variation in the interpleural distance during inspiration and expiration, as the visceral layer of the pleura exhibits a movement similar to a sinusoidal curve throughout the breathing cycle.
Lung ultrasound is superior at detecting pleural effusion and differentiating between fluid and consolidation compared to conventional chest X-ray. Studies show that lung ultrasound detects pleural effusion as effectively as CT7-8. Pleural effusion is common in ICU patients and is a frequent cause of atelectasis, which can lead to physiological effects such as reduced compliance, impaired oxygenation, and increased pulmonary vascular resistance1.
Video link for pleural effusion can be found here. Fast forward to 23:30.
In a sitting person, free pleural effusion accumulates in the lower regions of the pleural cavity. Therefore, the fluid is most easily seen in the posterior axillary line at the diaphragm level.3 On a conventional chest X-ray, rounding of the lateral costophrenic angle is a common sign of pleural effusion, but up to 500 ml of fluid may be present without this rounding being visible on X-ray. Rounding of the posterior costophrenic angle[6] can sometimes detect as little as 75 ml of fluid. The patient’s body habitus, the presence of lung tissue consolidation, and other factors may complicate assessment using conventional X-rays.4
Because the pleural space has a complex structure, it is difficult to find a definitive mathematical formula to estimate the volume of pleural fluid using LUS. Several formulas have been proposed that consider body surface area and multiplanar analysis of the pleural fluid. Balik et al. published a simplified formula in which pleural fluid was measured as the maximum separation (sep) between the parietal and visceral layers of the pleura at end-expiration. The simplified formula provides a quick estimate of fluid volume and can help the clinician decide whether to perform pleurocentesis. Note that the patients in the study were in a supine position with a 15-degree tilt of the head end.9
The volume (V) is calculated as follows: V (ml) = 20 x sep (mm)
In summary, lung ultrasound provides excellent opportunities to visualize pathological processes in the lungs of critically ill patients, allowing for rapid and easy diagnosis, making it well-suited for intensive care.
References
- Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Medicine. 2012;38:577-91.
- Enghard P, Rademacher S, Nee J, et al. Simplified lung ultrasound protocol shows excellent prediction of extravascular lung water in ventilated intensive care patients. Critical Care. 2015;19:36.
- Noble VE, Murray AF, Capp R, et al. Ultrasound assessment for extravascular lung water in patients undergoing hemodialysis. Time course for resolution. Chest. 2009;135:1433-9.
- Frassi F, Gargani L, Tesorio P, et al. Prognostic value of extravascular lung water assessed with ultrasound lung comets by chest sonography in patients with dyspnea and/or chest pain. Journal of Cardiac Failure. 2007;13:830-5.
- Lichtenstein D, Meziere G, Seitz J. The dynamic air bronchogram. A lung ultrasound sign of alveolar consolidation ruling out atelectasis. Chest. 2009;135:1421-5.
- Blaivas M, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Academic emergency medicine: Official journal of the Society for Academic Emergency Medicine. 2005;12:844-9.
- Ultrasound estimation of volume of pleural fluid in mechanically ventilated patients. M Balik et al. Intensive Care Med (2006) 32:318–321.
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