How to do lung ultrasound

European Heart Journal - Cardiovascular Imaging, vol. 23, pp. 447–449

Abstract

Increasing signs and symptoms of pulmonary congestion are the main reasons why patients with heart failure are admitted to the hospital. In addition, (sub)clinical pulmonary congestion at discharge is a very strong predictor of poor outcomes and readmission of these patients. Early detection of pulmonary congestion therefore seems essential to improve outcomes in heart failure patients. However, physical signs and even chest X-ray have limited sensitivity to detect pulmonary congestion.1 In addition, although elevated natriuretic peptides have a high negative predictive value to rule out heart failure in patients with acute dyspnoea, their specificity is rather low.1 Lung ultrasound (LUS) appears to be one of the most useful tests to confirm the presence of pulmonary congestion by detecting B-lines, which originate from extravasated fluid into the lung interstitium and alveoli.2 The European Association of Cardiovascular Imaging (EACVI), as well as the 2021 European Society of Cardiology (ESC) guidelines for the diagnosis and treatment of acute and chronic heart failure acknowledge the importance of LUS in suspected heart failure.3 The following article is intended to provide guidance on how to perform LUS and explain how it can easily be used as an add-on to transthoracic echocardiography to accurately assess pulmonary congestion in heart failure patients. Although a 5-MHz convex probe may provide optimal visualization of the lung parenchyma, a standard cardiac transducer with a 3.5–5.0 MHz frequency range allows for sufficient visualization of the pleura and subpleural structures and lowers the threshold to use LUS as an add-on to standard echocardiography. The small surface of the cardiac transducer also allows easy scanning along the intercostal spaces. Because no high temporal or spatial resolution is required, LUS can be performed with relatively simple equipment (e.g. handheld echocardiography device) which lowers the threshold for its use at the emergency department or for daily use at the heart failure hospitalization unit. In contrast to transthoracic echocardiography, the acoustic window for LUS is always accessible and patent. The patient can be examined in a sitting as well as supine position. For serial examinations, however, the position should be consistent because pulmonary fluid changes with posture. Using the anterior axillary line as a landmark, each chest wall can be divided into four lung regions (upper and lower parts of the anterior and lateral chest wall) that should be systematically analysed.2 Additional scanning of the posterior chest wall can be useful to evaluate the presence of a pleural effusion but is usually not necessary for the assessment of pulmonary congestion. When using a convex probe, the probe can be positioned perpendicular to the ribs, as well as along the intercostal spaces. When using a cardiac probe, however, the reasonable position is along the intercostal space, where the length of the probe can occupy the intercostal space without touching the ribs. Using the cardiac transducer offers the additional advantage of not having to change the probe after the echocardiographic examination. In a normally aerated lung, the pleural line will be the only structure that can be visualized with LUS. The pleural line is visualized as a hyperechogenic, thin and horizontal line that moves back and forth with respiration (so-called ‘lung sliding’) and results from the movement of the visceral pleura against the parietal pleura during the respiratory cycle. The perpendicular scanning approach, which is usually performed with a (micro)convex probe, visualizes the upper and the lower ribs, which are characterized by a posterior shadowing. Next, the pleural line can be located between both ribs (Figure 1A). The visualization of the upper and lower ribs in combination with the pleural line will identify the so-called ‘bat-sign’ (Figure 1B). However, this sign is not always noted with a cardiac probe. It is important to notice that the pleural line should be moving with in- and expiration. If a motionless horizontal line is visualized, it could be that the operator is looking at muscle fascia or, in cases of acute and unexplained dyspnoea, there may be an underlying pneumothorax.2 Some additional, hyperechogenic, horizontal lines can arise at regular intervals from the pleural line and are called ‘A-lines’ (Figure 1C). These A-lines are reverberation artefacts that represent a sign of normal or increased (e.g. chronic obstructive pulmonary disease) content of air in the lung. The sonographer should also always try to locate the diaphragm because pulmonary consolidations and pleural effusions are mostly found in dependent lung regions and can therefore be differentiated from intra-abdominal organs once the diaphragm has been identified (Figure 1D). Because air causes a complete reflection of the ultrasound beam, pulmonary parenchyma cannot be visualized in the normally aerated lung. (A) Lung ultrasound (LUS) showing the upper and lower ribs (red arrows) with the pleural line between them (blue arrow). (B) The ‘bat sign’ (white arrows) is created by two ribs forming the wings and the pleural line forming the back of the bat. (C) LUS showing the pleural line (blue arrow) with additional A-lines arising from the pleural line (red arrows). (D) Visualization of the diaphragm (white arrow) with a pleural effusion (blue arrow) and the liver (red arrow); a small part of the lung can be seen ‘floating’ in the pleural effusion (green arrow). (E) B-lines (red arrows) arising from the pleural line (blue arrow). (F) Severe pulmonary congestion with fusion of B-lines, creating confluent, hyperechogenic zones (red arrows). With increasing pulmonary congestion and aeration loss, lung density increases and the acoustic mismatch between the lung and the surrounding tissues is lowered. This creates vertical reverberation artefacts which are also known as ‘B-lines’. B-lines are well defined, laser-like, vertical artefacts that move in synchrony with the pleural line. They have a hyperechogenic appearance, arise from the pleural line and reach the edge of the screen2 (Figure 1E). Although less than one or two B-lines can sometimes be detected in the dependent pulmonary zones of normally aerated lungs, the presence of more than two B-lines is almost always a pathological sign. However, because B-lines are only an expression of lung aeration loss, they are not pathognomonic for lung congestion. To make the diagnosis of lung congestion, B-lines have to be multiple (two or more B-lines within a single scanning zone), diffuse (detection of B-lines within multiple scanning zones), and bilateral. B-lines that are restricted to one or two zones could be the sign of an underlying pneumonia or consolidation, whereas an inhomogeneous distribution with spared areas could be the sign of acute respiratory distress syndrome or pulmonary fibrosis. The number of B-lines correlates well with the degree of lung aeration loss. In addition, the number of B-lines decreases immediately after the initiation of decongestive treatment, making LUS a valuable tool to follow-up improvement of pulmonary congestion during diuretic treatment. When there are only few B-lines, they can be counted individually and the sum of the B-lines found in each lung region provides a total score that demonstrates the severity of the alveolar-interstitial syndrome and allows a more objective follow-up after the initiation of diuretic treatment.2 When B-lines are numerous, they tend to merge and form well-distinct, hyperechogenic, confluent zones (Figure 1F). In these cases, the percentage of the scanning site occupied by these ‘white zones’ can be used as a semiquantitative measurement.2 Besides B-lines, the presence of pleural effusion could also be a sign of progressive lung congestion. With LUS, it is easy to differentiate between pleural effusion and a consolidation, whereas with chest X-ray, it can sometimes be very difficult to distinguish between these two conditions. When looking for pleural effusions, the operator should first look at the dependent regions of both lungs. A pleural effusion will mostly be seen as a hypo- or anechogenic structure which is delineated by the chest wall and the diaphragm (Figure 1D). Although the aetiology of a pleural effusion cannot always be accurately detected by LUS, a transudate (observed in pulmonary congestion) is always anechogenic. Very severe lung congestion, lung oedema, or large pleural effusions compressing lung parenchyma can all induce an extensive loss of pulmonary aeration. In these patients, the acoustic window on the lung becomes completely open, showing the presence of compression atelectasis (Figure 1D). A compression atelectasis appears as a hyperechogenic tissue structure that may be visualized as solid parenchyma (such as the liver). LUS is an essential skill for every physician caring for heart failure patients and can be easily used as an add-on to standard transthoracic echocardiography. Adequate knowledge of LUS enables the timely diagnosis of pulmonary congestion and can help the clinician in the initial management as well as in the follow-up of heart failure patients presenting with pulmonary congestion. J.S. received funding from the European Society of Cardiology (ESC Training Grant App000064741). Conflict of interest: The Department of Cardiology of the Leiden University Medical Centre received unrestricted research grants from Abbott Vascular, Bayer, Biotronik, Bioventrix, Boston Scientific, Edwards Lifesciences, GE Healthcare and Medtronic. J.J.B. received speaker fees from Abbott Vascular.

Authors 2

1. Jan Stassen Aachen Department of Cardiology, Leiden University Medical Center

   Leiden University Medical Center

   Affiliation as printed

   Department of Cardiology, Leiden University Medical Center, Albinusdreef 2, 2300 RC Leiden, The Netherlands

2. Jeroen Joost Bax corresponding Aachen Department of Cardiology, Leiden University Medical Center

   Leiden University Medical Center · University of Turku · Turku University Hospital

   Affiliation as printed

   Department of Cardiology, Leiden University Medical Center, Albinusdreef 2, 2300 RC Leiden, The Netherlands

   Turku Heart Center, University of Turku and Turku University Hospital, Kiinamyllynkatu 4-8, FI-20520, Turku, Finland