Review
The use of lung ultrasound: A brief review for critical care physicians and pneumonologists Nektaria Xirouchaki, Dimitrios Georgopoulos
Registrar of Intensive Care Unit, University Hospital of Heraklion, Crete 2 Professor of Intensive Care Unit, Intensive Care Unit University of Crete and University Hospital of Heraklion, Crete 1
Key words: - lung ultrasound - lung sliding - B-lines - pleural line
SUMMARY. This is a review of the use of lung ultrasound (US) in routine clinical practice. Over the past few years, the use of lung US in critically ill patients has gained in popularity, and has been associated with very good outcomes. Lung US is the fastest, noninvasive, sophisticated diagnostic tool used in the Intensive Care Unit (ICU) and in other in-patient settings, free of complications and with minimal cost. It is common knowledge that the US beam cannot normally pass through air-filled structures, making the evaluation of lung parenchyma under the ribs impossible. This does not prevent the diagnosis of several abnormal conditions, including pneumothorax, consolidation, atelectasis, pleural effusion, and others. The aim of this review is to summarize the applications of sonographic imaging of the lungs and the findings associated with major respiratory disorders. Pneumon 2007; 20(2):134-141.
Introduction
Correspondence to: Dimitrios Georgopoulos Intensive Care Unit, University Hospital of Heraklion, Crete 711 10 Heraklion, Crete Tel.: 2810 392636 e-mail:
[email protected]
Until recently, the use of lung ultrasound (US) as a diagnostic tool was considered unjustifiable, on the grounds of conventional knowledge that lungs are filled with air, and that the US beam cannot normally pass through air-filled structures. This theory has been rejected and lung US is currently considered the fastest, non-invasive, sophisticated diagnostic approach in the Intensive Care Unit (ICU) and in other in-patient settings. It is associated with minimal complications and has low cost.1-5 The relevant literature suggests that the theory of non-feasibility was subverted as soon as thoracic specialists took over lung imaging.2,3 Lung US can be used in the diagnosis of several abnormal conditions (Table 1). The aim of this review is to provide an introduction to US imaging of the lungs, and to summarize the findings associated with basic respiratory disorders.
PNEUMON Number 2, Vol. 20, April - June 2007 Table 1. Use of lung ultrasound 1. Diagnosis of pneumothorax 2. Diagnosis of pneumonia 3. Diagnosis of atelectasis 4. Diagnosis of pleural effusion 5. Follow-up of resolving pneumonia and atelectasis 6. Ultrasound-guided aspiration of bronchoalveolar lavage from lesions with negative cultures 7. Diagnostic and therapeutic thoracocentesis of pleural effusion 8. Diagnosis of pulmonary oedema and interstitial lung disease 9. Biopsy of pleural lesions or peripheral lung tumours 10. Disorders of the diaphragm 11. Diagnosis of traumatic lung contusion 12. Diagnosis of pulmonary embolism
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tocols are in use for transthoracic lung examination. This review is based on the protocol followed at the University Hospital of Heraklio (Fig. 1). The patient lies in the supine position. The anterior surface of each lung is divided in two areas, superior and inferior, corresponding to the unilateral radiologic hilum. The lateral surface is defined by the anterior and posterior axillary lines, and is also divided into a superior and an inferior area. Finally, the posterior lung surface is defined by the posterior axillary and the paravertebral lines and is divided into a superior and an inferior area.1,5 During transthoracic lung examination the following basic signs must be recognized;
1. Bat sign (Fig. 2) The bat sign is one of the most important signs in lung US. It can be revealed by placing the probe vertically over the intercostal space. The resulting image depicts the superior and inferior ribs, curved like a bow (diagonal arrows), their acoustic shade, and the pleural line, 0.5 cm from an imaginary line connecting the ribs (rib line), indicated by the vertical arrow in the figure. The pleural line corresponds to the surface of the lung. The remain1
BASIC PRINCIPLES OF TRANSTHORACIC ULTRASONOGRAPHIC EVALUATION OF THE LUNGS Visualization of the lungs requires a 5 mHz transducer, appropriate for transthoracic examination. Various pro-
Intensive Care Unit, Un.H.H. Professor: Georgopoulos D. Patient Name: ……………………………………………………………… RN: ……………………………. Date: ……………………….
PNEUMOTHORAX
PNEUMOTHORAX
PLEURAL EFFUSION
PLEURAL EFFUSION
Figure 1. Protocol for lung examination followed in the Intensive Care Unit. C: Consolidation, A: Atelectasis, P: Pulmonary oedema, I: Infiltrations
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Figure 2. Bat sign. The diagonal arrows indicate the curved margin of the ribs and the vertical arrow indicates the pleural line.
ing lines parallel to the pleural line are the A-lines, which should not be confused with other lines or artifacts, since their location is constant.
2. Lung sliding This sign indicates the sliding movement of the lung towards the thoracic wall. It is maximized in the lower lung fields, as the lung descends towards the abdomen. On two-dimensional US, lung sliding is demonstrated as fleeting dots arising from the pleural line in accordance with lung movements during respiration. Lung sliding is not always clearly revealed on two-dimensional ultrasound, so M-mode sonography is the preferred method for lung movement imaging, producing the characteristic “seashore sign”1,12 (Fig. 3). Lung sliding becomes vague in pulmonary overexpansion and parietal emphysema, while it disappears in pneumothorax, complete atelectasis, pleural fibrosis and apnoea.
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Figure 3. Seashore sign. The sign appears at the right of the screen (time-motion mode). The arrow points to the pleural line, the parallel lines correspond to the static thoracic wall and the sandy pattern under the pleural line is produced by the lung parenchyma and is a dynamic artifact that disappears in pneumothorax (The parallel lines correspond to sea, the sandy pattern corresponds to shore, and their border corresponds to the pleural line, arrows).
preferred.8 Pathologic conditions associated with B-lines are discussed below.
PATHOLOGIC IMAGES ON LUNG ULTRASOUND ASSOCIATED WITH BASIC ABNORMAL CONDITIONS
4. Stratosphere sign The absence of lung sliding is called the stratosphere
3. B-lines Although B-lines can only be seen in pathologic parenchyma they are classified as constant image lung signs because they are very common in ICU patients, and their presence rules out certain abnormalities, such as pneumothorax. B-lines are vertical lines that always arise from the pleural line and spread uninterrupted up to the edge of the screen. They are also called comettail artifacts due to their mobility during respiration, and lung rockets when observed in groups1,7 (Fig. 4). In a recent review the term ultrasound lung comets was
Figure 4. A pattern of B-lines (white arrows).
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sign, and is diagnostic of pneumothorax with a sensitivity of 100% and a specificity of 78%.1,7,9,10 This sign is recognizable on time-motion US, where standard lung sliding disappears, as described above. It is replaced by parallel horizontal lines, suggesting complete absence of structures over and under the pleural line (arrow) (Fig. 5). Although this sign is not specific to pneumothorax, even an inexperienced operator can easily recognize it. It is particularly useful in the ICU, where pneumothorax is undetectable on chest radiography in 30%-40% of cases.
5. A-line sign The A-line sign has a sensitivity of 100% and a specificity of 60% for the diagnosis of pneumothorax. It corresponds to complete absence of B-lines and lung rockets, which is a strict feature of A-lines. The concomitant absence of lung sliding raises the specificity for complete pneumothorax to 96%10 (Fig. 5).
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exact pathologic location, resulting in the detection and successful drainage of the entrapped air (Fig. 7).
7. Lung pulse11 The lung pulse is a sign of complete atelectasis, with a characteristic pulsation of the lung according to the heartbeat.
8. Air bronchogram The air bronchogram is located inside a consolidation area and consists of straight or variously shaped hyperechoic blurred margins. According to its status, it is divided into static and dynamic air bronchogram. Static air bronchogram is usually produced in ate
6. Lung point The lung point is produced at the transition area between healthy and pathologic parenchyma. It is best revealed in M-mode sonography and its sensitivity and specificity in diagnosing pneumothorax are 79% and 100%,9,10 respectively (Fig. 6). The lung point represents the alternation of pathologic parenchyma during expiration with the normal pattern during inspiration, when lung volume increases. In one patient with no clinical suspicion, whose chest radiography indicated the presence of a small quantity of air, the lung point revealed the
Figure 5. Stratosphere sign. Replacement of the seashore sign with parallel lines indicating complete abolition of structures. The arrow corresponds to the pleural line. A-line sign (arrows).
Figure 6. Lung point (arrow). The lung point is used as a guide to locate entrapped air.
Figure 7. The same location as in Figure 6, after placement of a chest tube, resulting in drainage of a reasonable quantity of encapsulated air.
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lectasis. It is caused by entrapped air inside an area of the lung that is no longer aerated, thus creating static artifacts. Static air bronchogram can be revealed by the absence of dynamic lung movement during respiration1,11-13 (Fig. 8). Dynamic air bronchogram is produced in ventilated areas of the lung and is caused by the presence of air inside bronchi, which then move according to the patient’s breathing. It is never produced in atelectasis but it can be seen in 60% of cases of infectious alveolar consolidation.12,13
near the diaphragm or superficially. In severely ill ventilated patients these areas are usually found in the lower lobes. The size of the opacity can be evaluated using the consolidation index, which is calculated from two-dimensional measures, namely the distance between surface and core (from the bottom to the top of the screen) and the longitudinal diameter (from the left to the right of the screen).1,14,15 Alternatively, the maximal thickness of the consolidation area can be measured; it is considered small when thickness is 50 mm1,14,15 (Fig. 9).
9. Attendant signs of atelectasis
11. B-lines and their pathologic value
Atelectasis is a relatively common condition in the ICU, and it is characterized by the following signs: 1) a change in the imaging location of the heart,1,11,12 2) abolition of the diaphragm dynamic movement,1,11,12 3) change in the imaging location of the diaphragm, which is raised by at least 2 cm (in the supine position it corresponds with the nipple),1,11,12 and 4) the presence of an attendant small pleural effusion of about 250 ml.1,11,12 The absence of lung sliding indicates complete atelectasis.
As mentioned above, B-lines indicate the presence of pathologic parenchyma. A common disorder characterized by diffuse B-lines in all lung fields is pulmonary oedema, which is also associated with bilateral pleural effusion.1,8,16,17 The presence of B-lines with a distance less than 7 mm was found associated with wedge pressure and extra vascular lung water (EVLW), using the PICCO system.16 A positive correlation was found between the number of B-lines (comet score) and wedge pressure, as well as with EVLW.16 The pattern of diffuse B-lines in lung parenchyma with a distance
