Probe selection depends on the likely pathology, obesity, physician preference and availability of equipment (Fig. 17.1). Pleural disease is usually superficial and therefore a linear high frequency probe may be more suitable (a and b), which would provide the best resolution for diagnosis or exclusion of pneumothorax and alveolar interstitial syndrome (described in Sect. 17.3). A linear probe without a handle (b), or curved probe head such as with a curvilinear (c) or microconvex (d) probe facilitates reaching posteriorly to the lung bases in patients who are unable to sit up. This is where pleural effusions, atelectasis, consolidation, empyema and lung abscess are usually found. The curvilinear probe has the largest footprint, providing the largest field of view and is probably the best probe for assessing the diaphragm. The microconvex probe (d) fits well in the intercostal space but has less depth penetration, however is less useful for sonography of other organs. Transthoracic probes (e) have similar penetration but less pleural resolution than curvilinear probes, however are more versatile as they can be used to image the heart and are a good general purpose probe for cardiothoracic assessment.

Some authors have described scanning regions of the lung, with as many as 28 scanning regions, or zones, on each side [5]. However, these do not correspond to the anatomic regions of the lung. As it is feasible to scan the entire chest surface in under 5 min, we propose three scanning zones on each side, which correspond more closely to the anatomy (Table 17.2 and Fig. 17.2).

Refer to Figs. 17.3 and 17.4 and Video 17.2.

Many of the principles for reporting lung ultrasound are similar to echocardiography. However due to the lack of anatomic landmarks, if the images are to be reviewed by someone who did not perform the ultrasound, or if lung pathology is tracked over time, then it is important to label the images. Although the liver and spleen are recognisable landmarks for the right and lower lobe, they can still be confused with each other.

Figure 17.5 shown below is a sample report form that is used in the University of Melbourne Graduate Certificate of Clinical Ultrasound and Advanced iHeartScan^TM course. It is useful to record the patient position during the examination and whether they are mechanically ventilated and position of pleural drains and chest wall dressings.

The pleural surfaces are superficial and usually reached with a high frequency linear probe (8–12 MHz), which provides the best resolution, however lower frequency probes are usually sufficient. Placing the probe perpendicular to the chest wall and the ribs and sliding the probe superiorly and inferiorly will enable visualisation of two rib shadows that are intervened by horizontally placed intercostal space (Fig. 17.7). Below the intercostal muscles is a hyperechoic horizontal line, which represents the adherent parietal and visceral pleura, the pleural line. In the normal lung during respiration the two layers of pleura slide past one another resulting in subtle movement on the ultrasound display at the pleural line and is termed lung sliding sign (Fig. 17.8 and Video 17.4). Sliding sign has been described as the appearance of “crawling ants”. The aerated lung below the pleura appears as a grey speculated shadow, which changes appearance during respiration quite similar to television ‘white noise’. A normal appearance may include the occasional short vertical line seen to extend inferiorly from the pleural line, which move and disappear with respiration, which are referred to as Z–lines. As the pleural line is highly reflective it is often duplicated below as reverberation artefacts that are equally spaced – A-lines (Fig. 17.9).

Video 17.4. Normal lung sliding sign (Reproduced with permission. © University of Melbourne. https://​s3.​amazonaws.​com/​iTU/​LU+chapter/​Lung+sliding.​mp4).

Rapid diagnosis or exclusion of pneumothorax with ultrasound is a valuable skill for the anaesthetist and critical care physician. The detection of pneumothorax is simply based on the inability to see lung sliding sign, caused by air between the pleura (Compare lung sliding seen in Video 17.4 and no-lung sliding in Video 17.5). As air normally collects anteriorly in the supine patient, the probe should be initially placed vertically in the anterior zone below the clavicle in the mid-clavicular line and then moved inferiorly and laterally over the entire anterior zone (Fig. 17.11 and Videos 17.5 and 17.6). It is important to keep the probe motionless, as movements of the probe may be misinterpreted as lung sliding.

Video 17.5. Pneumothorax (no lung sliding) (Reproduced with permission. © University of Melbourne. https://​s3.​amazonaws.​com/​iTU/​LU+chapter/​No+lung+sliding.​mp4).

Video 17.6. Pneumothorax examination. Reproduced with permission. © University of Melbourne. https://​s3.​amazonaws.​com/​iTU/​LU+chapter/​Pneumo-thorax+examinati​on.​mp4.

Identification of lung sliding excludes the presence of pneumothorax in the scanned region with a sensitivity and specificity close to 100 % [4]. The presence of Z-lines or B-lines (described in the Sect. 17.3.7) also excludes pneumothorax because these signs rely on apposition of the two layers of pleura. Unfortunately, lack of lung sliding may be caused by other pleural or parenchymal pathology that prevents movement of the pleura and hence lung ultrasound is better at ruling pneumothorax out rather than ruling it in. Conditions that also cause lack of lung sliding include pleural adhesions, bullous disease and severe atelectasis or hypoventilation for example from endotracheal tube malposition or sputum obstruction, and interstitial diseases such as acute respiratory distress syndrome (ARDS) and pulmonary fibrosis. The ultrasound feature specific to pneumothorax is identification of a point where lung sliding abuts an area of no lung sliding, which identifies the point that the pleural surfaces regain contact, the contact point or lung point (Fig. 17.12).

Lack of lung sliding also has a characteristic appearance with M-mode (Fig. 17.13). The horizontal bars or waves extend all the way from the top to the bottom of the field without the area of pixelation due to lack of perceived lung movement, resulting in horizontal waves but no area of sand. This appearance has been termed stratosphere sign and bar code sign [9]. During M-Mode scanning with the linear probe in a horizontal position, if respiration results in the lung point moving from out of ultrasound field into the ultrasound field, a characteristic vertical linear demarcation appears, separating an area of lung sliding (seashore sign) from no long sliding (barcode sign) as shown in Fig. 17.14. This linear border represents the lung point or contact point, where the pneumothorax ends and the two layers of pleura contact each other and lung sliding recurs. Identification of this lung point is specific for pneumothorax, and may enable quantification of the size of a pneumothorax and allow tracking of the size of the pneumothorax over time, aiding in the decision whether or not to drain the pneumothorax. For example a pneumothorax with lung point located at the anterior axillary line would be smaller than one located at the posterior axillary line or not located at all. However this is not a very reliable indication of size as the position of the lung point is dependent on other factors, such as diaphragm position, patient posture and lung pathology such as collapse, consolidation and pleural adhesions. The lung point is usually sufficiently detected with the use of 2-D ultrasound, but M-mode enables documentation of the lung point, which can be printed and stored in the patient’s medical file.

When there is lack of lung sliding from complete absence of ventilation, pneumothorax may be still be excluded by observing a lung pulse [9]. When the probe is kept stationary, the movement of the heart may transmit oscillatory movements in the adjacent lung, resulting in very short bursts of lung sliding in time with the pulse (Fig. 17.15 and Video 17.7). Even though there is no ventilation, and no movement of the lung would occur with respiration, small movement of the lung is observed due to the cardiac motion displacing the lung synchronous with the electrocardiogram.

Video 17.7. Lung pulse (Reproduced with permission. © University of Melbourne. https://​s3.​amazonaws.​com/​iTU/​LU+chapter/​LungPulse.​mp4).

Surgical emphysema (occasionally occurring with a pneumothorax) will not only impair imaging of the pleura; but in minor cases B-lines may arise from air bubbles within the chest wall tissues (superficial to the pleura), which are referred to as E-lines [9].

The typical sonographic appearance of pleural effusion is an anechoic (black) area between the parietal and visceral pleura (Fig. 17.16), which usually changes size with respiratory movements. It is easy to detect with lung ultrasound because fluid is an effective transmitter of ultrasound, which has the added advantage of enabling direct visualisation of underlying lung pathology such as atelectasis and consolidation. The heart and aorta may also be visualised through an effusion. For detection of effusion, lung ultrasound is more accurate than supine chest X-ray [2] and is as accurate as computed tomography [2, 10]. For lung opacities found on chest X-ray, lung ultrasound is more accurate in distinguishing between effusion and consolidation [11]. Pleural effusions are commonly seen in patients with cardiac failure, malignancy and after cardiac or thoracic surgery.

Video 17.8. Pleural effusion (Reproduced with permission. © University of Melbourne. https://​s3.​amazonaws.​com/​iTU/​LU+chapter/​Pleural+effusion​+2.​mov).

A low frequency probe with a large footprint such as a curvilinear probe or TTE probe enables sufficient penetration, which is typically 5–8 cm but may need to be increased in large effusions. Mostly effusions are not loculated and will collect in the dependent zone of the chest. Therefore effusions are mostly detected in the posterior lower zone, whether the patient is supine or erect. It is important to routinely identify the diaphragm (Sect. 17.3.10.3) to avoid confusion of pleural fluid and peritoneal fluid (Fig. 17.17). Very rarely, there may be confusion between a vascular structure and free fluid, and the use of color flow or pulsed wave Doppler will easily identify the nature of the vessel.