Lung ultrasound: The basics

Overview

The lung and its pathologic states are amenable to ultrasound analysis by virtue of the interplay between air and fluid within the thorax, the varying proportions of which suggest distinctive processes. In fact, bedside lung ultrasound has proved to be more informative than portable chest radiography in supine, critically ill patients for detecting and qualifying pulmonary pathology, including pneumothorax, infiltrates, pulmonary edema, and pleural effusion. Although some technical limitations are inherent in ICU patients, the accessibility of real-time ultrasound for immediate goal-directed imaging and interpretation is a valuable tool in the approach to respiratory dysfunction in the ICU.

The normal lung

A low-frequency transducer, 3.5 to 5.0 MHz, is positioned in longitudinal orientation over the chest wall in an intercostal space with the probe marker held cranially by convention. Gain and depth settings are adjusted to focus attention at the space between the rib shadows. The pleural line ( Figure 19-1 ), a bright hyperechoic horizontal linear structure, is visualized between the rib shadows. A dynamic sign perceived as shimmering of the parietal and visceral pleura in contact is referred to as lung sliding and is part of the normal aeration pattern. Confirmation of lung sliding can be pursued with the use of M-mode . Directing the ultrasound beam between the rib shadows to transect the pleural line allows analysis of motion over time in the single-dimensional interface at that location. In the presence of normal pleural interaction, the resulting screen after initiating M-mode will display a pattern referred to as the seashore sign ( Figure 19-2 ). Horizontal lines superficial to the pleural surface indicate a lack of motion of the chest wall structures with respiration. Deep to this is a bright hyperechoic horizontal line (representing the pleural surface), and immediately deep to the pleural line is a granular pattern indicating a normal aeration pattern in an inflating and deflating lung.

Evaluation of the lung parenchyma is based on the presence of different artifact patterns, all of which emanate from the pleural line. The A-line ( Figure 19-3 ) is a reverberation of the pleural line, and given an adequate depth setting, several of these parallel lines can be seen at regular intervals. When visualized in the presence of lung sliding, an A-line–dominant pattern is said to occur and is suggestive of normal lung parenchyma. In addition, an A-line–dominant pattern is noted to correlate with low pulmonary capillary wedge pressure and hence represents a low likelihood for pulmonary edema.

Evaluation for pneumothorax

A sonographic diagnosis of pneumothorax begins with clinical suspicion. Chest trauma in a young, otherwise healthy patient, sudden alterations in ventilator airway pressure, and postprocedure hypoxemia are all settings in which a lack of normal lung ultrasound findings along with compatible vital signs and findings on physical examination would yield a diagnosis of pneumothorax with certitude. Depending on the clinical stability of the patient, action may be warranted based only on the ultrasound findings.

Either low- or high-frequency transducers can be used, although with older machines, use of a low-frequency transducer in a thin patient may produce insufficient near-field detail. In our practice a high-frequency (vascular) transducer is routinely used for evaluation of pneumothorax and is preferred in thin subjects. In a supine patient, examination of the anterior chest wall at six to eight positions in each hemithorax and identification of lung sliding at each point would essentially rule out pneumothorax. In contrast, the absence of lung sliding (representing parietal pleura without visceral pleura to slide against) or the presence of a nonspecific air artifact without the normal aeration pattern is suggestive of pneumothorax. Confirmation of lung sliding can be verified with M-mode. The area under investigation, between the ribs shadows, is centered on the screen. M-mode is initiated after aligning the vertical line through the area in question while holding the probe as still as possible to avoid motion artifact. The “barcode sign” (or “stratosphere sign”) ( Figure 19-4 ) is the appearance of parallel horizontal lines extending through the entire field of view and indicates lack of normal motion of an inflated lung.

If lung sliding is absent and the barcode sign is noted on M-mode, any condition separating the two viscera from their physiologic juxtaposition or impeding their normal movement should be suspected. Causes include massive pneumonia, effusion, atelectasis, apnea, severe acute obstructive lung disease, and previous pleurodesis or pleurectomy. Even though lack of lung sliding is not specific for pneumothorax, the presence of lung sliding effectively rules out pneumothorax at the intercostal locations under the applied ultrasound probe .

A lung point is a less commonly found but highly specific finding that rules in pneumothorax. ^, The operator would see findings compatible with pneumothorax (absent lung sliding) juxtaposed to aerated lung (lung sliding, A- or B-lines) and moving with each respiratory cycle. This finding not only makes the diagnosis but also, since a lung point defines the margin of the pneumothorax, helps quantify the size of the pneumothorax within the chest cavity. It can also be helpful for procedural guidance in evacuating a pneumothorax. A lung point can be viewed in M-mode ( Figure 19-5 ), with the screen fluctuating over time between the seashore and barcode (or stratosphere) patterns with the transducer stationary. The position on the hemithorax at which the lung point is noted is further informative. Extensive pneumothorax with retraction of the majority of lung would produce a lung point very posteriorly or not produce one at all because of lack of lung reaching the chest wall in any location. A small pneumothorax would generate a lung point at an anterior and caudal location. In general, the more lateral and posterior a lung point is noted, the larger and more clinically relevant the pneumothorax.

Evaluation for pulmonary edema (Interstitial Syndrome)

Lung ultrasound can contribute to the diagnosis of pulmonary edema when supported by a suggestive clinical scenario. A normal, A-line–dominant pattern, as described earlier, has been associated with low pulmonary artery occlusion pressure and is likely to represent a “dry” lung. Several authors have described a correlation between extravascular lung water and a specific air artifact pattern known as the B-line pattern. ^,

A B-line, or comet tail ( Figure 19-6 ), is a horizontal hyperechoic beam emanating from the inferior margin of the pleural line and extending through to the deep edge of the screen. The B-line fans out, obliterates visualization of A-lines, and moves with the motion of the pleural line. Vertical lines that do not meet these criteria are artifacts of no clinical value. When diffusely identified, a B-line pattern correlates with thickened interlobular septa or ground-glass areas on computed tomography (CT). Though not reproduced in the literature, two distinct B-line patterns have been described, each correlating with a specific pulmonary parenchymal process. The first pattern consists of multiple B-lines discernible on screen, separated by 3 mm at a smooth pleural line. Sometimes referred to as B3-lines, this pattern is thought to correlate with subpleural ground-glass lesions, as would be the case with pulmonary edema, whether cardiogenic or related to acute respiratory distress syndrome (ARDS). In the case of ARDS, a mixed pattern of B-lines anteriorly and consolidation at gravity-dependent regions of the lung is expected. The second pattern consists of fewer visible B-lines, separated by approximately 7 mm at a ragged pleural line. These B7-lines are thought to correspond to thickened subpleural interlobular septa, as would be the case with interstitial lung disease.

A single B-line in isolation or a B-line pattern confined to the last intercostal space is unlikely to be indicative of any pathology. The lung bases, which have a greater mass of lung parenchyma, will often display a B-line pattern, and therefore an upper lung field examination is more informative in directed evaluation for pulmonary edema.

Several comet tail artifacts can mimic B-lines. The latter are identifiable by several key features: arising and fanning out from the inferior edge of the pleural line, not fading and extending to the deep edge of the ultrasound screen, obliterating A-lines as they proceed, and moving in synchronization with lung sliding. E-lines, often seen in emphysema, are aligned comet tail artifacts arising from layers superficial to the pleural line and indicate a thin air layer formed within two parietal layers. The commonly found Z-line is another comet tail artifact that does arise from the pleural surface; however, it is ill defined, is of variable echogenic appearance, and fades quickly without obscuring A-lines.