Lung ultrasound in trauma

Overview

The great potential of ultrasound in the study of lung parenchyma lies in its ability to detect alterations in parenchymal density resulting from loss of alveolar air with or without increase of interstitial fluids. Moreover, pleural disorders can be studied by direct visualization or indirect detection of abnormal fluids or air collected in the pleural space. Ultrasound abilities are based not only on the capacity to visualize real images of disorders (e.g., direct visualization of pleural effusion or lung consolidations) but also on analyzing various artifacts. The latter are generated by the acoustic interface created between parietal pleura and lung parenchyma and enable the detection of pneumothorax and various interstitial disorders. In trauma patients, bedside lung ultrasound readily identifies life-threatening disorders, such as hemothorax and/or pneumothorax, thus facilitating prompt treatment. ^, In trauma patients who present without any life-threatening conditions, lung ultrasound can detect lesions that appear radio-occult on radiography (e.g., lung blast), thus optimizing the selection of patients undergoing whole-body computed tomography (CT) scans.

The technique

In a trauma scenario, the lung can be examined by any transducer, from high-frequency transducers well-suited for surface scanning to microconvex transducers that facilitate scanning between the tiny acoustic bone windows of the thoracic cage. Convex transducers represent the optimal mixture between a large spatial visualization and penetration depth achieved by low frequencies. Apart from lung scanning, microconvex transducers enable the examination of the abdomen, heart, veins, and so on, which is a time-saving feature in trauma cases.

Trauma patients usually lie supine and cannot be moved; hence emergency lung ultrasound can access the chest through its anterior and lateral areas. Scanning these areas is sufficient in most cases, but sometimes it is necessary to sweep the transducer above the diaphragm or toward the posterior aspect of chest. Initially, scanning is performed on longitudinal planes because the latter allow identification of the pleural line between two adjacent ribs. By slightly rotating the transducer along intercostal spaces, oblique views are obtained that allow visualization of a larger portion of the pleura and of the underlying lung parenchyma.

The normal pattern

Scanning a normally aerated lung permits visualization of the entire chest wall from skin to parietal pleura. The alveolar air creates an interface with the pleura and the soft tissues of the chest wall because of differences in acoustic impedance (air is an acoustic barrier). Hence visualization of the lung parenchyma below the pleural line is not feasible. The image corresponding to the normal lung is made by artifacts that represent chest wall’s reflections below the pleural line (“mirror effect” of the lung) ( Figure 21-1 ). A-lines are horizontal lines visualized at constant intervals in the zone of artifacts and represent multiple reflections of the pleural line.

Lung ultrasound is a dynamic technique that allows detection of lung movements against the chest wall. The normal lung is characterized by two movements. Lung sliding is a horizontal movement against the chest wall that is observed during normal respiratory efforts. Lung pulse is observed because of the transmission of cardiac beats to lung parenchyma against the chest wall and can only be visualized when patients with normal lungs, for some reason, stop breathing. Pulsation is horizontal or vertical, depending on the lung zone that is scanned.