Is There a Role for Lung Ultrasound During the COVID‐19 Pandemic?

Abstract

Lung ultrasound (LUS) has evolved considerably over the last years with respect to its theoretical and operative aspects. Consequently, its clinical application has come to be sufficiently known and widespread. One of the characteristic aspects of LUS is the ability to define the alterations affecting the ratio between tissue and air in the superficial lung. Normally, the lung surface mainly consists of air. Incident ultrasound (US) waves are thus generally completely back-reflected by the visceral pleural plane, especially when healthy. In this context, the scattering of US waves produces artifactual images characterized by horizontal reverberations of the pleural line (A-lines) and mirror effects. When the ratio between air, tissue, fluid, or other biological components is reduced, the lung no longer presents itself as an almost complete specular reflector. Hence, various types of localized vertical artifacts appear on the US images in relation to the alterations of the subpleural tissue. These artifacts have generally been called B-lines, but recently it has become clear that B-lines are very heterogeneous in their appearance. Moreover, their heterogeneity may be exploited as a means to characterize the alterations of the lung surface. Another well-known phenomenon linked to the increase in subpleural lung density (in the absence of consolidated tissue) is the coalescence of many vertical artifacts in more extended echogenic patterns, in which the individual artifacts are still recognizable or fused in a single homogeneous subpleural echogenic area (white lung). When the subpleural density goes toward the value of 1 g/mL (about that of the solid tissue), then consolidations appear. Therefore, the clinician, through the visual inspection of LUS images, can detect, at the subpleural level, nonconsolidative increases in the ratio between full (tissue) and empty (air) and assess them in a range between normal and consolidative. Topographic images of the lesions can also be acquired. Finally, the extent of these lesions on the lung surface, as well as their evolution or regression over time, can also be evaluated. The study of these patterns shows very high sensitivity in cases of interstitial and alveolar-interstitial lung diseases, which have a peripheral distribution. Numerous studies on acute respiratory distress syndrome (ARDS) confirm this. Other studies related to the 2009 pandemic influenza A (H1N1) epidemic confirm these hypotheses even in a virally infectious setting. The recent pneumonia outbreak spreading from Wuhan, China, in December 2019 is caused by the 2019 novel coronavirus infection, defined as new coronavirus disease (COVID-19). This epidemic currently involves many areas of the world, with particular incidence in Italy, representing a serious challenge to public health and the efficiency of the health care structures. The histopathologic appearance of initial COVID19 pneumonia is characterized by alveolar damage, which includes alveolar edema, while the inflammatory component is patchy and mild. Reparative processes with pneumocyte hyperplasia and interstitial thickening can occur. The advanced phases show gravitational consolidations similar to those of ARDS. There are hemorrhagic necrosis, alveolar congestion, edema, flaking, and fibrosis. An analysis of the available computed tomographic (CT) data from patients with COVID-19 pneumonia shows largely bilateral lesions that are patchy and also confluent, appearing as ground glass or with a mixed consolidative and ground glass pattern. Ten percent of lesions with a crazy-paving appearance are reported. The lesions often have a wedge-like appearance with a pleural base. Major consolidations may show air bronchograms. Pleural effusion is absent. Patchy or confluent lesions tend to be distributed along the pleura. The lobe most frequently affected is the lower right lobe, followed by the upper and lower left lobes. The posterior lung is involved in 67% of cases. Given that LUS can identify changes in the physical state of superficial lung tissue, which correlate with histopathologic findings and can be identified on CT but remain hidden in a large percentage of chest radiographs, the role of LUS can be relevant in the context of the COVID-19 epidemic. It should also not be underestimated that, in experimental models of ARDS, LUS has proved capable of detecting lung lesions before the development of hypoxemia. The current clinical evidence (although not yet represented in the literature), the theoretical bases of LUS in the aerated lung, and LUS findings of similar aspects in other diseases (ARDS and flu virus