Lung Ultrasound for Treatment of Patients With COVID‐19

Abbreviations

COVID‐19

coronavirus disease 2019

LUS

lung ultrasound

US

ultrasound

One of the most concerning characteristics of the severe acute respiratory syndrome coronavirus 2 pandemic is its fast contagion rate: the number of total confirmed cases grows approximately exponentially in many countries. ¹ In this rapidly evolving scenario, clear and unambiguous information on patient treatment strategies must be exchanged efficiently. This is particularly important regarding the use of valuable resources for diagnosis and evaluation of disease progression.

A bulk of recent literature is showing the usefulness of ultrasound (US) imaging for the assessment of lung interstitial diseases in general. ² This is even more true of point‐of‐care US in the context of the coronavirus disease 2019 (COVID‐19) pandemic. ³ In this context, lung ultrasound (LUS) is gaining momentum as a highly valuable and practical resource in the evaluation of COVID‐19 pneumonia and acute respiratory distress syndrome. In addition to the well‐known advantages of US imaging compared to other imaging modalities (use of nonionizing radiation, lower equipment cost, portability, real‐time imaging, and relatively easy disinfection of the equipment), LUS allows imaging of patients at the bedside, reducing complications from patient movement and the risk of staff exposure. ⁴ Various studies have suggested that LUS has superior performance to chest radiography to detect pneumonia. ⁵ , ⁶ The advantage of LUS may become more relevant in health systems with limited resources, such as those in low‐ and middle‐income countries.

Lung US differs from US imaging of other tissues in that there is not a one‐to‐one relationship between the appearance of lung parenchyma in the image and its structure. As a consequence of the high air content and low density of the lung, sound does not propagate in the parenchyma as it does in soft tissues. In particular, in the healthy lung, the presence of air‐filled alveoli leads to multiple US scattering, which prevents conventional B‐mode imaging but could be exploited for the assessment of interstitial diseases, as shown by Mohanty et al ⁷ As a result, LUS is mostly based on the detection and evaluation of artifacts that arise when the assumptions of the image formation process in the scanner (small variations in sound speed and single scattering) are not met. ⁸

Among the artifacts suggested to have relevance for the evaluation of COVID‐19 pneumonia and acute respiratory distress syndrome are vertical brightness strikes that extend axially from the pleural line. ⁹ The physical nature of B‐lines is not yet well understood. One of the current working hypotheses is that these artifacts represent propagation within zones of tissue with reduced aeration that trap sound. Demi et al ¹⁰ recently showed evidence of this hypothesis by recreating these artifacts in tissue‐mimicking phantoms with bubbles to mimic air‐filled alveoli. Moreover, similar results have been also shown in clinical data. ¹¹ Crucially, these phenomena are highly dependent on the US frequency. This suggests that LUS findings should be interpreted by considering the scanning frequency and related settings (focal depth and use of filters). In addition, other LUS features, such as the appearance of the pleural line, bronchograms, and consolidations, vary with the frequency (a higher frequency provides finer detail).

Another critical issue to consider while performing LUS examinations is the increased risk of inducing tissue damage associated with cavitation. Air pockets in lung tissue can oscillate with the pressure changes of the oscillating acoustic pulse and enter cavitation. Although no complications have been reported in humans under clinical settings, studies in animal models show evidence of pulmonary capillary hemorrhage at diagnostic frequencies. ¹² The likelihood of these effects increases with the acoustic output and scanning time. Thus, clinicians should closely monitor the mechanical index, which is associated with acoustic energy exposure and the likelihood of inducing cavitation.

On the basis of this evidence, we strongly advocate for authors of clinical studies and case reports to provide detailed information on the scanning settings, especially the scanning frequency and the mechanical index. As medical organizations and groups advance toward standardizing the use of LUS in the severe acute respiratory syndrome coronavirus 2 setting, ¹³ the inclusion of detailed information on the system settings, acoustic exposure, as well of anatomic views will help clinicians understand the reported findings and how they relate to the progression of the disease. This standardization in the reporting effort will contribute to making LUS more reproducible, thus facilitating its adoption worldwide. In this context, clinicians must abide by the as‐low‐as‐reasonably‐achievable principle, ¹⁴ exposing the patient to the minimum acoustic energy without compromising valuable information.