Lung Ultrasound in COVID-19

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Although lung ultrasound has been used clinically in respiratory failure since 1986¹ and was introduced into critical care by Lichtenstein and Axler² in the 1990s, in many ways the techniques and findings have only begun to be explored.³ Each year, a rapidly increasing number of new publications show alternative findings or slight modifications on scanning techniques. This process has exploded since the advent of COVID-19,⁴ but the usefulness of lung ultrasound has been described in multiple environments, with multiple pathologic characteristics, and with many different published scanning protocols. However, just as with Laennec’s stethoscope, the true diagnostic capabilities of lung ultrasonography are likely to be discovered only with iterative advances in application.

Answers From Stillness

The underlying truth of lung ultrasound as a collection of mostly artefactual findings has been well described,⁵ with most of the relevant information coming from the interfaces between the two layers of pleura and that between the visceral pleura and the first layer of alveoli. As such, pathologic features deep in the parenchyma, such as airway lesions or pneumonia deep to the pneumothorax, usually are invisible to lung ultrasound.

The traditional technique is to place the ultrasound device in a relevant area and to observe the artifacts generated by the relevant interfaces for one respiratory cycle. One then can see the A lines, B lines, consolidations, and lung sliding easily. This technique is the basis for several protocols, including the 2012 international consensus statement⁶ and a quantitative scoring system of 28 static points assigning values based on specific patterns.⁷ These scores were shown to have high interobserver reliability and to correlate with lung recruitability and CT scan findings and, recently, with disease severity in COVID-19.⁸ However, when one’s hand moves against the skin, the lung sliding artifact becomes lost in the motion and cannot be used reliably. Other plural artifacts, such as B-lines, plural irregularities, or subpleural consolidations, remain visible.

Why Sweep?

The 28-zone technique, although comprehensive, can be time-consuming: up to double the time of focused scanning.⁹ An alternative technique would be to slide across the surface of the thorax rapidly and to visualize large swathes of lung, instead of discrete points. This is analogous to an expert radiologist reviewing a chest CT scan, wherein instead of looking at a single slice, they might slide rapidly through multiple plans. In some ways, sweeping the surface of the chest allows a more holistic view of a broad area of the lung. However, the discrete points strategy could be more analogous to a protocolized investigation with more easily quantified clear diagnostic criteria.

In this issue of CHEST, a group of authors describe their experience with 179 patients seeking treatment in the spring of 2020 with COVID-19 for whom they performed sweep imaging of the lungs and contemporaneous CT scanning.¹⁰ They found that the technique could be performed quickly and showed a significant correlation between both the percent of lung involved and the severity of oxygenation impairment (as defined by Pao ₂ to Fio ₂ ratio). They also demonstrated the feasibility of applying this technique under the harrowing conditions of the early pandemic in Italy. The authors are to be commended for this work; a rapid, alternative approach to assessing disease severity using imaging could be very helpful in COVID-19 and perhaps in other forms of ARDS.

The actual clinical findings of the study are specific only to the disease state identified (COVID-19-induced respiratory failure in the era before vaccination) and may not be generalizable to other disease states or as the prevalence of COVID-19 (we hope) diminishes. Perhaps more important is the potential for the sweep imaging technique to change the way lung ultrasound is performed. This builds on the work of another group showing feasibility in a remote environment, granting unskilled scanners a more forgiving method to acquire images for offsite interpretation¹¹ and correlation with standard chest imaging.¹²

The Heart, or Lungs, of the Matter

Once again, lung ultrasound demonstrated value in this study. It is rapid, avoids the risk of transfer, and correlates well with CT imaging. It clearly brings value to an underresourced setting by providing inexpensive and portable high-quality chest imaging. But what is the genuine benefit of quantifying the extent of disease on lung ultrasound after the diagnosis has been confirmed by polymerase chain reaction testing? Clinical decision-making and therapeutic interventions should be based on signs and symptoms (Pao ₂ to Fio ₂ ratio, respiratory rate, fatigue) as opposed to imaging severity, so what does lung ultrasound really add beyond that? It may be that serial lung sweep imaging is a more rapid method for monitoring disease progression, as has been demonstrated with the lung ultrasound score,⁸ but these studies still need to be carried out.

Perhaps more exciting, Volpicelli et al¹⁰ have demonstrated the feasibility of a new approach to assessing lungs with ultrasound and have allowed us to take steps in a different direction from the classical approach of Lichtenstein⁵. With this study building on the prior work by Marini et al,11, 12 perhaps we are seeing the sort of evolution that took the stethoscope from a method for preserving personal space into a sophisticated diagnostic tool for rapidly assessing a patient.

It has been a decade since the 2012 consensus, and the modern practice of lung ultrasound includes more findings, multiple score systems, and a stronger evidence base than at that time. Now, we have an entirely different approach to acquiring images of the lungs, one that is more rapid and covers greater surface area than the standard. Time will tell as to whether lung sweep becomes an established technique, either to supplement or replace the traditional zone model. Breadth may be as important as depth, after all.