To the Editor:
We read with interest the excellent study by Arbelot et al.,1 evaluating the learning curve for focused, diagnostic lung ultrasound. The authors should be commended for their heroic effort to conduct a multicenter educational study at 10 intensive care units spanning three continents to address an important question that will inform future training guidelines. But although the study’s results are broadly consistent with those of other related publications on this topic,2,3 we take issue with one aspect of the study’s methodology: the authors’ unique classification of lung ultrasound pathology. Specifically, the authors asked learners to assign each lung ultrasound exam a score on a five-point scale “according to the worst parenchymal pattern” visible in the exam. The scores ranged from 1 for “normal aeration” to 5 for “lung consolidation.” Although the authors’ definitions for scores 1 and 5 conform to widely accepted norms,4 their definitions for scores 2, 3, and 4 contain some irregularities.
The authors define these intermediate points as follows: 2 = “interstitial-alveolar syndrome”; 3 = “interstitial syndrome”; 4 = “pulmonary edema.” For these three states, both the authors’ numerical ordering and their proposed definitions are problematic for several reasons. First, in the lung ultrasound literature, the terms “interstitial-alveolar syndrome” and “interstitial syndrome” are often used interchangeably to refer to the same condition: a state of pathologically increased lung density short of complete lung consolidation.4,5 Second, the authors’ own definitions for these states seem to overlap. For instance, the authors define “interstitial-alveolar syndrome” as “multiple B lines either spaced or coalescent” and “interstitial syndrome” as “more than two spaced B lines or coalescent B lines, detected in a limited portion of the intercostal space and issued from the pleural line or subpleural consolidations of at least 5 mm.” The only way for these definitions to not overlap is if the authors intended learners to think of “interstitial-alveolar syndrome” as having two or fewer B lines per interspace. However, such a definition would be inconsistent with the widely accepted definition of this term: the presence of three or more B lines per intercostal space.5 Third, if one did wish to use “interstitial-alveolar syndrome” (state 2) and “interstitial syndrome” (state 3) to distinguish between two conceptual conditions, then the terms would need to be flipped in the author’s spectrum of severity because interstitial edema precedes alveolar edema clinically.6 Fourth, in their definition of states 3 and 4, the authors present a description of B lines that contradicts accepted norms: Whereas the authors imply that a B line could emanate from a subpleural consolidation, the literature defines B lines as ring-down artifacts that originate only from the pleural line.4 In contrast, ring-down artifacts that originate from subpleural consolidations are termed “shred sign” and identify lung that has higher density than edema: consolidated lung.6 Fifth, the term “pulmonary edema” (state 4) is classically a part of the differential diagnosis of interstitial/interstitial-alveolar syndrome, rather than its own standalone category of lung ultrasound pathology severity.4 Notably, in addition to the manuscript, we have also reviewed the authors’ supplemental digital content: Although the supplemental content is generally excellent, it does not clarify any of the peculiarities described above.
The authors should be praised for conducting a study that will inform multiple specialties’ training guidelines. But precisely because the study is likely to be so influential, we think it is important to identify any seeming methodologic flaws.
Competing Interests
Dr. Bronshteyn has performed paid consulting for Teleflex/Arrow (Wayne, Pennsylvania) in 2020 unrelated to diagnostic ultrasound. The remaining authors declare no competing interests.