Lung Ultrasound Training in the Critically Ill: Reply

In their letter, Bronshteyn et al.¹  pinpoint a critical issue concerning lung ultrasound training² : the identification and interpretation of vertical artifacts, the so-called B lines.

Vertical artifacts have been indifferently termed comet tail artifacts, ultrasound lung comets, ring-down artifacts, and B lines.³  Multiple internal reverberations issued from subpleural airspaces surrounded by edematous tissue is the main biophysic mechanism producing B lines.⁴  In experimental models,^4,5  the generation of B lines requires two conditions: (1) The existence of acoustic traps combining transonic access channels and air bubbles. The multiple reflections between bubbles reradiate the incident wave to the probe, with a wavelength that depends on transonic access channels’ shape and size. (2) The acoustic trap should have a minimal size under which it cannot emit a B line. The ratio between the wavelength emitted by the acoustic trap and the probe, determines B lines spatial characteristics: When the ratio is greater than 1, B1 spaced lines are generated; when the ratio is less than 1, B2 coalescent B lines are emitted.⁴  Basically, the detection of B lines is indicative of an abnormal interface between alveolar gas and pulmonary tissue extending to the lung periphery.

The applicability of experimental models to clinical situations is illustrated in figure 1. When the lung is normally aerated (fig. 1A), there is no available acoustic trap because the normal interlobular septa is not thick enough to transmit the incident wave and induce reflections between airspaces. Only horizontal A lines are present (slide 3, https://links.lww.com/ALN/C148). Edematous or fibrotic interlobular septa characterizing interstitial syndrome open transonic access channels and generate multiple spaced B1 lines (fig. 1B). Because the size of the secondary pulmonary lobule varies from 10 to 30 mm,¹  multiple B1 lines may be regularly or irregularly spaced (slides 9 and 10, https://links.lww.com/ALN/C148). Ultrasound interstitial syndrome is defined as the presence of more than two spaced B1 lines, and not “as the presence of 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,” as falsely stated in the Method section (typographical error attributable to an automatic copy and paste).²  We thank Dr. Bronshteyn et al. for their careful reading of our article and agree that our ultrasound definitions were confusing because of this typographical error. Interstitial-alveolar syndrome (fig. 1C) is characterized by the coexistence of acini with interstitial edema and acini with pulmonary edema. Alveolar flooding increases the number and the size of transonic access channels and creates multiple interconnected small air bubbles (fig. 1D), two conditions resulting in coalescent B2 lines. Ultrasound interstitial-alveolar syndrome is defined as the simultaneous presence of B1 and B2 lines in adjacent lung regions (slide 4, https://links.lww.com/ALN/C149). An analog situation is created when alveolar flooding is on the border of a subpleural consolidation representing a foci of interstitial pneumonia (fig. 1E). According to the size of the subpleural infectious foci, which determines the size of the transonic access channel, either B1 or B2 lines can be detected. Therefore, B1 and B2 lines characterizing interstitial-alveolar syndrome can be issued either from the pleural line or subpleural consolidations (slide 11, https://links.lww.com/ALN/C148, and slide 3, https://links.lww.com/ALN/C149). Pulmonary edema, which is characterized by alveolar flooding involving all lung regions, creates conditions that generate diffuse coalescent B2 lines (fig. 1D). In hemodynamic pulmonary edema, B2 lines are issued from the pleural line (slide 5, https://links.lww.com/ALN/C149). In high-permeability type pulmonary edema, as observed in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) severe pneumonia,⁶  B2 lines can also be issued from subpleural consolidations (slide 6, https://links.lww.com/ALN/C149). Ultrasound pulmonary edema can be defined as the presence of coalescent B2 lines issued from pleural lines or juxtapleural consolidations, in all examined regions.

The ability to identify and correctly interpret B lines is a major, and difficult, part of the lung ultrasound training. We hope that our answer clarifies this complex issue. We thank Dr. Bronshteyn et al. for identifying a mistake related to a typographical error in the ultrasound definition of Interstitial syndrome.

The authors declare no competing interests.