(1)
Hôpital Ambroise Paré Service de Réanimation Médicale, Boulogne (Paris-West University), France
It was first necessary to carefully define the B-line. Now, we can do one more step – the essential one: lung rockets. Several names were given; here is the latest update (see Anecdotal Note 1).
Our 5-MHz microconvex probe is ideal for assessing lung rockets.
Lung Rockets, Preliminary Definitions
Ultrasound interstitial syndrome is defined by the visualization of three or more B-lines simultaneously visible between two ribs, in a longitudinal scan (Fig. 12.1). Three or more B-lines were coined lung rockets, since the pattern is reminiscent of a rocket at liftoff. Said differently, lung rockets include eight features: they are comet-tail artifacts, arising from the pleural line, moving with lung sliding, usually long, usually well-defined, usually erasing A-lines, usually hyperechoic, and multiple in one longitudinal scan.
Fig. 12.1
Septal lung rockets. Patient with cardiogenic pulmonary edema. Four B-lines are identified in this longitudinal scan of the anterior chest wall. Reminiscent of a rocket at liftoff, this pattern has been called lung rockets. The B-lines are separated from each other by an average distance of 7 mm; this is the septal variant of the lung rockets, labeled septal rockets. Lung rockets indicate interstitial syndrome. Anterior disseminated lung rockets in the critically ill usually mean pulmonary edema. They have a basic place in the BLUE-protocol and FALLS-protocol
By definition, lung rockets are plural. Less than three B-lines are not consistent with interstitial syndrome. The b-line (lower case): It is defined by a single B-line between two ribs. It cannot be assimilated to interstitial syndrome nor any disease. Can be the sign of a minor fissura (see Fig. 11.2).
Isolated lung rockets (i.e., visible at only one focal area) define focalized interstitial syndrome, of minor importance in the BLUE-protocol. Lung rockets disseminated to the whole lung define diffuse interstitial syndrome, i.e., a characteristic of most disorders seen in the emergency.
The Data of Our Princeps Study and the Real Life
Our princeps study, assessing 121 cases of patients with diffuse alveolar-interstitial syndrome on radiography, and comparing them with 129 patients without any alveolar-interstitial pattern, showed a sensitivity of 93 % and a specificity of 93 % for the disseminated lung rockets [1]. When CT was used as a reference, the concordance was complete with interstitial syndrome.
These data mean that no disorder can yield lung rockets, if not interstitial syndrome. This was published in 1997, and we wanted to see how these data would age. With time, we never saw diffuse lung rockets in the countless healthy models we have insonated during workshops, which shows that time passing, ultrasound sensitivity would be 100 %. With time, a case of interstitial syndrome without lung rockets will maybe be described, but this will be an extreme rarity. Extremely rare cases of lung rockets not related to interstitial syndrome should now be described (read Peculiar Note 1).
Pathophysiological Explanation of Lung Rockets, Clinical Outcome
The average distance between two B-lines in the septal variant is roughly 6–7 mm. This corresponds to the average size of a lobule. The polyedric shape of these lobules explains that the 6-/7-mm distance is an average; it can be less, depending on the section. Between two ribs in an adult, 2 cm of pleural line is visible, i.e., space for three or four subpleural interlobular septa, let us keep in mind the minimal value of three.
The question whether one may miss deep interstitial syndrome (without superficial extension) is solved by looking any CT: the subpleural interlobular septal thickening is a representative sample of deeper changes.
Characterization of the Lung Rockets in Function of Their Density: Morphological Patterns
There is a subtle gradation of the number of B-lines, with a dichotomy inside (Fig. 12.2). All data must be understood “between two ribs in longitudinal scans.” To the left, all patterns which are not lung rockets are first the O-line (a variant of A-line), then the A-lines, then the isolated B-line, dubbed b-line (lower case), then two B-lines between two ribs, and dubbed bb-lines (lower case). To the right, there are three B-lines or more, i.e., lung rockets, i.e., interstitial syndrome. The label B-lines does not infer a specific number. Some B-lines can be large with a fusiform pattern; this does not mean, without mistake, a severe form of edema (Fig. 12.3).
Fig. 12.2
From no B-line to countless B-lines, a continuum. This figure shows, from left to right, an O-line then an A-line, then a b-line then a bb-line. Then follow three types of lung rockets: septal rockets, ground-glass rockets, then the Birolleau variant (countless B-lines)
Fig. 12.3
Squirrel variant. These are typical (septal) lung rockets, with here two fusiform B-lines, at the center, reminiscent of a squirrel tail, the squirrel variant (unknown meaning, but no apparent link with the severity of the interstitial syndrome without mistake)
No Lung Rockets
O-lines mean A-lines.
One B-line means probably, when seen between the anterior BLUE-points, a minor fissura (see Fig. 11.2). For disseminated b-lines, an uncommon finding, we have not enough cases to conclude.
Two B-lines (labeled bb-lines) have no solid meaning yet. It is not enough for being assimilated to interstitial syndrome. This infrequent finding has still no pathological correlation.
Lung Rockets
Septal Rockets
This label specifies that B-lines are 6-/7-mm apart, i.e., space for three or four B-lines between two ribs. This is the anatomic distance between two subpleural interlobular septa in adults (Fig. 12.1). Septal rockets indicate thickened subpleural interlobular septa (probably a mild stage of edema). They appear as an ultrasound equivalent of the familiar Kerley B-lines [2] (Fig. 12.4).
Fig. 12.4
CT correlating with septal rockets. CT scan of massive alveolar-interstitial syndrome. Thickened interlobular septa are visible touching the anterior surface (arrows). In a normal subject, no dense structure (apart from fissurae, see Fig. 11.2) is visible abutting the surface. This is the CT appearance of the Kerley lines
Ground-Glass Rockets
This label indicates one more degree of severity. The B-lines are twice as numerous as septal rockets, i.e., separated by 3 mm from each other, i.e., space for 6–8 B-lines (Fig. 12.5). Ground-glass rockets indicate ground-glass areas on CT, a high-degree interstitial syndrome.
Fig. 12.5
Here, 6 or 7 comet-tail artifacts are visible. The distance between each B-line is roughly 3 mm. These lung rockets called ground-glass rockets correlate with CT ground-glass areas (arrows), i.e. severe stage of interstitial edema Arrowheads: thickened interlobular septa
The Birolleau Variant
This is an extreme variant of ground-glass rockets. B-lines are so contiguous that no anechoic space is managed between the two, and the Merlin’s space appears homogeneous and hyperechoic (Fig. 12.6). We suppose it corresponds to extremely severe edema. The correspondent disorder on CT is again a ground-glass lesion. This variant cannot be confused with an O-line (see Fig. 9.2), which also yields homogeneous Merlin’s space: O-lines make a dark space, and the Birolleau variant makes a white space (the Storti’s distinction).
Fig. 12.6
Extreme case of pulmonary edema. The B-lines are so contiguous that they shape a homogeneous hyperechoic Merlin’s space (Birolleau variant). The underlying CT pattern is a ground-glass disorder. The pleural line can be used as a reference tone. A lung consolidation would yield a less echoic pattern. An O-line would yield an anechoic Merlin’s space. Here, in the Birolleau variant, the Merlin’s space is as echoic as the pleural line (the Storti’s distinction). This demonstrates that in this zebra, the native, natural tone is dark