The radiologists often question the notion of ultrasound diagnosis of interstitial syndrome since this term involves many conditions and they are accustomed to high-resolution CT. Those who have heard about lung rockets argue that this sign is not specific, but they mean for distinguishing, for example, histiocytosis X from sarcoidosis. This is the typical misconception which can occur in medicine when a tool is not in the right hands. In the critically ill, the interstitial syndrome is limited to acute phenomena: nearly always pulmonary edema, either hemodynamic or permeability induced. Hemodynamic pulmonary edema includes fluid overload and cardiogenic pulmonary edema. Permeability-induced edema includes ARDS and any inflammatory syndromes surrounding infectious processes (bacterial, viral, etc.). Even in the rare cases of chronic interstitial syndrome seen in acute settings, the intensivist has tools for this diagnosis; see Chap. 35.

The auscultation? Two centuries old [5], it does not provide any sign of interstitial syndrome to our knowledge.

The bedside radiography? More than one century old [6], it rarely demonstrates interstitial changes in critical settings. It shows rough alveolar-interstitial patterns, but rarely the Kerley lines. Even in a good-quality radiograph taken in an ambulatory patient, this diagnosis is fragile: an imaging specialist can make different interpretations from one day to another [7].

CT? It has been available since the 1980s [8]. It can maybe describe interstitial patterns, but referring critically ill patients to this heavy technique for this diagnosis alone would be really questionable. What more is experience showed us that standard CT inconstantly demonstrates subtle interstitial changes.

Therefore, maybe for a lack of easy-to-access diagnosis, the intensivist has invested little for knowing whether this patient has, or not, interstitial syndrome: he or she never integrated this disorder in the medical thought process and got accustomed to do without, not aware of what could be done with, it.

The B-line was coined using alphabetic order. It was even elegantly called “BLUE-line” by a colleague in Bangalore (Dr. Gana without mistake), which may result in a new term with the advantage of decreasing the effort of memory (BLUE speaks more than B – the very principle of SLAM (Anecdotal Note 1)).

The “BLUE-line” or, say for the moment, the B-line is a hydro-aeric artifact, indicating a mingling of fluid and air and whose definition has been updated from article to article (Anecdotal Note 2). The most recent definition is given in this book (this was one reason among others to build this new edition, paradoxically faster than trying to submit 1000 manuscripts).

The B-line is an artifact having seven characteristic features (Fig. 11.1). Three are constant and four almost constant.

Constant features:

Almost constant features (93–97 %):

This updated, standardized, comprehensive definition allows immediate distinction with any other artifact that can be seen in the human being, mainly E-lines and Z-lines (see below). The risk of confusion is decreased by a factor 10 (even more) for each added criterium (read Anecdotal Note 3). Here is how to understand this concept. A comet-tail artifact that arises from the pleural line can be a B-line or many others. If lung sliding is absent, the 3rd criterium does not work; the probability is, say, 90 % at worst. Now adding the well-defined criterium, it climbs up to 99 %; adding the long criterium, to 99.9 %; adding the dominant criterium, to 99.99 %; and adding the echoic criterium, to 99.999 %. In medicine, this precision appears as clinically sufficient.