Pneumothorax should be learned “slowly.” The abolition of lung sliding, a first step, should be first recognized on nondyspneic patients quietly. Idiopathic cases seen in the ER and cases occurring on mechanical ventilation do not generate dyspnea (nor do patients in cardiac arrest, but it is not the quiet place for learning). In these “pure” conditions, one can see, first on real time, that the pleural line is completely, strikingly standstill. The slightest sign of activity at the pleural line or Merlin’s space should be considered (see again the variants of lung sliding in Chap. 10). Here, nothing is moving. This is definitely not normal at a vital organ, supposed to move all the time. The absence of parietal activity (severe dyspnea) makes the absence of lung sliding more obvious to detect. The absence of dyspnea is recognized on real time by simple observation (Video 14.1). Critical detail explained in Chap. 10: all filters should have been deactivated.

The M-mode analysis in LUCI shows two superimposed, rectangular areas: the upper Keye’s space and the lower M-Merlin’s space (MM-space). They are separated by the pleural line. In a pure pneumothorax, not dyspneic, the absence of motion from the chest wall makes a regular Keye’s space (upper square), and the absence of lung motion makes a regular MM-space (lower square). Consequently, both spaces have exactly the same pattern, resulting in one M-mode stratified pattern, strikingly homogeneous, from top to bottom (the opposite of the usual seashore sign). Reminiscent of stratospheric condensation phenomena of B-17 flying fortresses squadrons in high altitude, this sign was called the stratosphere sign [3] (Fig. 14.1, Video 14.2). Those who really want by any means to call it the barcode sign should see the Fig. 14.2, read its caption (Fig. 14.2), and make their opinion.

We highly advise to take some time for fixing (in one’s brain) the absence of lung sliding on real-time first. The M-mode helps for understanding the pathophysiology of pneumothorax and the difficult cases which will come soon or late. The control of the real-time pattern will be of major use for being operational in a few seconds, as requested in the first step of the SESAME-protocol (cardiac arrest).

Abolition of lung sliding, analyzed in a medical ICU with CT as reference, showed a sensitivity of 95 % [4]. The rules of reviewing are intangible [5]. Yet if rewritten today, the data would be 100 %; see Anecdotal Note 1. In other words, all cases of pneumothorax yield abolition of nondependent lung sliding. The negative predictive value is 100 % [4]. A normal, nondependent lung sliding confidently rules out pneumothorax.

Abolished lung sliding has a poor specificity: it is far from indicating pneumothorax (see Anecdotal Note 2). When ICU controls have no lung disease, ultrasound positive predictive value is 87 % [4]. This rate decreases to 56 % when the control population includes ARDS patients [6]. It falls to 27 % when patients in acute respiratory failure are selected [7]. In ARDS or extensive pneumonia, lung sliding is abolished in more than one-third of cases. For some who believe that abolished lung sliding means pneumothorax, Table 14.1 is a list of many other causes.

How to complicate the procedure?

The ultrasound diagnosis of pneumothorax is the science of standstillness. A dyspneic patient who tries to survive by recruiting the accessory muscles is not standstill at all. Examining the pleural line in such patients obeys in some sort to the rules of shooting from a mobile point to a mobile target (dogfight). A pneumothorax generates no movement, and a dyspnea generates hectic movements. This latter dynamics from superficial areas will parasite the characteristic standstill sign at the pleural line. The operator must detect a standstillness (at the pleural line) inside a hectic, moving area: an “absolute quandary?”

A standardized approach, using the M-mode and the concept of the Keye’s space, gives the answer. The phenomenon observed on M-mode can be related to a column of sand. The top of this column is located above the pleural line, inside the Keye’s space: this cannot be a seashore sign. A seashore sign would by definition begin at the very pleural line, not 1 mm above, not 1 mm below, spreading below homogeneously. Here, the dynamic comes from the contraction of the parietal muscles (pectoral, intercostal). The sand visible at the Keye’s space, meaning severe dyspnea, is called the Keye’s sign (see details in Chap. 10). Now, the whole of the sand column will be analyzed. In the case of pneumothorax, this sandy image will cross the pleural line without any change, even slight. This is the Avicenne sign (conceived in Avicenne Hospital) (Fig. 14.3, Video 14.3). The Avicenne sign demonstrates that, in spite of the diffuse movement coming from the muscular recruitment above the pleural line, lung sliding is definitely abolished.