Pneumothorax and the stratosphere sign. Left: same pattern as in Figure 2, i.e., pleural line with A-lines, indicating gas below the pleural line. Not visible on the left image, lung sliding is totally absent. Right: here on M-mode, the abolition of lung sliding is visible through the stratosphere sign (which replaces the seashore sign) and indicates total absence of motion. This suggests pneumothorax as a possible cause (see others in text). Arrows: location of the pleural line. The combination of abolished lung sliding with A-lines, at the anterior chest wall, is the A’-profile of the BLUE-protocol (as opposed to the A-profile, where lung sliding is present, ruling out pneumothorax). Extract from “Whole body ultrasonography in the critically ill” (2010 Ed, Chapter 18), with kind permission of Springer Science.