The word “pneumothorax” is used several times a day in no less than a dozen of disciplines, not only in ICUs. It is seen in trauma, pre-hospital medicine, emergency rooms, anesthesiology, pulmonology, pediatrics, thoracic surgery, after any procedure, including acupuncture, can be debated in internal medicine, geriatrics, even palliative care, spaceship medicine and world medicine again.

It touches the most vital organ. In trauma, bilateral cases are rapidly deadly. In the ICU, it is a frequent event [1]. The physicians know that severe cases can be radioccult. Traumatized and ventilated patients call for exceptional care, since the risk of a missed pneumothorax is major [2]. Some authors consider that any pneumothorax even occult should benefit from a chest tube before initiating mechanical ventilation [3]. How to make an immediate diagnosis is an issue, since bedside radiographies miss a number of cases. CT makes the diagnosis; this is true, yet two conflicting issues are not envisageable: first, sending all patients to CT, generating irradiation, delays, costs, and lost energy, and, second, losing a patient from such an “illegitimate” trouble. The dilemma is elegantly and perfectly solved by ultrasound. Providentially, the most accessible area is the anterior chest wall, and the A′-profile can be detected in a few seconds. What is difficult on radiography (anterior pneumothorax) is the easiest on ultrasound, which will detect the lung point quite always (when anterior). The most severely injured lungs (ARDS, etc.) are the ones giving the most striking signs ruling out pneumothorax. Ultrasound is a providence for these daily settings.

This is why we really consider that even if it may appear difficult to some (especially those who do not follow the rules), this approach offers so many advantages that a minor investment effort is valuable. The user will benefit from:

The interest of lung ultrasound for diagnosing pneumothorax is confirmed by so many works that it becomes impossible to quote all of them [4–13]. The community wakes up, at last, but is well awaken now.

The lung is an elastic structure not larger than a hand. It is held under negative pressure in order to be stuck against the pleural cavity. A rupture in this negative pleural pressure results in the physiological need of the elastic forces to come back to a stable status, with massive retraction of healthy lungs.

Idiopathic cases rarely generate acute respiratory failure. Trauma is the most obvious setting. Iatrogenic cases are a classical cause. Cases occurring under mechanical ventilation can lead to major concerns.

If the clinical diagnosis was easy, free from operator dependency, it would raise no problem. Yet in the usual conditions, the need for a confirmation test is quite constant.

Up to 30 % of cases are occulted by the initial radiograph [14–17], many of them evolving to tension pneumothorax [14]. Some tension cases remain even unclear in the bedside radiograph [18]. In dramatic situations, time is lacking for radiological confirmation [19]. CT, the gold standard [20], is a suboptimal option in these critically ill patients.

It is scary to see how often CT was still recently used in the follow-up of a pneumothorax, by doctors aware that the radiograph is not a sensitive tool.

In a dyspneic patient just after a trauma, this is completely part of the physical examination. In these noisy settings, an efficient auscultation is a quandary. This is why lung ultrasound is performed as soon as possible during the physical examination.

The first step is to apply the probe at the anterior BLUE-points. Detecting lung sliding or lung rockets rules out pneumothorax in a few seconds. If lung sliding is absent and no B-line is visible in this area (in one word, an A′-profile), finding a lung point confirms the diagnosis and indicates the volume of the pneumothorax. In the absence of lung point, read below the “Australian variant.”

The accuracy using our technique indicates an overall 66 % sensitivity (79 % for occult cases) and a 100 % specificity [21, 22]. This makes sensitivity highly superior to that of radiography for partial pneumothorax, especially anterior cases, regularly radioccult: few millimeters of air thickness are sufficient (read Anecdotal Note 1). The overall sensitivity may appear low, but note that 100 % of patients have the A′-profile (but only 2/3 have the lung point).

The A′-profile is immediately acquired and is highly suggestive. Remember that acute dyspnea can generate the Keyes’ sign, i.e., noise above the pleural line, which will not confuse a user following the rules.

Pulmonary edema (B-profile), pneumonia (B′-, C-, A/B-profiles), COPD and asthma (A-profile), and pulmonary embolism (A-profile), these diseases generate profiles distinct from the A′-profile.

Look again at the list of situations able to create an A′-profile in Table 14.​1 of Chap. 14 on pneumothorax.

Spending energy to distinguish an A′-profile from pseudo-A′-profiles is a good exercise, allowing to simplify the management of patients who have no pneumothorax.

Only the parietal pleura is visible at the pleural line. This generates abolished lung sliding. The visceral pleura, even very near (even 1 mm) to the parietal pleura, is hidden by the free gas in the pleural cavity. This generates a homogeneous pattern of the Merlin’s space, with regular reverberation of the pleural line, i.e., A-lines. The whole generates the A′-profile. A-lines can be replaced by O-lines without any damage to the concept. “O-lines are A-lines”.

The lung point is explained by even a slight increase in parietal contact when the lung inflates, i.e., on inspiration. We reiterate that any lung inflates during inspiration, whether normal or collapsed by a pneumothorax, whether spontaneously breathing, or under mechanical ventilation. This increases the lung volume, even very slightly. If the probe is applied at the boundary area, the very area where the lung increases its contact with the parietal pleura, thanks to real-time, instant response, and zero filter, the user will see sudden lung signs (lung sliding, B-lines) replacing the A′-profile, living air replacing dead air, to make it short.

The specificity of ultrasound is 100 %. The sensitivity depends on the existence of a lung point. This raises the following problem: how to do it in a critically ill patient who has an A′-profile, i.e., probably a pneumothorax, but no lung point?

Too much purism would kill patients: mandatorily requiring the lung point would classify cases of pneumothorax as false–negative.

Too much laxism would kill other patients. Considering the lung point as a futile sign not really useful would result in correctly managing these patients is true. Yet this simplification would generate false–positives of pneumothorax in patients with previous pleural history. To begin with, all these patients who previously received pleural talcage (poudrage) or pleurodesis for iterative pneumothorax are now visited for an acute thoracic pain. These patients would receive a chest tube insertion, not a good idea if they just needed a coronary desobstruction.