Movement of pleural line in and out of the frame is described as a lung point sign, and is commonly quoted as being pathognomonic for pneumothorax.1,2
If you scan too low on the thorax3 or too close to the pericardium,4however, you can get a scan that looks remarkably similar to a lung point. This is a pitfall for diagnosing pneumothorax.
To avoid this problem, make sure that you are well away from the diaphragm and heart, and make sure you can tell the difference between an actual lung point and a false positive lung point.
Visually, the key is to look at the image deep to the non-sliding pleural line. If you see movement or tissue and absence of a-lines, your scan does not represent a true positive lung point:
Lichtenstein DA. Relevance of Lung Ultrasound in the Diagnosis of Acute Respiratory Failure: the BLUE protocol. Chest. 134(1):117-. 2008. [article]
Lichtenstein D, Mezière G, Biderman P, Gepner A. The "lung point": an ultrasound sign specific to pneumothorax. Intensive care medicine. 26(10):1434-40. 2000. [pubmed]
Gillman LM, Alkadi A, Kirkpatrick AW. The "pseudo-lung point" sign: all focal respiratory coupled alternating pleural patterns are not diagnostic of a pneumothorax. The Journal of trauma. 67(3):672-3. 2009. [pubmed]
Soldati G. Occult Traumatic Pneumothorax . Chest. 133(1):204-. 2008. [article]