Over the past few years, ultrasound of lung parenchyma is gaining acceptance especially in the emergency and critical care setting. Point of care lung US (LUS) techniques are practical, easy to learn, and give valuable information especially in a compromised patient with respiratory distress. To a cardiologist, the lung had traditionally been our "enemy" because normal lung impedes the transmission of ultrasound waves and the normal pleural - lung parenchymal interface has a high acoustic gradient creating a "mirror-like" reflection. Typically, ultrasound images are based on the US waves actually penetrating tissue with similar acoustic impedances and images are constructed based on the reflection of the "echo's" returned to the probe from tissue interfaces. However, with lung ultrasound, rather than images constructed by penetration of the lung tissue, images of various lung pathologies are created as a result of ultrasound artifacts. These artifacts and findings are highly correlated or suggestive of specific pulmonary conditions providing rapid, non-invasive diagnostic information. Generally, for LUS to identify an abnormality, the lesion must be close to the periphery of the lung, essentially within a few mm of the subpleural space. "Lung" ultrasound is a bit of a misnomer as it is often just "pleural line" ultrasonography, only becoming "lung" ultrasonography when the lung becomes airless, either through compression by surrounding fluid or intra pulmonary pathology.
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