Lung ultrasonography is easy to learn, simple to perform, and has strong clinical utility for the critical care clinician. Interestingly, radiologists have not been instrumental in developing critical care applications of lung ultrasonography. Perhaps because lung ultrasonography in the intensive care unit (ICU) is a purely bedside technique, it required a frontline ICU clinician to develop the field. Dr. Daniel Lichtenstein is responsible for developing critical care lung ultrasonography. In the 1990s, he established the principles of the field and developed the semiology of lung ultrasonography that is in current use.1 Based on his original and continued work, in the past few years there have been numerous published studies from other groups, which have served to validate and expand the field. This section will review critical care applications of lung ultrasonography.
Basic Principles of Lung Ultrasonography
Air is the enemy of the ultrasonographer. There is a large difference in the acoustic impedance and velocity of ultrasound between tissue and air. This leads to complete reflection of the ultrasound wave at the first air–tissue interface. When combined with the unfavorable attenuation coefficient of air, this leads to a pattern of repeating horizontal lines consistent with a reverberation artifact or a homogeneous amorphic grayness that occupies the ultrasound screen deep to the tissue–air interface. This frustrates any attempt to scan through air to deeper body structures.
The alveolar lung parenchyma is normally filled with air; so well-aerated lung is not visible as a discreet structural entity with ultrasonography, as the ultrasound waves are blocked and reflected by air. When a disease process reduces the amount of air within the lung, ultrasound findings change in a predictable fashion. Atelectatic lung is airless so it appears as a discrete structure with tissue echogenicity. Likewise, lung that is consolidated from pneumonia appears as a well-defined hyperechoic structure. Lung that is edematous, though still aerated, has ultrasonographic findings that are different from normally aerated lung. One of the limitations of lung ultrasonography is that abnormalities that do not involve the pleural surface cannot be visualized, such as focal lesions surrounded by aerated lung. Fortunately, most lung processes that are of interest to the intensivist (e.g., pneumonia, hydrostatic pulmonary edema, lesional edema) extend to the periphery of the lung.
Lung ultrasonography may be performed with a wide variety of ultrasound machines with two-dimensional (2D) scanning capability. It was fully described using a machine manufactured in 1990. A 3.5–5.0 MHz transducer of convex sector design works well. Vascular transducers of higher frequency may also yield serviceable images, although the examination may be limited by a lack of penetration in the larger patient. A microconvex transducer has the advantage that it fits well between rib interspaces. As lung ultrasonography will generally be performed in the context of a whole body approach, many groups use a phased array cardiac transducer for general ...