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 beside 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 published a series of landmark articles that defined the important features of the field. He also developed the semiology of lung ultrasonography that is in current use. Based on his original 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.
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 a reflection of the ultrasound wave at any air–tissue interface. When combined with the unfavorable attentuation coefficient of air, this leads to the homogeneous amorphic grayness that occupies the ultrasound screen deep to a tissue–air interface. This frustrates any attempt to scan through air to deeper body structures.
The normal alveolar lung parenchyma is filled with air, so that lung is not visible as a discreet structural entity with ultrasonography. Air blocks the visualization of normally aerated lung. When a disease process reduces the amount of air within the lung, ultrasound findings change in a predictable fashion. Atelectatic lung is airless, and appears as a discrete structure with tissue density. 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 are surrounded by aerated lung cannot be visualized. 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. The effect that fluid accumulation has on ultrasonographic findings is summarized in Figure 22.1. Lung ultrasonography demonstrates a spectrum of patterns that depends on the ratio of air to fluid: from a normal aeration pattern to alveolar/interstitial edema and finally to a tissue density pattern. Each of these findings may have major implications for the management of the critically ill patient.
Pneumothorax and normal aeration pattern yields A line pattern (image 1) with a high air/fluid ratio. Lung edema cause B lines with a moderate air/fluid ratio (image 2). Pleural fluid and alveolar consolidation (seen together in image 3) with a very low air/fluid ratio. The amount of fluid relative to air results in the characteristic ultrasound patterns of thoracic ultrasonography.