Emergency Medicine News:
The Speed of Sound
Dr. Butts is the director of the division of emergency ultrasound and a clinical assistant professor of emergency medicine at Louisiana State University at New Orleans. Follow her @EMNSpeedofSound, and read her past columns at http://bit.ly/ButtsSpeedofSound.
The lungs have traditionally been thought of as difficult to assess because they are filled with air that conducts sound waves poorly. The beginning sonographer typically gets frustrated by the hazy interference of the lungs when attempting to evaluate the heart, but that cloudiness can actually convey valuable information about the cause of a patient's shortness of breath.
The RADiUS protocol (rapid assessment of dyspnea with ultrasound) utilizes an algorithmic approach for evaluating patients with dyspnea, with the examiner evaluates the heart, IVC, pleura, and lung parenchyma in a stepwise fashion.
A low-frequency transducer should be used when evaluating the lung parenchyma to obtain maximum penetration. A phased array transducer is ideal; its small shape allows it to image between the ribs, minimizing rib shadowing artifact. The lungs should be evaluated across the anterior and lateral chest, continuing onto the posterior aspect if patient stability and time permits. The exam should then be continued on the contralateral side.
The physician examining the acutely dyspneic patient should seek to identify the presence of one of two artifacts. Identifying the presence of A-lines or B-lines allows the clinician to assess quickly for several important causes of shortness of breath.
A-lines are artifacts created by the reverberation of the pleural line into the air-filled lung. They are seen as hyperechoic (white) horizontal lines emanating deep into the lung. (Image 1.) The presence of A-lines throughout the lung fields indicates normal lung parenchyma or the presence of a nonedematous condition, such as COPD or asthma.
B-lines are reverberation artifacts created in the presence of edema within the interstitium of the lung. They are seen as vertical, hyperechoic lines emanating from the pleura and appearing to move back and forth with respiration. (Image 2.) Typically, they are dense enough that they obscure the underlying A-lines. The presence of bilateral B-lines is associated with the alveolar interstitial syndrome, seen in conditions such as pulmonary edema and acute respiratory distress syndrome. Unilateral B-lines are consistent with more focal processes such as pneumonia or pulmonary contusion.
The costophrenic recesses can be assessed for the presence of pleural effusion as a final step in evaluating the lung parenchyma. Small amounts of fluid can be identified in these recesses by a quick assessment with the low-frequency transducer. Utilizing the RUQ and LUQ views of the FAST exam, the transducer is shifted toward the patient's head to reveal the diaphragm. Fluid will appear as an anechoic (black) collection in the space superior to the diaphragm. (Image 3.)
Next month, we will put the RADiUS protocol to use in evaluating a patient with acute dyspnea.
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