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. Read Dr. Butts' past columns at http://bit.ly/ButtsSpeedofSound.
The RADiUS protocol makes the evaluation of acute dyspnea straightforward by using an approach that evaluates the heart, the IVC, the pleura, and the lung parenchyma. Diagnosing pneumothorax can easily be assessed with high sensitivity by evaluating the pleura. Combing ultrasound findings with the overall clinical picture can identify other diagnoses such as pneumonia, pulmonary edema, and acute respiratory distress syndrome.
A high-frequency transducer, such as a linear array, is well suited to evaluate the pleura secondary to its high resolution. The motion of the pleura can be seen with a low-frequency transducer, but it lacks sufficient resolution to identify more subtle findings. Each side of the chest should be examined in a stepwise fashion, typically proceeding from the anterior chest near the mid-clavicular line to the posterior axillary line. The posterior lung fields should be evaluated as well if time and patient stability allows.
The normal pleura will appear as a hyperechoic (white) horizontal line deep to the musculature of the chest wall. (Image 1.) Orienting the transducer with the indicator pointing toward the patient's head will reveal the ribs in cross-section, making them a useful orientation point in identifying the pleura line.
The presence or absence of a pneumothorax should be the first goal of this section of the RADiUS protocol. I addressed the “sliding” motion of the pleura previously; a detailed explanation of this phenomenon can be found at http://bit.ly/PjVTNU. The lack of lung “sliding” and comet tails are reliable indicators of a possible pneumothorax, but the presence of a lung point adds to the sensitivity of these findings. The lung point represents the spot at which the normal sliding pleura meets a pneumothorax. This can best be visualized using M-mode. (Read about M-mode at http://bit.ly/GD7kLV.) The normal lung will exhibit the “seashore sign” until it abruptly changes to the “barcode sign,” indicating the beginning of a pneumothorax.
The parenchyma of the lung can be assessed as well, although not as thoroughly as with a lower frequency transducer. A-lines, which are reverberation artifacts created by normal, air-filled lungs, can be helpful in ruling out disease processes that result in interstitial edema, such as pulmonary edema or ARDS. (Image 2.) B-lines, which are dense, hyperechoic vertical lines that emanate from the pleura deep into the lung parenchyma, are associated with disease processes that result in interstitial edema. (Image 3.) Based on the distribution of the B-lines (bilateral or unilateral) and the clinical picture, a more definitive diagnosis can be established.
Next month: Evaluating the lung parenchyma as part of the RADiUS protocol.
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