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The Speed of Sound: Ultrasound Beats Other Imaging in Diagnosing Pneumonia

Butts, Christine, MD

doi: 10.1097/01.EEM.0000535022.70944.ab
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 on Twitter @EMNSpeedofSound, and read her past columns at http://bit.ly/EMN-SpeedofSound.

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I was introduced (on Twitter, of course) to this saying recently: Why just wonder when you can know? Meaning why speculate about a diagnosis when you can know for sure with ultrasound?

Chest x-rays are known to have significant deficiencies in diagnosing pneumonia. It's accepted that the radiographic appearance of infection lags behind the onset of symptoms, sometimes by several days. Of course, we evaluate the history and physical exam to determine whether the patient's symptoms represent early pneumonia or some other process. This approach works well most of the time, but it can be helpful in certain cases to know that pneumonia is present for sure. Patients at extremes of age or who are immunocompromised are at higher risk of pneumonia. Even healthy patients can go from relatively well-appearing to death's door in a matter of days in cases of post-influenza pneumonia. Establishing a definitive diagnosis of pneumonia can obviate the need for further testing (avoid that roll of the D-dimer dice) even in healthy patients. Differentiating pneumonia from viral syndromes can also separate those who will require antibiotics from those who will improve simply with supportive care.

Ultrasound has shown promise as a superior test compared with chest x-ray in identifying pneumonia. A 2012 international consensus statement found strong evidence supporting ultrasound use in a variety of thoracic conditions and in evaluating suspected consolidation in children and adults. (Intensive Care Med 2012;38[4]:577.) Ultrasound also seems to have an advantage over chest x-ray and the gold standard of CT in diagnosing these conditions.

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Ultrasound of the lungs should proceed in an organized fashion to avoid missing subtle findings. The examiner should use a low-frequency transducer (such as a cardiac transducer) to evaluate the upper and lower anterior chest wall, followed by the upper and lower lateral chest wall. This evaluation should be followed by a similar evaluation of the opposite side. A depth of at least 16 cm should be set to capture the lung parenchyma adequately.

A normal air-filled lung appears as a hazy, hypoechoic (gray) field deep to the hyperechoic (white) pleural line. (Image 1; video available at http://bit.ly/VideosSound.) Pneumonia is suspected by several ultrasound findings. As fluid replaces the normal air-filled lung, the lung will begin to resemble the liver, a finding known as hepatization or the tissue sign. The border between this area and the air-filled lung will often be irregular (the shred sign) with visible adjacent B-lines. (Image 2; video available at http://bit.ly/VideosSound.) A dynamic air bronchogram indicates linear air-filled areas that move with respiration, and is highly specific for pneumonia. (Image 2.)

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