A 45-year-old man presented to the emergency department with a stab wound to the chest. His vital signs were stable, and he had a large laceration to the left upper chest, near his shoulder. His breath sounds were diminished bilaterally. A bedside ultrasound over the anterior left chest wall was performed. (Image 1.) What is the diagnosis?
Answer: A large amount of subcutaneous air.
Placing the ultrasound over the anterior chest wall revealed what initially appeared to be a nonsliding pleural line, which was concerning for a possible pneumothorax given the patient's clinical history. A closer inspection, however, revealed no visible rib shadows, which are an important landmark for ensuring that the line visualized is, in fact, the pleural line. Movement of the transducer slightly inferiorly revealed the ribs and the true pleural line, which was sliding back and forth normally. An upright chest x-ray confirmed a large amount of subcutaneous air.
Ultrasound is very sensitive for picking up even a small pneumothorax. When a patient is lying on his back, air will rise to the highest point (the anterior chest wall), which is not well visualized by supine chest x-ray.
Normally, nothing separates the parietal and visceral pleural layers except for a small amount of pleural fluid, enabling ultrasound to visualize them sliding back and forth against each other as the patient breathes in and out. This is known as the slide sign. When air fills the pleural space, however, the visceral pleura becomes obscured, and only the parietal pleura is seen as a static, nonsliding layer. Use of M-mode in this case may reveal a bar code sign.
Subcutaneous air, particularly a large amount of it, can appear sheet-like and resemble a pleural layer, however. Because it will not slide, it will appear similar to a static parietal pleural layer and can be confused with a pneumothorax. Looking for the rib shadows will help confirm that you are looking at the pleural line. (Image 2.) Always take that extra step to make sure you don't misdiagnose a pneumothorax.
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Dr. Buttsis the director of the division of emergency ultrasound and a clinical associate professor of emergency medicine at Louisiana State University at New Orleans. Follow her on Twitter@EMNSpeedofSound, and read her past columns athttp://bit.ly/EMN-SpeedofSound.