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.
A 30-year-old man is brought to the ED after a motor vehicle crash. His GCS was 3 on scene, he was intubated by paramedics, and was unresponsive on arrival. Vital signs are heart rate of 120 bpm, blood pressure of 100/70 mm Hg, and SpO2 at 90%. Breath sounds are decreased to the left chest, but supine chest x-ray appears normal.
Supine chest x-rays have been shown to be significantly insensitive in diagnosing pneumothorax. CT scan is the gold standard, but may not be immediately available in all centers or prudent given a patient's hemodynamic status. Ultrasound has been demonstrated to be markedly better at diagnosing pneumothorax than supine chest x-ray in multiple head-to-head studies. A meta-analysis found that supine chest x-ray had a pooled sensitivity of 52% and a specificity of 99%. (Chest 2011;140:859.) Ultrasound in comparison had a pooled sensitivity of 88% and a specificity of 100%.
Ultrasound of the pleura for pneumothorax primarily looks for two important findings: the presence of lung “sliding” and of comet tail artifacts.
The visceral and parietal pleural layers should “slide” against each other with each breath when they are in approximation. This can be visualized easily with a high-frequency transducer placed in the mid-clavicular line at approximately the second to third intercostal space. Pointing the indicator toward the patient's head (rather than the patient's right) will yield the view shown in Image 1. The higher frequency yields excellent resolution of the chest wall, and allows the sonographer to identify the pleura and surrounding structures quickly. Evaluating the area throughout the respiratory cycle should demonstrate a back-an-forth motion of this area. (Video 1; see FastLinks.) This indicates that the pleural layers are still in opposition to each other. Absence of sliding may indicate the presence of pneumothorax. (??Video 2; see FastLinks.) M-mode can be used to evaluate this sliding motion further. (Images 1 and 2.) (See FastLinks for article about M-mode by Dr. Butts.)
A “comet tail” artifact is created by the visceral pleura, and is seen as short, hyperechoic vertical lines that arise from the pleura. (Image 1.) They tend to appear and disappear as the pleura slides back and forth. (Video 1.) The comet tails will be obscured when air disrupts the normal interface between the visceral and parietal pleura.
The absence of lung sliding has been identified as a sensitive marker for pneumothorax, but certain conditions such as the presence of a bleb or a patient with previous pleurodesis may have lack of sliding even in the absence of a pneumothorax. Assessing for the absence of comet tails and sliding increases the sensitivity of the exam for pneumothorax, as demonstrated by Lichtenstein et al. (Intensive Care Med 1999;25:383.)
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Questions? We Have Answers
Have an ultrasound question? A topic you'd like to see in future Speed of Sound columns? Send your questions and suggestions for Dr. Butts to firstname.lastname@example.org.
Watch two videos that demonstrate sliding of the pleura and comet tail artifacts in EMN's iPad app on Dec. 5 and on www.EM-News.com on Dec. 12.
* Read Dr. Butts' article on M-mode at http://bit.ly/GD7kLV.
* Read Dr. Butts' past columns at http://bit.ly/ButtsSpeedofSound.
* Comments about this article? Write to EMN at email@example.com.