The Speed of Sound
Last month's column introduced using ultrasound to differentiate rapidly between the causes of acute or severe dyspnea by using the RADiUS protocol. (Ultrasound Clinics 2011:6;261.) The algorithmic concept is similar to the RUSH exam in undifferentiated hypotensive patients or the FAST exam in trauma patients. The RADiUS algorithm utilizes a focused cardiac exam and assessments of IVC, pleura, and lung parenchyma to identify and rule out serious causes of undifferentiated dyspnea.
This month we delve into the first part of the RADiUS exam — a focused cardiac exam to identify pericardial effusion, impaired left ventricular function, and evidence of pulmonary embolism. All of these conditions represent life-threatening causes of acute dyspnea that can be rapidly diagnosed with bedside ultrasound.
A bedside cardiac scan typically includes the parasternal long and short, apical, and subxiphoid views. Variations in patient anatomy, habitus, and clinical condition may keep you from getting certain views (e.g., patient may not tolerate lying flat). Thankfully, pathology can be evaluated in any of the views. (See FastLinks for how to access detailed descriptions of each view.)
Pericardial effusion can be visualized in any of the four traditional cardiac views. It typically appears as an anechoic (black) area separating the myocardium from the bright stripe of pericardium. (Image 1.) Identifying the presence of an effusion can be helpful to identify the cause of a patient's dyspnea, and identifying signs of tamponade can be life-saving. Classically, in tamponade the right atrium is seen to collapse during systole and the right ventricle during diastole. (See FastLinks for how to access a video demonstrating this.) Identifying these findings may be difficult, especially in a tachycardic patient, and a diagnosis of tamponade should be strongly considered in patients with an effusion who are either in distress or hypotensive.
Evaluating the contractility of the left ventricle may seem intimidating, but can be quickly evaluated and can be helpful in assessing the dyspneic patient. Again, any of the four classic views can be used, although the parasternal long and short axis views are often the easiest. Quantifying the contractility of the left ventricle (LV) can be done a number of ways, but a quick visual assessment is usually sufficient. Does the LV have good squeeze (the walls are far apart then nearly touch over a cardiac cycle), poor squeeze (the walls barely move), or something in between? (See FastLinks for how to access videos demonstrating this.)
A hemodynamically significant PE can cause acute dilation of the right ventricle (RV), normally about a third the size of the left ventricle. This appears on ultrasound as an RV size approaching or even exceeding the LV size, and is best seen on the apical or subxiphoid views. (Image 2.) Other signs of acute PE are septal bowing as the RV pushes the septum toward the LV. (See FastLinks for how to access a video demonstrating this.)
We will break down each of the components of RADiUS over the next few months, and examine how to perform and interpret each section of the protocol. Next month: IVC.
BONUS! See One, Do One: Detailed descriptions of the parasternal long and short, apical, and subxiphoid views will be available, along with additional images and videos, on March 5 in the EMN iPad app and on March 12 in the Breaking News blog on www.EM-News.com, where the EMN app can also be downloaded for free.
- Read Dr. Butts' past columns at http://bit.ly/ButtsSpeedofSound.
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