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.
The RADiUS protocol, an algorithm that integrates aspects of bedside ultrasound for assessing patients with undifferentiated dyspnea, evaluates the heart, inferior vena cava, pleura, and lung parenchyma to focus a broad differential. Last month we explored focused cardiac ultrasound as part of the RADiUS protocol (http://bit.ly/1hGTTsW), and this month we focus on the inferior vena cava and its place in evaluating shortness of breath.
Ultrasound of the inferior vena cava (IVC) has been a hot topic over the past few months, with disagreement about how it should be interpreted. Size and collapsibility with respiration are generally agreed to correlate with central venous pressure (CVP), but this does not always exactly predict a patient's overall fluid status.
The IVC can typically be viewed from the subxiphoid area using a low-frequency transducer. Pointing the indicator toward the patient's head, the transducer should be placed in the midline and slowly moved toward the patient's right until the vessel is seen. (Image 1.) Angling the transducer cephalad or up toward the patient's chest will allow the IVC to be seen emptying into the right atrium. (Image 2.) Identifying this transition is helpful to avoid mistaking the aorta for the IVC. These structures may lie very close together in some patients, and a plethoric IVC may be difficult to differentiate from the aorta.
The IVC, once identified, should be evaluated for its overall size and change with respiration. Locating the hepatic vein will provide a landmark for performing this assessment. Utilizing M-mode allows for more precise measurements. (http://bit.ly/GD7kLV.) M-mode should be allowed to run long enough to capture a respiratory cycle so the maximum size during expiration and the minimum size during inspiration can be measured. (Image 3.) These measurements can then be used to calculate the percentage of collapse. A size of 2 cm can be used as a midpoint, with vessels larger than this having a higher CVP and vessels smaller than that having a lower CVP. Once size has been determined, the variability helps to assess CVP further. Vessels that have greater than 50% collapse correlate with a lower CVP. An IVC measuring 1 cm with greater than 50%, for example, would correlate with a CVP of 0-5 mm Hg.
The assessment of the IVC should be viewed in light of the other findings of the RADiUS exam. A finding of a large and poorly collapsing IVC indicates a high CVP, which combined with a finding of poor overall ventricular contractility would appear to support a diagnosis of heart failure with volume overload. Conditions such as mitral valve dysfunction, however, may elevate the CVP in euvolemic patients.
The IVC is used to estimate volume status in the RADiUS algorithm, but other methods such as Doppler evaluation of the brachial artery have shown promise as well. (http://bit.ly/NO2O1h.)
Next month: Assessment of the pleura as part of the RADiUS algorithm.
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