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The Speed of Sound

The Speed of Sound: A Great Pretender of a Positive FAST Exam

Butts, Christine MD

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doi: 10.1097/01.EEM.0000428933.80782.c9
    Figure. I
    Figure. I:
    mage 2. Free fluid surrounding the spleen. The free fluid (FF) is anechoic (black), and can be seen between the spleen (S) and diaphragm (arrowhead), around the tip of the spleen, and between the kidney (K) and spleen.

    A 35-year-old man is brought to the ED after a motor vehicle crash. He is lethargic and moaning. His blood pressure is 80/50 mm Hg, his heart rate is 120 bpm, and his respiratory rate is 30 bpm.

    He has some lacerations to his scalp without active bleeding. Breath sounds are decreased, but no deformity to the chest wall is seen. His abdomen is soft, but the patient groans with palpation. A FAST exam is performed.

    The FAST (Focused Assessment with Sonography for Trauma) has become a standard tool for emergency physicians to evaluate unstable blunt trauma patients. Done correctly, the FAST exam can reveal free fluid in the peritoneal space and guide patients quickly to laparotomy. A pericardial effusion can also be quickly discovered, allowing emergent intervention. The FAST exam is highly sensitive and can be life-saving in these critically ill patients.

    Most EPs have some familiarity with the FAST exam, and many use it routinely. Most operators, even those with minimal experience, can quickly identify the classic findings of abdominal free fluid and pericardial effusion. Some normal findings, however, may mimic the appearance of free fluid, causing confusion and misdiagnosis. This patient's case demonstrates evaluation of the left upper quadrant.

    Figure. I
    Figure. I:
    mage 1. Normal left upper quadrant with the diaphragm (arrowhead), spleen (S), and kidney (K).
    Figure. I
    Figure. I:
    mage 3. Image of a full stomach within the left upper quadrant. Note that the stomach (ST) is very posterior, lying just anterior to the vertebral bodies (V). It also appears rounded in shape, versus either a stripe of free fluid or a more amorphous larger collection.

    The diaphragm, spleen, and kidney can be seen when viewed in the sagittal axis (indicator toward the patient's head) in the normal left upper quadrant view. (Image 1.) These structures typically lie in close orientation to each other, with very little in between. A small amount of perinephric fat may surround the kidney, but this will typically appear hypoechoic (gray).

    Free fluid will appear anechoic (black), and it tends to collect between the diaphragm and spleen or between the spleen and kidney. Small amounts of fluid may resemble a stripe or thin line while larger amounts may appear more amorphous. (Image 2.)

    A full stomach in the left upper quadrant view may appear as a large, fluid-filled space adjacent to the spleen. (Image 3.) It can be misinterpreted as a collection of free fluid without careful evaluation. A full stomach will appear as a rounded collection of fluid deep to the spleen. It may appear at first glance similar to free fluid collections, but careful evaluation reveals that the position is much deeper in the body. The stomach can be seen to lie up against the vertebral bodies, behind the spleen. Particulate matter, which may be hyperechoic (white), may appear to float within this area.

    Decompressing the stomach with an orogastric or nasogastric tube may prove helpful in making the diagnosis when doubtful about whether a fluid collection represents free fluid or the stomach.

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