The FAST exam can be a tricky thing. Identifying free fluid should be fairly straightforward in theory, but a number of pitfalls can confuse even the most experienced examiner. One of the areas most fraught with error is the pelvis, particularly when it comes to whether the bladder is full or empty. The presence of fluid in the bladder can make or break the ability to pick up small fluid collections and distinguish free fluid from normal anatomy.
A normal pelvis contains air-filled loops of bowel, and ultrasound waves are frequently scattered in this area, resulting in poor image resolution and small amounts of free fluid being obscured. The bladder offers a perfect acoustic window into the depths of the pelvis because it allows sound waves to pass deeper into the pelvis without being scattered. Image 1 shows the bladder as a small, triangular-shaped object in a sagittal view.
Free fluid can typically be seen collecting in multiple areas, including deep to, anterior to, and lateral to the bladder. Image 1 presents only a hazy gray area around the bladder, initially suspected to be air-filled bowel loops. This patient was evaluated again after the initial negative-appearing FAST exam because he continued to complain of abdominal pain. A repeat image shows a clearer view of the bladder because it is more distended with urine. (Image 2.)
Now that it is providing a greater acoustic window, a well-defined free fluid collection can be seen posterior, or deep to, the bladder. Incidentally, a CT scan done at the time of the original FAST was read as positive for a small amount of free fluid in the pelvis. The examiner had greater difficulty recognizing this finding because the bladder was not full.
A full bladder also assists the examiner in identifying free fluid. Fluid collections may sometimes resemble the bladder if they appear regular in shape. This pitfall frequently occurs in the transverse view, especially when the bladder is small. It can easily be overlooked in a quick exam, causing the sonographer to misidentify the free fluid. The bladder is often recognized as separate from the other fluid collection, identifying the abnormality when the orientation is changed. (Image 3.)
This error can be minimized by evaluating patients when they are most likely to have a full bladder (prior to catheter placement) and by scanning the pelvis in the transverse and sagittal planes.
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