Many clinicians are challenged when evaluating patients for perisplenic fluid as part of the FAST or RUSH examination. Here are some common problems and how to fix them.
Fix probe location
- Make sure you are holding the probe in a longitudinal view, probe marker towards the patient’s head. Place the probe just above the costal margin, in the posterior axillary line. The knuckles of your probe hand should be touching the stretcher
Start too high (too cephalad)
- Starting with the very posterior probe position described above, slide towards the patient’s head until you clearly see pleura and rib shadows. Once you’ve established clear evidence you are over the thorax, slide the probe toward the patient’s feet along the same posterior axillary line until the pleura ends. Now you have found the diaphragm! Scan just caudal to the end of the pleura and you should see the diaphragm and spleen.
- Another way to simplify this- If you see pleura, slide towards the feet. If you see bowel gas (or “nothing”), slide towards the head.
Use a slightly oblique approach
When rib shadows obscure the view, use the “sonographic rib spreader” technique.
Rotate the probe slightly towards the patient’s back so the probe is slightly more parallel to the ribs. Do not go fully transverse.
This exposes more of the probe to the interspace, yielding a larger window through which to view the spleen.
For more tips on viewing the spleen, check out this post.
The FAST exam is generally described as a trauma assessment (hence the acronym). But it is often used as a metanym to mean any assessment of the peritoneum for fluid. In fact when I was a resident folks would often say, “let’s FAST that gallbladder,” or “get the FAST machine so we can put that central line in.” And we didn’t have Twitter.
Anyway, here are a few cases where the “FAST” was used in a non-trauma patient to assess the peritoneum:
Cirrhotic with abdominal pain and tenderness:
who was found to have ascites, and spontaneous bacterial peritonitis
Lower abdominal pain in pregnancy:
who was found to have hemoperitoneum from a ruptured ectopic pregnancy
Diffuse abdominal tenderness in a healthy ten-year-old:
who was found to have idiopathic seromas of the peritoneum, pleura, and pericardium!
Shortness of breath and abdominal distension:
which turned out to be massive abdominal abscesses
Diffuse abdominal tenderness and distension after hysteroscopy:
which was complicated by a bowel perforation; hence fecal material throughout the peritoneum
Take home points:
- Assessment of the peritoneum greatly aids medical and surgical diagnoses
- Fluid appears black (anechoic) on ultrasound. Very difficult to tell what TYPE of fluid by appearance alone
- Your clinical assessment must guide the differential diagnosis for your ultrasound findings
Ultrasound is quite sensitive in detecting even very small pleural effusions; it has been demonstrated to perform better than chest x-ray and nearly as well as CT scan. In order to assess for pleural fluid, the transducer should be directed through the liver (Right side) or spleen (Left side) and diaphragm. In a normal thorax, a mirror image artifact will generally be seen above the diaphragm. When effusion is present, fluid eradicates this artifact, creating an anechoic appearance in the costophrenic angle.
The image above demonstrates a common pitfall in abdominal and thoracic ultrasound. The liver is visible in the near field, and a dark anechoic structure is evident just deep to the liver. Some see this fluid and may note a positive FAST examination or free intraperitoneal fluid. Others may see this appearance and diagnose pleural effusion or hemothorax. While it is true the anechoic area represents fluid, there is a more correct response.
The inferior vena cava can generally be seen posterior to the liver, towards the patient midline. As it is filled with blood it will appear anechoic. below the diaphragm it will course parallel and to the [patient’s] right of the Aorta. Just above the diaphragm it will quickly merge into the Right Atrium.
As with most scanning, fanning through multiple planes will generally sort out the true anatomy. In the clip below we see the IVC as the operator sweeps medially, and the the pleural effusion is more evident in the lateral portions of the sweep. One (of many) giveaways is that the hepatic veins drain into the IVC, and even in this brief sweep through the IVC a hepatic vein is visible anteriorly, draining into the IVC.
Pleural effusion and mimic from Sinai EM Ultrasound on Vimeo.
One common source of confusion or false positives in the FAST exam is the assessment of the left upper quadrant. We’ve already covered some tips on improving your view of the spleen. This post will illustrate how the stomach can mimic free fluid to the unprepared.
The first image shows a perisplenic (left upper quadrant) view on ultrasound. Note the spleen (S), kidney (K), and diaphragm (D).
As the operator fans anterior and posterior to assess for fluid, anechoic fluid with some dirty shadows and slightly irregular margins is seen (bounded by arrows). This structure is often visualized anterior and medial to the spleen, and represents fluid within the stomach.
The video better illustrates fanning through a sagittal plane and encountering this common artifact.
Scan through the area carefully to ensure this fluid is all accounted for within the confines of the stomach, and does not layer out around the kidney, spleen, or highlight bowel loops at its margins.
The FAST exam is the prototypical application of emergency ultrasound. However, it is important to know that there are limited randomized controlled trials assessing the utility of the FAST exam. Despite this, let’s look at three good articles that all emergency residents should know.
1. In this Cochrane review, the authors’ conclusion was that there was insufficient evidence for the use of ultrasound-based clinical pathways in the initial diagnostic workup of patients with blunt abdominal trauma. Ultrasound was not sensitive and lacked diagnostic accuracy. However, the use of ultrasound did reduce the use of CT scans. The take-home message is that ultrasound should not be used as a single rule-out test for significant intra-abdominal injury and the ED resident should be aware of ultrasound’s limitations.
Stengel D, Bauwens K, Sehouli J, Rademacher G, Mutze S, Ekkernkamp A, Porzsolt F. Emergency ultrasound-based algorithms for diagnosing blunt abdominal trauma (Review). The Cochrane Collaboration. February 18th, 2008.
2. Interestingly, this review used the same methodology as the Cochrane review and found that an adequately performed FAST exam can predict the need to send a patient to the operating room, with a high degree of sensitivity (98.9%) and specificity (98.1%).
Melniker LA. The value of focused assessment with sonography in trauma examination for the need for operative intervention in blunt torso trauma: a rebuttal to “emergency ultrasound-based algorithms for diagnosing blunt abdominal trauma (review)”, from the Cochrane Collaboration. Critical Ultrasound Journal. 2009;1:73-84.
3. Thoracic ultrasound has become part of the standard assessment of the trauma patient. Ultrasound has been shown to be much more sensitive than CXR in the detection of pneumothorax when compared to CT as the gold standard. There are many studies proving this point and this is a good example.
Blaivas M, Lyon M, Duggal S. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med. Sep 2005;12(9):844-849