Pardon the Interference [Interactive Ultrasound Case Study]

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This seemingly routine ultrasound revealed a surprise finding

One of your first patients on the night shift is an elderly lady with RLQ pain. She states she was fine last night and this morning until about an hour after breakfast when she developed gradually worsening generalized abdominal pain followed by multiple episodes of non-bloody, non-bilious vomiting. The vomiting has continued, with a total of more than six episodes and the pain has continued to get worse, especially in the RLQ. She denies any fever or diarrhea and has no other complaints. Her past medical history is notable for hypertension and diabetes but no prior abdominal surgeries. This puts appendicitis in front of bowel obstruction on your differential.

On exam her vital signs are normal and she appears in no acute distress. Heart and lung exam is normal but on abdominal exam she is tender throughout but especially in both the RUQ and RLQ with a positive Murphy’s sign. Cholecystitis nudges above appendicitis in your differential. You feel for her aorta and note its pulsations; though she is pretty thin you think it feels enlarged. You worry about getting her imaged rapidly to find out for sure.


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Returning to your desk and logging on to the computer you note that protocol labs sent by the nurse are already resulted and show a WBC count of 4.3 with mildly elevated bilirubin and transaminases. Probably the gall bladder after all you think to yourself. You know she will need formal imaging but you decide to wheel over the ED bedside ultrasound machine in order to take an early look at both her aorta and gallbladder and maybe even see if you can find the appendix. You will use this information to decide if you should order a CT scan, a formal ultrasound or call the surgeon early if it is her aorta that seems to be the cause of the problem.

As you obtain the following image below (slide right for answers) while trying to find the aorta you receive a simultaneous critical value call from the lab for a band count of 21. What diagnosis should you suspect and what should your next move be?

You hopefully figured out that the ultrasound image does not show the aorta. What it does show is reverberation artifact, repeating bands of white and dark from reflection of the ultrasound beam back and forth at a large tissue/air interface. The white lines labeled “reverb artifact” are duplicated images of the peritoneal lining. The air just behind it looks black but also reflects the ultrasound beam rather than letting it reach deeper structures. It’s similar to the repeated reflection of yourself that you would see if you were standing between two parallel mirrors. To ultrasound, water acts like a window but air like a mirror. The clinical significance of this finding is that there is a lot of air blocking your view. With this degree of artifact continuing to look for the aorta will likely prove difficult or fruitless. You should instead immediately assume that the patient has a perforated viscus causing free air to leak into the peritoneal cavity.


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Your next steps include ordering an upright chest x-ray to confirm the free air, ordering broad spectrum antibiotics and IV fluid to hold back the tide of sepsis, and calling an emergent general surgical consult. Your chest x-ray confirms a large amount of free air, your antibiotics are started and your surgical consult of course asks… wait for it… “What does the CT show?” You reluctantly agree to order the CT, knowing that it may delay things, as long as the surgeon agrees to come right in to see the patient. You make sure to have your charge nurse bump your patient to the front of the CT list.

Your patient remains stable. The CT shows a large amount of both free air and free fluid but no obvious source. Your surgical colleague does take the patient to the OR STAT where a perforated gastric ulcer is diagnosed. Your patient does well post-operatively thanks to your expedited care, and with the help of point of care ultrasound (POCUS) of course.

Pearls & Pitfalls for Diagnosing Free Air On Abdominal Ultrasound

  1. Large Free Air Causes Reverb Artifact: Reverberation artifact, repeating bands of white and dark from reflection of the ultrasound beam back and forth between the probe and the peritoneal stripe, occurs at a large tissue/air interface. It’s kind of like the repeated reflection of yourself that you would see if you were standing between two parallel mirrors. The best place to look for this is epigastric or RUQ. The significance of this is that there is a lot of air blocking your view. With this degree of artifact continuing to look for the aorta will likely prove difficult or fruitless. You should instead immediately assume that the patient has a perforated viscus causing free air to leak into the peritoneal cavity. Sensitivity may be increased with the head of bed elevated to 10-20 degrees.
  2. Small Air Bubbles Cause Echogenic Foci with Comet Tail Artifact: This may be best seen withing ascitic fluid or between the right hemidiaphragm and the liver. Sensitivity can be increased by turning the patient into the left lateral decubitus position with the head of bed slightly elevated so that air bubbles rise to the right side above the liver. Normal air pockets within the intestinal lumen should move with bowel peristalsis.
  3. Consider Switching to the Linear Probe: When imaging the abdomen one usually uses the curvilinear probe. When free air is suspected, switching to a linear probe may improve resolution as air will usually rise to the near field in the supine patient.
  4. Pneumoretroperitoneum: If perforation occurs in retroperitoneal structures one may see air around the duodenum, head of the pancreas, or around the kidney. In addition, retroperitoneal vessels (aorta and IVC) may be difficult to visualize.
  5. Practice, Practice, Practice: The best way to minimize errors is through experience, so scan lots of normal anatomy. The more scans you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is. Our Soundings library at epmonthly.com is a great way to start.

REFERENCE


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  1. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3214511/

ABOUT THE AUTHOR

EMERGENCY ULTRASOUND SECTION EDITOR Dr. Pregerson manages a free online EM Ultrasound Image Library. He is the author of the Emergency Medicine 1-Minute Consult Pocketbook and the A to Z Pocket Emergency Pharmacopoeia & Antibiotic Guide (available at EMresource.org) and the Tarascon Emergency Department Quick Reference Guide (Tarascon.com).  

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