Mount Sinai Emergency Medicine Ultrasound

Winfocus 2010 at Policlinico Gemelli

WINFOCUS held the 6th world congress on Ultrasound in Emergency and Critical Care at the Policlinico Gemelli in Rome, Italy this year.  Sinai was represented by Jim Tsung and Phil Andrus.  The excitement included sessions devoted to development of consensus on point of care ultrasound for: vascular access and thoracic ultrasound.  There was an also an emphasis on the use of ultrasound in resuscitation – HCMC’s Rob Reardon led a session on integration of ultrasound into Adult resucitation and Sinai’s Jim Tsung led the counterpart Pediatric session.  The conference wrapped up with a very interesting research forum and an invitation to the next world congress in New Delhi, India.

Diagnosis?

What trip to Rome would be complete without some b-roll pics to supplement future EUS presentations?  A colleague suggested we obtain the following image.

Can you guess the diagnosis?

Finally, our days were long at Gemelli, but we managed to get out a bit.  We ate at Pier Luigi the first night.  Saw U2 at the stadio olimpico on the second night.  On the last day, Jim and I made it to Trastevere to meet up with friends at a Da Carlone for Cacio e Pepe and a Carbonara.  Our only sightseeing for the trip was the walk home after dinner the night before we flew back.  Arrivaderci, Roma.

Winfocus 2010 End of Conference

Crossing the Tiber

Dr. Tsung at the Trevi Fountain

While an uncommon condition, a leaking abdominal aortic aneurysm (AAA) is deadly if not recognized quickly.

How good is the physical examination for excluding AAA? Even if the physical exam is specifically directed to look for AAA, the sensitivity is still not good enough to rule out its presence (Sensitivity of 29% for AAAs of 3.0 to 3.9 cm, to 50% for AAAs of 4.0 to 4.9 cm, and 76% for AAAs of 5.0 cm or greater in diameter). So go pick up that ultrasound probe to look for it.

Lederle, F. A. et al. The rational clinical examination. Does this patient have abdominal aortic aneurysm? JAMA. 1999;281(1), 77.

Can ultrasound define the diameter of a AAA as accurately as a CT scan? Pretty close, though ultrasound will probably underestimate the diameter. Where it matters (at and around the bifurcation with a longitudinal view), ultrasound can come within 1cm of the CT diameter 95% of the time.

Knaut, A. L.et al. Ultrasonographic measurement of aortic diameter by emergency physicians approximates results obtained by computed tomography. The Journal of Emergency Medicine, 2005; 28(2), 119-26.

Finally, how good are emergency physicians in detecting AAAs? Pretty good it seems, with 100% sensitivity (95% CI 1⁄4 89.5 to 100), 98% specificity (95% CI 1⁄4 92.8 to 99.8), 93% positive predictive value (27/29), and 100% negative predictive value (96/96).

Tayal VS, et al. Prospective study of accuracy and outcome of emergency ultrasound for abdominal aortic aneurysm over two years. Acad Emerg Med. 2003;10:867-871.

This is a longitudinal view of trachea, the air-mucosa interface just beneath the tracheal wall. What are the structures “A” and “B”?

Answers:

• A – Reverberation artifact from air-mucosal interface
• B – Mirror image of cricoid cartilage.

Reflection at the tissue interfaces occur when there is a difference in acoustic impedance between two tissues. The greater the difference, the stronger the reflection, the brighter the image.

A . A transmitted ultrasound beam hits the air-mucosa interface and is reflected back to the transducer (1^st reflection). Based on the time taken for the reflected beam to return (assuming a constant speed of 1540 m/s), the machine calculated the distance this 1^st image is away from the transducer (at around 1.15cm) and registers it. The skin-transducer interface itself also results in the 1^st reflected beam being partially reflected back into the air-mucosa interface, which again gets reflected back to the transducer as a 2^nd reflection. This 2^nd reflection takes twice the time compared to the first; therefore the machine (assuming all beams travel only once to and from an object) registers a 2^nd image, the reverberation artifact, at twice the depth (around 2.3cm in this case). Lichtenstein called these artifacts “A” lines when they arise from the pleura.

B. A similar explanation accounts for the mirror image of the cricoid cartilage below the air-mucosa interface, only that the 2^nd reflection occurs at the cartilage-soft tissue interface.

What’s the difference between the two? The reproduction of tissue interfaces is called reverberation artifact; whereas the reproduction of objects is termed mirror image. Both artifacts follow the same principles:

1. They occur when there is a bright reflective surface
2. They are always deeper than the real image
3. They are always less distinct than the real image

Next time, look out for the mirror in the tracheal wall.

With each new course, rotation, or group of novice sonographers we often give the same advice on scanning. Although I don’t mind the repetition, I’ve codified some of the most common tips below so I don’t forget them.

These are mainly directed towards novices, but there may be something useful in there for everyone to remember. Note that I used self-restraint and did NOT list “clean the machine” among the tips. I assume everyone has already built up an impressive list of excuses for not cleaning the machine. That sounds like another post in itself!

• Start with one indication and become comfortable with it, then expand your repertoire
• Before picking up the probe, think about how the results of the scan will change your management and clarify your clinical question (good advice for any diagnostic test)
• Familiarize yourself with the most useful buttons first (every machine has these):
  • Power, probe selection, depth, gain, save/print
• Remember you are scanning three-dimensional structures- be sure to fan the ultrasound beam through several planes to visualize the full anatomy
• Practice, and keep practicing. Ultrasound IS operator dependent, just like everything else you do in your practice. So get good at it, just as you became proficient in EKG interpretation or laceration repair.
• When you can’t see anything:
  • Use more gel, find a better acoustic window, and check the common buttons (transducer, depth, gain)
• Proper hand position is crucial- hold the probe so you are comfortable and stable
• Check follow-up studies if they are performed, and compare your bedside results to CT scan, operative findings, etc.
• Position the patient, the machine, and yourself for optimal visibility and comfort whenever possible
• Share positive findings with your colleagues! Although pregnancies and gallstones are common, sharing aortic aneurysms or deep vein thromboses will be appreciated.
• Share ‘saves’ with your colleagues! Although most applications for bedside ultrasound are evidence-based, never underestimate the power of the anecdote in changing practice patterns.

Please leave YOUR best scanning tip in the comments.