“The suprasternal notch view is not difficult to get….positioning the patient correctly may be.”
-Someone
Dissection is almost always on the differential list when it comes to chest pain in the emergency room. Whether the medical student says, “does that mediastinum look wide?” Or the patient says, “the pain does go to my back a little.” It seems like we can never escape the worry of thoracic aortic dissection. Even though this diagnosis is always on our mind, it is relatively rare. One paper from 2015 looked at 9.5 million ED visits and only found 782 dissections overall. (1) Out of those 9.5 million visits 763,000 (8%) were for atraumatic chest pain. So that means for every 980 atraumatic chest pains you see, one of them will have a dissection. Now I know what you are thinking, “well that makes no sense, I’ve seen 4 dissections this month!” But give that some thought, have you? Most of the time we hear about a dissection from colleague or get a transfer from an outside hospital for a dissection. I bet if you really dig into it, you may have seen a dissection once or twice in the last few years. That being said, we worry, it’s our job. So what can we do to help us worry a little less? Well a prospective trial from 2014 looked at bedside ultrasound in the diagnosis of dissection and found that dilation of the ascending aorta was present in 70% of dissection cases. (2) They used a cutoff of 4 cm on transthoracic echo to define dilation. As the aorta progresses it actually tapers down and along the aortic arch the average size is 2.2 to 3.6 cm. (3) Here is a graphical representation of the average sizes found in normal adults.
So all that is great but what does that mean for me!? Well…lets take a look at the suprasternal notch view on ultrasound and see how that might be able to help. Most people I talk to say that they don’t use this view because it is too hard to get, the truth is, it’s really easy to get you just have to position the patient correctly. What does that mean, well the angle that we need the probe to be facing is very superficial in the chest, so the optimal patient positioning requires them to have their head out of the way. The easiest way to accomplish this is have the patient lay flat with a pillow underneath their upper back and their head against the bed. If you have an older patient who cannot tolerate extending their neck this much you can try having them extend as much as possible and side bend or turn to one side. Now place a generous amount of gel in the patients suprasternal notch and gently rest the phased array probe there angling towards the anterior of the patients chest by moving the probe tail close to their anterior neck. Now the other issue is that even after the patient is in the correct position people give up becuase they feel like they can’t get the “perfect” view. Here are 2 example views, the 1st one shows the ideal window with labels, in the 2nd view, while you cannot identify every structure the aortic arch is clearly visible. The second image was actually featured in Ultrasound Case # 1 where you were able to identify a dissection flap within the arch.
So what am I really suggesting here? Well if we know that aortic dissections are exceedingly rare (0.1% of atraumatic chest pain complaints)(1) in the emergency department, and we know that absence of aortic root dilation has a 92% negative predictive value (2) then it could be argued that visualizing even more of the aortic root increases that NPV. Therefore I feel that it is reasonable to say that in a patient who we already have a very low clinical suspicion for aortic dissection, like the patient with mostly chest pain who says, “O yea maybe it does go to my back a little,” we can combine a normal TTE without aortic root dilation, no effusion, and no visible flap with a suprasternal view without a flap and measuring <4 cm to say that the patient is sufficiently low risk to forego CTA looking for dissection. But who am I kidding, I’ll still scan them if the med student thinks the mediastinum looks wide.
References
- Alter SM, Eskin B, Allegra JR. Diagnosis of aortic dissection in emergency department patients is rare. West J Emerg Med. 2015;16(5):629–31.
- Nazerian P, Vanni S, Castelli M, Morello F, Tozzetti C, Zagli G, et al. Diagnostic performance of emergency transthoracic focus cardiac ultrasound in suspected acute type A aortic dissection. Intern Emerg Med. 2014;9(6):665–70.
- Evangelista A, Flachskampf FA, Erbel R, Antonini-Canterin F, Vlachopoulos C, Rocchi G, et al. Echocardiography in aortic diseases: EAE recommendations for clinical practice. Eur J Echocardiogr. 2010;11(8):645–58.