Today’s post focuses on the “fifth” acoustic window to the heart, the right sternal edge (RSE). The right sternal edge , or right sternal border, allows improved visualization of the mid to distal ascending aorta and potentially an improved angle of incidence for assessing aortic stenosis. Whilst some consider this a routine component to the scan, other sonographers are quite unsure of how to approach this.
The aorta begins its course just to the left of the sternum, travelling superiorly from the heart towards the suprasternal notch. The normal aorta passes from the left side of of the sternum at around the 3-4th intercostal space (aortic valve), heading behind the sternum and continuing to the right of the sternum at approximately the 2nd intercostal space before curving at the aortic arch just below the suprasternal notch, then heading inferiorly and posteriorly down the chest cavity as the descending thoracic aorta. As we get older and/or the aorta dilates, the artery also becomes longer (think of it like when the elastic goes from your socks and there seems to be a lot more material). The base of the heart is fixed in place by the pulmonary veins, effectively anchoring the aorta at its origin. The aorta is also anchored at the aortic arch by the ligamentum arteriousum. This means the angle needs to change to accommodate the increased length of the artery and we see a rightward angulation of the aorta. In this case, the majority of the ascending aorta then lies behind or to the right of the sternum, making imaging of the full length from a left sternal approach often impossible.
The RSE approach is critical for comprehensive assessment of ascending aortic morphology.
Patient positioning and transducer orientation
Position the patient steeply on their right side with the right arm extended towards the head of the bed with the left arm down by their side (right lateral decubitus position). Start with the probe close to, but to the right of, the sternum in around the 3rd intercostal space. As a rule of thumb it will be an intercostal space higher than where you obtained your left parasternal images. You should see the mid ascending aorta at this point as two parallel lines, with the right pulmonary artery in short axis. If you can see the aortic valve, you are too low on the chest and need to be an intercostal space higher. The transducer will be rotated to about 11 o’clock, but this will depend a lot on the amount of rightward angulation (young patients will be closer to vertical, older patients are rotated anticlockwise back to ~ 10 o’clock). And always remember the Golden Rule for obtaining an on-axis image: tilt for the center and rotate for the sides (and always in that order). You will need to rotate the probe to open out the walls of the aorta as two parallel lines.
Still struggling to get a good image?? Most commonly you need to tilt the patient steeper, or try imaging with a full breath out (the patient, not you!!!).
When/how should we image from RSE?
I obtain 2D images of the aorta from the RSE in the following patients:
- Dilated aorta measured in any other view, irrespective of image quality
- Bicuspid aortic valve
- Marfan’s syndrome
- Mitral valve prolapse
- Known aortopathy or other systemic condition associated with aortopathy
- Aortic stenosis
- When I haven’t seen the aorta well in the PLAX view. This includes when I can’t get high enough up the aorta or haven’t imaged the aorta as two parallel lines. Typically I aim to image / measure the mid ascending aorta at the point where we see the right pulmonary artery in short-axis. (Need image)
- And anytime that I am just about to use the non-guided CW probe from that window (this will help you find the correct window and save heaps of time)
This is not an exhaustive list, but you will note that the list also doesn’t leave many patients that wouldn’t fall into one of these categories. It is pretty rare that I don’t perform a RSE view.
Non-guided CW (PEDOF) probe
Non-guided CW (PEDOF) probe is a must for proper interrogation of native aortic valve stenosis and for aortic valve replacement (AVR) assessment. There is a common misconception that modern advances in transducer and processor technology has removed the need for a dedicated CW probe. In other words, there is a misguided belief that velocities just as high can be obtained with the regular imaging transducer using guided CW. The primary benefit of the PEDOF probe is the small angled transducer face which can be placed more effectively between the rib spaces, improving the ability to reduce the Doppler angle of incidence. An experienced sonographer will always get at least as high a velocity with the non-guided, and most times will obtain a significantly higher velocity with the PEDOF (compared to imaging transducer). It is estimated that velocities may be underestimated by as much as 20% by only using the imaging Tx.
in other words… YOU MUST USE PEDOF FROM MULTIPLE WINDOWS TO ADEQUATELY ASSESS AORTIC STENOSIS!!
Check out this example of how multiple acoustic windows were needed to accurately estimate mean aortic gradient in this patient with severe aortic stenosis.
Most common mistake when obtaining the aortic signal from the RSE.
Inexperienced sonographers are sometimes so excited/relieved to obtain a Doppler signal above the baseline that they anxiously hit freeze and store the signal. Remind yourself what qualities of the signal tell you that you have the correct signal from the apex…strong, dense signal with a clear opening and closing click. This often gets forgotten about from the RSE and some people store the aortic flow, rather than the flow through the actual valve orifice… If you are in the right spot, you will get clicks.
Similarly, If you are in the right spot, you don’t need a lot of Doppler gains. If you find you are increasing the gains to fill out the signal (and the background is now full of echoes) then you are probably not in the right space.
Practice, practice, practice…
RSE imaging, especially with the PEDOF probe, should be second nature. The addition of the RSE view should not add more than 1-2 minutes to the standard scan. Just keep practicing. I often hear people say that that you can only get a RSE view in patients with severe AS or a dilated aorta…this is simply not true. If you look you can get it in nearly everyone.
When do you use RSE imaging?? Got some tips for finding the perfect signal?? Add your experience to the comments below.
Update – 21/12/2015
A good RSE image will allow visualisation of the ascending aorta, at least up to the level of where the right pulmonary artery is in short-axis.
In reality though, images this nice are rare… and more commonly look like this…