Chapter: Proximal Aorta and Valve

Proximal Aorta Aortic Stenosis Aortic Regurg

The Aorta & Aortic Valve

A deep dive into the careful assessment of these two dovetailed entities with all aspects of Internal Medicine POCUS (Acquisition, Identification, Interpretation, and Clinical Integration)

This chapter concerns diseases that are frequent in clinic patients. The advanced IMBUS physician can offer these patients improved care, less resource use, and strong patient satisfaction. The danger lies in being only mediocre. These exam techniques and measures must be routinely performed to gain and maintain the skill.


PROXIMAL AORTA

IMBUS cannot see all parts of the proximal aorta. The best views for imaging the ascending aorta in long axis are a high-interspace PLAX, apical3, and an apical5. An upper interspace PSAX can give a short axis measurement. The difference in aorta size between systole and diastole is small so just get the clearest image of the ascending aorta and freeze to measure. Enlarged ascending aortas are usually spotted immediately with the eyeball method, but it takes experience measuring aortas to develop this ability. With bicuspid aortic valves, only the beginning portion of the proximal aorta may be aneurysmal, but the most common aneurysms do not involve this portion but have dilation beginning more distally at the sinotubular junction.

The IMBUS exam uses the “leading edge to leading edge” (LELE) method for measuring the aorta in all locations, including the ascending aorta. Measuring methods vary between labs, so expect variations of some millimeters between measurements and don’t think that these are real differences. LELE means the first caliper goes at the start of the hyperechoic anterior wall and the second caliper goes at the beginning of the posterior wall (end of the lumen). Even with views that are only fair, the aorta walls are echogenic and can be measured. Usually, just the ascending aorta is measured, unless the beginning aortic sinus looks much larger, when it would also be measured. Here is a typical PLAX that allowed a good aorta measurement that was larger than normal.

Here is a clip of the initial PLAX of a patient with a proximal aorta clearly larger than the RVOT or the left atrium.

The diameter of this aorta was 4.0 cm but a patient’s height must be considered when interpreting the aorta size. This woman was only 61 inches tall, so her physician was appropriately worried. Overall, aortic rupture is rare under 4 cm in average-sized adults. Aortic dissection can occur at any size.

 

Sinus of Valsalva

Ascending Aorta

Descending aorta

Aorta diameter (cm)

2.9 - 4.5

2.2 - 3.6

2.0 - 3.0

The descending aorta in short axis can often be seen, although not always well, in the PLAX (below the left atrium) and apical4 (below the lateral LA). Fortunately, enlarged descending aortas rarely occur in isolation from the ascending and an enlarged descending aorta is easier to see and measure. If an apical2 is fanned medial with  modest increased B-mode depth, the long-axis descending aorta may be seen below the heart. This can show dilation, dissection, or thrombus. Here is a modified apical2 showing the long axis descending aorta below the left atrium.

The supra-sternal view of the aortic arch is not easy and increasingly fails in elderly patients. Fortunately, the arch is also rarely enlarged in isolation so this view and measurement is not part of the IMBUS exam.


AORTIC STENOSIS (AS)

Under-estimating severe aortic stenosis is an increasingly serious mistake as the ability to correct AS continues to improve. The rate of AS progression is unpredictable and patient symptoms are not reliable for indicating severe disease. Unfortunately, even expert echocardiographers, using just visual estimation of AS severity (degree of calcification and amount of valve opening), had a sensitivity of only 60% and a specificity of 85% for severe AS compared to full Doppler evaluation (Quader, et al. J Ultrasound Med, 2015).

NOTE: The rest of this section concerns specifically measuring aortic valve (AV) flow, but standard B-mode observations and measurements are still important for AV analysis. Careful assessment for IVS hypertrophy, LV diastolic dysfunction, LV longitudinal systolic dysfunction, and LA enlargement are important. A patient who lacks all these abnormalities would almost never have severe AS. This is important in those patients in whom the measurements of AV flow are suboptimal or impossible.

The great majority of clinic AS patients are older with calcification of tri-leaflet aortic valves. In these patients, the severe AS is accompanied by severe sclerosis/calcification of the valve. Thus, observing only modest sclerosis of the AV in an elderly patient markedly lessens the chance of severe AS. However, severe calcification does not always mean severe AS. Here is a PLAX clip of a patient with obvious thickening and sclerosis/calcification of the AV along with some restriction to the opening of the valve, but this alone can’t determine the severity of the AS.

Sometimes the hyperechogenicity of a calcified aortic valve makes it difficult to see the valve leaflet movement. Experts have described using the TGC control to turn down the gain over the valve to better visualize leaflet movement.

Bicuspid AV disease is different in several ways. First, such patients develop trouble at an earlier age. Second, they can develop AS without having severe calcification. Third, they are more likely to develop dilation of the aorta at an early phase of the disease. Thus, these patients need to be followed regularly from an earlier age focusing both on the AV and on the aorta.

Distinguishing a bicuspid (or even unicuspid or quadrcuspid) AV from a tri-leaflet valve is often difficult, partly because the views of the valve in the upper PSAX may not be clear. Even with clear views, some bicuspid valves can appear tri-leaflet. But, statistically the vast majority of clearly tri-leaflet-appearing valves will be tri-leaflet. An apparent bicuspid valve in a patient in his or her 40s should always be confirmed by formal echocardiography. The fish-mouth of a bicuspid valve can be oriented various ways in the aorta. Here is a confirmed bicuspid aortic valve in the upper PSAX.

In contrast, here is a tri-leaflet aortic valve in PSAX in a younger patient with a family history of bi-leaflet valves. Slow motion playback was very helpful in determining that each leaflet fully opened and coapted.

There is no single ultrasound measurement that reliably estimates AS severity. Valve area calculations are time-consuming and more prone to error than many realize (see discussion at the end of this section of the chapter). The measurements we need are Doppler flow velocities. It is adequate to measure only peak velocities without obtaining mean velocities (Finegold et al, International Journal of Cardiology, 2013).

One essential peak velocity is right after the aortic valve (AVmax). Because this post-valve velocity is > 2 m/sec in aortic stenosis, continuous wave Doppler (CW) is used. However, the CW gate line must be as parallel as possible to the blood flow or there is under-estimation of the velocity. The apical5 view is the most likely to get close to parallel for CW. If the left ventricular outflow tract (LVOT) is 30 degrees or less off-parallel to the default CW gate line, angle correction on Venue will allow an accurate estimate of AV max. Here is a good apical5 view for measurement.

With the AV and LVOT positioned, activate Color and find the anterior/posterior plane that maximizes the flow through the AV. Then place the CW line with the area of interest right after the AV opening and activate it. Angle correct to be exactly parallel to flow. Here is a tracing from a patient with mild AS.

The flow through the outflow tract and valve shows below the line because it is going away from the probe, so the baseline must be moved up. Adjust the Scale so complexes occupy most of the screen. Then freeze the tracing in later expiration and measure the lowest point on the curve/spectrum, which is AVmax. Don’t measure a poor spectrum.

Next is a tracing from a patient with moderate AS. The spectrum is clear and the AVmax was 3.1 m/sec.

Before echocardiography, master clinicians thought they could detect severe aortic stenosis with fingers on the carotid pulse. Pulsus parvus et tardus (small and slow rising) was the sought-after finding. While there is a method to measure the time to peak of the the AVmax tracing (if it is clear), it needs to be corrected for heart rate and an IMBUS exam rarely needs this measure. We suspect that a prolonged corrected carotid flow time will identify a pulsus tardus just as well.

In many patients, the AVmax alone could be used to categorize the severity of AS, as in the following table.

Post AV CW (m/sec)

Severity

< 2

No AS

2-3

Mild AS

3-4

Mod AS

4-5

Severe AS

Dimensionless Index: Unfortunately, in an important minority of patients, AVmax can be misleading. This happens in patients with either reduced stroke volume (low flow/low gradient AS) or increased stroke volume (including patients with moderate or greater AR). To avoid these traps, look at the ratio of flow velocity before the valve (LVOTmax) to the AVmax. This is called the “dimensionless index”.

Infrequently, the orientation of the CW line will go perfectly parallel through the LVOT and the AV and the spectrum may then show both AVmax and LVOTmax in the same tracing. Here is such a spectrum in a clinic patient with mild AS and a calculated dimensionless index of 0.53.

However, it is probably best to use PW to separately measure the lower velocities of the LVOTmax, orienting the cursor parallel to flow and putting it just before the valve. It is best to turn Color off to see the gate placement exactly. Here is the LVOTmax in the previous patient with an AVmax of 3.1 m/sec. Venue calculated the DI of 0.36.

AV Dimensionless Index

 

1

No AS

Down to 0.5

Mild AS

0.5 - 0.3

Moderate AS

0.25

Severe AS

Because of technical measurement details with Venue, the IMBUS routine is to do PW measurement of LVOTmax first, followed by AVmax with CW in all patients suspected of having AS. We think there is safety in always knowing the DI when considering AS.

OVERALL APPROACH TO AS: With a first diagnosis of AS in an older clinic patient with a sclerotic/calcified valve, a formal echo may not be needed if the views are good and all evidence points to non-severe disease. Any equivocal finding mandates a formal echo. Patients can be followed in clinic at decreasing intervals as AS progresses beyond mild, watching for a meaningful lowering of the dimensionless index or clear change in septal hypertrophy, LV function, or LA size. Formal echo and valve center referral are needed when anything suggests disease greater than moderate. The criteria for replacing the AV in AS are in evolution and missing a worsening of disease is a potentially bad error.

AORTIC VALVE AREA (AVA): Our colleagues periodically ask us if we can determine a patient’s AVA and our answer should be, “We can, but we don’t need to. The Dimensionless Index tells us everything we need to know”. The AVA calculation uses the Dimensionless Index and the estimated area of the LVOT right before the AV in the following equation:

AVA = (Cross sectional area of the aorta at the valve) x Dimensionless Index

The assumptions are that a fully opening AV will have an area equivalent to the cross sectional area of the aorta at the valve and that this area is a circle. In fact, the assumption of circularity can be inaccurate. Nevertheless, the aorta diameter is measured right before the AV and half this diameter is used as the radius in the equation for the area of a circle. This area, when multiplied by the DI, gives the AVA for a patient in cm2.

In the formal echo lab, the DI for this calculation is obtained using mean velocities obtained from the velocity time integral (VTI) of the Doppler tracings. However, there should be clinically unimportant difference between the DI calculated from mean velocities and the DI obtained from AVmax and LVOTmax. The max values have been found to be more reproducible over time and probably have greater agreement between examiners than the mean values from the VTI.

Since we already have the DI, why not just get the distal aorta diameter and let Venue calculate the valve area? The non-circularity of some AVAs was already mentioned as a source of error. A second issue is that the AVA varies with the size of a patient so the absolute AVA can be misleading unless it is indexed for height. The DI does not vary with patient size. Even if a lab reports an AVAindex, many physicians learn absolute numbers and assume that a 1 cm2 AVA has a particular meaning for all patients, when it doesn’t. A third issue is that any inaccuracy in the LVOT diameter measurement is squared in the calculation of the circular area, disproportionately increasing the error in the AVA estimation. Studies have shown important variability in examiner measurement of the aorta diameter at the AV.

The DI alone categorizes a stenosis on the mild to critical spectrum and by being a ratio, it is robust to changes in body size and stroke volume. The DI obtained from well-performed AVmax and LVOTmax values is all the IMBUS physician needs to make decisions. Valve centers do need to know absolute AVA numbers as they plan for AV replacements because they need to implant correctly sized devices. The absolute AVA from a transthoracic echo may be part of this analysis, but it is not all that centers use because they know the errors inherent in the AVA calculation.


AORTIC REGURGITATION (AR)

AR is common and important, but sometimes difficult to assess for severity. There isn’t a specific measurement, either caliper or spectral Doppler, to reliably determine if AR is severe. Color and other B-mode information are used to assess severity. Important AR should have a hyper-dynamic LV and worry begins when the LV starts to dilate and develop decreased longitudinal systolic function. AR should be fixed while radial systolic function is still normal. An increase in LA size is also a warning sign.

The first evidence of AR may be fluttering of the anterior leaflet of the mitral valve in the PLAX. The fluttering may be subtle or prominent and indicates a partially eccentric jet. Here is a clip of the PLAX of a bicuspid AV patient with subtle anterior leaflet fluttering. There is also some sclerosis of the AV and at least mild restriction in AV opening.

When the AR jet impinges strongly on the mitral leaflet, it can restrict the leaflet opening. This would be seen in an EPSS measurement (with the M-mode tracing also showing the leaflet fluttering). This pathology is one theory of what causes the traditional Flint murmur of AR, a diastolic low-pitched rumble, like mitral stenosis.

Color analysis of AR requires optimal width of the Color sector box. The sector needs to cover from a little distal to the valve on the aorta side down through the outflow tract. Remember, the height of the box doesn’t matter. Here is a Color sector that is appropriately wide for a patient’s moderate AR.

The AR jet is assessed with Color in the PLAX, PSAX, apical5, and apical3. Eccentric jets are particularly difficult to assess and tachycardia makes things more challenging. Sometimes the width and length of the AR jet can help categorize, but there are problems with this.

ALIASING: This is the term used when a Doppler signal has gone beyond the Nyquist limits set for the exam type. With Color this helps our interpretation. If we see a uniform light blue or light red color without any mosaic of colors, the velocity of flow in the view is not high in the direction being studied. With high velocities, we see a mosaic of mixed colors.

More important than the width and length of the AR jet are the characteristics of the jet’s origin. It is important to optimize the image before looking at the jet (sometimes frame by frame). Look for the presence and size of a flow convergence zone (sometimes called PISA), which is the ball of color that appears at the start of the jet on the aortic side of the valve. Also look at the width of the narrowest portion of the jet as it comes through the valve, which is called the vena contracta. Here is a diagram from 123sonography representing the characteristics of an AR jet.

This table shows how the characteristics of the AR jet are used to determine the severity of the lesion. The jet width refers to the percentage of the outflow tract the jet occupies.

AR Severity

Jet width

Flow convergence

Vena contracta

Mild

< 25%

Not visible

thin

Moderate

25-65%

Small

medium

Severe

> 65%

Large

wide

Coanda effect: Eccentric, regurgitant jets that hug walls create something called the Coanda effect, which makes it hard to estimate the severity of the AR by jet length and width. Here is an apical4 that was initially misleading, because a red jet was seen going towards the apex of the heart in diastole, but arising about mid-ventricle.

When the view was moved to an apical5, it could be seen that this was an AR jet that was hugging the septum and then curving out into the ventricle.

Finally, the apical3 can occasionally be important for assessing AR. Here is a clip from a patient with severe AR in whom the apical3 gave the best view of the jet. The flow convergence zone is prominent, the vena contracta is wide, and the jet is wide, long, and strongly aliased.

 

Approach to AR: AR is a volume stress on the LV. Surgery is indicated for AR when the ventricle begins to dilate and LV longitudinal function starts to deteriorate to mid or low normal. The LVEDD, MAPSE, s’, and LA size need to be watched carefully. The severity of the AR jet itself can vary as a patient’s preload, afterload, and contractile state vary. When the LVEDD increases to upper normal for height, LV longitudinal function decreases, or the LA begins to enlarge with sinus rhythm, a formal echo is needed to be sure an AR patient is corrected at the optimal time.