Ascending Aorta & Aortic Valve

	Sinus of Valsalva	Ascending Aorta	Descending aorta
Aorta diameter (cm)	3.4, 4.0	3.6, 4.0	2.6, 2.9

The descending aorta in the short axis can often be seen, although not consistently 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 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 challenging and increasingly fails in elderly patients. Fortunately, the arch rarely enlarges in isolation, so this view and measurement are not part of the IMBUS exam.

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AORTIC STENOSIS (AS)

Under-estimating severe aortic stenosis is an increasingly grave 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, had a sensitivity of only 60% and a specificity of 85% for severe AS compared to a Doppler evaluation (Quader et al. J Ultrasound Med, 2015).

NOTE: The rest of this section focuses on measuring aortic valve (AV) flow, but standard B-mode observations and measurements are still crucial for AV analysis. Careful assessment for IVS hypertrophy, LV diastolic dysfunction, LV longitudinal systolic dysfunction, and LA enlargement is essential. A patient who lacks all these abnormalities would not have severe AS. This is important in those patients in whom the measurements of AV flow are suboptimal or impossible.

Most 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 prominent thickening and sclerosis/calcification of the AV, along with some restriction to the opening of the valve. However, 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 suggest using TGC to decrease gain over the valve to visualize leaflet movement better.

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

Distinguishing a bicuspid (or even unicuspid or quadracuspid) AV from a tri-leaflet valve is often challenging, 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. We should always confirm a possible bicuspid valve with formal echocardiography. A bicuspid valve's fish mouth can be oriented in 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 opened fully and coapted.

ADVANCED TIP: When it is essential to see the AV leaflets, but the PSAX view is suboptimal, a cross-sectional AV view can sometimes be found in the SCSAX. The following view was better than the PSAX in the patient. The orientation of the leaflets is rotated about 90 degrees clockwise compared to the PSAX, but three modestly sclerotic leaflets are seen to open fully.

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). The measurements we need are Doppler flow velocities. It is adequate to measure peak velocities without obtaining mean velocities from Velocity Time Integrals (Finegold et al., International Journal of Cardiology, 2013).

One essential peak velocity is right after the aortic valve (AV Vmax). Because this post-valve velocity is > 2 m/sec in aortic stenosis, continuous wave Doppler (CW) is used. However, the CW gate line should be as parallel as possible to the blood flow, or the velocity will be underestimated. The apical5 view, particularly with some RV enhancement, is likely to be close to parallel. The apical3 view may give even better tracings and measurements, so it should also be attempted in patients suspected of AS. 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 Vmax. Here is an acceptable 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, activate it, and angle-correct to parallel the flow. Here is a tracing from a patient with mild AS. The sharp, hyperechoic line at the end of the envelope is the closing valve “click” of the AV, which is unavoidable with CW.

The flow through the outflow tract and valve shows below the line because it is going away from the probe, so the baseline should be moved up. Adjust the complex size to occupy most of the graph. Then, freeze the tracing in later expiration and measure the lowest point on the curve/spectrum, AV Vmax. Don’t measure a poor spectrum. Experts suggest measuring spectral Doppler tracings closer to the well-defined “chin” and not to the last faint “tip of a beard,” The same principle applies to tracings above the baseline.

Next is a tracing from a patient with moderate AS. The spectrum is clear, and the AV Vmax was 3.1 m/sec. The sharp spike at the beginning of the envelope is an opening valve click.

In many patients, the AV Vmax 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 a significant minority of patients, AV Vmax 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 AR). To avoid these traps, look at the ratio of LVOT Vmax to AV Vmax. This is called the “dimensionless index” (DI).

Occasionally, the orientation of the CW line will be parallel through the LVOT and the AV, and the spectrum may then show separate AV Vmax and LVOT Vmax in the same tracing. Here is a spectrum in a clinic patient with mild AS and a calculated dimensionless index of 0.53.

However, this embedded LVOT velocity curve should be used cautiously because it includes the turbulent flow immediately proximal to the stenosis, making the LVOT Vmax falsely high. This would increase the DI, causing an underestimation of the severity of AS. We should use PW to measure LVOT Vmax separately, orienting the gate parallel to the flow and putting it away from the valve enough to avoid valve clicks in the tracing. Here is the LVOT Vmax in the previous patient; the DI was 0.36. The DI is interpreted in the following table.