Aortic Stenosis

Aortic Stenosis: Etiology and Clinical Context

Aortic stenosis (AS) is the most common primary valve lesion requiring intervention in developed countries. In patients over 65, calcific/degenerative AS (progressive leaflet fibrosis and calcification) is the dominant etiology. In younger patients (<65), a bicuspid aortic valve is the most common cause, with stenosis developing years to decades earlier than in trileaflet valves because of abnormal leaflet stress distribution. Rheumatic AS is rare in isolation in developed countries and almost always coexists with rheumatic mitral disease and commissural fusion.

Aortic sclerosis is a distinct, earlier stage: leaflet thickening/calcification without significant obstruction to flow (peak velocity ≤2.5 m/s). It is not a synonym for mild AS and should be reported separately, though it shares the same risk factors and can progress to true AS over time.

Severity Grading

AS severity is graded using an integrated, multiparametric approach — no single measurement should be used in isolation. The core hemodynamic parameters, derived from continuous-wave (CW) Doppler across the aortic valve and the continuity equation (see Chapter 6), are summarized below per current ASE/ACC-AHA criteria:

The dimensionless index (DVI), also called the velocity ratio, is the ratio of the LVOT velocity-time integral (VTI) to the aortic valve VTI (or peak LVOT velocity to peak AV velocity). Because LVOT area cancels out of the ratio, DVI is unaffected by errors in LVOT diameter measurement — a major source of error in the continuity-equation AVA calculation. A DVI <0.25 reliably indicates severe AS regardless of body size or LVOT measurement accuracy, and should always be checked against the calculated AVA for internal consistency.

Low-Flow, Low-Gradient Aortic Stenosis

Gradient and AVA can be discordant — a small AVA with a gradient below the severe threshold — when stroke volume is reduced. Flow status is defined by the stroke volume index (SVi): a value <35 mL/m² defines low flow, regardless of ejection fraction.

• Classical low-flow, low-gradient AS: reduced LV ejection fraction (<50%) reduces transvalvular flow and therefore the gradient, even when the valve is truly severely stenotic.
• Paradoxical low-flow, low-gradient AS: preserved EF (≥50%), but a small, concentrically hypertrophied, often restrictive left ventricle limits stroke volume despite normal systolic function.

Both patterns can produce a mean gradient <40 mmHg despite an AVA <1.0 cm², making it essential to distinguish true-severe AS (an anatomically severely stenotic valve with secondarily reduced flow) from pseudo-severe AS (a moderately stenotic valve that only appears severe by AVA because low flow prevents full leaflet opening).

Low-dose dobutamine stress echocardiography (DSE) is the standard workup for classical low-flow, low-gradient AS with reduced EF:

1. Dobutamine is infused in low, incremental doses to increase stroke volume.
2. If SV increases ≥20% (contractile/flow reserve present) and the AVA remains <1.0 cm² while the mean gradient rises to ≥40 mmHg → true-severe AS.
3. If SV increases ≥20% and the AVA increases to >1.0 cm² with a persistently low gradient → pseudo-severe AS.
4. If SV fails to increase ≥20% (no flow reserve), the study is inconclusive; adjunctive CT aortic valve calcium (AVC) scoring or clinical judgment is needed.

For paradoxical low-flow, low-gradient AS with preserved EF, dobutamine adds little (contractility is already normal). CT aortic valve calcium scoring is the preferred confirmatory test here: an AVC score ≥2000 Agatston units (AU) in men or ≥1200 AU in women makes severe AS likely.

Clinical Correlation

Severe AS with symptoms (angina, syncope, or heart failure) is a Class I indication for aortic valve intervention regardless of the specific gradient measured on a given study. Exercise stress testing is contraindicated in symptomatic severe AS but can safely unmask exertional symptoms or an abnormal blood-pressure response in asymptomatic severe AS, helping guide the timing of earlier intervention.

Technical Pitfalls in Doppler Assessment

Because CW Doppler measures the single highest velocity along the ultrasound beam, the transducer must be swept through multiple acoustic windows — apical, right parasternal (with the patient in the right lateral position), and suprasternal — to find the true maximal jet velocity. Relying on the apical window alone can miss a higher-velocity signal obtained elsewhere and lead to underestimation of severity; a dedicated dual-crystal (Pedoff) probe is often used at the right parasternal and suprasternal windows to improve alignment.

A second technical pitfall is pressure recovery: CW Doppler measures peak velocity at the vena contracta, the narrowest, highest-velocity point just downstream of the valve, while a small aortic root (<3 cm² cross-sectional area) converts much of that kinetic energy back into pressure over a short distance, so a catheter-measured gradient further downstream reads lower than the Doppler-derived gradient. Pressure recovery causes Doppler to overestimate severity in a small ascending aorta and can be corrected for using an energy loss index, which adjusts AVA for aortic cross-sectional area.

Key Testing Points

• Report peak velocity, mean gradient, AVA, and DVI together — discordance between AVA and gradient should always trigger a flow-state assessment.
• SVi <35 mL/m² defines low flow; this is independent of EF.
• Classical LFLG-AS = low EF; paradoxical LFLG-AS = preserved EF with a small, hypertrophied LV.
• Dobutamine stress echo distinguishes true-severe from pseudo-severe AS via the ≥20% stroke-volume flow-reserve response.
• Aortic sclerosis (peak velocity ≤2.5 m/s) is not the same as mild AS.