8.3 Aortic Stenosis: Velocities, Gradients, Valve Area, and Flow States

Build the hemodynamic dataset in the right order

CCI task C2 is to assess aortic stenosis (AS). Begin with valve morphology and color localization, then acquire LVOT PW Doppler and aortic-jet CW Doppler. The primary ASE/EACVI hemodynamic measures are peak aortic-jet velocity, mean transvalvular gradient, and continuity-equation aortic valve area (AVA). They answer related but different questions and are flow dependent in different ways. No single number should overrule obviously contradictory anatomy, flow, or acquisition quality. Record heart rate, rhythm, and blood pressure because they shape the hemodynamic context.

Align CW as parallel as possible to the jet. Interrogate apical, right-parasternal, suprasternal, and other appropriate windows with a dedicated small-footprint nonimaging transducer when needed. Use the highest valid dense velocity, not an average across windows, and document where it was obtained. A small angle error underestimates velocity and, because pressure uses velocity squared, causes a larger gradient error. Exclude mitral regurgitation, dynamic LVOT obstruction, noise, and fine linear transit-time artifact before tracing.

Trace the outer edge of the dense CW envelope through systole. The simplified Bernoulli equation gives an instantaneous gradient:

ΔP = 4V²

At 4.0 m/s, the peak instantaneous gradient is 64 mmHg. The machine calculates mean gradient by averaging all instantaneous 4V² values during ejection. It is incorrect to square the mean velocity. Check that the trace follows the dense modal border rather than faint feathering and that an irregular rhythm is averaged according to laboratory and disease-specific guidance.

These are standard guideline markers, not five independent verdicts. Cutoffs are not perfectly mathematically concordant, and high or low flow can move velocity and gradient without an equivalent change in anatomic valve narrowing.

Protect the continuity equation from compounded error

The continuity principle assumes stroke volume in the LVOT equals flow through the stenotic valve:

AVA = CSA(LVOT) × VTI(LVOT) / VTI(AV)

Measure LVOT diameter inner edge to inner edge in a zoomed parasternal long-axis view during mid-systole at the same anatomic level represented by PW sampling. Calculate circular area as π(D/2)². Because diameter is squared, a 10% diameter error produces about a 21% area error. The true LVOT is often elliptical, so a circular assumption can underestimate area and stroke volume. Do not move the diameter calipers simply to make AVA agree with gradient.

From an apical approach, place a 3–5 mm PW gate on the LV side of the valve just proximal to flow acceleration. Move it slowly apically until the envelope is smooth and narrow; broadening means the gate is too close to accelerated flow. Trace the dense modal LVOT VTI and use corresponding beats for AV VTI when rhythm varies. For D = 2.0 cm, LVOT VTI = 20 cm, and AV VTI = 80 cm, CSA is 3.14 cm² and AVA is about 0.79 cm². That calculation suggests a small effective orifice, but it still requires concordance review.

The dimensionless index equals LVOT VTI divided by AV VTI and avoids the LVOT diameter term. It is useful when diameter is uncertain, but it does not rescue a contaminated LVOT sample or misaligned AV envelope. Direct TTE planimetry is limited by calcific shadowing and by difficulty locating the smallest cusp-tip orifice; effective Doppler orifice area is not identical to anatomic planimetered area.

Resolve discordance before assigning severity

A concordant high-gradient severe pattern typically has Vmax at least 4.0 m/s or mean gradient at least 40 mmHg with a small AVA and supportive calcified restricted anatomy. Even then, check for reversible high-flow states such as anemia, sepsis, hyperthyroidism, significant regurgitation, or shunt. A high gradient is not interpreted in a vacuum.

When AVA is 1.0 cm² or less but velocity and mean gradient are below the severe-pattern thresholds, first repeat quality control: Doppler alignment and all windows, LVOT diameter and PW site, rhythm averaging, BP, body size, and arithmetic. Then calculate stroke volume index (SVi) and review LVEF. The 2017 ASE/EACVI framework defines low flow using SVi below 35 mL/m² and separates reduced EF from preserved EF.

Classic low-flow, low-gradient AS has reduced LVEF. Under an authorized, medically supervised low-dose dobutamine protocol, changes in flow, velocity, gradient, AVA, and contractile reserve can help distinguish truly severe from pseudosevere stenosis. Paradoxical low-flow, low-gradient AS has preserved EF, often with a small hypertrophied LV; measurement error and severe hypertension must be excluded, and CT valve calcium may add evidence. Normal-flow, low-gradient discordance commonly reflects moderate disease or cutoff/measurement inconsistency but still requires clinical review and follow-up.

Report the acquired facts and limitations: valve morphology and calcification, maximum-velocity window, Vmax, mean gradient, LVOT diameter and VTI, AV VTI, AVA, dimensionless index, SVi, LVEF, BP, rhythm, and associated LV or aortic findings. The interpreting clinician integrates symptoms and adjunct testing. Precision means making every input defensible—not making one threshold decisive.