Discordance between various measures of severity of aortic stenosis (AS) is considered as discordant grading of severity of aortic stenosis or simply discordant AS. Severe aortic stenosis has aortic Vmax ≥4.0 m/s, mean gradient ≥40 mm Hg and effective orifice area (EOA) ≤1.0 sq. cm. Peak aortic velocity and mean gradient are flow dependent measurements, while effective orifice area and Doppler velocity index are relatively flow independent.
About 20-30% of patients may have discordant measures of severity of aortic stenosis on echocardiography . Discordance is mostly between effective orifice area and Vmax/Doppler gradients. Low gradient and Vmax may occur with small EOA. So the usual discordant AS has EOA ≤1.0 sq. cm and mean gradient <40 mm Hg .
Low flow low gradient severe aortic stenosis has low stroke volume with stroke volume index <35 ml/sq. m. Low stroke volume index can occur with reduced left ventricular ejection fraction or with preserved ejection fraction. Lower stroke volume index is associated incrementally with mortality in low gradient severe aortic stenosis with preserved ejection fraction .
Most frequent cause of low gradient aortic stenosis is a low left ventricular outflow state. This could be due to reduced left ventricular ejection which is the classical low flow low gradient aortic stenosis. When there is low flow with preserved ejection fraction, it is paradoxical low flow low gradient aortic stenosis. There is a third category of normal flow low gradient aortic stenosis with small aortic valve area as well .
To differentiate between true severe aortic stenosis and pseudo severe aortic stenosis, two investigations are useful. Low dose dobutamine stress echocardiography is useful in classical low flow low gradient aortic stenosis with LV ejection fraction <50%. Aortic valve area increases as stroke volume increases with dobutamine in pseudo severe AS. But it may not be useful unless the left ventricular stroke volume increases by ≥20%.
Aortic valve calcium scoring and multi detector computed tomography (MDCT) are useful in case of low flow low gradient and normal flow low gradient aortic stenosis .
A calcium score (Agatston score) above 2000 in males and 1250 in females suggest the presence of true severe aortic stenosis . In general, low flow low gradient severe aortic stenosis has worse outcome following aortic valve replacement compared to those with high gradient. Still there is a survival benefit with aortic valve replacement. There is also a suggestion that transcatheter aortic valve implantation (TAVI) may be superior to surgical aortic valve replacement in this group .
Patients with low flow low gradient aortic stenosis have higher late gadolinium enhancement on cardiac magnetic resonance imaging suggesting more myocardial fibrosis. This is in comparison with high gradient aortic stenosis . But this was found irrespective of the flow reserve documented by dobutamine stress echocardiography.
While measuring left ventricular outflow tract (LVOT) for the continuity equation, measurement is better taken at the level of the aortic valve annulus rather than deeper into the LVOT. Velocity and gradient should be sampled from multiple windows like apical, right sternal border and suprasternal notch to avoid underestimation which can occur in up to 50% of cases. If the data are discordant even after these careful measurements, hybrid imaging may be considered to better define the LVOT cross sectional area . These include three dimensional echo and contrast enhanced computed tomography.