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Background: We previously reported that a new index of aortic stenosis (AS) severity derived from dobutamine stress echocardiography, the projected effective orifice area (EOAproj) at a normal transvalvular flow rate (Q), is superior to traditional Doppler-echocardiographic indices to differentiate true severe from pseudo severe stenosis in patients with low left ventricular ejection fraction (LVEF), low flow, low gradient AS. In this study, we examined the diagnostic and prognostic value of EOAproj in patient with low flow, low gradient AS and preserved LVEF (i.e. paradoxical low flow AS). This recently described clinical entity has been associated with worse prognosis.Methods: Forty four (44) patients with low flow, low gradient AS (AVA≤1cm2, and mean gradient [MG]≤40mmHg) and preserved LVEF (>50%) underwent stress echocardiography (exercise stress in 29 asymptomatic patients and dobutamine stress in 15 symptomatic patients). EOAproj was calculated by the simplified method: EOAproj=EOArest+(ΔEOA/ΔQ)×(250-Qrest), where "rest" indicates EOA and Q measured at rest and Δ indicates the increase in these parameters during stress echocardiography. The treating physician was not aware of the results of EOAproj. Twenty three (23) patients underwent aortic valvle replacement during follow-up and valve weight was measured in 9 of these patients.Results: EOAproj (Hazard Ratio [HR]=0.13 per one cm2 increase in EOAproj; p=0.04) and increase in mean gradient during stress echocardiography (ΔP) (HR=1.06 per one mmHg increase in ΔP; p=0.01) were the only univariate predictors of the combined end-point of AVR or death. EOAproj correlated well with the weight of the explanted aortic valve (r=0.87; p=0.01) whereas ΔP did not (r=0.22; p=0.64). A value of EOAproj≤1cm2 was the most accurate cut-off to identify true severe AS defined on the basis of examination of explanted valve.Conclusion: In patients with low flow AS and preserved ejection fraction, the measurement of EOAproj with the use of stress echocardiography is helpful to determine the actual severity of the stenosis and predict risk of adverse events. This stenotic index may contribute improving therapeutic decision making in this challenging population.