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Purpose: Reduced tranvalvular pressure gradient in patients with significant aortic stenosis (AS) and preserved systolic function has been attributed to fixed (true severe) valve stenosis with low cardiac output due to increased peripheral resistance. However, the possibility of a reduced valve opening due to low cardiac output has not been fully investigated.Methods: To investigate this possibility, we performed a dobutamine stress echo (DSE, dose 5-40 μgr/kgr/min ± atropine) in 9 consecutive patients with significant AS (aortic valve area- AVA ≤0.6 cm2/M2), EF ≥ 50 % and stroke volume index (SVI) ≤ 35 ml/M2, targeting to increase cardiac output at peak exercise. Total resistance (peripheral resistance+valvular resistance-Zva-) was calculated according to the following formula: Zva= (SBP + mean PG) / SVI.Results: All patients had increased Zva at rest (> 5.5 mmHg/ml/M2). During DSE cardiac output was increased (SVI > 35 ml/M2) and Zva was reduced in all patients. In 4 patients AVA was not increased or was increased to a value less than 1 cm2 (increase < 0.3 cm2), while in the other 5 patients AVA was increased to a value more than 1 cm2 (Table)Conclusions: Patients with significant AS, low transvalular pressure gradient and preserved ejection fraction constitute, as patients with reduced systolic function, a mixed population: they present as a fixed (true severe) stenosis or a reduced valve opening due to low cardiac output.