How to understand the mean aortic transvalvular pressure difference and valvular stenosis? In stenotic lesions, Doppler echocardiography applied to the simplified Bernoulli equation yields maximum transient and mean pressure differences across the stenosis, which, when combined with measurements of flow and pressure difference, allow calculation of the orifice area for evaluation of stenosis and obstruction severity. Most useful is the measurement of the mean aortic pressure difference, which is the average of all transient pressure differences on either side of the aortic valve orifice during systole. Although in most cases, the mean pressure difference obtained by Doppler echocardiography has become an accurate and reliable method for evaluating the extent of primary stenosis lesions. However, the differential pressures obtained by Doppler echocardiography are volume-dependent, and a severely calcified aortic valve may show a mean differential pressure of 20 mmHg or less if the patient’s left ventricular ejection fraction is below 30%. This so-called low-flow, low-pressure differential does not exclude hemodynamically significant valvular stenosis. A low-dose dobutamine test can reveal severe aortic stenosis in the setting of combined left ventricular insufficiency. In addition the stress test may be useful in identifying whether benefit can be obtained from valve surgery in patients with aortic stenosis combined with severe left ventricular insufficiency. If a low-dose dobutamine test fails to increase the volume per beat by more than 20% in patients with severe aortic stenosis, the surgical risk is high and the long-term prognosis after valve replacement is poor. In cases of left ventricular ejection fraction greater than or equal to 50%, the use of transvalvular velocity and pressure differential to evaluate the severity of aortic stenosis may be the right choice. In patients with low-flow, poor-pressure aortic stenosis, the use of a continuous equation to calculate aortic orifice area is recommended to evaluate the severity of stenosis.