Low-gradient aortic stenosis (AS) has been a vexing problem for cardiologists since its original description in 1980 by Carabello et al.1 In that study, 3 of 4 patients with low-gradient AS died at surgery, and the other patient had persistent severe heart failure postoperatively. Subsequent studies showed that although operative mortality is high, most patients survive aortic valve replacement (AVR), and some have dramatic improvement in symptoms and left ventricular (LV) function.2,3 The challenge facing clinicians is how to accurately distinguish those patients who will benefit from AVR from those who will not. The American College of Cardiology/American Heart Association guidelines for valvular heart disease recommend hemodynamic evaluation of low-gradient AS with dobutamine echocardiography to distinguish patients with fixed anatomic AS from those with flow-dependent (“relative”) AS in patients with LV dysfunction.4 In the latter case, aortic valve area (AVA) is spuriously low because there is not enough forward flow to fully open the leaflets.5–7 To understand how dobutamine infusion helps sort out the dilemma of low-gradient AS, it is useful to revisit the hemodynamic principles that govern the relationship between AVA, flow, and gradient. Article p 711 In 1951, Gorlin and Gorlin proposed the “hydraulic orifice equation” for calculation of valve area in valvular stenosis.8 The Gorlin formula was based on the fundamental equation: equation ![Formula][1] in which valve area (A) is equal to transvalvular flow (F), divided by transvalvular velocity (V). In AS, the Gorlin formula is used to calculate AVA as: equation ![Formula][2] Because transvalvular flow occurs during systole in AS, the numerator (flow) becomes the cardiac output (CO) divided by heart rate times systolic ejection period (SEP), or stroke volume divided by SEP. The denominator (velocity) could not be measured in 1951 (Doppler echocardiography did not exist), so it was calculated from the … [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif