**Abstract**

Hemodynamic classifications of aortic valve stenosis (AS) have important prognostic implications. In normal flow state, severe AS is defined as peak aortic velocity ≥ 4.0 m/s, mean transaortic gradient (MG) ≥ 40 mmHg, and aortic valve area (AVA) < 1.0 cm<sup>2</sup> . However, numerous studies have shown that severe AS (based on AVA < 1.0 cm<sup>2</sup> ) with low gradient (MG < 40 mmHg) is prevalent due to low flow state, in the setting of reduced and preserved left ventricular ejection fraction (LVEF). Thus, the hemodynamic classifications of AS with AVA < 1.0 cm<sup>2</sup> were expanded to include the transvalvular flow state and pressure gradients. These flow-gradient patterns include normal flow/very high gradient, normal flow/high gradient, low flow/high gradient, low flow/low gradient with reduced LVEF, low flow/low gradient with preserved LVEF, and normal flow/low gradient. Among these, the low-gradient AS subgroups are challenging, particularly to differentiate true-severe AS (where aortic valve replacement is necessary) and pseudo-severe AS (where conservative management is recommended). Additional diagnostic studies such as dobutamine stress echocardiography and/or cardiac computed tomography, as well as other parameters such as projected AVA and/ or valvuloarterial impedance may be helpful. This chapter will review diagnostic approaches and prognostic implications of different AS subtypes.

**Keywords:** aortic stenosis, classification, echocardiography, hemodynamics, low flow, prognosis
