**1. Introduction**

Aortic valve stenosis (AS) is the most common valvular heart disease in developed countries. When symptomatic, AS is known to have significant morbidity and mortality. While the prevalence of AS is expected to rise with the aging population, there is no pharmacological treatment option to prevent its progression at this time [1, 2]. Aortic valve replacement (AVR) is the only treatment demonstrated to improve survival and symptoms [3, 4]. Therefore, in the management of patients with AS, it is essential to accurately diagnose the disease severity and determine the proper timing of surgical referral. According to the ACC/AHA guidelines, AVR is class I indication for patients with symptomatic severe AS with high transaortic mean gradient (MG) ≥ 40 mmHg and left ventricular (LV) ejection fraction (LVEF) < 50% and/or who are undergoing another surgery [5]. Over the past decade, challenges due to discrepancies with grading


#### **Table 1.**

*Hemodynamic classification of severe aortic stenosis (AVA < 1.0 cm2 ).*

AS severity and the necessity of integrating the valve gradient with flow patterns were recognized when a significant subset of patients were found to have small AVAs suggestive of severe AS with lower gradients despite preserved LVEF [6]. As a result, under the umbrella of severe AS (based on AVA < 1.0 cm2 ), a new hemodynamic classification of AS was proposed which can be categorized into six subgroups based on LV flow state [normal flow (NF) vs. low flow (LF)] and pressure gradient [very high gradient (VHG) vs. high gradient (HG) vs. low gradient (LG)]. These six flowgradient patterns (NF/VHG, NF/HG, LF/HG, LF/LG with reduced LVEF, LF/LG with preserved LVEF, and normal NF/LG) have shown to represent distinct pathophysiologic types of severe AS with different clinical outcomes (see **Table 1**).
