Many patients with severe aortic stenosis have a "low-flow, low-gradient" aortic stenosis. The management of these patients can be quite difficult, as these patients often show impairment of the left ventricle, which can lead to false measurements of the severity of stenosis and also leads to a higher risk during aortic valve replacement. More diagnostic tools than only standard echocardiography are needed to correctly differentiate true severe aortic stenosis from pseudo severe aortic stenosis.

An important proportion of patients with aortic stenosis (AS) have a 'low-gradient' AS, i.e. a small aortic valve area (AVA <1.0 cm(2)) consistent with severe AS but a low mean transvalvular gradient (<40 mmHg) consistent with non-severe AS. The management of this subset of patients is particularly challenging because the AVA-gradient discrepancy raises uncertainty about the actual stenosis severity and thus about the indication for aortic valve replacement (AVR) if the patient has symptoms and/or left ventricular (LV) systolic dysfunction. The most frequent cause of low-gradient (LG) AS is the presence of a low LV outflow state, which may occur with reduced left ventricular ejection fraction (LVEF), i.e. classical low-flow, low-gradient (LF-LG), or preserved LVEF, i.e. paradoxical LF-LG. Furthermore, a substantial proportion of patients with AS may have a normal-flow, low-gradient (NF-LG) AS: i.e. a small AVA-low-gradient combination but with a normal flow. One of the most important clinical challenges in these three categories of patients with LG AS (classical LF-LG, paradoxical LF-LG, and NF-LG) is to differentiate a true-severe AS that generally benefits from AVR vs. a pseudo-severe AS that should be managed conservatively. A low-dose dobutamine stress echocardiography may be used for this purpose in patients with classical LF-LG AS, whereas aortic valve calcium scoring by multi-detector computed tomography is the preferred modality in those with paradoxical LF-LG or NF-LG AS. Although patients with LF-LG severe AS have worse outcomes than those with high-gradient AS following AVR, they nonetheless display an important survival benefit with this intervention. Some studies suggest that transcatheter AVR may be superior to surgical AVR in patients with LF-LG AS.

In recent years, transcatheter aortic valve implantation (TAVI) has become an established treatment option for selected high-risk patients with severe aortic stenosis (AS). Favorable results with regard to both hemodynamics and clinical outcome have been achieved with transcatheter valves. Aortic regurgitation (AR) remains a major concern after TAVI. Echocardiography is the imaging modality of choice to assess AR in these patients due to its wide accessibility and low cost. Mostly mild residual AR has been observed in up to 70% of patients. However, as even a mild degree of AR has been associated with a decreased survival up to two years after TAVI, accurate evaluation and classification of AR is important. AR in transcatheter valves can be divided into three types according to different pathophysiological mechanisms. Besides the well-known transvalvular and paravalvular forms of regurgitation, a third form termed supra-skirtal has recently been observed. A thorough understanding of AR in transcatheter valves may allow to improve device design and implantation techniques to overcome this complication. The aim of this review is to provide an overview of the three types of AR after TAVI focussing on the different pathophysiological mechanisms.