This editorial refers to ‘Multimodality imaging derived energy loss index and outcome after transcatheter aortic valve replacement’ by E.W. Holy et al., pp. 1092--1102
Aortic valve stenosis (AS) is the commonest valvular heart disease leading to surgery or catheter intervention worldwide.1 Echocardiography is the mainstay imaging technique to diagnose AS, providing information on aetiology and severity, presence of cardiac damage and concomitant valvular diseases that may also need intervention. International guidelines recommend the use of three main echocardiographic parameters to define AS severity: peak velocity, mean transvalvular gradient, and aortic valve area (AVA).2,3 Moreover, left ventricular ejection fraction (LVEF) and stroke volume should also be assessed to define the flow-status and identify the type of AS: high-flow high-gradient, low-flow low-gradient, and normal-flow low-gradient.2,3
The continuity equation is used to estimate the AVA when using echocardiography (Figure 1). The several limitations of this approach have been endlessly discussed in the literature.4 The use of the left ventricular outflow tract (LVOT) diameter to derive the LVOT area is the most important source of inaccuracy. By assuming a circular geometry of the LVOT when it is well-known that the shape is elliptical, we introduce in the continuity equation an error that is squared, leading to significant underestimation of AVA.4 When considering the mean transvalvular gradient as a measure of AS severity, it is important to remember the pressure recovery phenomenon that happens downstream of a stenotic aortic valve.5 If the magnitude of the pressure recovery phenomenon is significant, echocardiography may overestimate the transvalvular pressure gradient. This is frequently observed when the ascending aorta is <30 mm (as it may happen in congenital AS) and in bileaflet aortic valve mechanical prostheses. The pressure recovery phenomenon can be estimated calculating the energy loss index (ELI). The ELI includes in the formula the AVA calculated by the continuity equation and the cross-sectional area of the ascending aorta (Figure 1). Therefore, the ELI calculation may fall into the same source of errors as AVA calculation.
Estimation of AVA and ELI from TTE using the continuity equation. Ao, aorta; BSA, body surface area; LA, left atrium; LV, left ventricle; LVOT, left ventricular outflow tract; STJ, sinotubular junction; VTI, velocity time integral.
The superior accuracy of computed tomography (CT) to measure the aortic valve annulus to select the prosthesis size, and to assess the anatomic suitability of the femoral arteries of patients with severe AS who may be treated with transcatheter aortic valve implantation (TAVI) has led to an exponential increase of the use of this imaging technique in clinical practice. Several studies have shown the elliptical shape of the LVOT and the implications of using the CT-derived planimetered LVOT area in the continuity equation.6 Kamperidis et al.6 showed that the introduction of the CT-derived planimetered LVOT area into the continuity equation reclassified 16% of severe AS into moderate AS, and this percentage was significantly higher in patients with normal-flow low-gradient severe AS (52%).
In this issue of the European Heart Journal of Cardiovascular Imaging, Holy et al.7 showed that CT and echocardiographic data can be combined to improve the grading of AS severity. Of 197 patients diagnosed with severe AS based on echocardiography, if the CT-derived planimetered LVOT area was introduced into the continuity equation, 62 (31%) would have been reclassified as having moderate AS. The percentage of reclassification into moderate AS was higher than that reported by Kamperidis et al.6 This could be related to differences in patient population since the study by Kamperidis et al.6 only included patients with an LVEF ≥50% (63 ± 7%), whereas in the study by Holy et al.7 the mean LVEF was 51 ± 9.6%.
The novelty of the study by Holy et al. relies on the reclassification of patients with severe AS based on the use of CT to measure the cross-sectional area of the LVOT and ascending aorta and integrating them into the formula to calculate fusion ELI. Defining severe AS based on a fusion ELI <0.6 cm2/m2 led to the reclassification of 85 patients (43% of the study population) from severe AS to moderate AS. Furthermore, the study provides prognostic data and shows that patients with a fusion ELI >0.6 cm2/m2 (i.e. moderate AS) had better survival rates at 3 years after TAVI compared to those with a fusion ELI <0.6 cm2/m2 (i.e. severe AS).
These results highlight the fact that we may have been referring patients with moderate AS for aortic valve replacement who were diagnosed as having severe AS based on echocardiography. In addition, in this study, the patients with a less advanced valvular disease (i.e. moderate AS) seemed to benefit more from TAVI. It should be acknowledged that moderate AS may lead to symptoms which are usually underestimated particularly in patients with associated comorbidities, such as heart failure, coronary artery disease, or anaemia. However, to know whether TAVI may improve or not the outcomes of patients with moderate AS, we will need to wait the results of an ongoing randomized clinical trial.8 On the other hand, it has been suggested that when using CT data to grade the severity of AS based on AVA, a higher cut-off value (<1.2 cm2) should be considered as marker of severe AS.9
Another important message that the present study underscores is the need of multimodality imaging for accurate risk stratification of patients with severe AS.4 Current recommendations include the assessment of the aortic valve calcification with CT to diagnose severe AS when patients present with low-flow low-gradient severe AS and do not show flow reserve on low-dose dobutamine stress echocardiography.4 Furthermore, cardiac magnetic resonance with T1 mapping, extracellular volume estimation, and late gadolinium enhancement can identify diffuse or localized fibrosis that are known markers of adverse LV remodelling and may help to define the time of intervention.10 The ongoing EVOLVeD trial is currently recruiting patients with asymptomatic severe AS and mid-wall (non-ischaemic) fibrosis to randomize them to early intervention (aortic valve replacement) vs. routine care and will provide new evidences on the topic.11
Current recommendations consider three main parameters to define severe AS (mean transvalvular gradient > 40 mmHg, peak jet velocity > 4 ms, and an AVA < 1 cm2). When these three parameters do not coincide, it is important to search for potential sources of error measurements and correct them to provide an accurate diagnosis and appropriate treatment. In addition, the consequences of severe AS on the LV, left atrium, pulmonary vasculature, and right ventricle should not be ignored and can be accurately assessed with multimodality imaging. With current imaging technologies and good outcomes of surgical aortic valve replacement and TAVI, to get it right seems, therefore, easier than ever.
Conflict of interest: The Department of Cardiology of the Leiden University Medical Center received research grants from Abbott Vascular, Bioventrix, Medtronic, Biotronik, Boston Scientific, GE Healthcare, and Edwards Lifesciences. V.D. received speaker fees from Abbott Vascular, Medtronic, Edwards Lifesciences, MSD, and GE Healthcare. The remaining author has no conflict of interest to declare.
The opinions expressed in this article are not necessarily those of the Editors of EHJCI, the European Heart Rhythm Association or the European Society of Cardiology.
