Energy loss index in aortic stenosis: from fluid mechanics concept to clinical application.

aortic valve replacement (AVR) is considered a class I indication in patients with aortic stenosis (AS) if the stenosis is severe and the patient has symptoms or left ventricular ejection fraction <50%. Hence, accurate assessment of the hemodynamic severity of the valvular stenosis is crucial for clinical decision making. The stenosis severity is generally determined by measuring the transvalvular pressure gradient or the aortic valve effective orifice area (AVA); however, these conventional parameters do not account for the extent of pressure recovery that may occur downstream of the stenosis. In an article published in 2000 in Circulation, 3 we proposed a new Doppler echocardiographic parameter based on the energy loss concept to adjust the AVA for pressure recovery, and we postulated that this energy loss index (ELI) would improve assessment of stenosis severity and risk stratification in AS. Thirteen years later, Bahlmann and colleagues 4 publish in this issue of Circulation the first prospective study to demonstrate that ELI provides independent and incremental prognostic information to that derived from conventional measures of AS severity. In this elegant substudy of the SEAS (Simvastatin Ezetimibe in Aortic Stenosis) trial, the authors report that a 1 cm²/m² reduction in baseline ELI predicts a 2-fold increase in the risk of aortic valve events and of the composite of mortality and heart failure hospitalization after adjustment for peak aortic jet velocity or mean gradient. The current guidelines 1,2 make no distinction between cath-eterization and Doppler echocardiographic measurements, as though values for gradient and AVA measured by either technique were interchangeable (Figure 1). Yet Doppler estimates the maximum pressure drop through the valve from the maximum flow velocity recorded at the level of the vena contracta, whereas catheterization provides a measure of the net gradient between the left ventricle and the ascending aorta (Figure 1). However, as blood flow decelerates between the valve and the ascending aorta, part of the kinetic energy is converted back to static energy because of a phenomenon called pressure recovery , and hence, the maximum pressure gradient measured by Doppler overestimates the net gradient, that is, the " irreversible " gradient, recorded at catheterization (Figure 1). Likewise, the AVA obtained at catheterization by use of the Gorlin formula is derived from recovered pressures, such that its value is higher than the Doppler AVA derived by the continuity equation. The latter measures the actual area occupied by flow at the valvular level …