Usefulness of the energy loss index in the adjudication of low-gradient aortic stenosis severity

This editorial refers to ‘Clinical significance of energy loss index in patients with low-gradient severe aortic stenosis and preserved ejection fraction’, by A. Altes et al., pp. 608–615. Up to 40% of patients with aortic stenosis (AS) present with a low gradient [i.e. a mean transvalvular gradient (ΔP) < 40 mmHg] despite a small aortic valve area (AVA ≤ 1 cm2) at echocardiography or cardiac catheterization. This ‘discordant grading’ situation raises uncertainty about the true severity of AS and therefore about therapeutic decision-making.1 A thorough, integrative approach including assessment of flow status and quantitation of aortic valve calcium score by multidetector computed tomography (MDCT) has been proposed in the 2017 European guidelines for discriminating true vs. pseudo-severe AS in the patients with low-gradient AS.2 It is, indeed, estimated that 50–70% of patients with low-gradient AS have a true-severe AS and thus an indication (Class I or IIa) for aortic valve replacement. However, it remains crucial to identify patients with pseudo-severe AS, who should be managed conservatively. The predominant cause of the concomitance of a small AVA with a low gradient is the presence of a low-flow state. Indeed, for a given degree of AS severity, both gradients and AVA decrease with reduced left ventricular (LV) outflow, therefore amplifying the AVA-gradient discordance. Besides low-flow state, other factors, including the pressure recovery phenomenon, may lead to a low-gradient AS pattern. Downstream to the aortic valve, a portion of the pressure gradient initially lost at the level of the vena contracta is recovered (Figure 1).3 This phenomenon explains why the catheter-derived gradient is generally lower and AVA smaller than those measured by Doppler-echocardiography. Indeed, echocardiography measures the gradient and AVA at the level of the vena contracta, i.e. prior to pressure recovery, whereas catheter measures these parameters a few centimetres downstream to the vena contracta, i.e. after pressure recovery. For a given AVA, the magnitude of the pressure recovery (and thus of the echo-catheter discrepancy) is more important in a patient with a small vs. a large aorta. To account for this pressure recovery phenomenon, we proposed in 2000 to calculate the energy loss index (ELI) using the formula:4 ELI = [AVA × Aa/(Aa − AVA)]/BSA, where Aa is the cross-sectional area of the aorta at the level of the sinotubular junction and BSA is the body surface area. Of note, this parameter provides an estimation of the indexed AVA that would be obtained by cardiac catheterization and has been shown to be superior to the echo-derived indexed AVA to predict outcomes in patients with AS.5,6 Implication of the pressure recovery phenomenon in the adjudication of low-gradient AS severity. (A) Schematic representation of systolic blood flow and PLVOT, aortic valve and AA, and the corresponding invasive (cardiac catheterization) and non-invasive (Doppler echocardiography) measurements. Doppler echocardiography indirectly measures the ΔPmax at the VC, whereas cardiac catheterization measures aortic pressure and thus pressure gradient more distally (i.e. after pressure recovery; ΔPnet). The effective AVA, i.e. the cross-sectional area of the VC, calculated by Doppler-echocardiography is thus smaller than the ‘recovered’ aortic AVA calculated by catheterization. The energy loss index (ELI) takes into account the pressure recovery phenomenon and provides a non-invasive estimation of the AVA that would be obtained by catheterization. This parameter better represents the actual energy loss caused by the stenosis and the degree of pressure overload imposed on the left ventricle. (B) Proposed algorithm for the management of patients with low-gradient AS and preserved LVEF. *Particularly in patients with a small AA (i.e. sinotubular junction <3.0). AA, ascending aorta; AS, aortic stenosis; AVA, aortic valve area; AVAI, indexed AVA; AU, Agatston Units; AVC, aortic valve calcium score; BSA, body surface area; ELI, energy loss index; LVEF, left ventricular ejection fraction; PLVOT, pressure across in the left ventricular outflow tract; VC, vena contracta; ΔP, mean transvalvular pressure gradient; ΔPmax, maximum pressure gradient.The schemtic representation in panel A is adapted with permission from Pibarot et al.7 Implication of the pressure recovery phenomenon in the adjudication of low-gradient AS severity. (A) Schematic representation of systolic blood flow and PLVOT, aortic valve and AA, and the corresponding invasive (cardiac catheterization) and non-invasive (Doppler echocardiography) measurements. Doppler echocardiography indirectly measures the ΔPmax at the VC, whereas cardiac catheterization measures aortic pressure and thus pressure gradient more distally (i.e. after pressure recovery; ΔPnet). The effective AVA, i.e. the cross-sectional area of the VC, calculated by Doppler-echocardiography is thus smaller than the ‘recovered’ aortic AVA calculated by catheterization. The energy loss index (ELI) takes into account the pressure recovery phenomenon and provides a non-invasive estimation of the AVA that would be obtained by catheterization. This parameter better represents the actual energy loss caused by the stenosis and the degree of pressure overload imposed on the left ventricle. (B) Proposed algorithm for the management of patients with low-gradient AS and preserved LVEF. *Particularly in patients with a small AA (i.e. sinotubular junction <3.0). AA, ascending aorta; AS, aortic stenosis; AVA, aortic valve area; AVAI, indexed AVA; AU, Agatston Units; AVC, aortic valve calcium score; BSA, body surface area; ELI, energy loss index; LVEF, left ventricular ejection fraction; PLVOT, pressure across in the left ventricular outflow tract; VC, vena contracta; ΔP, mean transvalvular pressure gradient; ΔPmax, maximum pressure gradient.The schemtic representation in panel A is adapted with permission from Pibarot et al.7 Altes et al.8 reported that in patients with low-gradient AS (ΔP < 40 mmHg and an indexed AVA < 0.6 cm2/m2) and preserved left ventricular ejection fraction (LVEF), ELI was able to reclassify AS from severe (based on indexed AVA) to moderate in an important proportion of the cases. These patients reclassified as moderate by ELI had significantly lower rate of events (cardiovascular mortality and aortic valve replacement) compared to those confirmed to be severe. As opposed to the indexed AVA measured by echocardiography, the ELI has the advantage to account for pressure recovery and thus better estimate the actual LV pressure loss resulting from the interaction of the stenotic aortic valve and the aorta (Figure 1). However, the present study suggests that 40% of patients with low-gradient AS are reclassified from severe to moderate on the basis of ELI. If the vast majority of these reclassified cases were related to the pressure recovery phenomenon, one would expect much lower proportion of discordant grading cases and thus of low-gradient AS with cardiac catheterization vs. with echocardiography. This is, however, not the case and several studies reported that the proportion of patients with discordant grading were similar to echocardiography vs. catheterization.9,10 Hence, pressure recovery may not be the main factor explaining the reclassification from severe to moderate AS by ELI in the present study. A large proportion of the reclassification may rather be related to the fact that an ELI <0.6 cm2/m2 selected a subset of patients with more severe AS. Indeed, the ELI is, de facto, systematically larger than the indexed AVA measured by echocardiography5 and, in the present study, the same severity cut-point (<0.6 cm2/m2) was applied for both parameters. As a matter of fact, patients confirmed to be severe on the basis of ELI had significantly smaller indexed AVA and higher gradients than those reclassified to moderate. Furthermore, the stroke volume index was substantially lower in severe vs. reclassified moderate AS patients and all parameters used in this study, including peak aortic velocity, mean gradient, indexed AVA, and ELI, are inherently flow-dependent. To this effect, the main limitation of this study is the lack of any flow-independent parameter such as the MDCT aortic valve calcium score. In the present series, only one-third of the patients had AS-related symptoms. According to current guidelines,2,11 asymptomatic patients with low-gradient AS and preserved LVEF have no indication for valve replacement, regardless of their AS severity. Hence, the measurement of additional parameters or performance of additional tests to confirm the AS severity is probably less relevant in this subset of patients. In light of the findings of the present study8 as well as of previous studies,4,6,12,13 we would like to propose the following algorithm for symptomatic patients with low-gradient AS and preserved LVEF (Figure 1): (i) perform non-contrast MDCT to measure aortic valve calcium score;14 (ii) if aortic valve calcium score is severe (≥1200 AU in women and ≥2000 AU in men), consider aortic valve replacement (IIa indication); and (iii) if aortic valve calcium score is not severe or borderline, calculate ELI; if ELI is <0.6 cm2/m2, i.e. severe, consider aortic valve replacement; if ELI is >0.6 cm2/m2, consider close clinical and echocardiography follow-up. Conflict of interest: none declared 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.

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