Author + information
- Philippe Pibarot, DVM, PhD⁎ ( and )
- Jean G. Dumesnil, MD
- ↵⁎Reprint requests and correspondence:
Dr. Philippe Pibarot, Québec Heart and Lung Institute, 2725 Chemin Sainte-Foy, Québec QC G1V 4G5, Canada
Low-flow, low-gradient aortic stenosis (AS) is a highly challenging condition in terms of diagnosis and therapeutic management. The transvalvular pressure gradient is inversely related to the square of aortic valve area (AVA) and directly related to the square of flow. Hence, a patient with severe AS may nonetheless present with a low gradient if his or her left ventricular (LV) output is reduced. This situation is frequently observed in patients with depressed LV ejection fraction (LVEF) who often have a low transvalvular flow and thus a low gradient despite the presence of severe AS. Conversely, it is generally assumed that patients with severe AS and preserved LVEF should necessarily have a high transvalvular gradient. However, recent studies revealed that this perception is erroneous and that a substantial proportion of patients with severe AS may indeed have a low transvalvular flow and thus a low gradient despite a preserved LVEF (1–6).
In this issue of the Journal, Hermann et al. (7) present a comprehensive and compelling set of data including Doppler echocardiography, cardiac catheterization, magnetic resonance imaging, blood biomarkers, and myocardial biopsies in a series of 71 consecutive patients with severe AS having undergone aortic valve replacement (AVR) compared with a control group of 17 patients with moderate AS. Importantly, this study provides novel insights into the pathophysiology of low-flow, low-gradient AS and further confirms that patients with this entity are at a more advanced stage of their disease. In particular, they elegantly demonstrate that, compared with patients with classic, normal-flow, high-gradient AS, these patients have subendocardial myocardial fibrosis that is at least partially irreversible and that translates clinically into reduced LV longitudinal shortening measured using either systolic mitral ring displacement or tissue Doppler imaging. Such information is precious and should contribute to improve our understanding of pathophysiology as well as our diagnostic accuracy.
Clinical Spectrum of Severe AS
The results of Hermann et al. (7) add to other recently published studies (1–6) to confirm that the clinical spectrum of severe AS is more complex than previously believed and includes, in fact, 3 main entities (Table 1). The common denominator for these 3 entities is the presence of a small AVA (≤1.0 cm2 and/or indexed AVA of ≤0.6 cm2/m2), whereas they may differ markedly in terms of LVEF and transvalvular flow and gradient, thus complicating clinical diagnosis and therapeutic management.
Normal-flow, high-gradient AS
LV concentric hypertrophy develops in the majority of patients with severe AS in response to pressure overload. The cavity size is generally normal or mildly reduced compared with healthy subjects (Table 1). The increase in wall thickness associated with LV concentric hypertrophy results in a greater contribution of wall thickening to endocardial inward displacement (8). Consequently, LVEF remains in the normal range or may even be supranormal, which allows maintenance of normal LV pump function and transvalvular flow rate. These patients with severe AS and normal flow rate exhibit a high gradient. Hence, this entity does not present any particular challenge with regard to grading of AS severity and therapeutic management because all Doppler echocardiographic indexes of stenosis severity (i.e., AVA, ≤1.0 cm2; mean gradient, ≥40 mm Hg; peak aortic jet velocity, ≥4 m/s) are consistent with regard to the framework of the guidelines and indicate the presence of severe AS.
Low LVEF, low-flow, low-gradient AS
This entity represents approximately 5% to 10% of the AS population. These patients generally have a dilated LV cavity with markedly depressed myocardial systolic function, most often due to a concomitant cardiomyopathy in relation to ischemic heart disease and/or to afterload mismatch (Table 1). They have a poor prognosis if treated medically but a high operative mortality if treated surgically. Low-dose dobutamine stress echocardiography is particularly helpful in these patients: 1) to assess the presence of myocardial contractile reserve; and 2) to differentiate a pseudosevere from a true severe stenosis.
Preserved LVEF, low-flow, low-gradient AS
We recently reported that a restrictive physiology may develop in some patients with severe AS on the basis of the AVA (i.e., <1.0 cm2 and/or indexed AVA of <0.6 cm2/m2), resulting in lower transvalvular flow rates (i.e., stroke volume index <35 ml/m2 and/or mean transvalvular flow rate <200 ml/s) and lower than expected transvalvular gradients (i.e., <40 mm Hg) despite the presence of a preserved LVEF (i.e., ≥50%), and we named this clinical entity “paradoxical low-flow, low-gradient AS” (1) (Table 1). The results of Hermann et al. (7) further confirm that this entity may be present in a substantial proportion of patients, and indeed, the prevalence that they report for paradoxical low-flow, low-gradient, severe AS (15%) is consistent with the range of 14% to 24% observed in previous studies (1–4,6).
Pathophysiology of Preserved LVEF, Low-Flow, Low-Gradient Severe AS
Additional characteristics previously reported with regard to this entity include higher prevalence of women, older age, higher global LV hemodynamic load as reflected by higher valvuloarterial impedance, higher degree of LV concentric remodeling, smaller LV end-diastolic volume, and reduced LV mid-wall myocardial shortening (1–5). Furthermore, other studies have shown that LV longitudinal shortening is often decreased in patients with AS and that the extent of this decrease is directly related to the degree of LV concentric remodeling (8,9). In this regard, the study of Hermann et al. (7) provides important new information in revealing that these patients with paradoxical low-flow, low-gradient AS also have more extensive subendocardial myocardial fibrosis. Moreover, they show that the extent of tissue fibrosis is closely correlated with the levels of N-terminal pro–B-type natriuretic peptide and procollagen type III N-terminal peptide (r = 0.63 and r = 0.69, respectively; both p < 0.01) and inversely and closely correlated to the extent of myocardial longitudinal shortening as measured by mitral ring displacement (r = −0.79, p < 0.0001). Interestingly, the latter is also inversely related to the valvuloarterial impedance (i.e., the global LV hemodynamic load).
Hence, these observations offer further evidence that, compared with patients with normal flow and high gradient, those with paradoxical low flow, low gradient are at a more advanced stage of their disease, from both structural and functional standpoints. Indeed, it can be readily surmised that a greater and more long-standing increase in LV hemodynamic load results in more pronounced LV concentric remodeling, a smaller LV cavity, more extensive myocardial fibrosis, and thus more severe impairment of LV filling and intrinsic myocardial function. Furthermore, with these results, we now have direct evidence that longitudinal myocardial shortening is affected to a larger extent in these patients due to more advanced fibrosis in the subendocardial layer where the myocardial fibers are oriented longitudinally, as hypothesized more than 30 years ago (8). As well, these abnormalities are largely irreversible after surgery, which is in accordance with the concept that these patients are potentially operated on too late in the course of their disease due to an underestimation of their pathology in relation to the low gradient.
Clinical Recognition of Low-Flow, Low-Gradient Severe AS
Clinically, paradoxical low-flow, low-gradient AS is certainly the most insidious of the 3 clinical entities described here because the valve stenosis may appear less severe on the basis of a lower transvalvular gradient (Table 1). Moreover, LV systolic function may appear normal on the basis of the preserved LVEF, when, in fact, these patients have a severe stenosis and a more pronounced impairment of myocardial structure and function. Because of this particular mode of presentation, this entity is often misdiagnosed, which may lead to underestimation of disease severity and inappropriate delay of AVR. Hence, previous studies report that these patients have a 40% to 50% lower referral to surgery compared with those with normal flow (1,2,4), likely due to underestimation of stenosis severity in light of the relatively low gradient. Indeed, the presence of a high gradient (>40 mm Hg) and/or a high peak jet velocity (>4 m/s) has a high specificity to identify severe AS. However, because it is highly flow dependent, the gradient (as well as the peak velocity) lacks sensitivity and negative predictive value (e.g., 71% and 46%, respectively, in the study of Hermann et al. ). Hence, 1 important message that these findings reiterate is that the presence of a low gradient (<40 mm Hg) or velocity (<4 m/s) does not exclude the presence of a severe stenosis in patients with preserved LVEF and that particular attention should be paid to patients with discordant findings (i.e., with small AVA and low gradient), especially if they are symptomatic.
As well, the findings of Hermann et al. (7) also further demonstrate that parameters of LV systolic function based on endocardial displacement can remain normal despite significant myocardial damage (5,8,9). In this regard, it is important to underscore that LVEF is the only index that is included in the guidelines to identify LV systolic dysfunction, which is a Class I indication for AVR. Unfortunately, LVEF markedly underestimates the extent of myocardial systolic impairment in the presence of LV concentric hypertrophy such as is often the case in AS patients and particularly in those with paradoxical low flow (3,5,7–9). Indeed, the findings of Hermann et al. (7) provide further argument for the routine use of indexes of myocardial longitudinal function (e.g., mitral ring displacement, global longitudinal strain) in addition to LVEF to identify LV systolic dysfunction and eventually recommend AVR in patients with severe AS.
In this context, it should be emphasized that the main pitfall associated with the echocardiographic diagnosis of paradoxical low-flow and/or low-gradient AS is an error in the calculation of the stroke volume. Indeed, this measurement is included in the calculation of many parameters including AVA, systemic arterial compliance, and valvuloarterial impedance; moreover, it is derived from 2 separate measurements (i.e., LV outflow tract diameter and LV outflow tract time-velocity integral), each having their potential for error. Hence, a comprehensive Doppler echocardiographic evaluation including independent measurements as well as other diagnostic modalities including valve calcium scoring by computed tomography and plasma B-type natriuretic peptide dose have been advocated to differentiate paradoxical low-flow, low-gradient severe AS from other situations (i.e., measurement errors, pseudosevere AS) (4). Indeed, the findings of Hermann et al. (7) further contribute to this recommendation by showing that the indexes of myocardial longitudinal function should also be valid for this purpose and should thus improve our diagnostic accuracy.
Obviously, additional outcome studies are needed to determine the most appropriate modality and timing of treatment in patients with low-flow, low-gradient AS. However, in these future studies, it is important to emphasize that it would not be adequate to include AVR in the endpoints, given that this endpoint is essentially determined by the treating physician's perception of disease severity, which, in turn, is highly influenced by the magnitude of the gradient (or peak jet velocity), the latter being often pseudo-normalized in patients with low-flow states (with preserved or reduced LVEF). Hence, the most appropriate and robust endpoints for future studies should be the occurrence of heart failure and/or cardiovascular mortality, regardless of the rate of AVR (1,2,4).
The authors have reported that they have no relationships to disclose.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
- American College of Cardiology Foundation
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