Author + information
- Published online September 4, 2017.
- Federico M. Asch, MD∗ ()
- MedStar Cardiovascular Research Network at MedStar Washington Hospital Center, and Georgetown University School of Medicine, Washington, DC
- ↵∗Address for correspondence:
Dr. Federico M. Asch, MedStar Cardiovascular Research Network at MedStar Washington Hospital Center, 100 Irving Street, NW, Suite EB 5123, Washington, DC 20010.
There has been a significant evolution in the understanding of the role and function of the left atrium (LA) over the last few decades. Historically viewed as a mere conduit for oxygenated blood coming from the pulmonary veins into the left ventricle, it is clear now that this “conduit” is actually active and that left atrial contraction plays a significant role in maintaining an appropriate cardiac output. In line with such a simplistic view of its function, imaging of the LA has been limited for decades to measuring its size by M-mode or 2-dimensional (2D) echocardiography. The prognostic significance of such dimensions as markers of LA size has been validated in large cohorts of apparently normal individuals (1) and in patients with a variety of cardiovascular conditions such as hypertension (2), ischemic heart disease (3), or left ventricular (LV) dysfunction (4–6). The understandings that the LA size and shape were not well-depicted by simple diameters lead to the current recommendations of measuring LA volumes (7). Moreover, volumetric measurements performed throughout the cardiac cycle lead to the “functional” evaluation of the LA by calculation of LA ejection fraction and other indexes (see a recent review by Hoit for further details) (8).
Whether it is due to its conduit or pump function, evaluation of the LA is nowadays well-accepted as a marker of cardiovascular health and, therefore, an established biomarker for prognosticating cardiovascular outcomes. LA volumes were measured initially by the biplane method of disks from 2-dimensional echocardiography and more recently by more accurate 3-dimensional (3D) imaging modalities such as echocardiography, cardiac computed tomography, or cardiac magnetic resonance. Importantly, although data to validate measurements of each of these techniques have been building, there has been a lack of information on the prognostic value of these “gold standard” 3D techniques.
By evaluating LA volumes through 3D echo in a large, community-based cohort of elderly individuals and following them for an average of 7 years, Russo et al. (9) provide in this issue of iJACC, a new step toward the validation of 3D LA imaging and at the same time brings a new dimension to our understanding of LA function. First, although in theory direct 3D volumetric analysis should be more accurate than 2D (which is based on geometric assumptions and equations), it is difficult to match the wealth of data supporting the clinical and prognostic value of 2D (and even m-mode) measurements. Russo et al. (9) provide for the first time data on a large cohort with long follow-up. Their 3D echocardiographic analysis is novel in that measures maximal and minimal LA volume index and provides prognostic information for LA size. They also report on the use of volumetric analysis to infer functional indexes (surrogates for reservoir and pumping). Although not yet as robust as prior data from m-mode or 2D echocardiographic imaging, their findings provide a significant step toward such validation of 3D imaging as a prognosticator for cardiovascular events, which adds to recent data from the MESA (Multi-Ethnic Study of Atherosclerosis) study by cardiac magnetic resonance (10). Second, their data suggest that it is not the maximal LA volume (before mitral valve opening) but rather the minimal volume (before mitral valve closure) that predicts CV outcomes best. The minimal LA volume is likely to reflect the pumping function of the LA, in addition to LV diastolic pressures, with worse pumping resulting in larger minimal LA volume (and a lower LA emptying volume, also observed in this study). In contrast, the maximal LA volume is likely to reflect the function of conduit or reservoir (how much the LA is filled while the mitral valve is closed) (Figure 1). I find this observation very interesting and provocative; it raises the hypothesis that impairment of the LA contractile (pumping) function is an early marker of cardiovascular disease. It is important to notice, however, that LA function and size cannot be isolated from LV diastolic filling pressures, because both passive and active LA emptying ultimately depend on pressure gradients across the mitral valve. Therefore, LA volumes and function will be affected by most cardiovascular conditions altering the normal LV physiology. Interestingly, over the years we have focused our attention on maximum LA size (2D, m-mode, LA volumes) as currently recommended (7), consistent with our old understanding of the LA being a mere “conduit.” Maybe it is time to start acknowledging its importance and evaluate LA pumping function in addition to size as we currently do on a daily basis.
Looking at the perspectives opened by this study, it is important to point out a few aspects. First, to properly standardize 3D evaluation, normative data across different groups (age, sex, race, or body size) should be better defined. Although the current report from Russo et al. (9) provides a stab at normal values, we should take this aspect cautiously. Individuals enrolled in this study are not representative of the general population, but rather of an elderly group without heterogeneous racial/ethnic background. Studies such as NORRE (Normal Reference Ranges for Echocardiography) in Europe (11) or the ongoing WASE (World Alliance of Societies of Echocardiography) study will better define this important aspect of the evolution in 3D LA imaging. Similarly, knowing that LA size and function are influenced heavily by multiple cardiac conditions, such as diastolic and systolic LV dysfunction or valvular heart disease, the novel LA biomarkers should be tested rigorously in states of disease. Second, this study opens the door to opportunities to evaluate the prognostic value of LA function with newer techniques such as strain, 3D strain, and vector flow mapping. Finally, there remains a need to better determine if the prognostic value provided by 3D is incremental to that of older, better established 2D techniques.
I hope the questions herein raised are not taken as a critique to the study by Russo et al. (9). They rather highlight the importance and potential of their findings, an obligation to further pursue the investigation of novel LA measurements and of a new understanding of left atrial function… a new dimension in echocardiography with significance beyond imaging.
↵∗ Editorials published in JACC: Cardiovascular Imaging reflect the views of the authors and do not necessarily represent the views of JACC: Cardiovascular Imaging or the American College of Cardiology.
Dr. Asch has reported that he has no relationships relevant to the contents of this paper to disclose.
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