Author + information
- Saibal Kar, MD∗ ( and )
- Rahul Sharma, MD
- ↵∗Reprint requests and correspondence:
Dr. Saibal Kar, Heart Institute, Cedars-Sinai Medical Center, 8631 West 3rd Street, Suite 415E, Los Angeles, California 90048.
Mitral regurgitation (MR) is a common valvular disorder affecting more than 2 million people in the United States (1). The etiology of MR can be divided into primary MR, caused by pathology of the valve apparatus, and secondary MR, a functional consequence of ventricular dysfunction. The clinical course of MR is generally insidious, and if left untreated, leads to heart failure and ultimately death. The most challenging aspect in the management of MR is accurate quantification of severity and consequent decision regarding timing of intervention.
To date, echocardiography has been the gold standard tool of investigation (2). Despite technological advancements, there are limitations regarding its utility for the quantification of MR. First, obtaining accurate data is heavily operator dependent and limited by patient habitus and acoustic windows. Further difficulties are encountered with prosthetic valves or rings, which cause acoustic shadowing. Transcatheter mitral valve repair using the MitraClip (Abbott Vascular, Santa Rosa, California) to create a double orifice mitral valve has emerged as a treatment for selected patients with severe MR. Unfortunately, most qualitative and quantitative echocardiographic methods have not been validated in a double-orifice valve model. The creation of high-velocity jets on either side of the clip may give the appearance of significant residual MR, even though the actual regurgitant volume is low (3). In the EVEREST (Endovascular Valve Edge-to-edge REpair STudy), the MitraClip has shown less complete reduction of MR compared with the surgical arm. However, at 4 years, there was equivalent clinical benefit with evidence of favorable remodeling in both groups. It is quite possible that creation of a double orifice resulted in an overestimation of MR grade in the MitraClip arm (4). A further limitation of echocardiography is the significant degree of interobserver variability, which poses potential difficulties when monitoring response to treatment. Importantly, no single echocardiographic parameter is sufficient to accurately assess the severity of MR. Indeed, assessment is dependent on integrating a number of parameters based on anatomic and Doppler criteria. Finally, MR severity is traditionally described in a categorical fashion, encompassing mild, moderate, or severe—or graded from 1 to 4. Perhaps, however, MR severity should be considered as a continuous variable, akin to other standard echocardiographic parameters such as ventricular dimensions and ejection fraction. This should be based on an accurate assessment of regurgitant volume/fraction. Unfortunately, current echocardiographic methods limit accurate and reliable assessment of volumes.
The utility of magnetic resonance imaging (MRI) for the assessment of valvular disease is well established (5). Compared with echocardiography, MRI offers a number of advantages. First and foremost, MRI has superior spatial and temporal resolution, with less operator dependency than echocardiography, relying on complex analytical algorithms to enable measurements in a 3-dimensional fashion without making geometric assumptions (5). MRI is not dependent on body habitus and can be performed in various planes without limitation of acoustic windows. MRI quantification of MR is based on calculation of regurgitant volume, which is derived as the difference between aortic forward stroke volume (based on phase contrast imaging) and total left ventricular (LV) stroke volume (based on planimetry of LV end-diastolic and end-systolic contours) (Figure 1) (6). This approach has been shown to have low variability and excellent reproducibility in several studies, making it an optimal technique for serial assessment (5,7). However, MRI does have some limitations including limited availability, increased cost, lengthy scan sequences (difficult for patients with claustrophobia), and incompatibly with many ferromagnetic devices. As such, MRI is not as widely used in the real-world setting.
In this issue of the Journal, Uretsky et al. (8) uniquely compared the degree of postoperative negative remodeling with preoperative regurgitant volume, thereby providing a reference standard with which to support the greater accuracy of MRI compared with echocardiography. Furthermore, the methodology is robust, with studies interpreted in a central core laboratory by experienced, blinded readers and separate analyses performed in both groups to assess interobserver variability.
The majority of patients in this study had degenerative (primary) MR. Importantly, there were no significant differences in loading conditions as assessed by blood pressure and heart rate between MRI and echocardiography at the time of assessment. MRI demonstrated less interobserver variability with excellent reproducibility compared with echocardiography (90% vs. 61%). The concordance between both techniques was poor, improving only modestly when patients were reclassified as either severe or nonsevere. A staggering finding was that only 22% of patients deemed to have severe MR by echocardiography were graded accordingly by MRI. This discrepancy also was reflected to a similar degree in the studies assessed for interobserver variability. The degree of discordance was impressive, with 34% of patients classified as having severe MR by echocardiography, having only mild MR as assessed by MRI. On the other hand, in cases of mild MR as assessed by echocardiography, there was agreement by MRI. Somewhat disconcerting is that of the patients who underwent surgery based on Class I/IIa recommendation, only 30% actually had severe MR according to MRI assessment. As expected after surgery, LV volumes and LV ejection fraction decreased. There was a significant difference in the decrease in LV end-diastolic volume and regurgitant volume across all categories of severity by MRI, a finding not mirrored by echocardiography.
Despite using robust methodology in imaging assessment and highlighting an obvious disparity between echocardiography and MRI, this study did have 2 key limitations. First, despite being a multicenter study, the final number of patients who underwent mitral valve surgery and for whom complete imaging assessment was available was small, limiting generalizability. Second, the clinical relevance of the discordance between MRI and echocardiography in the assessment of MR severity and its subsequent impact on treatment was not assessed, and therefore no conclusions can be drawn regarding clinical outcomes.
Mitral regurgitation is a prevalent pathology with significant implications for morbidity and mortality when not treated appropriately. Recent guideline revisions identify asymptomatic patients with severe MR as those who may warrant surgical treatment. Echocardiography is a cost-effective, readily available, and well-established investigative tool. However, it is limited by patient anatomic factors, interobserver variability, and assessment of numerous factors to improve diagnostic accuracy. In contrast, MRI, although more expensive and less readily available, provides superior imaging with less interobserver variability, better reproducibility, and greater accuracy in the assessment of MR severity. Moreover, MRI appears to correlate better with improvements after surgery compared with echocardiography. However, the clinical relevance of these differences needs to be elucidated. Thus, these study results need to be reflected in larger studies to determine generalizability.
On balance, it would seem reasonable that echocardiography remain the first-line investigation for patients with suspected MR. Those with mild MR could be monitored clinically. In those with moderate or severe MR, particularly asymptomatic patients, MRI may be indicated to confirm the degree of severity before subjecting the patient to invasive therapy. After intervention, MRI may be more appropriate for serial monitoring. Finally, with improvements in all imaging modalities, we hope that in the future the severity of MR will not be characterized categorically as mild/moderate/severe but rather as a continuous variable incorporating regurgitant volume/fraction.
↵∗ 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.
Dr. Kar has received research grants from Abbott Vascular and Boston Scientific. Dr. Sharma has reported that he has no relationships relevant to the contents of this paper to disclose.
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