Cardiac CT and Percutaneous Mitral Annuloplasty: A Vision of the Future?

Mitral regurgitation (MR), a common valvular disorder, is a heterogeneous condition that can be broadly categorized into primary (affecting the valve apparatus) or secondary (functional) etiologies. For appropriately selected patients, surgical mitral valve repair, when possible, is the preferred strategy over mitral valve replacement.¹ The evidence for device/surgical treatment of functional mitral regurgitation is less well established for patients with severe heart failure and conveys increased operative risk.² Several investigational percutaneous devices are being developed that make use of the close anatomic relationship between the coronary sinus (CS) and the mitral valve annulus (MA) in an effort to alter the geometric alignment of the mitral valve and improve functional mitral regurgitation.³ While the accessibility of the coronary sinus makes development of coronary sinus devices attractive, several important anatomic limitations have tempered this excitement. Previous ex-vivo pathology studies and in-vivo imaging studies have described the variable anatomy of the coronary sinus and its relationship with the MA and the left atrium. Additionally, the intimate proximity of the left circumflex artery (LCX) to the coronary sinus raises the concern of LCX ischemia after device implantation.^4–6 Cardiac CT, a rapidly advancing imaging modality with high spatial resolution, provides an attractive noninvasive technique to precisely characterize cardiac anatomy and may be useful for evaluating patients prior to percutaneous mitral annuloplasty in the future. In this issue of the Journal, Gopal et al,⁷ using a novel systematic approach to reporting measurements, retrospectively studied the relationship between the coronary sinus, the LCX and the MA by way of a 64-multislice computed tomographic scanner (LightSpeed VCT, GE Healthcare, Waukesha, Wisconsin). Images selected from a cardiac CT database included those from 102 normal patients (46 females) and 27 (5 females) with severe ischemic MR. Participants were further classified on the basis of their coronary artery dominance. As a part of their systematic analysis, the authors have introduced 12 descriptive terms in order to characterize the CS-LCX-MA relationship. In the normal patient cohort, 30.4% had a right coronary dominance (RCD), 34.3% had a left coronary dominance (LCD), and 35.3% had a codominant circulation (CCD). In patients with a normal mitral valve, the LCX crossed under the coronary sinus/great cardiac vein (CS/GCV) in 74% with RCD, 83% with LCD and 97% with CCD, respectively. In the cohort with severe ischemic MR, all of the participants studied had a right dominant circulation and the LCX initially crossed under the CS-GCV 96% of the time. In the normal cohort, while the initial crossing of the LCX and CS-GCV did not differ between groups, the authors found that participants with LCD and CCD had a lengthier segment of arterial and venous proximity and that this proximity terminated closer to the CS os. Comparisons between the normal group and the severe ischemic MR group yielded some notable differences, namely that in the ischemic MR group, the CS os to GCV-anterior interventricular vein length increased, the CS os diameter increased and the diameter of the MA increased. However, wide variations in the proximal CS diameter in the ischemic severe MR group were noted and the difference compared to the normal group was not statistically significant. Lastly, and of possible importance to device implantation, the region of arterial and venous proximity was significantly farther from the CS os in patients with ischemic MR. The results of this retrospective study are in line with previous cardiac imaging studies suggesting significant anatomic variation between normal patients and those with severe heart failure and/or severe MR.^4,6,8 Consistent findings across studies show that patients with severe heart failure and/or severe MR have an increased length of the coronary sinus, mitral valve annular dilation and displacement of the coronary sinus in relation to the MA. Caution should be used when comparing these studies, as each included varied patient populations as comparison groups to normal controls — severe ischemic MR versus mitral valve prolapse versus severe heart failure (both ischemic and nonischemic). Variability in the measurement techniques between studies must also be acknowledged. Indeed, the authors of this retrospective analysis introduce and define several novel measurements. Further investigation and validation of these measurement techniques, with particular focus on populations that may ultimately undergo percutaneous annuloplasty, are needed to determine the reproducibility and clinical utility of these measurements. An important point of emphasis in this paper is the detailed analysis of the LCX and the coronary sinus in relation to coronary artery dominance. Similar to previous reports, the LCX crossed under the CS/GCV in a high percentage of patients regardless of arterial dominance, but at an increased proportion in those with LCD or CCD systems. Perhaps more importantly in patients with LCD or CCD, the presumed region most at risk for ischemic compromise, in this paper defined as the “proximity zone,” was significantly increased, raising the important question of whether patients with LCD or CCD can be safely approached with percutaneous mitral annuloplasty. Several challenges remain with the use of cardiac CT in relation to percutaneous mitral annuloplasty. First, as the authors correctly identify, many of the patients most likely to benefit from mitral annuloplasty may have additional factors, such as atrial fibrillation, that make cardiac CT imaging more technically challenging. Second, static measurements made with cardiac CT on image data reconstructed at specific intervals may not entirely explain the anatomic relationships within the heart that likely vary throughout the cardiac cycle. Third, one ultimate goal of cardiac CT would be to improve on patient selection for mitral annuloplasty. Data from the recently published AMADEUS trial, which investigated the CARILLON Mitral Annuloplasty Device in 48 patients with moderate-to-severe functional MR, failed to show the benefit of cardiac CT as a useful prescreening technique to determine safety or efficacy.^9,10 Indeed, further analysis of the data from this trial showed that the distance between the CS/GCV and the mitral annulus did not predict the degree of MR reduction with the device. Moreover, while the circumflex artery was initially crossed in 84% of the patients, only 14% of the patients in whom the implant was attempted did not receive the device due to arterial compromise. Thus, it remains unclear if cardiac CT will be able to predict procedural success or if the predictors of procedural success have not been fully defined. Ongoing research of additional investigational devices, some of which utilize the pre-screening acquisition of cardiac CT data, will hopefully clarify this question. Lastly, and most importantly, the clinical indications, safety and long-term efficacy of percutaneous MA have not yet been established. Further research is needed to define the role, if any, these techniques will have on meaningful clinical outcomes. Thus, our current vision of the future remains unclear, but there is hope given the rapidly advancing device and imaging technologies.

References

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__________________________________________ From the University of California, San Francisco, California. The authors report no conflicts of interest regarding the content herein. Address for correspondence: Paul Frey, MD, MPH, UCSF, Box 0124, San Francisco, CA 94143. E-mail pfrey@medicine.ucsf.edu