Author + information
- Ehud Schwammenthal, MD, PhD∗ ()
- Heart Center, Sheba Medical Center, Tel Hashomer, Sackler School of Medicine, Tel Aviv University, Ramat Aviv, Israel
- ↵∗Reprint requests and correspondence
: Dr. Ehud Schwammenthal, Heart Center, Sheba Medical Center, Emek Dotan Street, Tel Hashomer 52621, Israel.
A structurally normal mitral valve when abnormally tethered in a dilated ventricle will start to leak once the available closing force becomes insufficient to achieve effective systolic leaflet coaptation (1). Conversely, diastolic tethering may restrict leaflet mobility and thus limit complete opening of the valve (1–3). Restricted opening of the anterior leaflet in particular is frequently observed in functional mitral regurgitation, resulting in the typical pattern of inflow diversion toward the posterolateral wall rather than the apex (2,3) (Figure 1). Diastolic tethering has already been implicated in the mechanism of diastolic mitral regurgitation (3) and has now also been identified as the culprit for functional mitral stenosis after restrictive annuloplasty (4).
What is the mechanism of functional mitral stenosis? Undersized rings may reduce the leaflet area necessary to cover the orifice, move the leaflets closer together by reducing the anteroposterior (septolateral) annular diameter, and thus facilitate effective coaptation. Unfortunately, such annular compensation for a problem that is mainly ventricular in origin often remains a temporary fix, as the high recurrence rates of functional mitral regurgitation attest. Moreover, undersized rings may reduce effective orifice areas and increase transmitral gradients. This is traditionally attributed to a potentially excessive reduction of annular area by the ring. In contrast, in this issue of the Journal, Bertrand et al. (4) convincingly demonstrate that functional mitral stenosis and functional mitral regurgitation share a common root cause: subvalvular tethering.
The suspicion that it is indeed not simply the ring already arises from the following reasoning: Mean ring size in this study was 28 mm, which according to the vendor’s brochure encompasses an area of 3.8 cm2. To yield an effective orifice area of 1.50 cm2, the discharge coefficient (performance index) would have to be 0.39, well below what elliptical orifices are expected to provide (>0.75) (5) and even below commonly reported values for mechanical prostheses (0.4 to 0.65) with obstructing elements, such as tilting discs, in their flow path.
However, a tethered anterior leaflet with limited mobility may actually represent such an obstructing element. Put differently, flow passing through the ring is forced to converge toward the actual geometric orifice of the valve at the leaflet tips and even beyond (vena contracta effect), until the minimal cross-sectional flow area, the effective orifice area, is reached (Figure 2). The anterior leaflet opening angle determines both leaflet separation, and therefore the size of the geometric orifice, as well as the degree of flow contraction, thus compounding its effect on effective orifice area. Unlike a tilting disc, the anterior leaflet’s opening angle may increase again during exercise-induced flow augmentation, provided myocardial reserve is preserved (otherwise, tethering might even increase, which would result in worsening of functional mitral stenosis) (6). This will enlarge the geometric orifice area and reduce the degree of flow contraction, resulting in a profound increase in effective orifice area (Figure 2). In addition, improved wall motion during exercise may move the papillary muscles anteriorly, reducing tethering.
If it is not the ring but the tethering, why isn’t there functional mitral stenosis preoperatively? First, because with severe mitral regurgitation, augmented diastolic transmitral flow and elevated left atrial pressure provide a distending force that counterbalances the restraining force of the tethered subvalvular apparatus (no longer available once mitral regurgitation is abolished) (Figure 2). Second, because restrictive annuloplasty may actually increase tethering, because it brings in the posterior mitral annulus while leaving the papillary muscles behind (7). As a result, the posterior leaflet is often straightened and immobilized, with the anterior leaflet “doing all the work” while also restricted in its mobility.
What is the clinical impact of functional mitral stenosis? Despite concern that use of undersized rings might trade functional mitral regurgitation for functional mitral stenosis, studies largely failed to detect clear evidence for a clinical impact, potentially because it might be buried in more dramatic determinants of outcome (8). Bertrand et al. (4) carefully chose 2 independent methods for addressing the question of clinical relevance: Exercise capacity (using the gold standard of peak oxygen consumption) and clinical outcome over a significant observation period; indexed effective orifice area during exercise indeed emerged as a strong independent statistical predictor for both. However, the chain of proof seems firmer with respect to exercise capacity than prognosis. If, as the authors have convincingly shown, a smaller effective orifice area is the result of persistent tethering, it might simply be a more powerful marker of ventricular remodeling and thus be more closely related to poor prognosis. The fact that postoperatively assessed left ventricular volumes were not associated with a clinical endpoint does not really rule out this possibility, because after successfully abolishing mitral regurgitation, they may no longer reflect the long-standing volume overload that had burdened the ventricle and whose sequelae are likely to influence prognosis well after mitral valve repair. Outcome in these chronically ill patients is evidently not all about postoperative hemodynamics: Despite an overall favorable hemodynamic effect of restrictive annuloplasty performed in this center of excellence (pulmonary artery pressure decreased from a mean of 50 mm Hg to 40 mm Hg postoperatively, reaching preoperative values only during exercise), outcome was still far from satisfactory. Notably, of 76 initially screened patients with verified functional mitral regurgitation, 27 (35%) had died.
What can be done to improve results? Bertrand et al. (4) clearly and elegantly demonstrated that functional mitral stenosis after restrictive annuloplasty is for the same reason as dynamic as functional mitral regurgitation (abnormal mitral tethering by the subvalvular apparatus) and that it may influence exercise capacity. Therefore, relief of tethering is an important therapeutic target (1,9). It has already been shown that off-pump papillary muscle repositioning under echocardiographic guidance using a transventricular suture (RING+STRING technique), combined with less restrictive annuloplasty using a partial ring, relieves tethering, resuspends the mitral leaflets, and leads to sustained improvement of valve competency (9). Subvalvular repair, restoring the normal geometry of the mitral valve apparatus with its balance between closing and tethering forces, might be preferable to overstretching and undersizing. Currently, despite the beneficial effects of a well-performed restrictive annuloplasty in many patients, it appears that both the disease and its surgical therapy distort the mitral valve apparatus.
↵∗ 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. Schwammenthal is a proctor for Medtronic; and the founder and Chief Executive Officer of Magenta Medical, Ltd.
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