Effects of undersized mitral annuloplasty on regional transmural left ventricular wall strains and wall thickening mechanisms

Background - Undersized mitral annuloplasty, widely used for ischemic and functional mitral regurgitation (MR), has been proposed as an "annular solution to a ventricular problem." Beyond relief of MR, it is thought to improve global left ventricular (LV) shape, hence potentially reducing myocardial stress and promoting beneficial reverse LV remodeling. We previously observed that undersized annuloplasty inhibited systolic wall thickening at the LV base near the mitral annulus. In this study, we measured the effects of undersized annuloplasty on regional transmural LV wall fiber and sheet strains and wall thickening mechanisms. Methods and Results - Nine sheep had transmural radiopaque beadsets surgically inserted into anterobasal and lateral equatorial LV regions, with additional markers silhouetting the LV and mitral annulus. 4-Dimensional marker dynamics were studied with biplane videofluoroscopy before and after tightening an adjustable Paneth- type mitral annuloplasty suture. Transmural circumferential, longitudinal, and radial systolic and remodeling strains in the subepicardium (20% depth), midwall (50%), and subendocardium (80%) in both regions were computed. Fiber and sheet angles from quantitative regional histology allowed transformation of these strains into local fiber (f), sheet (s), and sheet-normal (n) coordinates. Further analysis calculated the transmural contributions of sheet extension (E-ssc), sheet thickening (E-nnc), and sheet shear (E-snc) to systolic wall thickening ( E-33). In the anterobasal region, undersized annuloplasty reduced systolic wall thickening (E33) by approximate to 50% at all transmural depths by inhibiting: (1) subendocardial systolic fiber shortening (-0.10 +/- 0.05 versus -0.04 +/- 0.05; P < 0.05); (2) subepicardial (0.16 +/- 0.15 versus 0.09 +/- 0.08; P < 0.05) and subendocardial (0.45 +/- 0.40 versus 0.19 +/- 0.18; P < 0.05) systolic sheet thickening; (3) midwall sheet extension (0.22 +/- 0.12 versus 0.11 +/- 0.06;P < 0.05); and (4) transmural sheet shear (subepicardium, -0.14 +/- 0.07 versus -0.08 +/- 0.07; midwall, 0.21 +/- 0.12 versus 0.10 +/- 0.11; subendocardium, -0.19 +/- 0.23 versus -0.11 +/- 0.16; P < 0.05). In the remote lateral equatorial region, fiber-sheet strains and E33 were unchanged. Conclusions - In this acute animal study, undersized annuloplasty inhibited systolic wall thickening in the anterobasal region by reducing subendocardial systolic fiber shortening and laminar sheet wall thickening, but had no effects in a more distant LV region. This suggests that undersized mitral annuloplasty may have potentially deleterious effects on local myocardial mechanics.