Mitral valve annulus tension and the mechanism of annular dilation: An in-vitro study

Background and aim of the study: The mitral annulus mechanics associated with annular dilation in pathologies such as mitral valve (MV) prolapse, ischemic and dilative heart diseases remain unknown. The study aim was to investigate annulus tension (AT) in the dilated annulus and in the displaced papillary muscles due to these pathological conditions. Methods: Ten porcine MVs were harvested and mounted on a MV closure test rig with the papillary muscles held in the slack, normal, and taut positions. The MV annulus tissue rested on top of a plastic ring on which it could slide freely. The annulus was held by strings in the periphery during valve closure under hydrostatic trans-mitral pressure. The string tensions were measured, and further divided by string spacing to obtain the AT. Three annuli of normal, 25% dilated and 50% dilated annulus sizes were used. Results: The AT was lowest at the commissural segment of the annulus, but increased towards the anterior and posterior segments, with a greater increase towards the anterior segment than towards the posterior. The AT increased with the increase of the apical PM displacement from the slack to normal, and then taut position in the commissural segment of the annulus. Although the AT increased with annulus area, the increase was less pronounced in the commissural segment than in the anterior and posterior segments. Conclusion: There is a significant difference in AT values between a normal and prolapsed MV, and between a normal MV and a MV with displaced papillary muscles. This difference suggests that annular dilation is a consequence and MV counteraction of imbalanced annular mechanics between the AT and myocardial force.