Mitral valve mechanism under diseased and repair condition

Date

2012-12

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Abstract

The mitral valve is an important valve located between left ventricle and atrium. The critical function of mitral valve is to control the blood flow between ventricle and atrium. When the heart relaxes, the valve opens and allows the blood flow from atrium to ventricle through it. When the heart contract, the valve closes and forbids the blood flow back to the atrium. The complete closure of mitral valve is determined by a delicate force balance of transmitral pressure and four components of the mitral valve, which are annulus, leaflets, chordae tendineae and papillary muscles. The abnormality of each component may result in incomplete closure of mitral valve. There are two main kinds of incomplete closure of leaflets: prolapsed leaflets and restricted leaflet. A simple and effective method was introduced and applied on the treatment of prolapsed leaflets, which is edge to edge repair. This technique involves approximating the anterior and posterior leaflets with sutures, creating a double-orifice or triple-orifice mitral valve. However the mitral valve ETER has been shown to alter normal mitral valve leaflet mechanics, which related to suture tension. So far little is known regarding the mechanical conditions of the leaflet during a cardiac cycle due to mitral valve ETER, especially under pathological papillary muscle (PM) positions, which simulate a dilated heart or prolapsed mitral valve. In this thesis, the effect of suture length and PM condition on the leaflet strain was investigated. A dynamic flow loop was utilized to simulate the blood flow in left ventricle and atrium. An ink marker array were put on the anterior leaflet and recorded by two high speed cameras. Biquadratic finite element interpolation was used to perform the surface fit to the resulting three-dimensional marker array, and the principal stretches were calculated at the center of the array. The major principal stretch during systole was significantly greater than that during diastole. There were no significant differences in the radial and circumferential, or areal stretches and stretch rates during diastole between the single suture and 6mm suture. The major principal stretch was significantly greater in the taut PM position than in the normal and slack PM positions during diastole. Minor change in suture length may not result in a significant load difference in the central region of the anterior leaflet during diastole. The load on the anterior leaflet during systole, rather than that during diastole, should be considered when evaluating ETER durability, especially in the taut PM position. The circumferential stretch during diastole was not influenced by the PM positions. The restricted leaflets, as in the functional/ischemic mitral regurgitation, were repaired with annuloplasty ring, which was considered as golden standard treatment. However a high recurrence up to 50% after 5 years was reported, which suggest a lack of understanding of coaptation mechanism of mitral valve. In this thesis, the geometry of the mitral valve leaflet free edges with asymmetric and symmetric displacement of PMs and a dilated annulus and coaptation mechanism of mitral valve were investigated in this study. Porcine mitral valves were sutured to a saddle-shaped dilated annulus with an area increase of 100%. Sonocrystals were attached to the porcine mitral valve leaflet free edges and leaflet gaps. Papillary muscles were adjusted to the normal position, and subsequently to positions of asymmetric and symmetric papillary muscle displacement. sonocrystal positions were measured in an in-vitro experiment to simulate valve closure with a transmitral pressure of 120 mmHg. Asymmetric PM displacement shifted the leaflet edges in the central and medial regions medially, apically, and posteriorly. Posterior relative displacement of free edges in the medial region increased gap size and medial relative displacement of free edges in the lateral region sometimes generated a gap in asymmetric PM displacement. Symmetric PM displacement shifted the leaflet free edges posteriorly and apically and increased the gap in the central region. Asymmetric PM displacement increases mitral valve tenting and impairs coaptation in the medial region, and jeopardizes the lateral region as well due to medial leaflet shifting. Symmetric PM displacement increases valve tenting and impairs coaptation in the central region. Closure of leaflet cleft between anterior and middle scallops of posterior leaflet was impaired by asymmetric PM displacement.

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Keywords

Mitral valve, Coaptation mechanism, Heart valve, Regurgitation

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