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Chordal force distribution determines systolic mitral leaflet configuration and severity of functional mitral regurgitation

Sten Lyager Nielsen, MD^* , Hans Nygaard, DMSc^* , Arnold A. Fontaine, PhD, J. Michael Hasenkam, MD, DMSc^* , Shengqui He, MD, Niels T. Andersen, PhD and Ajit P. Yoganathan, PhD

^* Department of Cardiothoracic and Vascular Surgery, Skejby Sygehus, Aarhus University Hospital, Aarhus, Denmark
Institute of Experimental Clinical Research, Skejby Sygehus, Aarhus University Hospital, Aarhus, Denmark
Institute for Bioengineering Bioscience and School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
Institute of Biostatistics, Aarhus University, Aarhus, Denmark

View larger version (162K): [in a new window]   Figure 1 Photograph of the left ventricular model showing the mitral valve mounting mechanism consisting of an annulus ring and two papillary muscle mounting rods. Chordal force transducers were attached to the primary (major fixing) chordae.

View larger version (38K): [in a new window]   Figure 2 Schematic showing the distribution of the chordal force transducers and the papillary muscle adjustments in normal (N), apical (A), posterolateral (PL) and apical posterolateral (APL) directions. Eight papillary muscle settings were tested. Scaled photograph of the transducer: 1 unit = 1 mm. CTAA = chorda from the anterolateral papillary muscle (APM) to anterior leaflet; CTAP = chorda from APM to posterior leaflet; CTPA = chorda from the posteromedial papillary muscle (PPM) to anterior leaflet; CTPP = chorda from PPM to posterior leaflet; AL = anterior leaflet.

View larger version (21K): [in a new window]   Figure 3 (A) Free body diagram of the mitral valve complex. In an equilibrium condition of the closed mitral valve complex in systole, mitral leaflet force balance (below the dashed line) requires that the transmitral pressure forces (Fp,i) are counteracted by the chordal tethering force, FT (equal to –FC) and the annular tethering force, FA. Chordal force balance (above the dashed line) requires that the coapting force component FC is counteracted by an equal and oppositely directed tethering force from the papillary muscle attachment, FT. Transmitral pressure forces and FA are only presented for schematic and conceptual purposes and were not assessed in the study. (B) The chordal force measure of each chorda tendinea consisted of the chordal tethering force component (FT) and the chordal coapting force component (FC). The difference of the force components, (FC – FT), defined the resulting (valvular directed) force of the chorda tendinea acting on the leaflets at the point of insertion.

View larger version (27K): [in a new window]   Figure 4 Diagram of a midsystolic tented mitral leaflet geometry (left top) with a typical leaflet contour imaged by two-dimensional echo from the apical view (below). Left ventricular side in z direction. The occlusional leaflet area, OLA, was calculated as the sum of fractions of a cone produced from four apical scanning planes rotated around an axis through the midpoint of the annulus (right top; one fraction of a cone is marked). Notice leaflet edge separation due to leaflet asynergy creating a regurgitant orifice. Variables to describe leaflet configuration are illustrated (r and h). The intersection of the anterior leaflet extension on the posterior leaflet gives the horizontal length r4 and perpendicular distance h4 of the posterior leaflet involved in mitral orifice occlusion. AL and PL = systolic leaflet coaptation angle of the anterior and posterior leaflet; DAL, DPL = perpendicular distance from anterior and posterior leaflet tip to the annular plane; LAL, LPL = horizontally projected lengths of the anterior and posterior leaflet.

View larger version (20K): [in a new window]   Figure 5 Schematic representation of the three-dimensional reconstruction of the mitral leaflet geometry, using one short-axis view of the annulus plane (top) and two parasternal long-axis views at the midpoint of the half mitral coaptation line (bottom). The primary chordae connected the tip of the papillary muscles and the corresponding midpoint of the half mitral coaptation line (asterisks). The central papillary muscle lines (LAPM and LPPM) are illustrated as double arrows from the papillary muscle tips through the chordae attachment (asterisks) to the annular plane. APM = anterolateral papillary muscle; PPM = posteromedial papillary muscle.

View larger version (15K): [in a new window]   Figure 6 A linear correlation was demonstrated between the occlusional leaflet area (OLA) and regurgitant fraction (RF) (dependent variables) and the total chordal netforces, [FC – FT]S (independent variable). Data represent one valve at all test conditions. Trends are representative of all valves (compare with Table 3). Solid circles = HR: 70; LVP – LAP: 90; solid squares = HR: 120; LVP – LAP: 90; open circles: HR: 70; LVP – LAP: 150; open squares = HR: 120; LVP – LAP: 150. HR = heart rate; LVP – LAP = transmitral pressure difference.

View larger version (11K): [in a new window]   Figure 7 Diagram showing mitral leaflet remodeling (thin arrows) caused by redistribution of the chordal tethering (thick black arrows) and coapting forces (dotted arrows) during apical (A) and posterolateral (PL) papillary muscle (PM) displacement. Apical displacement of the papillary muscles resulted in an apical shift of the leaflet coaptation line. Posterolateral displacement of the papillary muscles caused a posterior shift of the leaflet coaptation line. Therefore, the chordal coapting force component of the anterior leaflet increased, generating a nonuniform regurgitant orifice area. N = normal.