Integrated mechanism for functional mitral regurgitation: Leaflet restriction versus coapting force: In vitro studies

Background: Functional mitral regurgitation in patients with ischemic or dilated ventricles has been related to competing factors: altered tension on the leaflets due to displacement of their papillary muscle and annular attachments, which restricts leaflet closure, versus global ventricular dysfunction with reduced transmittal pressure to close the leaflets. In vivo, however, geometric changes accompany dysfunction, making it difficult to study these factors independently. Functional mitral regurgitation also paradoxically decreases in midsystole, despite peak transmittal driving pressure, suggesting a change in the force balance acting to create a regurgitant orifice, with rising transmittal pressure counteracting forces that restrict leaflet closure. In vivo, this mechanism cannot be tested independently of annular contraction that could also reduce midsystolic regurgitation. Methods and Results: An in vitro model was developed that allows independent variation of papillary muscle position, annular size, and transmittal pressure, with direct regurgitant flow rate measurement, to test the hypothesis that functional mitral regurgitation reflects an altered balance of forces acting on the leaflets. Hemodynamic and echocardiographic measurements of excised porcine valves were made under physiological pressures and flows. Apical and posterolateral papillary muscle displacement caused decreased leaflet mobility and apical leaflet tethering or tenting with regurgitation, as seen clinically. It reproduced the clinically observed midsystolic decrease in regurgitant flow and orifice area as transmitral pressure increased. Tethering delayed valve closure, increased the early systolic regurgitant volume before complete coaptation, and decreased the duration of coaptation. Annular dilatation increased regurgitation for any papillary muscle position, creating clinically important regurgitation; conversely, increased transmittal pressure decreased regurgitant orifice area for any geometric configuration. Conclusions: The clinically observed tented- leaflet configuration and dynamic regurgitant orifice area variation can be reproduced in vitro by altering the three-dimensional relationship of the annular and papillary muscle attachments of the valve so as to increase leaflet tension. Increased transmitral pressure acting to close the leaflets decreases the regurgitant orifice area. These results are consistent with a mechanism in which an altered balance of tethering versus coapting forces acting on the leaflets creates the regurgitant orifice.