Left Ventricular Length-Force-Shortening Relations Before and After Surgical Correction of Chronic Mitral Regurgitation

Abstract

Objectives. We tested the hypothesis that postoperative left ventricular (LV) systolic wall stress can be predicted from the change in LV diastolic dimension and ejection fraction (EF) after surgical correction of chronic mitral regurgitation (MR). We used a simple mathematic model to predict postoperative systolic stress from end-diastolic dimension and EF. The validity of this model was assessed using data from 21 patients undergoing mitral valve replacement (MVR) for chronic MR.Background. The decline in EF after MVR for chronic MR is traditionally thought to be a consequence of a postoperative increase in afterload, caused by closure of a low resistance runoff into the left atrium. However, consideration of the Laplace relation suggests that afterload does not necessarily increase after the operation.Methods. A spherical mathematical model of the left ventricle was used to define the relations between LV end-diastolic dimension, systolic wall stress and EF. To test the validity of this model, clinical and echocardiographic data were obtained from 21 patients with chronic MR before and 10 to 14 days after MVR. These echocardiographic data were examined with reference to plots derived from the mathematical model.Results. Patients were categorized as those in whom end-diastolic dimension declined after the operation (group I, n = 15) and those with no reduction in end-diastolic dimension (group II, n = 6). Group I patients were subclassified into those undergoing MVR with chordal preservation (group Ia) and those undergoing MVR with chordal transection (group Ib). In groups Ib and II, there were significant reductions in EF (56 ± 3% to 48 ± 3% in group Ib and 50 ± 2% to 40 ± 3% in group II, both p < 0.05), but the changes in end-diastolic dimension and wall stress differed. In group Ib, end-diastolic dimension decreased and systolic wall stress was unchanged; in group II, end-diastolic dimension was unchanged and wall stress increased. In contrast, group Ia patients experienced a substantial reduction in end-diastolic dimension, no change in EF and a reduction in stress. The corresponding length-force-shortening coordinates closely approximate those predicted from a mathematic model relating end-diastolic dimension to EF and systolic wall stress.Conclusions. Concordant echocardiographic and mathematical model results indicate that postoperative changes in systolic stress are directly related to changes in chamber size and that LV afterload may fall when chordal preservation techniques are used in combination with MVR.