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Purpose: Adaptive increase of mitral leaflet area (MLA) has been recently reported in a model of ischemic mitral regurgitation (IMR). We aimed to elucidate geometric parameters associated with leaflet adaptation and factors determining MLA.Methods: MLA was measured at mid-systole (closure area, CMLA) and diastole (total area, TMLA) using real-time 3-D echocardiographic data in normal controls (n=12) and patients with inferior or posterior wall motion abnormalities (WMAs, n=23, left ventricular [LV] ejection fraction 41±9%). Tenting volume (TNV) was also measured at mid-systole.Results: TMLA was increased by 39±26 % in patients with WMAs compared with normal controls. Each TMLA and CMLA showed positive correlations with LV mass (r=0.682, p<0.001) and TNV (r=0.939, p<0.001), respectively. Of 23 patients with WMAs, 12 patients showed significant IMR defined as proximal isovelocity surface area radius ≥4 mm at the aliasing velocity of 40 cm/s. Although TMLA was not different between groups (18.8±5.0 vs 18.2±3.6 cm2, p=0.976), patients with significant IMR were characterized by older age (71±7 vs 62±11 years, p=0.009), larger CMLA (12.3±4.3 vs 9.1±1.6 cm2, p=0.016), smaller ratio of TMLA-to-CMLA (1.6±0.3 vs 2.0±0.5, p=0.011), and lager TNV (3.9±2.1 vs 2.4±0.8 cm3, p=0.043). Multivariate analysis showed that the ratio of TMLA-to-CMLA was the only determinant of significant IMR (odds ratio = 0.004, 95% confidence interval = 0.001-0.514, p=0.026).Conclusions: In vivo measurement of MLAs made it possible to appreciate both adequacy of leaflet adaptation and tethering geometry. TMLA shows parallel increase with change of LV mass, whereas CMLA represents tethering. Similar TMLA increase despite larger TNV in patients with IMR suggests that inadequate leaflet adaptation is the main mechanism of IMR.