MOST OVINE MITRAL ANNULAR 3-DIMENSIONAL SIZE-REDUCTION OCCURS BEFORE VENTRICULAR SYSTOLE AND IS ABOLISHED WITH VENTRICULAR PACING

Background Conventional surgical thinking indicates that mitral annula r (MA) size reduction plays a key role in mitral valve closure, and mo st MA size and shape changes are thought to occur during left ventricu lar (LV) systole. The influences of left atrial (LA) and LV systole on MA size and shape, however, remain debated. Methods and Results Eight radiopaque markers were placed equidistantly around the MA and imaged using high-speed simultaneous biplane videofluoroscopy in seven close d-chest, sedated sheep before and during asynchronous LV pacing. Marke r images were used to compute the three-dimensional coordinates of eac h marker every 16.7 ms throughout the cardiac cycle, allowing calculat ion of three-dimensional MA area, septal-lateral (SL) dimension, and c ommissure-commissure (CC) dimension under control and LV pacing condit ions. Maximum MA area occurred in early diastole, and minimum MA area near end-diastole; maximum area reduction was 12+/-1% (P less than or equal to.001). Interestingly, 89+/-3% of area reduction occurred befor e LV systole. During this ''presystolic'' period, SL decreased by 8+/- 1% and CC by 2+/-1%; the SL/CC ratio fell from 0.73+/-0.02 to 0.69+/-0 .01 (P less than or equal to.005), indicating a less circular shape at end-diastole. With LV pacing, total MA area reduction was similar (13 +/-2 versus 12+/-1%, P=NS versus control); however, all MA area reduct ion occurred during LV systole with minimum MA area occurring at end-s ystole. Presystolic shortening in both SL and CC dimensions was lost, and presystolic ellipticalization disappeared. Conclusions Changes in MA size and shape coincident with LA systole included area reduction a nd shape change prior to the onset of LV contraction. These presystoli c changes vanished when LA systole was absent (LV pacing). Thus, LA sy stole plays a pivotal role in MA size reduction and shape alteration. The unexpected timing of these MA dynamics should be taken into accoun t during mitral valve reparative procedures.