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The exact times of mitral valve opening and closure were determined in dogs under varying hemodynamic conditions in 143 cardiac cycles (five experiments). Radiopaque markers had been sutured to the cusps and the valve annulus 7-124 wk before the studies. Valve opening and closure times were correlated with simultaneously obtained high-fidelity intracardiac pressures. Closure of the mitral valve was completed 5-105 ms after the atrial-ventricular pressure crossover; the time interval between the onset of ventricular systole and the instance of complete valve closure varied less (10-40 ms). These observations suggest that in the intact heart alpha, rapid mitral cusp closure at the end of diastole is initiated and completed by ventricular systole alone, and beta, the ventricular isovolumic contraction period might be shorter than assumed. Opening of the valve during ventricular relaxation was characterized by 1) initial separation of the markers placed on the free edges of the cusps, of variable duration, apparently due to alterations in ventricular geometry, and 2) a rapid opening motion which clearly preceded the diastolic pressure crossover by 5-60 ms. This finding would suggest that ventricular isovolumic relaxation might be shorter than generally accepted, although the mechanism of early opening is not adequately explained by the data.

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