The relationship of the first heart sound to mitral valve closure in dogs

Abstract

The relationship between the first heart sound (S1), mitral valve (MV) closure, and left atrial (LA) and left ventricular (LV) hemodynamics was investigated in dogs during right-heart bypass. High speed cineradiography (~350 frames/sec) of lightweight clips attached to the free edge of the anterior and posterior MV leaflets permitted study of MV motion with high time resolution. LA and LV pressures were recorded, along with simultaneous phono- and echocardiograms, on a multichannel strip-chart recorder. S1 occurred 25.5 ± 3 msec after LA-LV pressure crossover and coincided with MV closure measured either echocardiographically or cineradiographically. Atrial-ventricular (AS-VS) sequential pacing was performed to vary the interval between atrial and ventricular systole. At AS-VS intervals of 0-50 msec, S1 and MV closure occurred simultaneously approximately 25 msec after the onset of LV systole as described above. At an AS-VS interval of 100 msec, S1 and MV closure occurred simultaneously but closer to the onset of ventricular ystole. At an AS-VS interval of 150 msec, MV closure occurred before S1, just preceding the onset of LV systole, and S1 closely followed the onset of LV systole. At longer AS-VS intervals (200-300 msec) MV closure occurred at progressively earlier points in diastole without a detectable heart sound. Later in diastole, the MV reopened and then closed a second time after the onset of LV systole; S1 was recorded with this closure. The amplitude of S1 varied with the degree of separation of the mitral valve leaflets at the onset of LV systole. The results show that the S1 occurs with MV closure when it follows the onset of LV systole. The timing and amplitude of S1 are related to the degree of separation of the MV leaflets at the onset of LV systole and to the relative timing of LA and LV systole.