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

The relationship between the first heart sound (Sl), mitral valve (MV) closure, and left atrial (LA) and left ventricular (LV) hemodynamics was investigated in dogs during right-heart bypass.Highspeed 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 phonoand echocardiograms, on a multichannel strip-chart recorder.S, 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 per- formed to vary the interval between atrial and ventricular systole.At AS-VS intervals of 0-50 msec, S, 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, S, and MV closure occurred simultaneously but closer to the onset of ventricular systole.At an AS-VS interval of 150 msec, MV closure occurred before Sl, just preceding the onset of LV systole, and Si 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; S, was recorded with this closure.The amplitude of SI varied with the degree of separation of the mitral valve leaflets at the onset of LV systole.The results show that the S, occurs with MV closure when it follows the onset of LV systole.The timing and amplitude of SI 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.DESPITE 150 years of experimental study, debate about the genesis of the first heart sound (S,) contin- ues.' The two conflicti-ng theories most frequently proposed are based on either a ventricular or a mitral valvular origin of the sound.