Temporal relation of the first heart sound to closure of the mitral valve.

The objective of this study is to describe the mechanism of production of the first heart sound (S). The temporal relation between the major components of S, and (1) cessation of flow across the mitral valve, (2) closure of the valve, and (3) time of peak rate of rise of left ventricular pressure (peak LV dP/dt), were investigated in open-chest dogs. Phasic flow across the mitral valve was recorded by a supraannular electromagnetic flow probe, and timing of valve closure was determined by synchronized cinefluorogram of the motion of the valve cusps after they were made radio opaque. Closure of the mitral valve did not occur at the crossing point of atrial and ventricular pressures (at the beginning of systole), but 20-40 msec later due to inertia of mitral flow. The first major component of the first sound coincided with two simultaneous events: cessation of mitral flow and closure of the valve. It did not show a fixed temporal relationship to time of peak dP/dt.