The mechanism of the physiologic disappearance of the third heart sound with aging.

The third heart sound (S3) is often present in children and adolescents but is not present in most adults. Applying at the left ventricle a mathematical model, the mechanism of the disappearance of S3 was studied employing the frequency analysis of the sound and echocardiographic data. The existence of a significant correlation between the spectrum energy of S3 and the diameter and thickness of the left ventricle at the moment of S3 in 25 healthy subjects (aged 21 +/- 7 years) allowed us to interpret the origin of S3 based on a viscoelastic oscillating system. Once the left ventricle starts vibrating it behaves as a simple physical model composed of a mass (m), a spring (k) and a viscous element. The abrupt deceleration of the blood mass (m) against the left ventricular walls (k) triggers the vibration of the system according to the equation Fd = 1/2 pi.square root of k/m.square root of 1 - zeta 2, where Fd is the natural damped frequency and zeta is the damping factor. The equation shows that the vibrating system can oscillate only if zeta is < 1. During the growth of the individual the increased myocardial mass may lead to augmented viscous forces causing a gradual increase in zeta until the system becomes overdamped and consequently unable to vibrate causing the disappearance of S3.