THE MECHANISM OF THE PHYSIOLOGICAL DISAPPEARANCE OF THE 3RD HEART-SOUND WITH AGING

The third heart sound (S3) is often present in children and adolescent s but is not present in most adults. Applying at the left ventricle a mathematical model, the mechanism of the disappearance of S3 was studi ed 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 s ystem. Once the left ventricle starts vibrating it behaves as a simple physical model composed of a mass (m), a spring (k) and a viscous ele ment. The abrupt deceleration of the blood mass (m) against the left v entricular walls (k) triggers the vibration of the system according to the equation Fd = 1/2 pi . root k/m . root 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. Du ring the growth of the individual the increased myocardial mass may le ad to augmented viscous forces causing a gradual increase in zeta unti l the system becomes overdamped and consequently unable to vibrate cau sing the disappearance of S3.