1996 Volume 37 Issue 2 Pages 215-226
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=1/2π√4k/m•√1-ξ2, where Fd is the natural damped frequency and ζ is the damping factor. The equation shows that the vibrating system can oscillate only if ζ is <1. During the growth of the individual the increased myocardial mass may lead to augmented viscous forces causing a gradual increase in ζ until the system becomes overdamped and consequently unable to vibrate causing the disappearance of S3.
