A non-invasive study of the third heart sound in children by phono and echo-cardiography.

There has been in the past and still is controversy over the genesis of the third heart sound (S3). Recent studies, however, strongly suggest that S3 is a manifestation of a sudden intrinsic limitation in the expansion of the L.V. The present study has aimed to explore that hypothesis further, using a spectral analysis technique. A FFT technique was used to determine the spectral distribution of S3 in 14 child subjects between the ages of 2 and 19 years. Spectral energies in 15 Hz frequency bandwidths were correlated with various 2D-echocardiographically derived parameters. This study has shown that the spectral energy of S3 is distributed in the lower frequency bands. Fifty percent (47 +/- 16%) is distributed in the 0-15 Hz band. Also it was found that: (i) as mitral orifice size increases, energy in the 0-15 Hz band of S3 decreases. (r = -0.53, p less than 0.05), (ii) S3 occurs earlier in the cardiac cycle with increase in age of the subject. (r = -0.76, p less than 0.005), (iii) the energy distribution of S3 tends towards the higher frequencies with increase in age. (r = 0.52, p less than 0.05). Further, the above results support the hypothesis that S3 is the result of an intrinsic limitation to the expansion of the L.V. due to an early diastolic pressure rise caused by an increased viscoelasticity of the myocardium. This, and the relation of the results in certain pathologies are also discussed.