Diastolic mechanics and the origin of the third heart sound.

The third heart sound (S3) is observed for various hemodynamic conditions in both the normal and diseased heart. A theory is proposed in which myocardial viscoelasticity is primarily responsible for S3. A mathematical model is developed based on the mechanical aspects of diastolic function: nonlinear elasticity, viscoelasticity, and pressure generation. The model is provided as an electrical analogy of the left ventricle and circulatory system. S3 is predicted for the normal heart and the heart with dilated cardiomyopathy. An elevation of S3 intensity is indicated for cardiomyopathy, as is often observed in the clinic. S3 is produced experimentally by volume loading of the open-chest canine preparation and mathematically by imposing the conditions of volume loading on the model. Consistency of theory and experiment imply that it is valid to attribute S3 to myocardial viscoelasticity. The animal whose heart possessed the largest constant of viscoelasticity produced the greatest level of S3, in both cases. Nonlinear ventricular compliance is not found to be an essential requirement for sound generation, although increased compliance led to an increase in sound. S3 is predicted to change in response to venous return, ventricular stiffness, contractility, heart rate, and duration of contraction, as observed by others. In general, the coupling of these quantities to S3 is explained in terms of an excitation of viscous properties of the ventricle.