Relationship of the third heart sound to transmitral flow velocity deceleration

Background: The third heart sound (S₃) occurs shortly after the early (E- wave) peak of the transmitral diastolic Doppler velocity profile (DVP). It is thought to be due to cardiohemic vibrations powered by rapid deceleration of transmitral blood flow. Although the presence, timing, and clinical correlates of the S₃ have been extensively characterized, derivation and validation of a causal, mathematical relation between transmitral flow velocity and the S₃ are lacking. Methods and Results: To characterize the kinematics and physiological mechanisms of S₃ production, we modeled the cardiohemic system as a forced, damped, nonlinear harmonic oscillator. The forcing term used a closed-form mathematical expression for the deceleration portion of the DVP. We tested the hypothesis that our model's predictions for amplitude, timing, and frequency of S₃ accurately predict the transthoracic phonocardiogram, using the simultaneously recorded transmitral Doppler E wave as input, in three subject groups: these with audible pathological S₃, those with audible physiological S₃, and those with inaudible S₃. Conclusions: We found excellent agreement between model prediction and the observed data for all three subject groups. We conclude that, in the presence of a normal mitral valve, the kinematics of filling requires that all hearts have oscillations of the cardiohemic system during E-wave deceleration. However, the oscillations may not have high enough amplitude or frequency to be heard as an S₃ unless there is sufficiently rapid fluid deceleration (of the Doppler E-wave contour) with sufficient cardiohemic coupling.