S3 occurs at the end of the rapid filling phase of diastole (75,76). The column of blood entering the ventricle during this phase is under the pressure head provided by the v wave pressure in the atrium. This phase of diastolic expansion is generally rapid and vigorous for reasons discussed earlier. But in almost all hearts this rapid expansion suddenly changes to a period of slower expansion. Thus, there is a tendency almost in all hearts for the moving column of blood entering the ventricle during the rapid-filling phase to decelerate somewhat toward the end of this period (Fig. 32). When the transition becomes more abrupt, this will be expected to affect the moving column of blood, causing it to decelerate more abruptly. The factors that are likely to make the transition more abrupt in general are those that decrease the compliance of the ventricle (77-80).
Fig. 32. Stop frame of a two-dimensional echocardiogram taken from a normal subject in the parasternal long axis at the end of the rapid filling phase of diastole when the moving column of blood (arrow) entering the left ventricle (LV) from the left atrium (LA) is suddenly decelerated because of the fact that the rapid expansion suddenly changes to a period of slower expansion. Factors that make the transition more abrupt tend to produce an S3 (see the text).
The energy achieved by the moving column of blood during the active rapid filling phase of diastole is related to the rate of relaxation, the velocity, and the volume of blood entering the ventricle and the pressure head provided by the v wave peak in the atrium. When the momentum achieved by the moving column of blood is significant and the transition from the early rapid filling phase to the slow filling phase more abrupt because of decreased ventricular compliance, however brought about, then the deceleration will occur more suddenly and the dissipation of energy will result in the production of an audible sound within the ventricle. The sound will obviously occur at the peak of the rapid filling wave, which is the S3. Intraventricular pressure and transmitral flow studies in dogs have demonstrated a small but consistent reverse transmitral gradient to always accompany this deceleration (81). In addition, the sounds accompanying the flow deceleration could be recorded inside the ventricles as well as over the epicardial surface ofthe exposed ventricles, ruling out the external origin theory of S3 (82). The whole hemic mass, including the blood, as well as the ventricular wall and the papillary muscles probably participate in the vibration.
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