Third heart sound- theories behind its genesis

The Third Heart Sound was first described by Obrastzow in l905. Since then, various theories have been proposed to explain the genesis of S3. Three major theories that are used these days include:

1. Valvular theory

2. Ventricular theory

3. Impact theory

Valvular theory:

Earliest theory proposed regarding the origin of S3 is the valvular theory according to which S3 is produced by the diastolic tenting of the atrioventricular valves at the end of rapid ventricular filling phase. This theory was rejected by various investigators after it was demonstrated that S3 continued to occur in patients who had their valves replaced by mechanical prostheses.

Ventricular theory:

The most accepted theory explaining the origin of S3 is the ventricular theory. According to this theory, these sounds originate within the left or right ventricle or their walls. S3 occurs when the ventricle suddenly reaches its elastic limit and abruptly decelerates early diastolic blood flow, thereby setting the entire cardiohemic system into vibration.

The major factor here is the dynamic interplay between the force of delivery of blood into the ventricle and the ability of the ventricle to accept this flow.

This theory also partly explains the 'low frequency' nature of the S3 because the the vibrating structure (ventricle) producing S3 has a great mass in proportion to its small elasticity.

Impact theory:

According to this theory proposed by Reddy and associates, S3 is produced by the dynamic impact of the heart against the chest wall. The intensity of S3 is dependent on the force of impact which in turn is decided by the size of the heart, its movement within the thorax and, the chest wall configuration. This theory explains the occurrence of S3 in hyperdynamic states.

Theories apart, presence of S3,especially if persistent, in a patient with cardiomyopathy or an MI is an ominous sign associated with high mortality needs aggressive treatment of the underlying disease state!!