Third heart sound is a unique heart sound because its perfect physiology to hear it in the young , while the same may denote serious LV dysfunction in patients with myocardial disease.
It is a low pitched early diastolic sound usually correspond to the end of rapid filling phase.The mechanism of genesis of this sound has been debated for many years .(Still I think it is unsettled !)
We know genesis of intracardiac sound is contributed by three factors
- The blood flow
- The valve motion
- The myocardial contractile and relaxyl property
The above three is collectively called cardio-hemic system . When this system vibrates heart sounds are generated .In the genesis of S3 all the three may be important . The only difference is , in physiological S3 the valvular and hemic component play a major role . In pathological S3 the myocardial component has a pivotal role .The distended LV facilitates chest wall impact during the rapid filling phase . It is now accepted LV S3 is generated outside the LV . It was proved elegantly by Shaver et all with sound recording on either side of LV /Chest wall.
It is to be emphasized the mechanism of genesis of S3 is diagonally opposite in physiology vs pathological S3 in some conditions . Rapid AV filling can rarely be responsible for pathological S3 associated with severe LV dysfunction , while chest wall impact can contribute in both physiological as well as pathological S 3 .
One can understand the complexity of genesis of S 3 , as there are too many determinants that contribute in varying degree of acoustics.
Factors determining the intensity of S3 is complex
Rapidity of flow across the atrio-ventricular valve,
Rate of early diastolic relaxation
Distensibility of the ventricle,
Ventricular cavity size,
Diastolic momentum of the heart,
Degree of contact (coupling) with the chest wall, thickness
Character of the chest wall
The position of the patient.
It is ironical, pathological S3 which is a diastolic sound though , still its genesis is largely determined by the systolic function of the heart .This mystery is partially solved as we recognise now , LV S3 is equally common in severe degrees of diastolic dysfunction. In fact , while we were studying the relationship of LVS3 in DCM , it has strongly predicted the presence of severe restrictive pattern in them .
1.Multimedia of S3
2.Importance of S3 in cardiology NEJM 2001 article
3.Chest wall impact theory of S3 by Shaver
Shaver JA, Salerni R. Auscultation of the heart. In: Hurst, ed. Heart. 8th ed. New York, NY: McGraw-Hill, Inc; 1994:291.