The third heart sound (S3) is a low-frequency, early diastolic sound that occurs during the rapid filling phase of the ventricles. It is typically associated with high ventricular filling pressures, often due to conditions like heart failure, volume overload, or mitral regurgitation. However, the S3 sound is generally absent or not prominent in mitral stenosis. Here’s why:
- Pathophysiology of Mitral Stenosis:
- Mitral stenosis involves a narrowing of the mitral valve, restricting the flow of blood from the left atrium to the left ventricle during diastole. This narrowing impedes the rapid early filling of the left ventricle, reducing the pressure difference between the left atrium and ventricle during early diastole.
- Decreased Rapid Filling:
- The reduced flow of blood through the narrowed mitral valve results in diminished rapid filling of the ventricle during early diastole. The reduced velocity of blood flow entering the ventricle limits the generation of the S3 sound, which is primarily associated with rapid ventricular filling.
- S3 Generation and Ventricular Compliance:
- The generation of S3 is often associated with increased ventricular compliance, allowing rapid filling of blood into the ventricle during early diastole. In mitral stenosis, the compliance of the left ventricle is reduced due to the stenotic mitral valve, resulting in a diminished or absent S3 sound.
- Dominant Late Diastolic Sound (S4):
- In contrast, mitral stenosis is often associated with an S4 sound, which occurs in late diastole and is related to atrial contraction against a non-compliant left ventricle. The increased resistance to atrial contraction due to the narrowed mitral valve contributes to the generation of the S4 sound.
In summary, the pathophysiological changes in mitral stenosis, particularly the narrowed mitral valve and decreased ventricular compliance, lead to reduced rapid ventricular filling during early diastole. This reduction in rapid filling limits the generation of the S3 sound, making it generally absent or less prominent in individuals with mitral stenosis.