First hea sound is soft in all, except -

Causes of the First and Second Hea Sounds. The earliest explanation for the cause of the hea sounds was that the "slapping" together of the valve leaflets sets up vibrations. However, this has been shown to cause little, if any, of the sound, because the blood between the leaflets cushions the slapping effect and prevents significant sound. Instead, the cause is vibration of the taut valves immediately after closure, along with vibration of the adjacent walls of the hea and major vessels around the hea. That is, in generating the first hea sound, contraction of the ventricles first causes sudden backflow of blood against the A-V valves (the tricuspid and mitral valves), causing them to close and bulge toward the atria until the chordae tendineae abruptly stop the back bulging. Myocardial inflammation may affect electrical conduction pathways, leading to P-R interval prolongation (firstdegree AV block or rarely higher-level block) and softening of the first hea sound. Sho PR interval causes loud S1 except n WPW syndrome. Ref Harrison 20th edition pg 1583