S1: start of systole; caused by the tricuspid and mitral valves closing.

S2: diastole; aortic and pulmonic valves close

 

S3: inrushing blood from the atria causes oscillation of the blood back and forth in the walls of the ventricles. Occurs in the middle of diastole because initially there is not enough blood in the ventricles to cause the reverberation.

Pathologic causes:

-rapid ventricular filling:

-mitral regurgitation (which means atria will be overfilled in systole and can thus send more blood to ventricles during diastole and thus cause rapid ventricular filling)

-elevated left atrial and left ventricular filling pressures (from a stiff and dilated left ventricle)

            -ventricular septal defect (hole allows rapid filling from other side)

-poor left ventricular function

            -post-MI (dead tissue is stiff and does not relax so filling is quick)

            -dilated cardiomyopathy (thin and stiff walls)

 

S4: atria contracting forcefully to overcome stiff or hypertrophic ventricles. (stiff from post-MI fibrosis, hypetrophy from aortic stenosis, hypertension, overloading).

Occurs immediately before S1. Sometimes heard in healthy children and trained athletes.

If pathologic, will be best heard at apex.