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Third Heart Sound S3
Rapid rate of ventricular filling due to volume overload heard immediately after S2. Can be normal in young children and athletes.
Causes: MR, VSD, Post MI, and dilated cardiomyopathy

Fourth Heart Sound S4
Atrial contraction against stiff ventricles. Left sided S4 heard best at the apex in left decubitus position while right side S4 is heard best at the sternal border in supine position. It's always pathologic.
Causes: Stiff ventricle, such as LVH, AS, Ischemic heart disease, and Hypertension
 
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