Third heart sound (S3)

The third heart sound or S3 is a rare extra heart sound that occurs soon after the normal two "lub-dub" heart sounds (S1 and S2).

Segmentation of heart sound with S3. [Source]

A double apical impulse can represent an auscultated third heart sound (S3). Just as with the displaced point of maximal impulse, a third heart sound is not sensitive (24 percent) for heart failure, but it is highly specific (99 percent).

Pulsus Alternans

Pulsus Alternans. [Source]

Pulsus alternans is found in patients with acute left ventricular failure. Alternate weak and strong pulses occur, which are regularly placed. A pathological third heart sound is usually associated. The condition may be associated with heart failure or pericarditis for example, but is not usually seen in association with pericardial effusion.