First Heart Sound
The first heart sound (S1) is produced by the closure of the mitral and tricuspid valves in early systole, and is loudest near the apex of the heart. It is described as a Lubb, is more complex, is louder, and lasts longer than the second sound.
Although mitral closure usually precedes tricuspid closure, they are separated by only 0.01 seconds, such that the human ear appreciates only a single sound through the stethoscope. An exception to this occurs in patients with right bundle branch block (where the electrical impulse does not travel as quickly through the right ventricle as the left), whereby the sounds from the mitral and tricuspid valves may be split enough for the physician to determine two sounds (due to the delayed closure of the tricuspid valve).
S1 may be accentuated when the heart rate is more rapid than normal (i.e. tachycardia), because diastole is shortened, and therefore ventricular contraction forces together the tricuspid and mitral leaflets (cusps) from relatively wide positions, because they have had insufficient time to drift together.
The first heart sound can be most clearly heard in the area of the apex of the heart.
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