S1 is caused by the closure of the tricuspid and mitral values. Generally, the closure will be simultaneous so that a single S1 is heard, loudest at the apex. In some cases, both physiological and pathological, a split S1 is heard. With a split S1 the closure of the two valves does not occur together. Splitting of the S1 is best heard in the tricuspid area and is not effected by respiration or other maneuvers which increase intrathoracic pressure.
An split S1 in an otherwise healthy individual is generally not a reason for alarm. Split S1 can be associated with pathological conditions, such a right bundle branch block. If you think you hear a split S1, listen carefully to make sure you are not confused a split S1 with an S4 (left or right sided) followed by an S1, an S1 followed by an aortic ejection sound, or an S1 followed by an aortic click.
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(17 October 2006)
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