First Heart Sound (S1)

First Heart Sound (S1)


S1 results from mitral and tricuspid valve closure. It is a duller, lower-frequency sound than S2, and occurs at the beginning of ventricular systole.

Listening Areas

Listening Areas for S1

Loudest at the Apex. Can be heard with the stethoscope bell or diaphragm.

Associated Conditions

Loud S1:

Soft or Absent S1:

See Also


S1 is usually duller, longer in duration, and softer than S2. (The popular "lub-dup" mnemonic of S1 and S2 accurately conveys this contrast in sounds.) This sound occurs at the beginning of ventricular systole, and indicates mitral (M1) and tricuspid (T1) valve closures.

In early systole, blood from the contracting ventricles attempts to exit back through the open tricuspid and mitral valves, filling the valve leaflets like parachute canopies, and causing the valves to close. This closure sound is reflected retrograde, back toward the apex.

At the apex, S1 is usually louder than S2 (sounding like "LUB dup," "LUB dup"). Usually, M1 and T1 are nearly synchronous, and difficult to detect as separate events (called a split S1 when it occurs).

Note: a first heart sound followed by a louder ejection sound is often confused with a split S1. The louder ejection sound can even overwhelm the softer S1, so that it is mistaken for S1.