First Heart Sound (S1)
- Mitral Stenosis
- Ejection Sound masquerading as S1
Soft or Absent S1:
- 1° Atrioventricular Block
- Aortic Regurgitation: Acute (from preclosure of mitral valve)
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).