What heart sound is created during isovolumic contraction?

• A. The first heart sound, Lubb (S1), produced by closing of the AV valves.
• B. The second heart sound, Dubb (S2), produced by closing of the SL valves.
• C. A third heart sound, S3.
• D. A fourth heart sound, S4.

Answer: (A)

During isovolumic contraction, the ventricles begin to contract with all heart valves closed. As the ventricular pressure rises quickly, the atrioventricular valves (mitral and tricuspid) snap shut. That sudden valve closure creates vibrations in the surrounding heart tissues, which we perceive as the first heart sound, Lubb (S1). This marks the very start of ventricular systole, before the semilunar valves open to begin ejection.

In contrast, the second heart sound (Dubb, S2) comes from the closing of the semilunar valves at the end of systole, when ejection finishes and the great arteries back pressure closes those valves. The other two sounds, S3 and S4, arise from different phases (early diastolic rapid filling and late diastolic atrial contraction against a stiff ventricle, respectively) and are not tied to the moment of isovolumetric contraction.