You are listening to a typical example of a fourth heart sound, or S4. The late stage of diastole is marked by atrial contraction, or kick, where the final 20% of the atrial output is delivered to the ventricles. If the ventricle is stiff and non-compliant, as in ventricular hypertrophy due to long-standing hypertension, the pressure wave generated as the atria contract produces an S4. It is heard best with the bell of the stethoscope at the apex.

     A good mnemonic to remember the cadence and pathology of an S4 is:

a-STIFF’-wall                  a-STIFF’-wall

^{S4     S1         S2                          S4     S1           S2}

Practice saying this aloud as you listen to this rhythm.

Compare this to a third heart sound.