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Heart Sounds During Cardiac Cycle


  • Four heart sounds are produced during a cardiac cycle:

    • First heart sound (S1)

    • Second heart sound (S2)

    • Third heart sound (S3)

    • Fourth heart sound (S4)

  • First and second heart sounds...

    • are classical or major or physiological heart sounds.

    • can be heard normally by stethoscope.

  • Third and fourth heart sounds...

    • cannot be heard with a stethoscope under normal conditions.

First Heart Sound (S1)

  • Coincides with closure of atrioventricular valves.

  • Mechanism:

During diastole, the atrioventricular valves are open

At the beginning of the systole, the contraction of ventricles causes sudden backflow of blood at atrioventricular valves

Valves to close and bulge towards the atria

Chordae tendineae abruptly stop this back-bulging motion

The blood that was trying to go back into the atria is bounded back into the ventricles

Sets up vibrations in blood, valves, and the ventricular wall

The vibrations travel through the adjacent tissues

Heard as sound

  • Note: Although it's a common belief, the sound is not produced due to slapping together of leaflets of valve because the blood in between the leaflet cushions the slapping.

  • Resembles spoken word 'Lub'

  • Duration: 0.14 sec

Second Heart Sound (S2)

  • Coincides with closure of semilunar valves

  • Mechanism:

    • is almost similar to first heart sound.

At the end of the systole, the backflow of blood from major arteries to ventricles

Closes the semilunar valves and makes them bulge into the ventricles

But the elastic stretch of the valves recoils the blood back into the arteries

This produces vibration in blood, valves, and wall of large arteries

The vibrations are transmitted to adjacent tissues

heard as second heart sound

  • Resembles spoken word 'Dub'

  • Duration: 0.11 sec

Third Heart Sound (S3)

  • Heard during early diastole (rapid ventricular filling).

  • Produced by rapid inrushing of blood from atria into ventricles.

  • Hear physiologically in:

    • Children

    • Young Adults.

  • In older people its indicative of pathology.

  • Protodiastolic Gallop or Ventricular gallop

When the third heart sound gets added to the physiological S1 and S2

Resultant three sound sequence 'S1-S2-S3' sound like a galloping horse

Its called protodiastolic gallop or ventricular gallop

Fourth Heart Sound (S4)

  • Heard late during diastole (atrial contraction)

  • Mechanism:

Pathological conditions in which ventricular compliance is decreased

i.e. ventricles do not dilate easily

Atria contract more forcefully to fill ventricles

Produces fourth heart sound

  • Presystolic Gallop or Atrial Gallop

When fourth heart sound is added to the physiological S1 and S2

The resultant sequence 'S4-S1-S2' also sound like a galloping horse

Its called presystolic gallop or atrial gallop


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