top of page

Heart Sounds During Cardiac Cycle



Introduction

  • 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

 

Next >>

Comments

bottom of page