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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
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At the beginning of the systole, the contraction of ventricles causes sudden backflow of blood at atrioventricular valves
โ Valves to close and bulge towards the atria
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Chordae tendineae abruptly stop this back-bulging motion
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The blood that was trying to go back into the atria is bounded back into the ventricles
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Sets up vibrations in blood, valves, and the ventricular wall
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The vibrations travel through the adjacent tissues
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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
โ Closes the semilunar valves and makes them bulge into the ventricles
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But the elastic stretch of the valves recoils the blood back into the arteries
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This produces vibration in blood, valves, and wall of large arteries
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The vibrations are transmitted to adjacent tissues
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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
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Resultant three sound sequence 'S1-S2-S3' sound like a galloping horse
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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
โ i.e. ventricles do not dilate easily
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Atria contract more forcefully to fill ventricles
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Produces fourth heart sound
Presystolic Gallop or Atrial Gallop
When fourth heart sound is added to the physiological S1 and S2
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The resultant sequence 'S4-S1-S2' also sound like a galloping horse
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Its called presystolic gallop or atrial gallop