QS Study

Mechanism of Production of Heart Sound

The vibratory motion of heart during the different phases of cardiac cycle produces a special audible sound known as heart sound. Many pathologic cardiac conditions can be diagnosed by auscultation of the heart sounds.

Classification: 4 types

(i) 1st heart sound (Audible by stethoscope)

(ii) 2nd heart sound (Audible by stethoscope)

(iii) 3rd heart sound (Detected by phonocardiogram)

(iv) 4th heart sound (Detected by phonocardiogram)

Causes of production of heart sound

  • The vibration of leaves or the valves during the closure.
  • An occurrence of turbulence during a rapid rush of blood from the atrium to the ventricle.

Fig: production of heart sound graph

(A) 1st heart sound

Definition: The dull and prolonged sound like L-U-B-B that occurs at the onset of ventricular systole is called 1st heart sound. The first heart sound results from the closing of the mitral and tricuspid valves.

Duration: 0.1 — 0.17 sec.

Clinical identification

  • Nature: Dull like the word L-U-B-B.
  • Low pitched and prolonged duration (0.1 -0.17) second.
  • Best heard over the Lt 5th intercostal space medial to midclavicular line.
  • It coincides with apex beat and with the commencement of the carotid pulse.
  • It comes just before the radial pulse.

Cause/Mechanism/physiological basis of production of 1st heart sound

  • The sudden closure of the AV valve at the onset of ventricular systole.
  • Oscillation of blood in the ventricular chamber.
  • The turbulence of blood through aorta & ventricle.
  • Contraction of the thickened ventricular muscle.


(B) 2nd heart sound

Definition: The short and sharp sound like D-U-B just after the end of ventricular systole is known on 2nd heart sound. The second heart sound is produced by the closure of the aortic and pulmonic valves.

Duration: 0.1 – 0.14 sec.

Clinical identification

  • Nature: Sharp like D-U-B.
  • High pitched and short duration.
  • Occurs just after the apex beat and carotid pulse.
  • Occurs after 1st heart sound.
  • Best heard in the aortic and pulmonary area.

Cause/Mechanism/physiological basis of 2nd heart sound

  • The sudden closure of semilunar valves at the onset of ventricular diastole.
  • A vibration of the aorta and pulmonary trunk.

(C) 3rd heart sound

Definition: The heart sound takes place just after 2nd H. S and an opening of A. V. valve. The third heart sound, also known as the “ventricular gallop,” occurs just after S2 when the mitral valve opens, allowing passive filling of the left ventricle.

Duration: 0.04 sec.

Clinical Identification: Soft, Low-pitched and Can only be recorded by phonocardiogram.

Cause/Mechanism/physiological basis of 3rd heart sound: Sudden inrush of blood into ventricle when A-V valve opens.


(D) 4th heart sound or Atrial heart sound

The low pitched sound during a rush of blood into ventricle during atrial contraction.

Clinical identification:

  • Low pitched,
  • It occurs just before 1st Heart sound.
  • Can be recorded by phonocardiogram.

Cause/Mechanism/Physiological basis of 4th H. S: The contraction of the atria and consequent rush of blood into the ventricle.


Causes of Heart sounds

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