First Heart Sound

A loud first heart sound reveals when the leaflets are wide open at the foot of the ventricular diastole and shut forcefully at the beginning of the ventricular systole. Possible causes of a loud first heart sound include late atrioventricular closure. 

Characteristics of first heart sound

  1. Nature: Dull, low pitched, prolonged duration: 0.15 sec.
  2. Sound like the word: L-U-B.
  3. Frequency: 25-45 HZ
  4. Area: it is best heard over the left 5th inter-coastal space just medial to mid-clavicular line.
  5. It coincides with the apex beat and carotid pulse and the spike of R-wave of ECG.


  1. It indicates the clinical onset of ventricular systole.
  2. The duration and intensity indicate the condition of the myocardium.
  3. A clear first heart sound indicates that A-V valves are closing properly. Ie. there is no incompetence.

First Heart Sound (Loud)

The first heart sound is made up of several components, although the most audible components heard at the bedside are the high-frequency vibrations related to mitral and tricuspid closure. Generally, the louder sound of mitral closure drowns out the softer sounds of tricuspid closure.

This audio may take some time for loading. keep full sound and use headphones for a better experience! The possible causes of loud first heart sound are:

  • Atrial fibrillation
  • Tachycardia
  • Hyperdynamic circulation
  • Mitral stenosis
  • Wolff-Parkinson-white syndrome
  • Reduced PR interval

A very loud first sound can be an indication of the mitral stenosis.

First Heart Sound (Minimally Split)

Minimally split the first sound is the normal version of the first heart sounds.

Dyssynchrony then occurs, resulting in the left ventricle contracting before the right ventricle, thus the pressures in LV rise before that of the right ventricle.

This delays the closure of the tricuspid valve, resulting in a minimally split of the first heart sounds. A left bundle branch block has the opposite effect on the first heart sound (S1).

First Heart Sound (Markedly Split)

If the splitting of the heart sounds more than about fifty milliseconds, then it is known as the first heart sounds markedly split.

The delay between the aortic and pulmonic sounds (A2-P2) is observed as markedly splitting of the second heart sound. Prolonged or fixed S2 splitting can be a sign of heart diseases such as an atrial septal defect or right bundle branch block. Note the fixed second heart sound split in the following example: learning to appreciate S2 splitting can be difficult.

First Heart Sound (Decreased Intensity)

The decreased intensity of first heart sound (S1) occurs in conditions that cause the Artio-ventricular (AV) valves to close before ventricular systole or if there is a reduction in the rate of intraventricular pressure development.

First-degree heart block is the most common cause of decreased intensity of the first heart sound. The sound can be also produced by the viral infections of the heart and decreased thyroid function. It is due to a decrease in the ability of contraction of the left ventricle.

Fourth Heart Sound Plus First Heart Sound

The fourth heart sound, also known as atrial gallop. The fourth heart sound is a low-pitched sound synchronous with a late diastolic filling of the ventricle due to atrial contraction.

Even though it is also called the atrial sound, its’ extension requires a productive atrial contraction. The fourth heart sound is the result of vibrations inaugurated within the ventricle. The fourth heart sound (S4) is of lesser frequency than the first heart sound (S1). You need to listen to the frequency of the initial sound in order to differentiate between the fourth heart sounds and the first heart sound. If it is lesser in frequency than the S2 it is and fourth heart sound. If the two sounds are identical it is a first heart sound (Split).

First Heart Sound plus Aortic Ejection Click

Aortic ejection clicks are high-pitched sounds that occur at the moment of maximal opening of the aortic or pulmonary valves. They are heard just after the S1 sound.

The cause of aortic click is a thickening of the aortic valve leaflet. It is heard in the congenital abnormality condition like a bicuspid aortic valve where someone is born with two aortic valve leaflets instead of the normal three. It is heard at the aortic valve area where the splitting of first heart sound is not heard.

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