The First Heart Sound

NORMAL S₁

Valves involved in S₁ sound production

Auscultatory area for S₁

The cardiac vibrations associated with the closure of the mitral and tricuspid valves produce the first heart sound, S₁. (♦ Sound 2) Normally, only two components of the first heart sound are audible. The first component, referred to as M₁, is associated with the closure of the mitral valve; the second component, T₁, is associated with the closure of the tricuspid valve. Both valves close at the beginning of ventricular systole, but the mitral valve usually closes slightly ahead of the tricuspid valve.

Auscultatory area and relationship to ECG

M₁ and T₁ are usually perceived as a single sound called S₁, which is heard best near the apex of the heart over the mitral area, using the diaphragm of the stethoscope. A single sound is heard because left-sided heart sounds are normally more intense at this site. S₁ occurs just after the QRS complex on the electrocardiogram (ECG) waveform.

As you inch the stethoscope from the mitral area toward the tricuspid area, without losing track of S₁, the M₁ and T₁ components of S₁ may become evident. Expiration makes them easier to hear. (♦ Sound 3) T₁ trails M₁ slightly and is softer; it’s heard best near the left sternal border. The timing of M₁ and T₁ with the QRS complex remains the same. In many patients, only the first heart sound is heard because M₁ and T₁ are separated by a 20-millisecond—or shorter—pause, which the human ear perceives as one sound. (See PCG and ECG showing normal S₁ split, page 26.)