Phonocardiography

Classification of acoustic phenomena in the heart

The physiological nature of the sounds recorded by the phonocardiogram lies in mechanical oscillations. These vibrations involve the valve cusps, the walls of large blood vessels, and the walls of the various chambers of the heart itself during the cardiac cycle.

All acoustic phenomena captured during the examination are broadly divided into two categories depending on the conditions of their occurrence:

• Sounds — acoustic phenomena that normally occur in the heart, i.e., during the physiologically correct functioning of the organ.
• Murmurs — acoustic phenomena whose appearance always indicates the presence of pathology in the cardiovascular system.

Normal physiology distinguishes exactly 4 heart sounds. Their occurrence in healthy individuals varies:

1. 1st and 2nd heart sounds — the main acoustic complexes that are recorded absolutely every time during every cardiac contraction.
2. 3rd and 4th heart sounds — additional acoustic phenomena that are recorded only in a small minority of people (typically children and young individuals with a thin chest wall).

Correlation between electrocardiography and phonocardiography

Electrocardiography (ECG) and phonocardiography (PCG) are the principal instrumental methods for examining the heart. Comparing them in parallel is of crucial diagnostic importance, as it allows electrical processes (excitation and repolarization) to be linked with mechanical and acoustic manifestations (myocardial contraction and valve function).

The graphs clearly trace the relationship between electrical and acoustic phenomena throughout the entire cardiac cycle. For instance, the time interval between the end of ventricular electrical systole and their actual mechanical relaxation with the snapping shut of the valves is approximately 40 milliseconds (ms).

Characteristics of the 1st heart sound

The first heart sound is closely associated with the onset of ventricular contraction. On the electrocardiogram, it coincides strictly with the ventricular depolarization complex — the QRS complex.

The time of occurrence of the 1st sound is recorded almost immediately after passing the peak of the R wave.

The physiological sequence of events leading to the formation of the 1st sound is as follows:

1. Electrical excitation of the ventricular myocardium occurs (the QRS complex forms on the ECG).
2. There is a transition from electrical excitation directly to the mechanical contraction of muscle fibers.
3. Increasing pressure in the ventricular cavities leads to the closure of the atrioventricular (AV) valves.
4. Rapid tension and vibration of the apposed AV valve cusps, as well as muscular trabeculae, form the sound wave recorded by the device as the 1st sound.

Mechanism of the 2nd heart sound

The second sound reflects processes occurring at the beginning of heart relaxation. Unlike the 1st sound, it occurs after the end of the T wave on the ECG, which marks the process of ventricular repolarization.

The precise time of the sound's appearance falls within the period following the so-called protodiastolic period after the completion of the T wave.

The cause and mechanism of the 2nd sound's formation are strictly sequential:

1. The phase of mechanical relaxation of the ventricles begins.
2. Pressure in the ventricular cavities drops precipitously, causing a retrograde blood flow from the aorta and pulmonary trunk back toward the relaxed ventricles.
3. This retrograde blood flow catches the cusps of the semilunar valves (aortic valve and pulmonary valve) and snaps them tightly shut.

The main acoustic component of the 2nd sound is caused precisely by the sharp mechanical oscillations of the walls of the closed semilunar valves under the impact of the retrograde blood flow.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Origin and diagnostic significance of the 3rd and 4th sounds
• Detailed phase structure of the cardiac cycle
• Analysis of pathological murmurs on the phonocardiogram
• Auscultation points and their projection onto the chest wall

Other topics in the "Cardiac Physiology" section

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