Classification of Cardiac Acoustic Phenomena
The physiological basis of the sounds recorded on a phonocardiogram involves mechanical vibrations. These oscillations encompass the valve cusps, large vessel walls, and the walls of the cardiac chambers throughout the cardiac cycle.
All acoustic phenomena detected during the examination are broadly divided into two categories based on their origin:
- Heart sounds — acoustic phenomena that occur normally within the heart during physiologically regular function.
- Murmurs — acoustic phenomena whose presence always indicates pathology within the cardiovascular system.
Normal physiology distinguishes exactly 4 heart sounds. Their prevalence among healthy individuals varies:
- S1 and S2 — the primary sound complexes registered during every single cardiac contraction.
- S3 and S4 — additional sound phenomena registered only in a minority of individuals (typically children and young adults with a thin chest wall).
Correlation Between Electrocardiography and Phonocardiography
Electrocardiography (ECG) and phonocardiography (PCG) are primary instrumental methods for cardiac evaluation. Their simultaneous comparison is of critical diagnostic importance because it links electrical events (depolarization and repolarization) with mechanical and acoustic manifestations (myocardial contraction and valvular function).
The graphs allow for a clear visualization of the relationship between electrical and acoustic phenomena across the cardiac cycle. For example, the time interval between the end of ventricular electrical systole and actual mechanical relaxation with valve closure is approximately 40 milliseconds (ms).
Characteristics of the First Heart Sound (S1)
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 onset of S1 is recorded almost immediately after the peak of the R wave.
The physiological sequence of events leading to the formation of S1 is as follows:
- Electrical excitation of the ventricular myocardium occurs (forming the QRS complex on the ECG).
- Transition takes place from electrical excitation directly to the mechanical contraction of muscle fibers.
- Rising intraventricular pressure leads to the closure of the atrioventricular (AV) valves.
- Rapid tension and vibration of the approximated AV valve cusps, along with muscular trabeculae, create a sound wave registered by the device as S1.
Mechanism of the Second Heart Sound (S2)
The second heart sound reflects processes occurring at the onset of ventricular relaxation. Unlike S1, it occurs after the end of the T wave on the ECG, which marks ventricular repolarization.
The exact time of sound appearance follows the protodiastolic period after the completion of the T wave.
The cause and mechanism of S2 formation follow a strict sequence:
- The phase of mechanical ventricular relaxation begins.
- Intraventricular pressure drops rapidly, creating a retrograde pressure gradient that drives blood from the aorta and pulmonary trunk back toward the relaxed ventricles.
- This retrograde blood flow catches the cusps of the semilunar valves (aortic and pulmonary valves) and snaps them shut.
The primary acoustic component of S2 is generated specifically by the sharp mechanical oscillations of the closed semilunar valve walls struck by this retrograde blood flow.