The first heart sound in normal and pathological conditions
Insights
The first heart sound originates from left ventricular dynamics, not valve closure. Its components are linked to ventricular wall tension and aortic valve events, offering insights into cardiac health.
Area of Science:
- Cardiology
- Biophysics
- Physiology
Background:
- The exact mechanism of the first heart sound has been debated, with older theories focusing on valve closure.
- Recent evidence suggests the first heart sound is generated by left ventricular dynamics.
Purpose of the Study:
- To elucidate the physical basis and mechanism of the first heart sound.
- To analyze the components of the first heart sound and their relationship to cardiac events.
- To investigate how various cardiac conditions affect the first heart sound.
Main Methods:
- Analysis of cardiac contraction and sound generation principles.
- Correlation of sound components with specific cardiac events (e.g., ventricular tension, aortic valve opening).
- Examination of first heart sound characteristics in conditions like bundle branch blocks, atrial septal defects, AV block, mitral stenosis, and arrhythmias.
Main Results:
- The normal first heart sound originates solely from the left ventricle, caused by accelerations and decelerations.
- Three components of the first sound are identified, related to ventricular tension, aortic valve opening, and peak aortic pulse.
- Abnormalities like left bundle branch block alter the sound's amplitude and timing, while right bundle branch block shows less significant changes.
- Conditions such as AV block, mitral stenosis, and catecholamine influence demonstrably alter the first heart sound, indicating its role as a contractility index.
Conclusions:
- The first heart sound is a complex event driven by left ventricular mechanics, not merely valve closure.
- Variations in the first heart sound provide valuable diagnostic information for various cardiac conditions and contractility assessment.
- The first heart sound's components and modulations offer a window into myocardial function and electrophysiological status.
Abstract:
Considerations of the physical basis of cardiac contraction and sound generation explain the mechanism of the first sound. Older theories examining this sound as the result of valve closure or stiffening are refuted. It has been demonstrated that the normal first sound originates in the left ventricle alone and that accelerations and decelerations, "timed" by mitral and aortic valves events, are its cause. Three components have been recognized in the first sound: a occurs when the left ventricular wall and septum have reached a certain tension; b when the aortic valve opens; c when the peak of the aortic pulse has been reached. The ventricular septum is an integral and essential part of the left ventricle. In left bundle branch block, abnormal activation of the septum transforms this into a passive structure resulting in a slower rise of left ventricular pressure and a longer isovolumic period. This causes a small and delayed first sound, whose components, however, are still separated by normal intervals. In right bundle branch block, the first sound has a normal amplitude and its components are separated by normal intervals. If there is a larger late component, it is a c component, similar to that of normal elderly subjects. A larger c component may also be found in atrial septal defect. The cannon sound of AV block is caused by more rapid deceleration due to higher atrial pressure at the onset of ventricular contraction resulting in intense vibrations. The first sound of arrhythmias varies in the different conditions and even in different subjects, due to the effect of several variable factors. Elevated left atrial pressure, stiffening of the mitral valve in mitral stenosis, causes a slow onset and a more rapid rise of LV pressure. This results in a delayed, but larger, first sound. The action of catecholamines on the myocardium dramatically increases the first sound. The latter can be considered as an index of contractility and may be of great interest during stress tests.
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