The intensity of S1 is highly variable and diagnostically valuable. It primarily depends on three factors: (1) the position of the AV valve leaflets at the onset of systole, (2) the structural integrity of the valve, and (3) the force of ventricular contraction.
Because the left ventricle contracts slightly before the right ventricle (due to earlier electrical activation and faster pressure rise), the mitral valve closes before the tricuspid valve. This physiological asynchrony is normally too brief to be heard as a split S1. However, in conditions that —such as a right bundle branch block (RBBB), which delays right ventricular activation and thus tricuspid closure—the M1 and T1 become separated enough to be appreciated as a widely split S1 on auscultation, best heard at the left lower sternal border. what is s1 heart sound
A hallmark of atrial fibrillation (irregularly irregular rhythm) is a changing intensity of S1, because the variable diastolic filling periods result in inconsistent positioning of the AV valve leaflets at the start of each systole. Similarly, in complete heart block , the “cannon” waves of atrial contraction against closed AV valves produce unpredictably loud S1 sounds. The intensity of S1 is highly variable and
The first heart sound (S1) is a succinct but rich auditory marker of the onset of ventricular systole, arising from the vibration of the closing mitral and tricuspid valves and adjacent structures. Its normal physiological splitting, though rarely audible, becomes a key finding in conduction delays like RBBB. More importantly, the intensity of S1—whether loud, soft, or variable—offers a non-invasive, real-time window into valvular integrity, cardiac rhythm, and ventricular contractility. For the skilled clinician, listening to S1 is not merely identifying the “lub”; it is a fundamental act of cardiovascular assessment that guides differential diagnosis and directs further investigation. This physiological asynchrony is normally too brief to
S1 is generally lower-pitched and longer in duration than the second heart sound (S2).