The INCORRECT Statement is B
Statement B is incorrect: In severe mitral stenosis, the intensity of the first heart sound (S1) typically INCREASES, not decreases. This is a classic finding in mitral stenosis with pliable, non-calcified valves.
Analysis of Each Statement
A) Produced by closure of atrioventricular valves - CORRECT
The first heart sound is generated primarily by closure of the mitral and tricuspid valves 1. The mitral component (M1) coincides with mitral valve closure in 100% of patients, while the tricuspid component may contribute variably 1.
B) Intensity decreases in severe mitral stenosis - INCORRECT
This statement is false and represents the answer to the question. In mitral stenosis with pliable valves, S1 is characteristically loud and accentuated 2, 3. The mechanism involves:
- Increased dp/dt at the moment of mitral valve closure - Studies demonstrate a considerable increase in the rate of pressure change (dp/dt) at valve closure in mitral stenosis patients compared to controls, which directly correlates with increased S1 amplitude 2
- Delayed but forceful valve closure - The stenotic but mobile valve closes from a more open position with greater force 4
- Pliable valve leaflets - When the valve body remains pliable with minimal calcification, it produces loud, "snappy" S1 and opening snap 3
Important caveat: S1 intensity may decrease only in end-stage mitral stenosis when the valve becomes heavily calcified and immobile 3. However, the statement refers to "severe" mitral stenosis generally, where the classic finding is an accentuated S1.
C) Greater intensity than S2 at cardiac apex - CORRECT
At the cardiac apex, S1 is normally louder than the second heart sound (S2). This is a standard auscultatory finding in normal hearts and is preserved or accentuated in conditions like mitral stenosis.
D) Intensity decreases in cardiogenic shock - CORRECT
In cardiogenic shock, reduced cardiac contractility and decreased stroke volume lead to diminished intensity of heart sounds 5. The severely compromised hemodynamic state results in weaker valve closure forces and reduced S1 intensity.
E) Intensity decreases with PR interval prolongation - CORRECT
This is a well-established clinical finding. When the PR interval is prolonged (first-degree AV block), the mitral valve drifts toward the closed position during the prolonged diastole before ventricular systole begins 6. Studies using phono-echocardiography demonstrate:
- Close correlation between S1 intensity and mitral valve position at the onset of ventricular systole (P < 0.001) 6
- When the valve is nearly closed at the start of systole (due to prolonged PR), there is less excursion and softer S1
- Conversely, with short PR intervals, the valve closes from a more open position, producing louder S1
Clinical Pitfall to Avoid
Do not confuse severe mitral stenosis with mobile valves (loud S1) with end-stage calcified mitral stenosis (soft S1). The classic teaching and the correct answer for this question refers to the typical presentation of severe mitral stenosis where the valve remains pliable - in this scenario, S1 is characteristically accentuated, making statement B incorrect.