## Opening Snap in Mitral Stenosis ### Timing and Mechanism **Key Point:** The opening snap (OS) occurs in **early diastole**, immediately after aortic valve closure (A~2~), and is caused by the **abrupt halting of mitral valve leaflet opening** when the stenotic orifice reaches its maximum excursion. ### Pathophysiology In mitral stenosis, the leaflets are thickened and fibrotic but remain pliable early in the disease. When the left atrial pressure exceeds left ventricular pressure in early diastole, the leaflets open rapidly until the narrowed commissures and stenotic orifice mechanically limit further opening. This sudden deceleration of leaflet motion generates a high-frequency sound. ### Clinical Significance - **A~2~-OS interval**: Inversely related to severity of stenosis - Severe MS: A~2~-OS = 40–60 ms (very short) - Mild MS: A~2~-OS = 80–120 ms (longer) - **Absence of OS**: Suggests either calcified, immobile leaflets (severe chronic MS) or absence of MS ### Differentiation from Other Sounds | Finding | Timing | Cause | |---------|--------|-------| | Opening snap (MS) | Early diastole (after A~2~) | Abrupt halting of stenotic leaflet opening | | S~3~ gallop | Early diastole | Rapid ventricular filling in dilated ventricle | | Pericardial knock | Early diastole | Constrictive pericarditis | **High-Yield:** The presence of an OS with a short A~2~-OS interval is a **hallmark of hemodynamically significant mitral stenosis** and indicates pliable leaflets suitable for balloon mitral valvuloplasty. **Clinical Pearl:** Loss of the opening snap in a patient with known MS suggests progression to severe, calcified disease and portends a worse prognosis. 
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