In mitral stenosis, the opening snap occurs at what point in the cardiac cycle and is caused by which mechanism?

## Opening Snap in Mitral Stenosis ### Timing and Mechanism **Key Point:** The opening snap (OS) is a high-pitched, early diastolic sound that occurs 0.06–0.12 seconds after the aortic component of S₂ (A₂). **High-Yield:** The OS is caused by **abrupt halting of mitral leaflet opening** due to: - Commissural fusion and fibrosis - Reduced leaflet mobility and elasticity - Tensioning of fused commissures at the limit of opening This is distinct from a normal mitral valve, which opens silently and gradually. In stenotic mitral valves, the leaflets are stiff and fused, so they snap open suddenly when the left atrial pressure exceeds left ventricular pressure in early diastole. ### Clinical Correlation **Clinical Pearl:** The presence of an OS indicates a **pliable, non-calcified mitral valve**—a favorable prognostic sign for balloon mitral valvulotomy. Absence of OS suggests severe calcification or a rigid valve. ### Differential Timing | Sound | Timing | Cause | | --- | --- | --- | | **Opening Snap** | Early diastole (0.06–0.12 s after A₂) | Abrupt halting of stenotic mitral leaflet opening | | **S₃ (ventricular gallop)** | Mid-diastole (0.14–0.16 s after A₂) | Rapid ventricular filling in dilated ventricles | | **S₄ (atrial gallop)** | Late diastole (before S₁) | Atrial contraction into a stiff ventricle | **Mnemonic:** **OS = Open Stop** — the leaflets snap open and then stop abruptly due to fusion.