The opening of an atrioventricular valve is normally silent and occurs about 100 msec after S2. This is about as long as it takes to say ''ma-ma'' quickly. An opening snap is a diastolic event that is the sound of the opening of a pathologically deformed atrioventricular valve. The sound is sharp and high-pitched. The mitral opening snap of mitral stenosis occurs after A2; the tricus-pid opening snap of tricuspid stenosis occurs after P2.
The interval between S2 and the opening snap is termed the S2-OS interval and has specific significance as to the severity of the stenosis. As mitral stenosis worsens, the resistance and obstruction to flow increase. Pressure in the left atrium increases, and the gradient across the mitral valve increases. The mitral valve, therefore, opens earlier than normal when the left ventricular pressure falls below left atrial pressure. The S2-OS time shortens as the severity increases. Try saying ''ma-da'' as quickly as possible. This interval is about 50 to 60 msec and approximates a very short S2-OS interval, as heard in severe mitral stenosis.
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