Diastolic murmurs: MS/TS AR/PR
Diastolic murmur mnemonics: MS ARD (Mitral Stenosis Aortic Regurgitation Diastole)
Diastolic murmur mnemonics by Qt: MS(TS) AR(PR) D
Diastolic murmurs occur during the diastole phase in which the Atrium is pushing blood from the atrium to the ventricle. This means, during the normal diastole, the M valve will stay open completely while the A valve is closed completely. Hence, murmur occurs when M valve fails to open completely (MS/TS) or the A valve fails to close completely (AR/PR).
AORTIC REGURGITATION
Aortic regurgitation occurs when the blood flow backwards. Note that in Mitral valve, the normal blood flow is from atrium to ventricle. In mitral valve regurgitation, the backward blood flow thus means the blood from ventricle to atrium. Similarly, in normal aortic valve, the normal blood flow is from ventricle to aorta. Thus, in abnormal aortic valve regurgitation, the blood flows backwards from the aorta to the ventricle. Note that, during diastole, the pressure in the left ventricle is low allowing the filling of the blood from the atrium to the ventricle. Therefore, the previously pumped blood from the previous systole is high leading to high blood pressure in the aorta. As aortic regurgitation occurs, the high pressure blood leakage of blood from aorta to the ventricle causing a high murmur sound heard. Then, as the ventricle is filling up with blood, the sound of the murmur is less and less audible (descendo). Thus, the term early diastolic murmur.
Note that early diastolic murmur (aortic regurgitation) is heard best in the left sternal border instead of the Aortic 2nd right intercostal space because the left seternal border is where the blood is regurgitating back not the Aortic valve location of the 2nd ICS.
PULMONIC REGURGITATION
Pulmonic regurgitation is similar to aortic regurgitation in mechanism. The only difference is that it's heard best in the upper left side of the sternal border. Pulmonic regurgitation is also a diastolic murmur; more specifically, it's an early diastolic murmur.
MITRAL STENOSIS
Mitral stenosis occurs during diastole in which it's supposed to be completely open to get blood flow from the atrium to the ventricle. However, in mitral stenosis, the mitral valve is too stenotic to allow blood flow from the atrium to the ventricle.
It takes the atrium to build up some more pressure than normally to force open the stenotic mitral valve. Thus, it will open loudly (Opening Snap) as the mitral valve is 'cracked' open instead of nicely quietly open in normal valves. The onset of the the opening of the mitral valve is about some msec after the S2 (closure of the A&P valves).
When the blood coming from the atrium to the ventricle is flowing in, at first, this pressure is very high (recall it takes a lot of pressure to 'crack open' this stenotic valve). Then, when the way is made, the pressure slows down.
At the last effort, the atrium starts to squeeze all the blood to the ventricle nearing the end of the diastole. This results in the louder sound at the end of the diastole. Thus, it's named 'rumbling sound' at the mid-diastolic.
Thus, this murmur is termed 'Opening snap + mid-diastolic' with rumbling sound.
TRICUSPID STENOSIS
Tricuspid stenosis is similar to mitral stenosis: opening snap + mid diaslotic rumbling sound.
References:
Info & videos mostly from Khan Academy: https://youtu.be/ZUHpAaVpiY8
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