Valsalva and squatting

[faisal 2000]

why cardiac murmurs and sounds increase in intensity during Valsalva maneuver in case of hypertrophic obstructive cardiomyopathy and MVP ?
what is the mechanism behind that squatting decreases cyanosis in any case of cyanotic heart defect?

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[yankees527]

During phase 2 of valsalva (straining phase), there is compression of the veins decreasing venous return. Decreased venous return=decreased preload. Remember that anything that decreases preload in HCM increases the outflow obstruction to the aorta (because the interventricular septum is allowed to bulge into the LV even more with that decreased blood volume in the LV), therefore increasing the murmur.

Compare this to increases in LV preload, which would push the interventricular septum towards the RV, relieving the obstruction of aortic outflow.