Murmur increasing with Valsalva manouvre

Ref: Harrison's 19th E pg 1568 All murmurs decreases with valsalva maneuver except that of HOCM and MVP. In hocm, blood acts as a physical barrier between hyperophied septum and anterior mitral leaflet. During valsalva, there is decreased venous return and subsequently decreased blood reaching LV. As blood flow reduces, there is more chances of obstruction, it increases the murmur (ejection systolic murmur)