What causes the murmur in hypertrophic obstructive cardiomyopathy to increase?

In hypertrophic obstructive cardiomyopathy (HOCM), the characteristic murmur is caused by the dynamic left ventricular outflow tract (LVOT) obstruction that occurs during systole. This obstruction is primarily due to the hypertrophied interventricular septum and the mitral valve apparatus, leading to turbulent blood flow.

When preload decreases, there is reduced ventricular filling, ultimately leading to a smaller end-diastolic volume. This condition can lead to increased obstruction in the outflow tract for two reasons. First, with less blood in the ventricle, a more significant proportion of the remaining blood can be directed towards the obstructive area, increasing turbulence and enhancing the intensity of the murmur. Second, a reduction in preload can result in a higher velocity of blood flow as the heart contracts against the smaller volume, which also contributes to a more pronounced murmur.

Other factors like increased afterload and increased heart rate can have varying effects in patients with HOCM, but they do not lead to the same significant increases in murmur intensity as decreased preload. Increased stroke volume might result from compensation mechanisms but does not directly increase the intensity of the murmur associated with the specific dynamics of the obstruction in HOCM.