In hypertrophic cardiomyopathy, what effect does decreased venous return have on murmur intensity?

In hypertrophic cardiomyopathy (HCM), the dynamic left ventricular outflow tract obstruction is influenced by several factors, including venous return. When venous return is decreased, such as during maneuvers that reduce preload (e.g., standing or valsalva maneuver), the intensity of the heart murmur actually increases.

This increase in murmur intensity occurs because decreased venous return reduces the volume of blood in the heart, leading to a smaller ventricular cavity. This smaller chamber size can exacerbate the obstruction created by the thickened interventricular septum and the anterior mitral valve leaflet. As the pressure gradient across the outflow tract becomes more significant due to the reduced volume, the turbulence of blood flow increases, thus intensifying the murmur.

Understanding this relationship is key in the clinical assessment of patients with HCM, particularly when evaluating changes in murmur intensity during different positional changes or physiological maneuvers. This dynamic nature of the murmur is a hallmark of HCM and plays an important role in diagnosing and managing the condition effectively.