Final answer:
Heart murmurs increase during the Valsalva maneuver due to a decrease in venous return, which decreases ventricular volume and accentuates the sound of the murmur, especially in conditions like hypertrophic cardiomyopathy.
Step-by-step explanation:
The question 'Why do murmurs increase with Valsalva?' relates to the effects of the Valsalva maneuver on heart murmurs. During the Valsalva maneuver, there is an increase in intrathoracic pressure which decreases venous return to the heart. This reduction in preload leads to a decrease in ventricular filling (volume), which in turn reduces the ventricles' size and makes the heart murmur more audible. Conditions such as hypertrophic cardiomyopathy will show a pronounced increase in murmur intensity due to the reduced ventricular volume further obstructing outflow.
Additionally, Starling's Law of the Heart explains how the decrease in venous return and subsequent reduced stretch of the ventricular muscle fibers during Valsalva diminishes stroke volume and cardiac output, but can accentuate the sound of certain heart murmurs. This is most notably the case with murmurs that are due to dynamic outflow obstructions, as less blood in the ventricle exacerbates the obstruction.