The same physical and physiological principles responsible for pathologic heart murmurs are responsible for innocent heart murmurs. This principle is Turbulent Flow.
Innocent (flow)murmurs are heard in settings of:
Physiologically increased turbulent flow, as in relatively narrow ventricular outflow tracts, high cardiac output states like anemia, and false tendons in the left ventricle
Relative vessel narrowing, as seen with vessels stretching during periods of rapid linear growth
With lung expansion after birth as seen in peripheral pulmonic stenosis
In acute rheumatic fever, a middiastolic murmur over the LV impulse, a Carey-Coombs murmur, has been attributed to acute mitral valvulitis. However, first-degree atrioventricular block (prolonged PR interval) is common in rheumatic carditis and an increased flow due to earlier atrial systole coinciding with the rapid filling phase may contribute to a Carey-Coombs murmur. Thank you.