Hemodynamics
The murmur of Ventricular Septal defect is created by left to right shunt of blood through the septum. The velocity of the shunt will follow the intraventricular pressure, but may also be demoted or promoted based on the pulmonary vascular resistance. At birth this resistance is high, and the murmur may be low in amplitude. As the lungs develop the pulmonary resistance will be lower, allowing the velocity to increase.