One way of thinking about innocent murmurs is to consider the vascular system as a series of tubes and compartments that are joined to each other. At these junctions there can be a normal degree of turbulence as the blood traverses from one tube or chamber to the next. This normal turbulence can set the surrounding tissues into vibration and this vibration is transmitted out through the chest and to the stethoscope.
Following the course of the blood we can first discuss the normal and positional low pitched continuous rumble of the blood draining through the jugular veins and the superior vena cava into the right atrium. This "venous hum" murmur disappears with turning of the head or change from the sitting to supine position. Next, the blood may be mildly turbulent as it flows from the right ventricular out flow tract across a normal pulmonary valve and into a rapidly growing main pulmonary artery or branched pulmonary artery. This pulmonary flow type murmur is common in infants, has a medium pitch, a soft 2/6 intensity and is heard best along the left upper sternal border. It generally does not radiate well through out the back. Importantly, this innocent murmur is not usually associated with an early systolic ejection click (but mild pulmonary artery stenosis may.) When the diagnosis of benign pulmonary flow type murmur is made, extra care must be taken to ensure that there is a normal physiologic split of the second heart sound with respiration, and that the precordial activity is normal (absence of right ventricular prominent lift). Atrial septal defect commonly presents with a pulmonary flow type murmur of increased flow across a normal pulmonary valve with associated fixed splitting of the second heart sound and a prominent lift.
In the first two months of age it is common to have a mild degree of turbulence at the branch point from the main pulmonary artery to each of the branched pulmonary arteries. This innocent murmur of mild branched pulmonary artery turbulence (also termed mild peripheral pulmonary artery turbulence) has a medium pitch and radiates though out the precordium with specific radiation to the axillae. The blood then traverses the lungs and returns to the left atrium and left ventricle. As the ventricle squeezes blood is ejected though the left ventricular out flow tract past the aortic valve to the aorta.
The flow of blood within the left ventricle may set in vibration support tissue within the left ventricle (left ventricular muscle cords) or the support structures to the mitral valve near the left ventricular out flow tract such as the cordae and papillary muscle attachments to the left ventricle. This causes a soft grade 2/6 crescendo-decrescendo (ejection) type murmur with a medium pitch (relatively pure uniform frequency of 70- 130 cycles per second) and a characteristic musical vibratory quality "like plucking a taught piece of string under water". This is the "Still's-Murmur (George F. Still 1909) " and is frequently also termed benign ventricular ejection type murmur.
Innocent flow murmurs which are related to mild turbulence across a normal aortic valve radiate slightly to the right upper sternal border, are not associated with an ejection click, are crescendo-decrescendo and systolic, have a non-harsh quality and are termed the murmur of aortic bruit. These murmurs must be separated from the murmur of mild aortic stenosis with or without bicuspid aortic valve. Finally, blood turbulence across normal aortic branches to the arms and head (brachiocephalic systolic murmur or supra- clavicular systolic murmur), or chest (mammary souffle) may cause innocent murmurs.