Innocent murmurs are commonly of two types. One, heard in children and young adults, is best heard at the second left intercostal space and is thought to originate from vibrations within the main pulmonary artery. The other, audible between the apex and lower sternal edge, is apt to have a buzzing or vibrating quality and is thought to originate from vibrations of normal pulmonary leaflets. Both murmurs are midsystolic, usually no louder than grade 2/6 or 3/6, and common in young healthy people.
Associated findings help in distinguishing an innocent from a pathologic murmur. For example, wide splitting of the second heart sound in conjunction with a basal systolic murmur favors the diagnosis of atrial septal defect or congenital pulmonary valve stenosis. A sharp, early systolic clicking sound (ejection sound) at the left second interspace suggests pulmonic stenosis. A similar click at the cardiac apex indicates congenital aortic stenosis. Single or multiple clicks that occur in mid or late systole are common with mitral valve prolapse and are best heard at the cardiac apex.