Mid diastolic murmurs occur when an atrioventricular valve opens. They represent either obstruction at that valve or increased flow into the ventricle during rapid ventricular filling. Let us listen to and observe the mid diastolic murmur at the lower left sternal edge in a patient with a large atrial septal defect, an abnormality that causes increased flow first into the right atrium and then into the right ventricle. This diastolic murmur is caused by high volume flow into the right ventricle at the time of rapid ventricular filling.