The two most common causes of a continuous murmur are a patent ductus arteriosus and severe aortic valve regurgitation.

A patent ductus arteriosus causes a continuous murmur since there is a constant pressure gradient in both systole and diastole, forcing blood from the aorta into the pulmonary artery. The normal aortic systolic/diastolic pressure is 120/80 mm Hg and the normal pulmonary arterial pressure is 25/5 mm Hg. Thus in systole, there is an average of a 95 mm Hg gradient causing a left to right shunt, and during diastole, there is a 75 mm Hg gradient causing a similar shunt.

Severe aortic regurgitation causes an early diastolic murmur which can be short, but when heart rates are faster and diastole is shortened, it can sound holodiastolic. There is a systolic flow murmur present as well due to the high volume of blood ejected across the aortic valve (normal LV forward volume from blood that filled via the left atrium PLUS the regurgitant volume). This effectively creates the effect of a continuous murmur throughout systole and diastole.