A patent ductus arteriosus (PDA) is a persistent connection between the aorta and the pulmonary artery resulting in left to right shunting. This connection is supposed to close shortly after birth, however if it is large and remains open, significant hemodynamic consequences can ensue and heart failure can develop.
Prostaglandin E1 is responsible for keeping the ductus open and thus in certain cyanotic congenital heart defects where the only mixing of arterial and venous blood is through a PDA, prostaglandin analogs are infused to keep the PDA from closing. Similarly, non-steroidal anti-inflammatory drugs like Indomethacin block prostaglandin production and can be given to close the PDA if it remains patent.
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 mmHg and the normal pulmonary arterial pressure is 25/5 mmHg. Thus in systole there is an average of a 95 mmHg gradient causing a left to right shunt and during diastole there is a 75 mmHg gradient causing a similar shunt.
The murmur of a patent ductus arteriosus is continuous throughout systole and diastole. Often the S2 heart sound is difficult to detect. The murmur begins just after S1 and crescendos peaking at S2, the decrescendos to S1.