Persistent ductus arteriosus (PDA) is the name of the tube that carries blood between the aorta and the pulmonary artery in the foetus.
This tube normally closes at or shortly after birth. Sometimes, however, the mechanism of closure does not work.
When the connection remains open, it causes blood to enter your lungs. This means your heart and lungs have to work harder.
If the PDA is small, it doesn't have to be closed because it doesn't make the heart and lungs work harder.