PATENT DUCTUS ARTERIOSUS (PDA) -- is in every mammal at birth.  It is a duct that blood flows through while the fetus is still using the mothers oxygen supply.  When the baby is born and starts to breath this duct is supposed to close.  It should close with in a few hours to a few days.  PDA is the most common congenital heart defect in dogs.  It  is usually detected by your veterinarian when he hears a continuous machinery heart murmur during a check-up.  Then there are tests that are done to confirm it.   All of the information that I have gathered give one to several, reasons that it might not.   Some are genetic, and some are classified as random events.   Some are even classified as polygenetic.