Patent ductus arteriosus (PDA) is the most common congenital cardiovascular defect in canines. PDA represents a failure of the fetal ductus arteriosus to properly close after birth. PDA is usually recognized in young dogs, but is occasionally seen in middle aged to older animals. Four clinical presentations of PDA have been described and likely represent a continuum of severity. They are based on the size of the patency and the direction of the shunt. Many dogs are asymptomatic at the time of diagnosis. Approximately 50% of untreated cases will die of congestive heart failure within the first year of life.