A 5-year and 11-month-old, intact female Toy Poodle, diagnosed with a heart murmur, was presented for cardiac evaluation. Echocardiography revealed a patent ductus arteriosus (PDA) in the main pulmonary artery, accompanied by folded-back blood flow in the opposite direction of the ductal flow. Computed tomography scan was performed to investigate this folded-back blood flow, and found no blood vessel abnormalities with the exception of a PDA 4.6 mm in diameter. Open thoracotomy was performed as treatment for the PDA, and intraoperative transesophageal echocardiography confirmed shunt blood flow and folded-back blood flow disappearance after ligation. At Day 78, the dog was in good condition, without shunt blood flow or folded-back blood flow findings. These results indicate that an unusual blood flow may develop depending on the direction and amount of shunt flow in the PDA.