Heart Defect (Congenital) in Dogs

The ductus arteriosus is a blood vessel that connects the two main arteries of the body – the aorta and the pulmonary artery. Aorta is the main blood artery, providing oxygenated blood from heart to the rest of the body. Pulmonary vein carries oxygenated blood from lungs to the left atrium of the heart.

Patent Ductus Arteriosus (PDA) is a common congential heart disease in dogs. It is characterized by an open or "patent" "ductus arteriosus" (a blood vessel in a fetus that bypasses pulmonary circulation by connecting the pulmonary artery directly to the descending aorta). To understand this condition, it is important to know how blood flows through the body of an animal as well as of fetus in the mother's womb.

In a normal heart, the blood comes from the body through superior vena cava and inferior vena cave into the right atrium and then travels to the right ventricle through tricuspid valve. From right ventricle, this venous (blue) or deoxygenated blood travels to the pulmonary artery (artery carrying blood to the lungs) where the blood picks up oxygen and other nutrients and goes to the left atrium through pulmonary veins. From left atrium, the blood passes to the left ventricle and then to the aorta which sends the blood to the rest of the body. This is normal blood flow.

However, in an unborn animal, the lungs are filled with fluid rather than air and are not functioning. The blood gets oxygen from mother's blood. To prevent the blood from entering lungs, there is a tubular vessel present between pulmonary artery and aorta which is knonw as ductus arteriosus which shunts the blood to the aorta. The ductus arteriosis is the fetal connection between the descending aorta and the main pulmonary artery, allowing the shunting of fetal oxygenated placental blood from the pulmonary artery to the systemic circulation bypassing the atelectic non-functional fetal lung.  

Normally at birth, this connection is no longer patent (open). Pulmonary artery opens up to allow blood to travel to the lungs, get oxygen and return to heart to be distributed to the rest of the body. In patent ductus arteriosis, the connection remains open resulting in the blood getting re-circulated in the heart, making the heart work overtime. In some cases, the blood flow reverses, causing a portion of blood that has not been oxygenated by the lungs to flow to the body. Most commonly there is a shunt from the left to the right side of the heart , with blood from the higher pressure aorta continuously shunted to the main pulmonary artery. This means an increased volume of blood to the lungs which results in fluid build-up (pulmonary edema) and volume overload to the left heart. Less commonly, there is a right-to-left shunt. This causes some deoxygenated blood from right side of the body to bypass the lungs entirely. When this happens, the result is circulation of poorly oxygenated blood in the body.

Symptoms of ductus arteriosus (DA) are those of heart disease and depend on severity of the condition. A loud, continous heart murmur, referred to as machine murmur, is the most common and obvious sign. The dog may show respiratory distress indicated by coughing, increased breathing rate and exercise intolerance. Symptoms seen in right to left shunting PDA include weak hindlegs during exercise, thicker than normal blood causing irregular heart beats (arrhythmias), right to left blood clot and bluish gums and skin. Other signs of PDA include stunted growth, left-sided congestive heart failure and rapid, irregular heart beat. 

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