Case Studies In Small Animal

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Case 6

Patent Ductus Arteriosus Chapter from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

Patent Ductus Arteriosus

Pathophysiology

Right-to-Left Shunt: Most dogs and cats with a right-to-left shunting PDA have a large patent ductus that provides no resistance to blood flow. In dogs with a grade 6 PDA, the ductus functionally does not close at all. This results in a communication between the aorta and the pulmonary artery that is the same size as these great vessels (0.83 - 1.25 times the size of the descending aorta). In this situation there is no or very little resistance to blood flow between the aorta and pulmonary artery. Consequently, blood flows from the left and right sides of the heart through both vascular beds in proportion to the resistance to flow in each vascular bed and pressures in the aorta and pulmonary artery equalize. We would expect pulmonary blood flow to be approximately five times systemic flow in this situation since pulmonary vascular resistance is normally about 1/5 that of systemic vascular resistance. This amount of flow would result in a massive and overwhelming left heart volume overload shortly after birth. The massive volume overload would result in peracute left heart failure, pulmonary edema, and death. Since these dogs live, pulmonary vascular resistance must not decrease to normal in this situation following birth but must remain partially elevated to prevent massive shunting. That is, the normal maturation of the pulmonary vasculature toward a low resistance vascular bed does not occur, probably because it is continuously exposed to high pressure and flow after birth. Normally, pulmonary vascular resistance decreases dramatically after birth as ventilation begins and hypoxic vasoconstriction of the pulmonary vasculature is released. The smooth muscle in the media of the pulmonary arterial resistance vessels is increased during fetal life and gradually thins toward normal adult thickness after birth. This process is delayed in infants with large left-to-right shunts. In sheep with experimentally placed aorticopulmonary shunts (2:1 Qp/Qs) placed during late gestational life, the media of the pulmonary vasculature remains increased in thickness at one month of age.

With flows matching resistances on both sides of the circulation, blood pressure equalizes in the aorta and the pulmonary artery as a result of the increase in pulmonary blood flow leading to severe pulmonary hypertension. At this stage, pulmonary vascular resistance is probably only moderately increased and this increase is not due to pulmonary vascular pathology but rather is due to vestigial medial hypertrophy and possibly reversible vasoconstriction. There is no pulmonary vascular disease. The increase in pulmonary artery pressure combined with the increase in pulmonary blood flow creates pathologic responses in the pulmonary arteries over time. The exact mechanism for this process is unknown but probably involves injury to the endothelial cells that activates growth factors. These factors induce smooth muscle cell hypertrophy and hyperplasia and promote connective tissue protein synthesis. The pathologic response consists primarily of medial hypertrophy and intimal proliferation in the medium and small pulmonary arteries. This narrows the lumina of these vessels, increasing pulmonary vascular resistance. As pulmonary vascular resistance increases, blood flowing from the aorta to the pulmonary artery through the ductus decreases. Systemic and pulmonary arterial pressures remain equalized. As blood flow through the ductus decreases, blood flow velocity decreases. When blood flow velocity approaches 2 m/sec, turbulence disappears and so the murmur disappears. In poodle crosses with grade 6 PDA, a continuous murmur is heard during the first days to weeks of life but disappears before the eighth week of life. Presumably by this time pulmonary vascular resistance has increased to the point that left-to-right flow through the ductus has already decreased markedly. Pulmonary vascular resistance continues to increase as pulmonary vascular disease worsens over the dog's life. Generally by the time a dog with a grade 6 lesion is three months to three years of age, pulmonary vascular resistance exceeds systemic vascular resistance. This creates a clinically significant right-to-left shunt. When the shunt is clinically significant, a large amount of deoxygenated blood shunts from the venous circulation (the pulmonary artery) into the aorta. This results in a significant decrease in arterial oxygen tension beyond the region where the ductus joins the aorta, either at rest or with exercise. Caudal body arterial oxygen tension is commonly between 30 and 45 mmHg at rest in clinically affected dogs and always <40 mmHg with exercise if cyanosis is present. Exercise results in a decrease in systemic vascular resistance whereas pulmonary vascular resistance is fixed because of the pulmonary arterial disease. Consequently, exercise produces an increase in right-to-left shunting and exacerbation of cyanosis.

 

 

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.