| In fetal life, oxygenated blood within the main pulmonary artery is shunted into the descending aorta through the ductus arteriosus, bypassing the nonfunctional lungs. At birth, several factors mediate closure of the ductus, which effects separation of the systemic and pulmonary circulatory systems. Inflation of the lungs allows the pulmonary circulation to function as a low-pressure system, and closure of the ductus prevents shunting of blood from the high-pressure systemic
circulatory system into the pulmonary artery. |
| Pathophysiology: |
| Persistence or patency of the ductus with an otherwise normal systemic and pulmonary circulatory system results in significant shunting of blood from left to right, ie, systemic to pulmonary. Because the systemic vascular resistance is always higher than that of the pulmonary circulation, shunting is continuous. The result is volume overload of the pulmonary arteries and veins, left atrium, and left ventricle. Left atrial and left ventricular dilatation may result in cardiac
arrhythmias. Chronic volume overload and dilatation of the left-sided cardiac chambers usually result in signs of left-sided CHF. Therefore, most untreated cases develop refractory CHF. Animals with a small ductus may reach adulthood without signs of heart failure but are at an increased risk of infective endocarditis. In some animals with a large PDA, increased pulmonary blood flow may induce pulmonary vasoconstriction and development of pulmonary hypertension, which has several
important implications: shunting through the ductus slows and reverses, which causes disappearance of the murmur and occurrence of caudal cyanosis (differential cyanosis); the right ventricle becomes dilated and hypertrophied as a result of pulmonary hypertension; and perfusion of the kidneys with deoxygenated blood causes excessive release of erythropoietin and subsequent polycythemia. Thus, if the ductus shunts right to left, clinical signs of polycythemia will predominate.
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| Clinical Findings and Treatment: |
| In animals with a PDA that shunts from left to right, a prominent, continuous, machinery-like murmur is present. The murmur is usually loudest at the time of the second heart sound, heard best over the aortic valve area, and is often associated with a precordial thrill. The diastolic component is softer and heard best over the pulmonic valve area, occasionally best at the axillary area (in some cases, the ductus remains open for several days after birth; therefore, a continuous
murmur may be detected during examination of the neonate). Occasionally the diastolic component may be inaudible in late diastole. Femoral pulses are typically bounding. Most young animals do not demonstrate clinical signs. Those with a large shunt and older animals often have signs of left-sided CHF. Electrocardiography frequently demonstrates tall R waves in lead II, indicative of left ventricular enlargement. A spectrum of cardiac arrhythmias may also be seen, including both
atrial and ventricular premature complexes. Radiographic abnormalities depend on the size of the ductus and, in left to right shunting PDA, may demonstrate left atrial and left ventricular enlargement, prominent pulmonary vessels, aortic and pulmonic aneurysmal dilatations, and variable degrees of pulmonary edema. Echocardiography is valuable in ruling out concurrent congenital cardiac defects as well as documenting presence of the PDA. Continuous turbulence in the main pulmonary
artery is characteristic of a left to right shunting PDA. Left ventricular and left atrial dilatation are typically noted, and mild mitral regurgitation may be present. |
| Surgical ligation of the ductus in patients with left to right shunting PDA is usually curative and is almost always indicated. If present, CHF should be medically managed (with diuretics, vasodilators, etc) before anesthesia and surgery are performed. Interventional closure is an alternative to surgical ligation. This can be accomplished by transcatheter occlusion through placement of a device (eg, Gianturco helical coils, Gianturco-Grifka vascular occlusion device) in the PDA
that results in clot formation or physical occlusion of the ductus. |
| In animals with a PDA that shunts from right to left, there is usually a history of lethargy, exercise intolerance, and collapse. Careful examination may reveal differential cyanosis. The second heart sound may be split, and there may be a soft diastolic murmur of pulmonic insufficiency. A continuous murmur is not present, and femoral pulses are not bounding. The finding of polycythemia in a young animal with the above clinical signs should prompt further diagnostic evaluation
of the heart. Electrocardiography demonstrates severe right ventricular enlargement and occasional arrhythmias. In reversed PDA, right ventricular enlargement and aneurysmal dilatation of the descending aorta can be noted. Echocardiography is indicated in these cases and will demonstrate right ventricular dilatation and hypertrophy. The right ventricular outflow tract is enlarged. Contrast echocardiography can be used to confirm the diagnosis. After the injection of agitated
saline into a peripheral vein, microbubbles will be seen within the abdominal aorta but not within the heart. Ligation of the ductus is contraindicated because this results in an increase in pulmonary hypertension (by causing an increase in flow through the already high and fixed pulmonary vascular resistance) and typically death. Therapy in theses cases involves control of polycythemia through periodic phlebotomies. Longterm prognosis is poor. |
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