In the fetus, the ductus arteriosus functions to bypass the pulmonary circulation. It is a short arterial connection arising from the sixth aortic arch which carries blood from the pulmonary artery to the aorta and systemic circulation. The ductus normally closes in the first few weeks after birth to form the ligamentum arteriosum. Closure is regulated by prostaglandin synthetase inhibition and also by the changes which occur in blood oxygen saturation.
Patent ducius arteriosus (PDA) occurs as a graded defect varying in severity according to the diameter of the dueial lumen. In some cases a ductus diverticulum forms when the ductal lumen closes at its pulmonary artery end but remains open over the rest of its length.
A ductus which remains patent after birth allows the shunting of blood during systole and diastole from the high pressure in the aorta to the pulmonary artery. patent ducius arteriosus therefore represents an artcriovenous fistula since oxygenated blood from the aorta mixes with venous blood of the pulmonary artery. Shunting of blood through the right side of the heart leads to pulmonary overcirculation, left ventricular volume overload, pulmonary venous and arterial hypertension and ultimately signs of left-sided congestive heart failure. Left ventricular and left atrial enlargement leads to mitral regurgitation which contributes to the volume overload and pulmonary venous engorgement. Pulmonary hypertension leads to pressure overload in the right ventricle and the direction of the shunt can reverse. The incidence of left heart failure and pulmonarv hypertension, and the rapidity of onset of clinical signs, is associated with the size of the ductal lumen. Occasionally patent ducius arteriosus occurs in association with other congenital cardiac defects, for example ventricular septal defect and pulmonic stenosis.
The incidence is highest in miniature poodles. German shepherds. Border collies. Shetland sheepdogs, pomeranians and Irish setters and a predisposition for females has been reported. A polygenic mode of inheritance is suspected in most of these breeds.
Most dogs with patent ducius arteriosus show clinical signs before one year of age and only a few cases reach adulthood undiagnosed. The clinical signs are those of left-sided heart failure. A few animals may experience syncopal episodes. Advanced cases may show signs of biventricular failure. The characteristic feature of patent ducius arteriosus is the presence of a continuous ‘machinery-type’ murmur over the aortic / pulmonic valve region which may radiate to the thoracic inlet. Most * machinery-type* murmurs are confined to a very narrow region and are associated with a palpable precordial thrill over the cranial thorax; in many cases an additional systolic murmur associated with mitral regurgitation can be located over the mitral valve region. The femoral pulse becomes jerky (‘water hammer’ pulse) because of the sharp fall off in arterial pulse pressure.
Once pulmonary hypertension develops the direction of the shunt reverses. Initially the diastolic component of the murmur disappears but as the shunt reverses the murmur may disappear completely. Shunt reversal is associated with differential cyanosis. Since the patent ductus joins the aorta distal to the aortic arch the blood supply to the head and neck is preserved and only the caudal extremities become cyanotic giving rise to hindlimb weakness.
Wide P waves, tall R waves and prolonged QRS complexes reflect left atrial and left ventricular enlargement. The presence of deep Q waves and S waves in leads I, II, III and aVF is indicative of right ventricular enlargement. T waves are often deep and negative in leads II, III and aVF. Arrhythmias (atrial fibrillation and ventricular premature complexes) may be noted especially in older dogs showing severe signs of decompensation. The mean electrical axis, in most cases, is within normal limits.
Left atrial and left ventricular enlargement is usually associated with signs of pulmonary over-circulation (enlargement of both the pulmonary arteries and veins) and pulmonary oedema. Classically, three knuckles or bulges may be present on the dorsoventral projection ; these represent (I) the dilated aortic arch at the one o’clock position, (2) the enlarged pulmonary artery segment at the two o’clock position and (3) the enlarged left auricular appendage at the three o’clock position. Pulmonary hypertension may result in right ventricular enlargement.
Echocardiography will confirm left atrial and left ventricular enlargement. In advanced cases there may be evidence of right ventricular enlargement and chronic volume over load may result in decreased left ventricular contractility. Sepcal motion may be exaggerated. The patent ductus is often difficult to image. Continuous flow disturbance or turbulence and high velocity retrograde flow toward the pulmonic valve can be detected if a pulsed Doppler sample gate is placed in the main pulmonary artery.
Angiocardiography and intracardiac pressure studies
A selective injection of contrast material into the ascending aorta or aortic root should result in simultaneous opacification of the pulmonary artery and aorta. A non-selective study, using a large diameter intravenous catheter placed into the jugular vein will demonstrate a left to right shunting patent ducius arteriosus only after contrast has reached the aorta; for this reason serial radiographs should be taken 1,5 and 10 s after the injection of the contrast agent. Pressure studies can be performed beforehand. Pressures in the right ventricle and pulmonary artery are increased with the pulmonary artery pressure being greater than that in the right ventricle. The pressures in the left heart may be normal. Pulmonary artery PO2, is usually increased and is greater than the PO2 in the right ventricle.
The prognosis for a young animal showing no clinical or radiographic signs of heart failure is good and surgical correction of the patent ducius arteriosus may result In near normal life expectancy. The prognosis becomes less favourable for dogs showing signs of cardiac decompensation, especially where echocardiography demonstrates decreased myocardial contractility.
Patent ductus arteriosus: Treatment
Treatment of patent ducius arteriosus usually involves double libation of the patent vessel. A large diameter patent ducius arteriosus may occasionally recanalize after 11 gat ion. Surgery is generally contraindicated if there is evidence of a right to left shunt. Congestive heart failure should be managed appropriately and the animal stabilized before surgery.