Case Studies In Small Animal

Cardiovascular Medicine

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Case 17

Case Discussion

This dog had a left-to-right shunting patent ductus arteriosus (PDA). This is a congenital and heritable cardiac abnormality seen in numerous species. In the dog, it has been proven in miniature poodles that the lesion is caused by a lack of smooth muscle in the ductal wall. Usually at the aortic end of the ductus, there is little to no smooth muscle while at the pulmonary artery end there is enough smooth muscle to result in partial closure. This results in a funnel shape to the patent ductus arteriosus. The narrowed end creates resistance to flow and causes high velocity flow into the pulmonary artery. Since the pressure in the aorta is always greater than the pulmonary artery, flow from the aorta to the pulmonary artery is continuous. Peak flow occurs as the left ventricle pumps blood into the aorta in systole.

Schematic drawing of a patent ductus arteriosus (PDA)

1 = PDA (from http://www.pediatriccardiacinquest.mb.ca/ch02/pda.html)

The diagnosis of a patent ductus arteriosus (PDA) is usually suspected by identifying the characteristic continuous heart murmur. This murmur is usually heard best cranially along the left thorax, forward from the area of the left heart base and in the left axillary region. Chico's heart murmur had been missed for a year probably because no one had listened this far cranially. In most dogs the murmur can also be heard over the left base and sometimes even over the left apex. However, in some dogs it is only heard cranially and it is in these dogs that the diagnosis is most commonly missed. Luckily for Chico, his PDA was moderate in size so he did not develop fulminant heart failure before the diagnosis was made.

Chico had a classic descending aortic bulge ("ductus bump" or "ductal aneurysm") on his DV radiograph (DA on image below). This is due to an aneurysm that occurs in the aorta at the origin of the ductus. He also had a tall R wave in lead II on his ECG indicative of left ventricular enlargement.

309161~1.jpg (41749 bytes)

Although the diagnosis was obvious after auscultation and thoracic radiography, echocardiography was also performed in this case. It revealed volume overload of the left ventricle and left atrium and a markedly enlarged main pulmonary artery. In addition, the PDA was visualized on two-dimensional echocardiography and on color flow Doppler echocardiography. The continuous wave Doppler echocardiogram showed that the peak velocity of the jet was approximately 5 meters/second. This translated into a pressure gradient of approximately 100 mm Hg. Since peak systolic pressure in the aorta is normally about 120 mm Hg and in the main pulmonary artery is 20 mm Hg, this velocity and pressure gradient are expected in a dog without any changes in systemic or pulmonary artery pressures. Finding a jet with this velocity rules out any pulmonary hypertension in this dog. A lower jet velocity would suggest that pulmonary hypertension was present. The echocardiography also ruled-out any concomitant congenital cardiac abnormalities.

 

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