Case 17 — Text

Text from “Small Animal Cardiovascular Medicine”

Pathophysiology of PDA

Left-to-Right Shunt

Grade 3, 4, and 5 ductal abnormalities result in shunting of blood from the systemic circulation into the pulmonary circulation (left-to-right shunt). Flow occurs in both systole and diastole. Because the ductus constricts at the pulmonary artery end, it provides resistance to flow and there is a pressure gradient between the aorta and pulmonary artery, with mean pressures usually remaining normal. The consequences are purely related to the amount of blood flow crossing the ductus, which depends on the size of the smallest orifice and the relative resistances of the systemic and pulmonary circulations.

Whatever blood flows through the ductus returns to the left heart and must be pumped out again. To accommodate this increased return, the left ventricle grows larger through volume overload (eccentric) hypertrophy. In PDA there is an absolute increase in circulating blood volume proportionate to the size of the shunt, recirculating within the ductus, pulmonary vasculature, left heart, and proximal aorta.

If the ductus undergoes only moderate constriction (grade 4), the chronic volume overload can lead to myocardial failure over several years. In a grade 5 ductus, shunt flow commonly overwhelms the ability of the left heart to compensate, resulting in left heart failure most commonly between a few weeks and six months of age.

The left ventricle and left atrium grow in size primarily in proportion to the shunt. Both end-diastolic and end-systolic diameters increase, usually almost equally, resulting in a normal shortening fraction. The increase in chamber size with normal wall motion results in an increased stroke volume.

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