Arteriovenous catheterization produces a shunt of blood from the aorta to the pulmonary artery (left to right). The amount of the shunt depends on the thickness of the catheter caliber and the pressure step difference between the aorta and pulmonary artery. Shortly after birth, the resistance of the pulmonary artery is still high and the pressure is high, so the left-to-right shunt is low or is only present during systole. Thereafter, the resistance of the pulmonary artery gradually decreases, the pressure is significantly lower than that of the aorta, and the shunt flow increases. As the pulmonary artery receives both right ventricular drainage and catheter shunts, the amount of blood returning from the pulmonary veins to the left ventricle increases, increasing the load on the left ventricle and leading to enlargement, hypertrophy, and even failure of the left ventricle. When too much blood flows through the mitral orifice, relative stenosis of the mitral valve can occur. Impaired pulmonary venous blood drainage and increased pressure can lead to interstitial pulmonary edema. The lumen of the ascending aorta and aortic arch is enlarged due to increased blood flow through them; this is also reflected by an increase in pulmonary artery blood flow. Long-term increase in pulmonary blood flow can cause reflex spasm of small pulmonary arteries, which can lead to thickening and sclerosis of the walls of small pulmonary arteries, thinning of the lumen, and increase in resistance to pulmonary circulation. As pulmonary circulatory resistance increases and pulmonary hypertension develops, left-to-right shunt flow gradually decreases, and eventually reverse (right-to-left) shunt occurs, lowering the oxygen content of the arteries in the lower part of the torso and causing cyanosis at the ends of the toes. Long-term blood flow impulse can make the duct wall thin and brittle to the point of aneurysm or calcification. It also predisposes to infection and endarteritis. The proximal pulmonary artery may show aneurysmal enlargement due to increased intraluminal pressure.