If the orifice is a tiny crevice up to 50-60% of the aortic diameter, orifice size limits maximal flow and the right ventricle pressure doesn't reach systemic. Blood flow through the defect is proportional to defect size. If the defect approaches the diameter of the aorta, there is equalization of left ventricle and right ventricle pressures.
The direction and magnitude of blood flow is controlled only by the resistances of the pulmonary and systemic vascular beds. Low pulmonary resistance means massive left to right shunting, until pulmonary congestion and edema cause vasoconstriction. Pulmonary vascular obstructive disease (i.e. hypertension) and congestive heart failure are inevitable unless the condition is corrected.
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