A condition in which the subject suffers from a defect in one of the dividing walls (septum) of the HEART and this is accompanied by PULMONARY HYPERTENSION. The defect allows blood low in oxygen to flow from the right to the left side of the heart and be pumped into the aorta, which normally carries oxygenated blood to the body. The patient has a dusky blue appearance, becomes breathless and has a severely restricted exercise tolerance. There is an increase in red blood cells as the body attempts to compensate for the lowered oxygen delivery. The condition may be avoided by early surgical repair of the septal defect, but once it is evident, surgery may not be possible.