septal defect [L, saeptum, fence, defectus, failure] , a defect in the wall separating the left and right sides of the heart. Depending on the size and the site of the defect, various amounts of oxygenated and deoxygenated blood mix, causing a decrease in the amount of oxygen carried in the blood to the peripheral tissues. The defect is usually congenital. Kinds include atrial septal defect, ventricular septal defect.
