The ventricles are separated by the interventricular septum, which divides the left and right ventricles. Typically, after blood is ejected from the left ventricle and oxygenated by tissues, it returns to the right ventricle. The blood from the right ventricle gets oxygenated through the pulmonary artery before it can return to the left ventricle. These two should be isolated from each other before passing through the lungs. The interventricular septum consists of the membranous and muscular parts, and a defect in either part can affect oxygenation. Therefore, whether the defect is in the membranous or muscular portion is not an indicator for diagnosing the severity of the interventricular septum defect; rather, the size of the defect should be considered. Generally speaking, a defect of about 10mm in the interventricular septum, especially when accompanied by other conditions such as Tetralogy of Fallot, is considered severe. Moreover, when the defect reaches over 10mm, surgical intervention is required as minimally invasive surgery would not be feasible; these are considered severe interventricular septum defects. Also, there are cases with multiple complex defects or larger defects that should be surgically treated as soon as possible.