Illustration of a ventricular septal defect (VSD). VSD is a heart defect where there is a hole in the cardiac interventricular septum which separates the left and right ventricles. This results in deoxygenated blood from the right ventricle mixing with oxygenated blood from the left ventricle. This can lead to pulmonary hypertension and breathlessness. VSD is usually a congenital defect, present at birth. It can close and heal without intervention, though in severe cases surgery may be necessary to repair the defect. Here, the left part of the heart has been removed to allow a view of the hole in the interventricular septum.