An opening between the two ventricles, allowing oxygenated and unoxygenated blood to mix. Shunting of blood occurs due to the higher pressure on the left side of the heart. Ventricular septal defects are classified as membranous or muscular according to location in the septum.

(From Ashwill, JW & James, SR: Nursing Care of Children: Principles and Practice. WB Saunders, Philadelphia, 2007.)