|Ventricular septal defect||
Ventricular septal defect (VSD) is the most common cause of acyanotic (where the blood contains enough oxygen but it is pumped abnormally around the body) heart disease. In cases of VSD, there is a hole between the left and right ventricle. As blood pressure is higher on the left-hand side of the heart, blood is pushed out of the left ventricle through the hole and into the right ventricle. The heart then has to work harder to pump the excess blood out of the right ventricle, which can put a strain on the heart. The excess level of blood can also increase the blood pressure in the lungs and damage them (pulmonary hypertension).
Definition from NHS Choices: Ventricular septal defect