Interfering with the latter leads to insufficient leftward movement of the ventricular outflow tract over the atrioventricular canal, which in turn can result in a VSD or, in the most extreme cases, a double outlet right ventricle with one. A ventricular septal defect arises when the superior part of the interventricular septum, which separates the right and left ventricles of the heart, fails to fully develop. The right ventricle pumps blood to the lungs to get oxygen, while the left ventricle pumps blood to the rest of the body to provide oxygen to tissues. A ventricular septal defect results in the mixing of oxygen-rich blood with oxygen-poor blood, increasing strain on the heart and lungs. Pathophysiology During ventricular contraction, or systole, some of the blood from the left ventricle leaks into the right ventricle, passes through the lungs and reenters the left ventricle via the pulmonary veins and left atrium. This has two net effects. First, the circuitous refluxing of blood causes volume overload on the left ventricle. Second, because the left ventricle normally has a much higher systolic pressure (~120 mmHg) than the right ventricle (~20 mmHg), the leakage of blood into the right ventricle therefore elevates right ventricular pressure and volume, causing pulmonary hypertension with its associated symptoms. In serious cases, the pulmonary arterial pressure can reach levels that equal the systemic pressure. This reverses the left to right shunt, so that blood then flows from the right ventricle into the left ventricle, resulting in cyanosis, as blood is by-passing the lungs for oxygenation. This effect is more noticeable in patients with larger defects, who may present with breathlessness, poor feeding and failure to thrive in infancy. Patients with smaller defects may be asymptomatic. Four different septal defects exist, with perimembranous most common, outlet, atrioventricular, and muscular less commonly. Diagnosis A VSD can be detected by cardiac auscultation. Classically, a VSD causes a pathognomonic holo- or pansystolic murmur. Auscultation is generally considered sufficient for detecting a significant VSD. The murmur depends on the abnormal flow of blood from the left ventricle, through the VSD, to the right ventricle. If there is not much difference in pressure between the left and right ventricles, then the flow of blood through the VSD will not be very great and the VSD may be silent. This situation occurs a) in the fetus (when the right and left ventricular pressures are essentially equal),