Diagnosis is based on clinical signs (particularly if cyanosis is present) and echocardiography. 2DE shows a VSD (usually quite large), an overriding aorta and RV hypertrophy, with thick walls and marked trabeculation (Figure 5.4). Pulmonary stenosis may be identified if present; pulmonary hypoplasia is more difficult to see. Contrast studies are useful to demonstrate right to left flow. Injection of echo-contrast into the jugular vein results in opacification of the RA, RV and aorta. Hypoxaemia may be identified by blood gas analysis; a haemamay reveal polycythaemia.