causes only one side of the heart to be capable of pumping blood to the body and lungs effectively. Hypoplasia of the heart is rare but is the most serious form of CHD. It is called hypoplastic left heart syndrome when it affects the left side of the heart and hypoplastic right heart syndrome when it affects the right side of the heart. In both conditions, the presence of a patent ductus arteriosus (and, when hypoplasia affects the right side of the heart, a patent foramen ovale) is vital to the infant's ability to survive until emergency heart surgery can be performed, since without these pathways blood cannot circulate to the body (or lungs, depending on which side of the heart is defective). Hypoplasia of the heart is generally a cyanotic heart defect. Obstructive defects Obstructive defects occur when heart valves, arteries, or veins are abnormally narrow or blocked. Common defects include pulmonic stenosis, aortic stenosis, and coarctation of the aorta, with other types such as bicuspid aortic valve stenosis and subaortic stenosis being comparatively rare. Any narrowing or blockage can cause heart enlargement or hypertension. Septal defects The septum is a wall of tissue which separates the left heart from the right heart. Defects in the interatrial septum or the interventricular septum allow blood to flow from the left side of the heart to the right, reducing the heart's efficiency. Ventricular septal defects are collectively the most common type of CHD, although approximately 30% of adults have a type of atrial septal defect called probe patent foramen ovale. Cyanotic defects Cyanotic heart defects are called such because they result in cyanosis, a bluish-grey discoloration of the skin due to a lack of oxygen in the body. Such defects include persistent truncus arteriosus, total anomalous pulmonary venous connection, tetralogy of Fallot, transposition of the great vessels, and tricuspid atresia. Defects Aortic stenosis Arrhythmogenic right ventricular cardiomyopathy Atrial septal defect (ASD) Atrioventricular septal defect (AVSD) Bicuspid aortic valve Cardiomyopathy Complete heart block (CHB) Dextrocardia Double inlet left ventricle (DILV) Double outlet right ventricle (DORV) Ebstein's anomaly Early Repolarization Syndrome Holmes heart Hypoplastic left heart syndrome (HLHS) Hypoplastic right heart syndrome (HRHS) Mitral stenosis Myocardial bridge Persistent truncus arteriosus Pulmonary atresia Pulmonary stenosis Rhabdomyomas (Tumors of the Heart) Transposition of the great vessels dextro-Transposition of the great arteries (d-TGA) levo-Transposition of the great arteries (l-TGA) Tricuspid atresia Ventricular septal defect (VSD) Wolff–Parkinson–White