**6. Right-to-left shunts (cyanotic heart defects)**

In cyanotic congenital heart defects systemic venous blood bypasses the pulmonary circulation and gets shunted across into the left side of the heart. Thus, there is systemic arterial desaturation. By definition, cyanotic congenital heart disease does not include cyanosis due to intrapulmonary right-to-left shunting and pulmonary venous desaturation secondary to congestive heart failure. There are usually multiple defects of the heart causing right-to-left shunt. Obstruction to pulmonary blood flow (for example tetralogy of Fallot), complete admixture of pulmonary and systemic venous returns (for example, total anomalous pulmonary venous return and double-inlet left ventricle) and parallel rather than in-series circulation (transposition of the great arteries) are the causes of right-to-left shunts and cyanosis. The most important of the cyanotic CHDs are what are called "5 Ts" and are listed in table 2.


Table 2. Common Cyanotic Congenital Heart Defects (5 Ts)

Three of these defects, namely tetralogy of Fallot, transposition of the great arteries and tricuspid atresia will be reviewed in this chapter.
