Cyanosis is caused by lesions that lead to right-to-left shunting of blood, in which blood coming from the right ventricle bypasses lungs to various degrees before entering systemic circulation.
These lesions can be remembered as the 5 Ts:
1. Tetralogy of Fallot (most common cause of early cyanosis)
2. Transposition of the great vessels
3. Truncus arteriosus
4. Total anomalous pulmonary venous return
5. Tricuspid atresia
Squatting increases left-sided pressure or systemic vascular resistance (SVR) by compression of femoral arteries; this can make SVR higher than PVR (pulmonary vascular resistance, or right-sided pressure) and thus may decrease right-to-left shunting and allow more blood to pass through the pulmonary circulation before entering the systemic circulation, alleviating symptoms of cyanosis.