Tetralogy of Fallot: Decreased SpO2
Cyanosis occurs in Tetralogy of Fallot (TOF) when right to left shunting occurs across the ventricular septal defect (VSD). The amount of shunting is dynamic, and is determined by the degree of right ventricular outflow obstruction. Right ventricular outflow tract obstruction can increase acutely, in "hypercyanotic" or "tet" spells. Classically, this was thought to be caused by increased contractility of the infundibulum (infundibular spasm), but the etiology is likely influenced by a combination of factors, including peripheral vasodilation, hyperventilation, and the stimulation of mechanoreceptors in the right ventricle.