Most common cyanotic CHD manifesting in post-infancy period
Acute episodes of hypoxia and cyanosis caused by right-to-left shunting across the VSD ('Tet Spells')
Patients may present with irritability, agitation, grunting, crying, and central cyanosis
May have history of squatting during spells (increases systemic vascular resistance)
During cyanotic spells, there is either:
During the spell there is hypercarbia and hypoxemia (which further increases pulmonary vascular resistance).  The process compounds itself creating worsening right-to-left shunting, hyperpnea, right outflow tract obstruction and increased systemic venous return
VSD
RV outflow obstruction (pulmonic stenosis)
Overriding aorta
RV hypertrophy
Tetralogy of Fallot
Seizure
Transposition of the great arteries
Breath-holding spell
Ebstein anomaly
Hyperventilation syndrome
Tricuspid atresia
Pregnancy 