Hypercyanotic ("Tet") Spell

A special concern in the management of infants and young children with tetralogy of Fallot is the risk of hypercyanotic, or hypoxemic, spells, also called "tet" spells. These represent a serious, sometimes life threatening event. They occur more often in a patient with mild infundibular obstruction, who ordinarily is minimally hypoxemic. This patient can develop rather sudden increase in the degree of obstruction as a result of contraction of the musculature that surrounds the infundibulum. This abruptly increases the right-to-left shunt, dropping systemic oxygen saturation and arterial oxygen tension. Drop in arterial pO2 has a direct effect on the pulmonary vascular bed, causing vasoconstriction, exacerbating the pulmonary obstruction and further increasing right-to-left shunting. Once started, therefore, the cycle can be self perpetuating, progressing if unchecked to severe hypoxemia, metabolic acidosis, brain damage and death.

Circumstances Increasing Risk of Hypercyanotic Spells

Some circumstances increase the risk of hypoxemic spells. Dehydration reduces circulating blood volume, thereby, reducing ventricular end diastolic volume. This brings the walls of the already narrow ventricular outflow tract closer together, thereby, increasing the obstruction. Systemic arteriolar vasodilation decreases systemic vascular resistance. With no change in pulmonary vascular resistance, this augments the right-to-left shunt through the ventricular septal defect. Similarly, increase in pulmonary vascular resistance with no change in systemic resistance, enhances the right-to-left shunt. Iron deficiency anemia in an already hypoxemic patient increases the risk if a spell develops, because the body's compensatory mechanism increasing red blood cell mass is impeded and, therefore, arterial oxygen content is chronically low.