What is a common symptom of tetralogy of Fallot when experiencing a “tet spell”?

Tetralogy of Fallot is a congenital heart defect characterized by four specific structural abnormalities that result in reduced blood flow to the lungs and oxygenated blood flowing directly into the systemic circulation. A "tet spell" refers to a period of acute cyanosis and respiratory distress that occurs in infants with this condition, typically triggered by factors such as exertion, feeding, or crying.

During a tet spell, one of the most notable symptoms is desaturation, meaning that there is a significant drop in oxygen saturation levels in the blood. This happens because the right-to-left shunt of blood occurs due to increased right ventricular outflow obstruction. When a baby is active or becomes agitated, the pulmonary blood flow can decrease further, exacerbating the lack of oxygenated blood. The resulting cyanosis is a hallmark of these episodes, making it clear that desaturation during such spells is a key symptom.

While other symptoms may be present in different contexts, like increased appetite or persistent coughing, they do not directly relate to the physiological effects of a tet spell. Similarly, a decreased respiratory rate would be atypical during a tet spell, as the infant typically exhibits tachypnea (increased respiratory rate) in response to hypoxia. Understanding these mechanisms clarifies why