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Hypercyanotic "Tet" spells

Infants and children with unrepaired tetralogy of Fallot may develop hypercyanotic, or "tet," spells. Patients with prominent subvalvar muscle bundles and/or conal tissue in the right ventricular outflow tract are thought to be at higher risk for hypercyanotic spells. These are acute spells characterized by profound cyanosis, often in the setting of agitation or tachycardia, that may progress to loss of consciousness or cardiac arrest if not aborted.: 200 This may be initiated by any event – such as anxiety, pain, dehydration, or fever – that leads to an increase in dynamic muscular obstruction of the right ventricular outflow tract. This in turn leads to decreased blood flow through the right ventricular outflow tract to the lungs and increased shunt of deoxygenated blood from the right ventricle to the left ventricle and, subsequently, to the systemic circulation. The pathophysiology of these episodes is multifactorial; increased sympathetic activation (from pain, agitation, fever, etc.) leads to increased myocardial contractility, which worsens dynamic muscular obstruction of the right ventricular outflow tract, and increased heart rate (tachycardia), which allows less time for right ventricular diastolic filling. Right ventricular outflow tract obstruction is more likely to occur in a relatively underfilled ventricle with increased contractility of the outflow tract myocardium. A relative decrease in systemic vascular resistance, as may be observed in distributive or neurogenic shock, may also precipitate hypercyanotic spells by increasing shunt from the right ventricle to the left ventricle.