Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are:
pulmonary stenosis, which is narrowing of the exit from the right ventricle; a ventricular septal defect, which is a hole allowing blood to flow between the two ventricles; right ventricular hypertrophy, which is thickening of the right ventricular muscle; and an overriding aorta, which is where the aorta expands to allow blood from both ventricles to enter. At birth, children may be asymptomatic or present with many severe symptoms. Later in infancy, episodes of bluish colour to the skin typically occur due to a lack of sufficient oxygenation, known as cyanosis. When affected babies cry or have a bowel movement, they may undergo a "tet spell", where they turn cyanotic, have difficulty breathing, become limp, and occasionally lose consciousness. Other symptoms may include a heart murmur, finger clubbing, and easy tiring upon breastfeeding. The cause of tetralogy of Fallot is typically not known. Maternal risk factors include lifestyle-related habits (alcohol use during pregnancy, smoking, or recreational drugs), medical conditions (diabetes), infections during pregnancy (rubella), and advanced age of mother during pregnancy (35 years and older). Babies with Down syndrome and other chromosomal defects that cause congenital heart defects may also be at risk of tetralogy of Fallot. Tetralogy of Fallot is typically treated by open-heart surgery in the first year of life. The timing of surgery depends on the baby's symptoms and size. The procedure involves increasing the size of the pulmonary valve and pulmonary arteries and repairing the ventricular septal defect. In babies who are too small, a temporary surgery may be done with plans for a second surgery when the baby is bigger. With proper care, most people who are affected live to be adults. Long-term problems may include an irregular heart rate and pulmonary regurgitation. The prevalence is estimated to be from 0.02 to 0.04% in the general population. Though males and females were initially thought to be affected equally, more recent studies have found males to be affected more than females. It is the most common complex congenital heart defect, accounting for about 10% of cases. It was initially described in 1671 by Niels Steensen. A further description was published in 1888 by French physician Étienne-Louis Arthur Fallot, after whom it is named. The first total surgical repair was carried out in 1954.
Signs and symptoms
Tetralogy of Fallot (TOF) results in low oxygenation of blood due to a mixing of oxygenated and deoxygenated blood in the left ventricle via the ventricular septal defect and preferential flow of the mixed blood from both ventricles through the aorta because of the obstruction to flow through the pulmonary valve. The latter is known as a right-to-left shunt. Infants with TOF – a cyanotic heart disease – have low blood oxygen saturation. Blood oxygenation varies greatly from one patient to another depending on the severity of the anatomic defects. Typical ranges vary from 60 to around 90%. Depending on the degree of obstruction, symptoms vary from no or mild cyanosis to profound cyanosis at birth. If the baby is not cyanotic, then TOF is sometimes referred to as a "pink tet". Other symptoms include a heart murmur, which may range from almost imperceptible to very loud, difficulty in feeding, failure to gain weight, retarded growth and physical development, labored breathing (dyspnea) on exertion, clubbing of the fingers and toes, and polycythemia. The baby may turn blue with breastfeeding or crying. Those born with TOF are more likely to experience psychiatric disorders such as attention deficit hyperactivity disorder in later life, potentially due to underlying genetic changes that predispose to both conditions.
Hypercyanotic "Tet" spells Infants and children with unrepaired TOF 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. 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. Clinically, hypercyanotic spells are characterized by a sudden, marked increase in cyanosis and may progress to syncope. Older children often squat instinctively during a hypercyanotic spell. This increases systemic vascular resistance and allows for a temporary reversal of the shunt. It increases pressure on the left side of the heart, decreasing the right-to-left shunt. The decreased shunt volume results in a decrease in deoxygenated blood flow entering the systemic circulation and an increase in deoxygenated blood flow antegrade through the obstructed right ventricular outflow tract.
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