Makindo Medical Notes

Related Subjects: |Congenital Acyanotic Heart Disease |Congenital Cyanotic Heart Disease |Cardiac Embryology |Cyanosis - Central and Peripheral |Down's syndrome (Trisomy 21) |Tetralogy of Fallot |Patent Foramen Ovale (PFO) |Ventricular Septal defect (VSD)

Tetralogy of Fallot (TOF) is a congenital heart defect characterized by four key anatomical abnormalities. It is one of the most common cyanotic congenital heart defects, accounting for about 10% of all such defects. Many babies with TOF appear "pink" at birth, but they often develop progressive cyanosis over the first few months as right ventricular outflow tract (RVOT) obstruction worsens.

About

• TOF is the most common cause of congenital cyanotic heart disease.
• May not be apparent immediately after birth, leading to delayed diagnosis in some cases.
• Progressive worsening of RV outflow obstruction typically occurs between 6 to 9 months of age, causing increasing cyanosis.
• Acute cyanotic episodes or "Tet spells" are caused by RVOT obstruction, often triggered by adrenergic stimulation (e.g., crying, feeding).
• In contrast, cyanosis present at birth is more suggestive of other conditions like transposition of the great arteries.

Structure: The 4 Classic Defects

• Overriding Aorta: The aorta is positioned directly over the ventricular septal defect (VSD), allowing oxygen-poor blood from the right ventricle to enter the systemic circulation.
• Right Ventricular Hypertrophy (RVH): Results from increased pressure load on the right ventricle due to RVOT obstruction.
• Sub-Pulmonary Stenosis: Obstruction at or below the pulmonary valve limits blood flow to the lungs, contributing to cyanosis.
• Ventricular Septal Defect (VSD): A large, typically unrestrictive VSD allows blood to flow between the right and left ventricles, leading to mixing of oxygen-poor and oxygen-rich blood.

Diagram

Aetiology

• Right-sided outflow tract obstruction reduces blood flow to the lungs, leading to low oxygen levels in the blood.
• The large VSD causes more blood to be shunted from left to right initially, but as right ventricular pressures increase, the shunt reverses to right-to-left.
• As RV pressure exceeds left ventricular pressure, deoxygenated blood is pushed into the systemic circulation, causing cyanosis.
• This right-to-left shunting with increased pulmonary vascular resistance is also seen in conditions like Eisenmenger syndrome, leading to central cyanosis.

Clinical Features

• Fatigue: Often occurs during feeding in infants or during activity in older children.
• Cyanosis: May not be apparent at birth but becomes more noticeable as RVOT obstruction worsens.
• Murmur: Harsh ejection systolic murmur heard best at the left sternal border due to turbulent flow through the narrowed RVOT. Absent P2 sound due to reduced pulmonary flow.
• Tet Spells: Characterized by sudden episodes of deep cyanosis, irritability, and potential loss of consciousness due to increased right-to-left shunting.
• Finger and Toe Clubbing: Common in older children as a sign of chronic hypoxia.

Investigations

• CXR: Shows a "boot-shaped" heart (coeur en sabot) due to right ventricular hypertrophy and pulmonary oligaemia (reduced lung markings due to decreased blood flow).
• ECG: Right ventricular hypertrophy (RVH) and right axis deviation are common findings.
• Echocardiography: The primary diagnostic tool for visualizing the four anatomical defects and assessing the severity of RVOT obstruction and VSD size.
• Cardiac MRI or CT Scan: Provides detailed anatomical information, especially useful for surgical planning.

Management

• Acute Management:
  • Prostaglandin E1: Administered to newborns with severe RVOT obstruction or pulmonary atresia to maintain ductal patency (patent ductus arteriosus), allowing blood flow to the lungs.
• Tet Spells:
  • Position the child in a knee-chest position to increase systemic vascular resistance and reduce right-to-left shunting.
  • Administer 100% oxygen to improve oxygenation.
  • Intravenous morphine to calm the child and reduce respiratory drive.
  • Beta-blockers (e.g., propranolol) may be used to reduce heart rate and improve filling time.
• Surgical Management:
  • Complete Repair: Typically performed within the first year of life. This involves closing the VSD with a patch and relieving the RVOT obstruction, often by widening the pulmonary valve and/or artery.
  • Palliative Surgery: For very small infants or those not suitable for immediate full repair, a Blalock-Taussig shunt can be created to increase pulmonary blood flow until complete repair is feasible.
  • Surgical intervention should ideally occur before 5 years of age to optimize long-term outcomes.
• Post-Surgical Outcomes: Most children have a good prognosis after surgical repair, but lifelong follow-up is required to monitor for arrhythmias, residual defects, and right ventricular dysfunction.
• Long-Term Monitoring: Includes regular echocardiograms, ECGs, and potentially cardiac MRI to assess cardiac function and detect any late complications.

References

• American Heart Association: Tetralogy of Fallot
• ACC Review on Tetralogy of Fallot