Tetralogy of Fallot

Question 1

Definition

Answer 1

= the most common cyanotic congenital heart disease, consists of 4 congenital abnormalities:

• ventricular septal defect
• overriding aorta (aorta positioned over the top of the VSD so that when right ventricle contracts, deoxygenated blood is forced out of aorta)
• Obstruction to RV outflow i.e. pulmonary stenosis
• Right ventricular hypertrophy

Question 2

Pathophysiology

Answer 2

• Stenosis of pulmonary valve obstructs RV outflow into the pulmonary arteries & encourages blood to flow through the VSD from right→ left or aorta via a path of less resistance.
• Right ventricular hypertrophy (due to increased afterload)
• Ventricular septal defect (blood is shunted from right → left as pulmonary stenosis makes right sided pressures higher, results in deoxygenated blood being pumped out of overriding aorta causing cyanosis)
• Overriding aorta (aorta positioned over the top of the VSD so that when right ventricle contracts, deoxygenated blood is forced out of aorta into the systemic circulation & bypasses the lungs, resulting in cyanosis.)

These cardiac abnormalities cause aright to left cardiac shunt. Deoxygenated blood bypasses the lungs (so is not oxygenated) & enters the systemic circulation causingcyanosis (bluish discoloration due to low PaO2).

Question 3

Risk factors

Answer 3

• Maternal rubella infection
• Maternal age> 40 years

Question 4

Clinical presentation

Answer 4

SIGNS
- Decreased SpO2
- Central cyanosis (bluish discolouration of lips & tongue due to hypoxemia)
- Clubbing
- Systolic ejection murmur (due to pulmonary stenosis causing RV outflow obstruction)
- Classic sign of ‘Tet spell’ = knee to chest position in infants or squatting position in children (this helps relieve the Tet spell)

SYMPTOMS

• Failure to thrive
• difficulty feeding

Question 5

Explain what a ‘Tet spell’ is

Answer 5

‘Tet spell’ = hypercyanotic episodes caused by physical exertion (e.g. crying) when the right to left shunt worsens (MORE deoxygenated blood bypasses the lungs & is pumped through the overriding aorta into the systemic circulation)

• During physical exertion, CO2 is produced. CO2 is a systemic vasodilator & pulmonary vasoconstrictor so systemic vascular resistance decreases & pulmonary vascular resistance increases. This results in MORE deoxygenated blood being shunted from right→ left through the aorta into the systemic circulation, so HYPOXAEMA worsens (O2 saturations drop) & cyanosis WORSENS.

Question 6

How might a baby in a tet spell present & why?

Answer 6

Child squats OR baby adopts knee to chest position. This occludes the femoral arteries in the groin→ systemic vascular resistance increases & systemic blood pressure increases → left sided heart pressures increase to pump blood against a higher SVR into the aorta. This causes the shunt to reverse & go left→ right, so oxygenated blood is pumped to the lungs instead. This relieves the cyanosis slightly.

Question 7

Diagnosis

Answer 7

• CXR- shows boot shaped heart (due to right ventricular hypertrophy)- not useful diagnostically except during medical school exams.
• Echocardiogram = diagnostic

Question 8

Management

Answer 8

1st line definitive Tx = full surgical repair by open heart surgery during first 2 years of life (due to progressive cardiac morbidity).