A 4-month-old boy presents to the emergency department with his mother, who has noticed that for the past 2 weeks his lips and extremities are turning blue when he cries or feeds. The infant has also been fussier than usual and does not seem to be putting on weight, which she attributes to him feeding now less than usual. The mother did not have any prenatal care, but the infant was born at term with no complications. Oxygen saturations are at 88% on room air and he is tachypneic with respirations 45/min. On physical examination, the infant is irritable, and is noted to have a cyanotic episode when he is agitated. He is below average weight for his age and height. Tetralogy of Fallot is suspected.
Which of the following abnormalities would not be expected to be found on further investigation?
A) Differential cyanosis
Differential cyanosis is when the lower limbs have lower oxygenation than the upper limbs. Most commonly, this occurs in severe pulmonary hypertension with a patent ductus arteriosus. Deoxygenated blood from the patent ductus enters the aorta distal to the subclavian artery, sparing the upper limb. An increased pressure gradient due to pulmonary hypertension forces deoxygenated blood into the aorta. The mixed blood then goes to the lower limb. A patent ductus is not part of the Tetralogy of Fallot, so differential cyanosis would not be expected.
Tetralogy of Fallot (ToF) is made up of pulmonary stenosis, right ventricular hypertrophy, an overriding aorta, and a ventricular septal defect. A right to left shunt is present in ToF, secondary to right ventricular outflow tract obstruction leading to right ventricular hypertrophy and overall increases in right heart pressure overcoming left heart pressure. Pulmonary stenosis causes a harsh systolic ejection murmur on the left sternal edge. Ventricular septal defects are often too large to cause a murmur. Right ventricular hypertrophy can cause right axis deviation on ECG. Option C is the hyperoxia test, used to distinguish between cardiac and respiratory causes of hypoxia. In lung disease, oxygenation is expected to improve, while the opposite is true for cardiac causes.
Key Learning Point
Tetralogy of Fallot (ToF) is made up of pulmonary stenosis, right ventricular hypertrophy, an overriding aorta, and a ventricular septal defect. Pre-ductal congenital heart defects do not present with differential cyanosis.
