A 2-year-old boy with cyanotic congenital heart disease suddenly becomes irritable, hyperpneic, deeply cyanotic, and unresponsive while crying at the pediatric clinic. The mother states that he often squats down during these episodes. Auscultation reveals disappearance of a previously loud systolic ejection murmur. What is the most likely underlying congenital heart defect and the physiological explanation of squatting?

Answer and explanation

Correct answer: C. Tetralogy of Fallot; squatting increases systemic vascular resistance, decreasing right-to-left shunting across the VSD

  1. Step 1: The child has Tetralogy of Fallot (overriding aorta, VSD, pulmonary stenosis, right ventricular hypertrophy) experiencing a hypercyanotic 'tet spell'.
  2. Step 2: Tet spells occur when infundibular spasm or decreased systemic vascular resistance (SVR) increases right-to-left shunting of deoxygenated blood through the VSD into the aorta.
  3. Step 3: Squatting compresses femoral arteries, thereby raising systemic vascular resistance. This shifts the intracardiac shunt from right-to-left toward left-to-right, directing more deoxygenated blood through the stenotic pulmonary artery to the lungs.
Answer
Tetralogy of Fallot; squatting increases systemic vascular resistance, decreasing right-to-left shunting across the VSD.
Why not the others: 'Transposition of the Great Arteries' presents with profound neonatal cyanosis at birth dependent on mixing lesions; 'Coarctation of the Aorta' is an acyanotic lesion causing upper-to-lower extremity blood pressure gradients; 'VSD alone' causes a left-to-right shunt without cyanosis unless Eisenmenger syndrome has developed over decades.

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