A 4-year-old child with tetralogy of Fallot assumes a squatting position during a cyanotic spell. The physiological benefit of squatting is best explained by:

• A Decreased systemic vascular resistance, favouring forward left ventricular output
• B Reflex bradycardia prolonging diastolic filling of the pulmonary artery
• C Increased venous return to the right atrium, improving pulmonary blood flow directly
• D Kinking of femoral arteries raising systemic vascular resistance, which reduces the right-to-left shunt through the VSD ✓

Explanation

Squatting compresses the femoral arteries and increases afterload, raising systemic ventricular pressure above right ventricular pressure for more of the cycle, so less deoxygenated blood shunts right to left across the VSD into the aorta. Systemic venous return transiently falls initially, contradicting option C. This manoeuvre is a classic examination correlation of congenital cyanotic heart disease physiology.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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