A 4-year-old cyanotic child squats frequently after exertion. He has a parasternal heave, single S2, and grade 3/6 ejection systolic murmur. Chest X-ray shows boot-shaped heart. This picture of Tetralogy of Fallot. What is the mechanism of squatting benefit?

• A Squatting decreases venous return, reducing right heart preload and pulmonary flow
• B Squatting reduces heart rate via vagal stimulation
• C Squatting increases intrapleural pressure, reducing pulmonary congestion
• D Squatting increases systemic vascular resistance by compressing femoral vessels, reducing R-to-L shunting and increasing pulmonary blood flow ✓

Explanation

In Tetralogy of Fallot, the right-to-left shunt through the VSD is driven by the ratio of pulmonary to systemic vascular resistance. Squatting acutely increases systemic vascular resistance (SVR) by mechanically kinking/compressing large femoral arteries, raising aortic pressure and reducing the pressure gradient driving blood from right to left across the VSD. This increases pulmonary blood flow and improves systemic oxygenation. It is a learned compensatory reflex unique to TOF among cyanotic CHDs.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.