During a cyanotic spell in Tetralogy of Fallot, which option best describes the physiologic effect of knee-chest positioning?

During a cyanotic spell in Tetralogy of Fallot, which option best describes the physiologic effect of knee-chest positioning?

• A. Increases venous return to the right heart
• B. Decreases systemic vascular resistance
• C. Increases pulmonary vascular resistance
• D. Increases systemic vascular resistance

Answer: (D)

During a cyanotic spell in Tetralogy of Fallot, the problem is a large right-to-left shunt through the ventricular septal defect caused by obstruction of the right ventricular outflow tract. Placing the child in knee-chest position raises systemic vascular resistance. This higher resistance increases left ventricular afterload, making left ventricular pressure relatively greater than right ventricular pressure. That shift reduces the amount of blood crossing the VSD from right to left and promotes more blood flow into the pulmonary circulation, increasing pulmonary blood flow and oxygenation. The main beneficial effect is the rise in systemic vascular resistance, which alleviates the cyanosis. Worsening cyanosis would occur if systemic vascular resistance were reduced or if pulmonary vascular resistance were increased.

During a cyanotic spell in Tetralogy of Fallot, the problem is a large right-to-left shunt through the ventricular septal defect caused by obstruction of the right ventricular outflow tract. Placing the child in knee-chest position raises systemic vascular resistance. This higher resistance increases left ventricular afterload, making left ventricular pressure relatively greater than right ventricular pressure. That shift reduces the amount of blood crossing the VSD from right to left and promotes more blood flow into the pulmonary circulation, increasing pulmonary blood flow and oxygenation. The main beneficial effect is the rise in systemic vascular resistance, which alleviates the cyanosis. Worsening cyanosis would occur if systemic vascular resistance were reduced or if pulmonary vascular resistance were increased.