Squatting-Induced Improvement in Cyanosis in Tetralogy of Fallot
Squatting improves cyanosis in tetralogy of Fallot by increasing systemic vascular resistance, which reduces right-to-left shunting across the ventricular septal defect during hypercyanotic episodes. [1] Reduced right-to-left shunting increases pulmonary blood flow and arterial oxygen saturation. [1]
Pathophysiology of Cyanosis in Tetralogy of Fallot
Tetralogy of Fallot physiology includes right ventricular outflow tract obstruction, which promotes right-to-left flow across an interventricular communication. [1] During hypercyanotic (“tet”) spells, pulmonary blood flow decreases and right-to-left shunt increases, causing worsening arterial oxygen saturation. [2]
Hemodynamic Effects of Squatting
Squatting increases systemic vascular resistance, counteracting the fall in systemic resistance that otherwise favors right-to-left shunt during spells. [1] The increased systemic resistance shifts flow patterns away from the right-to-left direction, thereby decreasing the proportion of deoxygenated blood entering the systemic circulation. [1] The reduction in right-to-left shunting increases flow through the pulmonary circulation and improves oxygenation. [1]
Role of Systemic-to-Pulmonary Vascular Resistance Ratio
Right-to-left shunting during spells is influenced by changes in the systemic vascular resistance to pulmonary vascular resistance relationship. [2] Squatting increases systemic vascular resistance, which restores a more favorable systemic-to-pulmonary resistance ratio for pulmonary blood flow. [2]
Supporting Evidence for Increasing Systemic Vascular Resistance
Acute pharmacologic increases in systemic vascular resistance with phenylephrine decrease right-to-left shunt and improve systemic oxygen levels in tetralogy of Fallot. [3] This effect supports systemic vasoconstriction as a mechanism for reducing right-to-left shunting during hypoxic episodes. [3]
Clinical Implications for Positioning
Knee-to-chest positioning and squatting are used as nonpharmacologic maneuvers to raise systemic vascular resistance and reduce right-to-left shunting during hypoxic spells. [1] Other systemic vasodilation conditions worsen tetralogy of Fallot shunting by lowering systemic resistance, highlighting the importance of maintaining elevated systemic vascular resistance. [2]