A 15-month-old boy with a known, unrepaired congenital heart defect is brought to the pediatric cardiology clinic for a follow-up evaluation. The parents report that earlier today, the child became extremely fussy, began breathing rapidly, and developed a deep bluish discoloration around his lips during a crying spell. The mother immediately placed the toddler on his back and pushed his knees firmly against his chest, which rapidly resolved the cyanosis and calmed him down. This physical maneuver improved the patient's condition primarily by causing which of the following physiologic changes?
- An increase in systemic vascular resistanceAnswer
- BA decrease in pulmonary vascular resistance
- CAn increase in right ventricular contractility
- DA decrease in right ventricular afterload
- EAn increase in heart rate
Answer
An increase in systemic vascular resistance
The clinical presentation describes a hypercyanotic 'tet spell' in a child with Tetralogy of Fallot. During these episodes, dynamic right ventricular outflow tract obstruction worsens, increasing the right-to-left shunt of deoxygenated blood across the ventricular septal defect into the systemic circulation. The knee-to-chest maneuver (or squatting in older children) mechanically compresses the femoral arteries. This sharply increases systemic vascular resistance, thereby elevating pressure in the left ventricle and aorta. The increased left-sided pressure decreases the right-to-left shunt and redirects more blood through the pulmonary circulation for oxygenation, rapidly resolving the cyanosis.
Step-by-Step Solution
Key Concept
The knee-to-chest maneuver (or squatting) rapidly increases systemic vascular resistance, which decreases the right-to-left shunt across the ventricular septal defect in patients with Tetralogy of Fallot.
Estimated Time:45s