In Tetralogy of Fallot, the degree of cyanosis is primarily determined by which anatomical lesion?

Explanation

## Determinants of Cyanosis in Tetralogy of Fallot **Key Point:** The severity of pulmonary stenosis (RVOT obstruction) is the primary determinant of the magnitude of right-to-left shunting and thus the degree of cyanosis in TOF. ### Pathophysiological Mechanism 1. **Pulmonary stenosis increases RV pressure** — The narrowed pulmonary valve creates resistance to RV ejection. 2. **RV pressure exceeds LV pressure** — When RV systolic pressure surpasses LV pressure, blood preferentially shunts right-to-left across the VSD. 3. **Deoxygenated blood enters systemic circulation** — Unsaturated blood from the RV bypasses the lungs and enters the aorta (which overrides the VSD), causing cyanosis. 4. **Severity of stenosis determines shunt magnitude** — Mild stenosis → minimal shunt → acyanotic ("pink tet"); severe stenosis → large shunt → profound cyanosis. ### Why Other Lesions Are Secondary | Lesion | Role | Effect on Cyanosis | | --- | --- | --- | | VSD size | Allows shunting pathway | Must be large; size alone does not determine shunt direction | | Pulmonary stenosis | **Drives pressure gradient** | **PRIMARY determinant — severity directly correlates with cyanosis** | | Aortic override | Receives shunted blood | Anatomical consequence; does not generate the pressure gradient | | RV hypertrophy | Compensatory response | Secondary change; does not initiate shunting | **High-Yield:** The VSD in TOF is always large and non-restrictive, so RV and LV pressures equalize. The *direction* of shunting (right-to-left vs. left-to-right) depends on which ventricle has higher pressure — determined by the RVOT obstruction, not the VSD size. **Clinical Pearl:** Infants with TOF may present with "tet spells" (hypercyanotic episodes) during crying or feeding, when increased RV contractility and systemic vascular resistance further increase the pressure gradient and worsen right-to-left shunting. Squatting posture increases systemic vascular resistance and temporarily improves oxygenation by reducing the pressure gradient. [cite:Harrison 21e Ch 282]