## Pathophysiology of Meconium Aspiration Syndrome ### Primary Mechanism **Key Point:** Ball-valve obstruction is the PRIMARY and most characteristic mechanism in MAS. Meconium particles lodge at the level of terminal and respiratory bronchioles, acting as one-way valves — air enters during inspiration but cannot exit during expiration, leading to air trapping, hyperinflation, and subsequent barotrauma. ### Secondary Mechanisms While surfactant inactivation and chemical pneumonitis DO occur in MAS, they are secondary phenomena: | Mechanism | Timing | Clinical Significance | |-----------|--------|----------------------| | Ball-valve obstruction | Immediate (first hours) | Hyperinflation, air leaks, barotrauma | | Surfactant inactivation | Hours to days | Persistent atelectasis, increased work of breathing | | Chemical pneumonitis | 24–72 hours | Inflammation, increased airway reactivity | | Pulmonary edema | Variable | Contributes to oxygenation failure | ### Clinical Correlates **High-Yield:** The ball-valve mechanism explains why MAS infants present with: - Barrel-shaped chest (hyperinflation) - Flattened diaphragm on CXR - Air leak syndromes (pneumothorax, pneumomediastinum) — seen in 20–40% of severe MAS - Patchy atelectasis alternating with hyperinflated zones ("classic" appearance) **Clinical Pearl:** Ventilation strategies in MAS must account for air trapping — permissive hypercapnia and gentle ventilation are preferred over aggressive hyperventilation, which worsens air leaks. [cite:Nelson Textbook of Pediatrics 21e Ch 102] 
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