Meconium Aspiration Syndrome

Meconium-stained amniotic fluid (MSAF) means that fetal stool has entered the fluid before birth. MSAF is a risk marker for abnormal transition, but it is not the same as meconium aspiration syndrome (MAS). MAS is diagnosed when a newborn born through MSAF develops respiratory distress that is not better explained by another condition. Meconium can obstruct the airway, especially when it is thick. A partial obstruction can act like a one-way valve: air enters during inspiration but cannot leave effectively during expiration. The trapped air causes hyperinflation and may rupture alveoli, producing pneumothorax or pneumomediastinum. A complete obstruction can instead cause atelectasis and impaired ventilation. The injury is not purely mechanical. Meconium interferes with surfactant, reducing lung compliance and increasing the work of breathing. It also irritates the airways and alveoli, triggering chemical pneumonitis, inflammatory mediator release, and pulmonary vasoconstriction. If pulmonary vascular resistance remains high after birth, blood may continue to bypass the lungs through right-to-left shunting. This produces persistent pulmonary hypertension of the newborn (PPHN) and severe, sometimes labile, hypoxaemia. Fetal stress can contribute to meconium passage, but...