Meconium Aspiration Syndrome



Meconium aspiration syndrome (MAS) is a clinical diagnosis defined as respiratory distress in a newborn delivered through meconium-stained amniotic fluid (MSAF) with no other explanation for clinical symptoms. Severity of MAS can be (i) mild: requiring <0.4 Fio2 for <48 hours; (ii) moderate: requiring ≥0.4 Fio2 for >48 hours with no air leak; and (iii) severe: requiring assisted ventilation or if associated with persistent pulmonary hypertension of the newborn (PPHN).


  • Frequency of MSAF: 10–15% of all pregnancies
    • MSAF is rare in premature infants and almost nonexistent before 31 weeks’ gestation.
    • 2–9% infants born through MSAF develop MAS (0.2–1.4% of all live births).
  • MAS accounts for 10% cases of respiratory failure in all newborns.

Risk Factors

  • Fetal hypoxia (in utero aspiration)
  • Postmature gestation
  • Thick consistency of meconium
  • 1- and 5-minute Apgar <6
  • Small for gestational age (SGA)
  • Chorioamnionitis
  • African American or South Asian ethnicity


Meconium aspiration creates ventilation/perfusion (V/Q) mismatch by the following variable effects on the airways, leading to hypoxemia, hypercarbia, acidosis, and cardiopulmonary failure.

  • Mechanical obstruction of airways
    • Complete (atelectasis +/− consolidation)
    • Partial (patchy atelectasis and hyperinflation)
    • Air leaks (due to the ball-valve phenomenon)
  • Meconium-associated pulmonary inflammation
  • Inactivation of existing surfactant
  • Decreased production of surfactant
  • Meconium-induced lung apoptosis
  • Coexisting pulmonary hypertension

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