Description

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).

Epidemiology

• 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

Pathophysiology

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