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 range from mild to severe, with severe cases requiring assisted ventilation or association with persistent pulmonary hypertension of the newborn (PPHN).

EPIDEMIOLOGY

• Frequency of MSAF: 8–27% of term pregnancies
  • MSAF is rare in premature infants (5%) and almost nonexistent before 31 weeks’ gestation.
  • 2–9% infants born through MSAF develop MAS (0.1–1.8% of all live births in developed countries).
• MAS accounts for 10% of cases of respiratory failure in all newborns.

RISK FACTORS

• Fetal hypoxia (in utero aspiration)
• Abnormal fetal heart rate patterns
• Low umbilical cord pH <7.2
• 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 obstruction of the airways with ventilation/perfusion (V/Q) mismatch, decreased lung compliance, and parenchymal lung injury leading to hypoxemia, hypercarbia, acidosis, and cardiopulmonary failure.

• Mechanical obstruction of airways
  • Complete (atelectasis with V/Q mismatch)
  • Partial (patchy atelectasis and hyperinflation)
  • Air leaks (due to the ball-valve phenomenon)
• Meconium-associated pulmonary inflammation and necrosis
• Inactivation of existing surfactant
• Decreased production of surfactant
• Coexisting pulmonary hypertension