Meconium Aspiration Syndrome
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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.
- Fetal hypoxia (in utero aspiration)
- Postmature gestation
- Thick consistency of meconium
- 1- and 5-minute Apgar <6
- Small for gestational age (SGA)
- 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