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Meconium Aspiration Syndrome
Intrapartum meconium aspiration can cause inflammatory pneumonitis and mechanical bronchial obstruction, causing a syndrome of respiratory distress. Findings include tachypnea, rales and rhonchi, and cyanosis or desaturation. Diagnosis is suspected when there is respiratory distress after delivery through meconium-tinged amniotic fluid and is confirmed by chest x-ray. Treatment is vigorous suction immediately on delivery before neonates take their first breath, followed by respiratory support as needed. Prognosis depends on the underlying physiologic stressors.
Physiologic stress at the time of labor and delivery (eg, due to hypoxia caused by umbilical cord compression or placental insufficiency or caused by infection) may cause the fetus to pass meconium into the amniotic fluid before delivery; meconium passage is noted in about 10 to 15% of births. During delivery, perhaps 5% of neonates with meconium passage aspirate the meconium, triggering lung injury and respiratory distress, termed meconium aspiration syndrome. Postterm infants delivered through reduced amniotic fluid volume are at risk of more severe disease because the less dilute meconium is more likely to cause airway obstruction.
The mechanisms by which aspiration induces the clinical syndrome probably include
Underlying physiologic stressors also may contribute. If complete bronchial obstruction occurs, atelectasis results; partial blockage leads to air trapping on expiration, resulting in hyperexpansion of the lungs and possibly pulmonary air leak (see page Pulmonary Air-Leak Syndromes) with pneumomediastinum or pneumothorax. Persistent pulmonary hypertension can be associated with meconium aspiration as a comorbid condition or because of continuing hypoxia (see page Persistent Pulmonary Hypertension of the Newborn).
Neonates also may aspirate vernix caseosa, amniotic fluid, or blood of maternal or fetal origin during delivery, which can cause respiratory distress and signs of aspiration pneumonia on chest x-ray.
Signs include tachypnea, nasal flaring, retractions, cyanosis or desaturation, rales, rhonchi, and greenish yellow staining of the umbilical cord, nail beds, or skin. Meconium staining may be visible in the oropharynx and (on intubation) in the larynx and trachea. Neonates with air trapping may have a barrel-shaped chest and also symptoms and signs of pneumothorax, pulmonary interstitial emphysema, and pneumomediastinum (see page Soft-Tissue Injuries : Pneumomediastinum).
Diagnosis is suspected when a neonate shows respiratory distress in the setting of meconium-tinged amniotic fluid. Diagnosis is confirmed by chest x-ray showing hyperinflation with variable areas of atelectasis and flattening of the diaphragm. Initial x-ray findings can be confused with the findings of transient tachypnea of the newborn (see page Transient Tachypnea of the Newborn). Fluid may be seen in the lung fissures or pleural spaces, and air may be seen in the soft tissues or mediastinum. Because meconium may enhance bacterial growth and meconium aspiration syndrome is difficult to distinguish from bacterial pneumonia, cultures of blood and tracheal aspirate also should be taken.
Routine suctioning of neonates delivered with meconium-stained fluid has not been shown to improve outcome. However, if the neonate's breathing appears obstructed, suctioning is done. If the neonate is nonvigorous at delivery (ie, with poor muscle tone or absent or depressed respiratory effort) or is bradycardic (< 100 beats/min), the trachea should be intubated and suctioned with a meconium aspirator. Suction is maintained while the endotracheal tube is removed. Reintubation and continuous positive airway pressure (see page Soft-Tissue Injuries : Continuous positive airway pressure (CPAP)) are indicated for continued respiratory distress, followed by mechanical ventilation (see page Soft-Tissue Injuries : Mechanical ventilation) and admission to the neonatal ICU as needed. Because positive pressure ventilation enhances risk of pulmonary air-leak syndrome, regular evaluation (including physical examination and chest x-ray) is important to detect this complication, which should be sought immediately in any intubated neonate whose BP, perfusion, or O2 saturation suddenly worsens. See page Pulmonary Air-Leak Syndromes for treatment of air-leak syndromes.
Additional treatments may include surfactant for mechanically ventilated neonates with high O2requirements, which can decrease the need for extracorporeal membrane oxygenation (ECMO—see page Soft-Tissue Injuries : Extracorporeal membrane oxygenation (ECMO)), and antibiotics (usually ampicillin and an aminoglycoside). Inhaled nitric oxide at 20 ppm and high-frequency ventilation are other therapies that are used if refractory hypoxemia develops; they also may decrease need for ECMO.
About 5% of neonates with meconium passage aspirate the meconium, triggering lung injury and respiratory distress.
Suspect the diagnosis when respiratory distress occurs in neonates who had meconium-tinged amniotic fluid.
Neonates may have tachypnea, nasal flaring, retractions, cyanosis or desaturation, rales, rhonchi, and visible meconium staining in the oropharynx.
Do cultures of blood and tracheal aspirates to exclude pneumonia.
After delivery, suction infants who have signs of obstructed breathing; if there is weak respiratory effort or bradycardia, insert an endotracheal tube and suction using a meconium aspirator.
Severe cases require mechanical ventilation and sometimes antibiotics, inhaled nitric oxide, or ECMO.
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