Meconium aspiration syndrome

Background

Image showing release of meconium into amniotic fluid (top), its progress into the mouth middle), and subsequently into the lung (C).

• Presentation ranges from mild respiratory distress to life-threatening respiratory failure
• Incidence is 2-10% of infants born through MSAF (meconium-stained amniotic fluid)
• Thought to be associated with fetal hypoxia and post-term delivery
• Causes hypoxemia and acidosis via airway obstruction, chemical irritation/inflammation, infection, and surfactant inactivation
• Associated with persistent pulmonary hypertension of the newborn (PPHN)

Newborn Vital Signs

Age	Pulse^	Respiratory Rate	Systolic BP
Preterm < 1 kg	120-160	30-60	36-58
Preterm 1 kg	120-160	30-60	42-66
Preterm 2 kg	120-160	30-60	50-72
Newborn	126-160	30-60	60-70

Min of life	Target sat^^
1 min	60-65%
2 min	65-70%
3 min	70-75%
4 min	75-80%
5 min	80-85%
10 min	85-95%

^Fever directly causes an increase in heart rate of 10 beats per minute per degree centigrade ^^Hyperoxia can be harmful

Clinical Features

• Usually develop symptoms within 15 minutes after birth
• Tachypnea
• Cyanosis
• Accessory muscle use (intercostal/subxiphoid retractions, paradoxical breathing, grunting, nasal flaring)
• Barrel-shaped chest
• Rales and rhonchi on lung auscultation
• Pneumothorax
• Pneumomediastinum

Differential Diagnosis

• Sepsis
• Pneumonia
• Delayed transition from fetal circulation
• Pneumothorax
• Pulmonary edema
• Blood aspiration
• Pulmonary hypertension
• Congenital heart disease

Newborn Problems

• Newborn resuscitation
• Hypoxia
  • Primary apnea
  • Secondary apnea
• Hypothermia
• Hypoglycemia
• Meconium aspiration syndrome
• Anemia (abruption)
• Infant scalp hematoma
• Transient tachypnea of the newborn
• Respiratory distress syndrome
• Congenital pneumonia
• Congenital heart disease
• Neonatal sepsis
• Pneumothorax
• Pulmonary hypertension

Evaluation

Chest X-ray of neonate with meconium aspiration.

Workup

• CXR
  • Streaky, linear densities
  • Hyperinflated lungs and flattened diaphragm
  • Diffuse patchy opacities (may appear similar to RDS if severe)
• ABG
• Echocardiography
• Blood and sputum cultures

Diagnosis

• Clinical diagnosis based on the following:
  • Evidence of meconium on infant
  • Respiratory distress shortly after birth
  • Characteristic CXR findings
  • If intubation required, meconium visualized in trachea
• Ways to differentiate between other causes of respiratory distress in a neonate:
  • Transient tachypnea of the newborn is more common in late preterm infants (34-37 weeks) and RDS in preterm infants, whereas MAS more common in postmature infants (>41 weeks)
  • Delayed transition from fetal circulation symptoms improve more quickly than those of MAS
  • Congenital cyanotic heart disease is differentiated by physical exam (murmurs, hepatomegaly), CXR (cardiac size/shape), and echocardiogram (cardiac anatomy and function)

Management

• Supportive care: see newborn resuscitation
  • Adequate oxygenation and ventilation
    • Supplemental oxygen to keep saturation >99% and PaO2 55-90
    • Assisted ventilation with CPAP if FiO2 exceeds 0.4 to 0.5
    • High frequency oscillatory ventilation or ECMO for those who fail conventional mechanical ventilation
    • Goal PaCO2 50-55 mmHg
  • Maintain blood pressure and perfusion
    • Umbilical lines to monitor blood gases and BP
  • Correct metabolic abnormalities
• Empiric antibiotics while awaiting culture results (because of difficulty differentiating between pneumonia initially)
• Surfactant administration for severe disease requiring mechanical ventilation, FiO2>0.5, and mean airway pressure >10-12

Disposition

• Admit to NICU

Prevention

• Fetal heart rate monitoring intrapartum to reduce risk of fetal hypoxia
• Induction of labor after 41 weeks gestation to reduce postmature delivery
• Amnioinfusion with isotonic fluid to dilute thick meconium is NOT routinely recommended

See Also

• Newborn resuscitation