Meconium aspiration syndrome

Background
  • 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[1]
Age Pulse^ Respiratory Rate Systolic BP
Preterm < 1 kg120-16030-6036-58
Preterm 1 kg120-16030-6042-66
Preterm 2 kg120-16030-6050-72
Newborn126-16030-6060-70
Min of lifeTarget sat^^
1 min60-65%
2 min65-70%
3 min70-75%
4 min75-80%
5 min80-85%
10 min85-95%

^Fever directly causes an increase in heart rate of 10 beats per minute per degree centigrade[2] ^^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

Newborn Problems

Evaluation

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


References
  1. National-Model-EMS-Clinical-Guidelines-23Oct2014
  2. Davies P, Maconochie I. The relationship between body temperature, heart rate and respiratory rate in children. Emerg Med J. 2009 Sep;26(9):641-3. doi: 10.1136/emj.2008.061598.
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