Meconium aspiration syndrome (MAS) is an important cause of respiratory distress in term infants, with a local incidence of around 1.5 per 1,000 live births.
The prelude to MAS is the passage of meconium at or prior to delivery, a circumstance encountered in 10-20 per cent of all deliveries at term and more frequently beyond term.
Meconium-stained amniotic fluid (MSAF) is rarely seen at preterm delivery; its presence raises the possibility of fetal infection (particularly listeria).
Delivery room management of the infant where liquor is meconium stained is described in a separate section.
Characteristics of MAS
Characteristics of MAS include the following:
- MAS is characterised by early onset of respiratory distress and hypoxaemia in a meconium-stained term or near-term infant.
- Most babies with MAS require nothing more than oxygen therapy and general supportive care; oxygen should be used liberally in this condition.
- Ventilator support should be instituted where there is refractory hypoxaemia or respiratory acidosis.
In an infant born through MSAF, the risk and severity of MAS is influenced by:
- the severity of concurrent asphyxia
- the degree of contamination of the amniotic fluid with meconium
- the presence of meconium in the airways (nasopharynx, trachea) at delivery.
Of these, asphyxia is the single most important risk factor for MAS and is presumed to relate to the influx of MSAF into the lung during hypoxic fetal gasping. MAS can occur, however, in meconium-stained infants who are in good condition at birth.
The perinatal events leading to the inhalation of meconium are outlined below.
Once in the lung, meconium quickly migrates down the tracheobronchial tree inducing a complex lung disease including:
- large and small airway obstruction
- chemical pneumonitis
- proteinaceous alveolar oedema
- surfactant dysfunction or inactivation.
The resultant impairment of gas exchange is often severe and manifests as hypoxaemia with or without hypercarbia.
Persistent pulmonary hypertension (PPHN) with right to left ductal and foramen ovale shunt frequently compounds the oxygenation difficulty.
Clinical features of MAS
Clinical features of MAS include the following:
- MAS is characterised by early onset of respiratory distress (within two hours) in a meconium-stained infant.
- Tachypnoea, cyanosis and variable hyperinflation are the main clinical findings.
- Ausculation reveals widespread ‘wet’ inspiratory crackles, occasionally with expiratory noises suggesting ball-valve airway obstruction.
- Radiologically, the typical progression is from global atelectasis in early x-rays to a widespread patchy opacification accompanied by areas of hyperinflation and/or atelectasis.
- Blood gas analysis invariably shows hypoxaemia accompanied by hypercarbia in those infants with significant airway obstruction or severe respiratory failure.
Management of MAS respiratory care
Guidelines for administration of oxygen:
- Oxygen should be administered early and liberally in any baby suspected of having inhaled meconium. The suggested target range for oxygen saturation is 91-95 per cent; target PaO2 60-90 mmHg.
- Note: The use of oxygen has a risk to benefit equation just as for any other drug.
- The pulmonary vasculature in a term infant is exquisitely sensitive to oxygen tension and failure to overcome hypoxaemia almost inevitably will lead to progressive pulmonary hypertension.
Consider CPAP as an interim measure in infants with MAS where there is moderate respiratory distress and hypoxaemia.
Intubation and positive pressure ventilation indications
Intubation is indicated when infant displays:
- persistent hypoxaemia (SaO2 < 90%, PaO2 < 50) despite CPAP and FiO2 of > 60%
- respiratory acidosis with pH < 7.20.
Intubation guidelines include the following:
- Other than in the delivery room, term infants with MAS require deep sedation and preferably muscle relaxation prior to intubation.
- Infants with MAS who need intubation often require high peak inspiratory pressures (30-35 cm H2O) to achieve gas exchange.
- Ventilation should be aimed at increasing oxygenation while minimizing the barotrauma that can lead to air leaks.
- Most evidence favours a high positive end-expiratory pressure (6-8 cm H2O) and a long expiratory time. The latter can be achieved using ventilator rates of 40-60 breaths per minute, with an inspiratory time of 0.5-0.6 seconds.
- If gas trapping is noticed, expiratory time can be increased and PEEP decreased.
- Particularly where there is concomitant pulmonary hypertension, deep sedation should be maintained after intubation and muscle relaxation should be continued if the disease is severe.
- Ventilation aims to maintain a pH of 7.3–7.4, with a PaO2 targeted between 60 and 80 mmHg and a PaCO2 of 40–50 mmHg.
Points to note regarding surfactant use:
- Use of bolus surfactant therapy in MAS is still controversial. Meta-analysis showed reduction in the severity of respiratory illness and decrease in the number of infants with progressive respiratory failure requiring extracorporeal membrane oxygenation (ECMO). However, there was no significant difference in mortality, hospital stay, length of ventilation, duration of oxygen use, pneumothorax, pulmonary interstitial emphysema or chronic lung disease.
- Studies using lung lavage with dilute surfactant (Survanta) in ventilated infants with severe MAS does not decrease the duration of respiratory support but may produce a reduction in mortality, especially in units not offering ECMO.
- Supportive therapy for pulmonary hypertension.
Points to note regarding suctioning:
- Routine intrapartum oropharyngeal and nasopharyngeal suction is no longer recommended.
- Non-vigorous infants with depressed respiratory effort, poor muscle tone and HR < 100 born through MSAF are recommended to have direct endotracheal suctioning prior to stimulation.
- All infants with MAS should have the stomach contents evacuated and an in-dwelling nasogastric tube inserted.
- In those intubated the trachea should be suctioned by small volume saline lavage if there is clinical evidence of build-up of meconium or secretions in the large airways.
General supportive care
General care for MAS includes:
- cardiovascular support - volume and inotrope therapy
- fluid restriction
- antibiotic therapy should be continued until primary bacterial infection is excluded
- IV therapy and nil orally until the respiratory distress is resolving.
Consider the following when making a differential diagnosis:
- Dargaville P, Copnell B. The Epidemiology of Meconium Aspiration Syndrome: Incidence, Risk Factors, Therapies & Outcomes. Pediatrics 2006;117;1712
- Jobe A, Kallapur S. Long Term Consequences of oxygen therapy in the neonatal period. Semin Fetal Neonatal Medicine. 2010 August; 15(4) 230-235
- Swarnam K,Soraisham A, Sivandan S. Advances in the Management of Meconium Aspiration Syndrome. International Journal of Paediatrics. Volume 2012, Article ID 359571
- Wiswell TE, Bent RC. Meconium staining and the meconium aspiration syndrome. Unresolved issues. Pediatr Clin North Am 1993; 40: 955-81.
- A. I. El Shahed, P. Dargaville, A. Ohlsson, and R. F. Soll, “Surfactant for meconium aspiration syndrome in full term/near term infants,” Cochrane Database of Systematic Reviews, no. 3, Article ID CD002054, 2007.
- P. A. Dargaville, B. Copnell, J. F. Mills et al., “Randomized controlled trial of lung lavage with dilute surfactant for meconium aspiration syndrome,” Journal of Pediatrics, vol. 158, no. 3, pp. 383–389, 2011.
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First published: August 2013
Last web update: October 2018
Review by: December 2020
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Page last updated: 27 Nov 2018