Meconium aspiration syndrome

Characteristics of MAS

MAS is characterised by early onset of respiratory distress and hypoxaemia in a meconium-stained term or near-term infant.1

Pathogenesis

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.4

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.4 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.4

The resultant impairment of gas exchange is often severe and manifests as hypoxaemia with or without hypercarbia.5

Persistent pulmonary hypertension (PPHN) with right to left ductal and foramen ovale shunt frequently compounds the oxygenation difficulty.

Differential diagnosis

• Birth asphyxia with pulmonary hypertension and/or haemorrhagic pulmonary oedema.
• Transient tachypnoea of the newborn.
• Surfactant deficiency (especially after elective caesarean delivery).
• Sepsis/pneumonia.
• Pneumothorax.
• Congenital diaphragmatic hernia.

Clinical features

• MAS is characterised by early onset of respiratory distress (within two hours) in a meconium-stained infant.4
• Tachypnoea, cyanosis and variable hyperinflation are the main clinical findings.
• Auscultation reveals widespread ‘wet’ inspiratory crackles, occasionally with expiratory noises suggesting airway obstruction.
• Radiologically, the typical appearance is of widespread patchy opacification accompanied by areas of hyperinflation and/or atelectasis.4
• Blood gas analysis invariably shows hypoxaemia accompanied by hypercarbia in those infants with significant airway obstruction or severe respiratory failure.5

Respiratory management

Infants suspected of having MAS and requiring respiratory support should be discussed with PIPER.

Oxygen: guidelines for administration

• Oxygen should be administered early in any infant 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.6
• The pulmonary vasculature in a term infant is highly sensitive to oxygen tension and failure to overcome hypoxaemia will likely lead to progressive pulmonary hypertension.

Nasal CPAP: guidelines for administration

• 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 (SpO2 < 90 per cent, PaO2 < 50) despite CPAP and FiO2 of > 60 per cent
• respiratory acidosis with pH < 7.20.

Intubation guidelines:

• Other than in the delivery room, term infants with MAS require sedation and muscle relaxation during 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 (PEEP) (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.

Surfactant therapy

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.7

Studies using lung lavage with dilute surfactant (Survanta) in ventilated infants with severe MAS do not decrease the duration of respiratory support but may produce a reduction in mortality, especially in units not offering ECMO.

Suctioning

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.10

Care and complications of MAS

General care for MAS includes4:

• cardiovascular support - volume and inotrope therapy (link to blood pressure disorders)
• fluid restriction
• maintenance of optimal thermal environment
• stomach contents evacuated and an in-dwelling nasogastric tube inserted
• antibiotic therapy until primary bacterial infection is excluded
• IV therapy and nil orally until the respiratory distress is resolving
• minimal handling.

Complications of MAS may include:

• persistent pulmonary hypertension
• air leak syndromes including pneumomediastinum, pneumothorax and cystic lung disease
• pulmonary haemorrhage
• asphyxia (affected infants may develop complications of asphyxia including encephalopathy, seizures, oliguria, coagulopathy and thrombocytopenia).

1. Dargaville PA, Copnell B. The epidemiology of meconium aspiration syndrome: incidence, risk factors, therapies, and outcome. Pediatrics. 2006 May 1;117(5):1712-21. https://doi.org/10.1542/peds.2005-2215
2. Clausson B, Cnattingius S, Axelsson O. Outcomes of post-term births: the role of fetal growth restriction and malformations. Obstetrics & Gynecology. 1999 Nov 1;94(5):758-62. https://doi.org/10.1016/S0029-7844(99)00387-7
3. Brabbing-Goldstein D, Nir D, Cohen D, Many A, Maslovitz S. Preterm meconium-stained amniotic fluid is an ominous sign for the development of chorioamnionitis and for in utero cord compression. The Journal of Maternal-Fetal & Neonatal Medicine. 2017 Sep 2;30(17):2042-5. https://doi.org/10.1080/14767058.2016.1236246
4. Swarnam K,Soraisham A, Sivandan S. Advances in the Management of Meconium Aspiration Syndrome. International Journal of Paediatrics. Volume 2012, https://doi.org/10.1155/2012/359571
5. Garcia-Prats JA. Clinical features and diagnosis of meconium aspiration syndrome. Martin R, ed. UpToDate. Waltham, MA: UpToDate Inc https://www.uptodate.com (accessed November 2019).
6. Jobe AH, Kallapur SG. Long term consequences of oxygen therapy in the neonatal period. InSeminars in Fetal and Neonatal Medicine 2010 Aug 1 (Vol. 15, No. 4, pp. 230-235). WB Saunders. https://doi.org/10.1016/j.siny.2010.03.007
7. El Shahed AI, Dargaville PA, Ohlsson A, Soll R. Surfactant for meconium aspiration syndrome in term and late preterm infants. Cochrane Database of Systematic Reviews. 2014(12). https://doi.org/10.1002/14651858.CD002054.pub3
8. Dargaville PA, Copnell B, Mills JF, Haron I, Lee JK, Tingay DG, Rohana J, Mildenhall LF, Jeng MJ, Narayanan A, Battin MR. Randomized controlled trial of lung lavage with dilute surfactant for meconium aspiration syndrome. The Journal of pediatrics. 2011 Mar 1;158(3):383-9. https://doi.org/10.1016/j.jpeds.2010.08.044
9. Perlman JM, Wyllie J, Kattwinkel J, Wyckoff MH, Aziz K, Guinsburg R, Kim HS, Liley HG, Mildenhall L, Simon WM, Szyld E. Part 7: neonatal resuscitation: 2015 international consensus on cardiopulmonary resuscitation and emergency cardiovascular care science with treatment recommendations. Circulation. 2015 Oct 20;132(16_suppl_1):S204-41. https://doi.org/10.1161/cir.0000000000000276
10. Dargaville PA, Copnell B, Tingay DG, Gordon MJ, Mills JF, Morley CJ. Refining the method of therapeutic lung lavage in meconium aspiration syndrome. Neonatology. 2008 Oct 1;94(3):160. https://doi.org/10.1159/000143394