Meconium Aspiration

Meconium aspiration syndrome occurs when a neonate inhales thick, particulate meconium. This is usually secondary to fetal hypoxia which causes increased peristalsis, relaxation of anal sphincters and reflex gasping. Most meconium deliveries involve some meconium staining of the liquor but the babies are vigorous, needing no further intervention. However, significant aspiration of thick meconium can induce 3 major pulmonary effects, viz: airway obstruction , surfactant dysfunction, and chemical pneumonitis.¹

Studies of tracheal aspirate confirm an inflammatory response, with increase in inflammatory cell count and the level of pro-inflammatory cytokines, with a corresponding decrease in lung function. These changes begin to resolve after the first six hours of life, with consequent improvement in lung function.²

Incidence

The figure quoted for infants born with meconium-stained liquor is 5-15% of births.¹ However, improvements in obstetric practice have resulted in a reduction in the incidence of meconium aspiration syndrome (MAS) over recent years. One unit reported a four-fold decrease from 1990-1992 to 1997-1998 (5.8% to 1.5% of meconium-stained infants more than 37 weeks).³

• Obvious presence of meconium or dark green staining of the amniotic fluid
• Green or blue staining of the skin at birth
• Baby appears limp, with low Apgar score
• Breathing is rapid, laboured, or absent
• Signs of post-maturity (eg. peeling skin) are present
• Fetal monitor may show bradycardia

• Blood gas analysis showing low blood pH, increased pCO₂, decreased pO₂.
• Chest Xray shows patchy infiltrates, course streaking of both lungs, increased AP diameter and flattening of diaphragm (due to hyperinflation).

• Suction - NICE do not recommend routinely suctioning of the nasopharynx and oropharynx prior to birth of the shoulder and trunk. However, they advise that the upper airways may be suctioned if thick or tenacious meconium is present in the oropharynx. If the baby has depressed vital signs, laryngoscopy and suction under direct vision should be carried out by a healthcare
  professional trained in advanced neonatal life support.⁶
• Oxygen - depending on the degree of respiratory distress, respiratory support should be provided with oxygen via a nasal cannula, continuous positive pressure ventilation, conventional mechanical ventilation, or high-frequency oscillatory ventilation.¹
• Antibiotics (e.g. gentamicin) - these may be useful in ventilated cases, but in non-ventilated babies there is little evidence to support their use.⁷
• Surfactant - meconium flowing into the lung deactivates the activity of surfactant, causes a rise in surface tension, and presaging the onset of respiratory distress.⁸ Surfactant lavage combats this effect and significantly improves infant mortality and morbidity.⁹ Modern synthetic surfactants are to be preferred to animal extracts due to their more consistent content of surfactant protein.¹⁰ Recently therapeutic surfactant lung lavage has been used to with good result in neonates affected by severe MAS.¹¹ Combination of surfactant with continuous positive pressure airways administration has also proved successful.¹²
• Inhaled nitric oxide - this is useful in the management of pulmonary hypertension associated with MAS. It is thought to act by relaxing smooth muscles in the pulmonary vessels causing vasodilatation, as well as promoting bronchodilation.¹³ It is often more effective when combined with high-frequency oscillatory ventilation.¹⁴
• Extracorporeal membrane oxygenation (ECMO) - this uses a heart-lung machine to take over the work of the lungs.¹⁵ Veno-venous ECMO seems as effective as veno-arterial ECMO.¹⁶
• Steroids - inhaled or systemic - have been used to good effect in some studies.¹⁷

In mild cases, respiratory distress usually subsides in 2-4 days, although tachypnoea can persist for longer. Rarely, more prolonged respiratory damage can occur which can persist for many years.¹⁸ This is more likely if ventilation has been required.^1,19 Cerebral hypoxia may lead to long-term neurological damage.¹ The mortality rate is approximately 5%. Risk factors for mortality include resuscitation outside hospital, first born babies, shock, pneumothorax, pulmonary hypertension, and renal failure.²⁰

In one study of 2,603 deliveries 11.6% had meconium-stained amniotic fluid, Of these infants, 21.1% developed MAS. Of pregnancies in which meconium-stained amniotic fluid was a feature, severity of meconium, low Apgar score at 5 min and non-reassuring fetal heart rate tracing was associated with MAS.²¹

More frequent diagnosis of abnormal fetal heart rate patterns and the avoidance of post-mature delivery by elective caesarean section have both been shown to reduce the incidence of MAS.³