Meconium Aspiration Syndrome (Mas) Primarily

Meconium Aspiration Syndrome (MAS) primarily affects newborns, and those that are premature are particularly at risk. Within the United States, it is estimated that 12% of live births have a complication with meconium in the amniotic fluid. Of those, 35% will actually develop MAS. Out of those babies, 30% will need mechanical ventilation, 10% will develop pneumothroaces, and 4% will die, so it can be seen that this is a serious condition that plagues obstetricians and pediatricians. Also related to MAS is persistent pulmonary hypertension, as it is thought that over 65% of these hypertension cases are linked to MAS in some way. Generally, MAS should be suspected in any birth where there is meconium staining in the amniotic fluid, especially if the infant is suffering from any kind of respiratory distress. Strictly speaking, MAS is defined as having the presence of meconium below the location of the vocal chords. However, this definition is not what would be considered the clinical definition. More specifically, the clinical definition is meconium-stained amniotic fluid, aspiration of meconium from the tracheobronchial tree (below the vocal chords), and a chest radiograph that is also consistent with a diagnosis of MAS. Not all cases of MAS are severe, as transient respiratory distress is sometimes seen in mild cases. For more severe cases, it is expected that one would see hypoxemia, acidosis, and pulmonary hypertension. Usually, MAS is not seen before 38 weeks of gestation, and any reported fetus suspected of having MAS before 34 weeks of gestation should be investigated carefully to determine whether the problem is actually related to something else instead (pedsccm.wustl.edu, n.d.).

The pathophysiology of MAS is also important to discuss. Meconium comes from the Greek for 'Opium-like' and the term was actually developed by Aristotle, since he believed that it induced the fetus to sleep. Meconium is actually sterile, and is a mixture of various chemicals including swallowed vernix caseosa, mucous glycoproteins, gastrointestinal secretions, bile, minerals, plasma proteins, lipids, and pancreatic and liver enzymes. The dry weight of meconium is made up of 80% mucopolysaccharides. How many enzymes from the liver and pancreas are actually present will vary somewhat with gestational age and there is no actual way to know which one of the many chemicals produces the severe inflammatory response that is seen in even mild cases of MAS. It is thought, however, that bile salts are mainly responsible for this problem. When biopsies are taken, they reveal both macrophage infiltration and polymorphonuclear cell infiltration. Soon, alveolar edema occurs, along with the formation of hyaline membranes. It is possible that the surfactant may be inactivated, but it can also be secreted, and pulmonary hemorrhaging may also occur. Some research samples have also shown the presence of microthrombi located in the pulmonary vasculature, as well as a more pronounced muscularization and thickness of the distal arterioles in the lungs (pedsccm.wustl.edu, n.d.).