Meconium is a viscous green-black substance that consists of denuded intestinal epithelial cells, ingested lanugo hair, mucus, digestive enzymes, bile acids, and water. The term meconium is derived from the Greek word mekonion, which means poppy juice or opium, presumably either because of its tarry appearance or because of Aristotle's belief that it induced sleep in the fetus.
Meconium constitutes the first stool of a newborn infant. The passage of meconium typically occurs within 48 hours after birth; however, it can occur in utero. Intrauterine meconium passage has been linked to fetal hypoxia and is associated with fetal acidosis, abnormal fetal heart tracings, and low Apgar scores. 
In preterm pregnancies, intrauterine meconium passage has been associated with fetomaternal stress and infection.  In term and postterm infants without fetal distress, intrauterine meconium passage may result from normal gastrointestinal (GI) maturation or from vagal stimulation caused by head or cord compression.  Meconium staining of the amniotic fluid occurs in approximately 13% of live births; this percentage increases with increasing gestational age at delivery. 
Meconium aspiration syndrome (MAS) occurs when meconium-stained amniotic fluid (MSAF) is aspirated into the lungs of an infant before, during, or immediately after birth (see the image below). Intrauterine gasping, resulting in aspiration of meconium, has been demonstrated in animal models exposed to hypoxia. [4, 5, 6] MAS occurs in approximately 5% of infants born through MSAF.  Even with modern neonatal intensive care, mortality from MAS remains high, in the range of 3-5%. [1, 7]
Many perinatal risk factors have been associated with meconium aspiration, including placental insufficiency, maternal hypertension, maternal diabetes mellitus, preeclampsia, oligohydramnios, and maternal tobacco use. [3, 8, 9] Perhaps the most significant risk factor, however, is postterm delivery. In a prospective clinical study, a decrease in the incidence of MAS from 5.8% to 1.5% over an 8-year period was attributed to a reduction in births at more than 41 weeks' gestation. 
MAS occurs along a continuum from mild to severe.  Mild MAS is seen in infants born through MSAF who have mild respiratory symptoms; it probably reflects mild parenchymal irritation from aspirated meconium. Moderate MAS presents with more pronounced pulmonary symptoms, including moderately high oxygen requirements and a possible need for mechanical ventilation; it may reflect a more significant meconium load or the aspiration of thicker meconium into the lungs. 
Infants with severe MAS require mechanical ventilation with high settings and may need alternative therapies, such as inhaled nitric oxide (NO) and, possibly, extracorporeal membrane oxygenation (ECMO).  These cases probably represent a combination of meconium aspiration and persistent pulmonary hypertension of the newborn (PPHN). PPHN in these cases is thought to arise from chronic fetal compromise (hypoxia) with resultant pulmonary vascular remodeling.
There is evidence to suggest that chronic in utero hypoxia with resultant PPHN, rather than the aspiration of meconium per se, may be the primary pathologic problem in newborn infants diagnosed with severe MAS. [13, 14]
The indication for selective intubation and tracheal suctioning of an infant born through MSAF includes any infant who is nonvigorous. The Neonatal Resuscitation Program (NRP) defines a nonvigorous infant as an infant who meets 1 or more of the following conditions  :
Depressed muscle tone
Heart rate below 100 beats/min
The consistency of the meconium in the amniotic fluid (thin versus thick) is no longer used to determine the need for tracheal suctioning. 
A contraindication to selective intubation and tracheal suctioning of an infant born through MSAF is apparent vigor. The NRP defines a vigorous infant as one with all of the following  :
Strong respiratory effort
Good muscle tone
Heart rate above 100 beats/min
Several procedures have been used in the past to prevent MAS; none are supported by strong evidence of proven benefit. This article focuses on the 1 preventative procedure that is still supported by the American College of Obstetricians and Gynecologists (ACOG) and the NRP—namely, selective intubation and tracheal suctioning of the nonvigorous infant. [16, 15]
Selective intubation of nonvigorous infants, as opposed to the earlier practice of universal intubation of all infants born through MSAF, is based primarily on a large prospective, randomized, controlled trial involving 2094 apparently vigorous infants born through MSAF, which showed no decrease in MAS with universal intubation compared with no intubation.  A review of 4 other randomized or quasi-randomized trials also found no difference in the incidence of MAS between intubated and nonintubated apparently vigorous newborns. 
Several other preventive measures that were commonly used in the past to prevent MAS, including amnioinfusion and intrapartum oronasopharyngeal suctioning, have now been largely abandoned as a result of findings from randomized, controlled trials.
Amnioinfusion involves the infusion of isotonic fluid (either normal saline or lactated Ringer solution) into the amniotic cavity via a transcervical intrauterine pressure catheter in an attempt to dilute the MSAF. The results of previous trials using amnioinfusion to prevent MAS have been mixed. However, given the heterogeneity of the studies and the small number of patients in each study, results must be interpreted with caution. 
A 2002 Cochrane review concluded that amnioinfusion was effective in reducing the incidence of MAS, especially in centers where perinatal surveillance was limited.  ; however, a 2010 update of this study found that substantive improvements in perinatal outcome were restricted to settings where facilities for perinatal surveillance are limited.  In a multinational, multicenter, randomized controlled trial involving 1998 women with thick MSAF, amnioinfusion had no significant effect on the incidence of MAS or death. 
These findings and the supposition that a large number of infants born through MSAF will have aspirated meconium before amnioinfusion can be performed prompted an ACOG opinion stating that "routine prophylactic amnioinfusion for the dilution of meconium-stained amniotic fluid should be done only in the setting of additional clinical trials."  In this same statement, however, the ACOG noted that "amnioinfusion remains a reasonable approach in the treatment of repetitive variable decelerations, regardless of amniotic fluid meconium status." 
Intrapartum oropharyngeal and nasopharyngeal suctioning with a DeLee suction catheter before delivery of the shoulders, combined with intubation after delivery, has been the standard practice for preventing MAS since the late 1970s. Although routine suctioning of the oropharynx is appropriate, current evidence argues against the efficacy of intrapartum suctioning of meconium with a DeLee suction catheter. 
In a randomized controlled trial involving 2514 full-term women with MSAF, Vain et al did not find intrapartum suctioning with a suction catheter to have a beneficial effect on need for endotracheal intubation, incidence of MAS, need for mechanical ventilation, and neonatal mortality.  The ACOG now recommends that "infants with MSAF should no longer receive intrapartum suctioning. If meconium is present, and the newborn is depressed, the clinician should intubate the trachea and suction meconium from beneath the glottis." 
Because meconium aspiration can occur before delivery as a consequence of chronic asphyxia and infection, perhaps the most important strategy for preventing MAS is good prenatal care, including the detection and prevention of fetal hypoxemia and the avoidance of postterm deliveries.
The equipment required for the management of meconium aspiration includes the following:
Laryngoscope with blade (size 1 for a term infant)
Uncuffed endotracheal tube (size 3.5-4.0 for a term infant)
DeLee suction catheter (12-14 French)
Medical suction device set to a continuous pressure of –80 to –120 mm Hg
Anesthesia is not required for the selective intubation and tracheal suctioning of infants born through meconium-stained amniotic fluid (MSAF).
For intubation and tracheal suctioning, the infant should be placed on a flat surface, ideally under a radiant warmer. The head should be in midline position, with the neck slightly extended into the so-called sniffing position (see the image below). Placing a roll of towels under the infant’s shoulders may help maintain slight neck extension. Do not flex or hyperextend the neck; this raises the glottis above the line of sight and makes intubation more difficult.
Intubation and Tracheal Suctioning
Hold the laryngoscope in the left hand, and stabilize the infant's head with the right hand. Introduce the laryngoscope blade into the right side of the mouth, and sweep the tongue to the left with the laryngoscope blade. To make introduction of the laryngoscope blade easier, try using the right index finger to open the newborn’s mouth. Advance the laryngoscope blade until the tip is positioned on top of the epiglottis or immediately anterior to the epiglottis in the vallecula (see the image below).
Gently lift the laryngoscope blade upward to elevate the epiglottis and tongue to reveal the vocal cords (see the image below). Do not "rock back" with the laryngoscope blade; doing so may damage the alveolar ridge. Identify the vocal cords, and attempt to verify the presence of meconium below the level of the cords. If meconium is present, the posterior pharynx may have to be suctioned with a DeLee suction catheter to improve visualization of the cords.
Maintain direct visualization of the vocal cords as an assistant places the endotracheal tube into your right hand. Advance the endotracheal tube through open vocal cords until the vocal cord guide on the endotracheal tube is at the level of the cords (see the image below). If the vocal cords are approximated, wait for them to open. Do not force the endotracheal tube through closed vocal cords; doing so can damage the cords.
Stabilize the endotracheal tube in position by using an index finger to hold the tube against the hard palate. While holding the endotracheal tube in place, remove the laryngoscope blade. Ideally, the entire process of tracheal intubation should take less than 20 seconds. 
Attach a meconium aspirator, connected to a medical suction device supplying a continuous pressure of –80 to –120 mm Hg, to the endotracheal tube. Occlude the suction-control port on the meconium aspirator to apply suction (see the image below).
While applying suction, withdraw the endotracheal tube over a period of 3-5 seconds. If a substantial amount of meconium is returned by suction, intubation and suction should be repeated until the aspirated material is cleared or the newborn’s heart rate falls below 100 beats/min.
Complications of Procedure
Complications of intubation and tracheal suctioning include the following:
Trauma to lips, alveolar ridge, and tongue
Vocal cord injury or avulsion
Fractures and dislocation of arytenoids
Regurgitation of gastric contents