Any episode of hypoxia in the unborn baby may result in relaxation of the anal sphincter and passage of meconium into the amniotic fluid. Meconium is the fetal stool that is in the baby's intestines before birth. It is thick, tarry, viscous, and greenish brown to light brown. It contains bile pigment, bile salts, fetal hair , squamous cells, mucopolysaccarides, and cholesterol.
The amniotic fluid is normally clear but when there is meconium in it the color will be greenish or yellowish. When meconium is first passed it will be thick and have particulate matter in the amniotic fluid. The amniotic fluid is being continually renewed. About a third of the water in the amniotic fluid is replaced each hour. This dilutes the meconium over a period of time so that the fluid is stained but the particulate matter has been absorbed.
A good example of how this works is orange juice concentrate. It would be similar to placing orange juice concentrate in a pitcher and then adding some water to it. It would be thick. Then if a little more water was added and a little juice was poured off it would be diluted a little. If the process of adding water and removing was continued gradually the orange juice would be more and more diluted.
The presence of meconium in the amniotic fluid shows that the baby may have been in distress. The lightly stained fluid (usually yellow) indicates that the distress occurred some time in the past. It takes about 4 to 6 hours for the placenta and the baby to become meconium stained after meconium has been passed.
About 10% of pregnancies will have meconium stained fluid. It is not always a sign of fetal distress. It is important to monitor the baby closely and watch for other signs of distress. The presence of meconium in the amniotic fluid is an indication that there may have been an episode of fetal stress at some time prior, but it is not necessarily an ongoing situation. Many babies are born with meconium stained amniotic fluid and have no problems at all.
In the rare case of a baby who has Rh-isoimmunization, the amniotic fluid may be golden-yellow due to an excess of bilirubin. This is not the same as meconium staining.
At term the lungs of the newborn contain about 100 ml of fluid. About one-third of this is expelled during the birth and the rest is absorbed by the lymphatics and blood vessels as air moves into the longs.
Meconium is not prominent in acute emergencies such as prolapse of the umbilical cord, placenta previa, or abruptio placenta. It is seen more commonly in the following situations.
- Chronic stress
- Prolonged gestation
- Intrauterine growth retardation
- Tetanic uterine contractions
- Difficult or traumatic delivery
- Umbilical cord wrapped around the baby's neck
In breech presentations the passage of meconium is caused by pressure of the uterine contractions on the fetal intestines and is not accepted as a sign of fetal distress. It is common for the breech baby to expel meconium before birth.
It is unusual for preterm infants to pass meconium even when stressed. Meconium staining is almost always seen in term or postterm babies.
Meconium Aspiration Syndrome
If amniotic fluid containing meconium is aspirated into the lungs it is called meconium aspiration syndrome (MAS). When particulate meconium is present in the lungs it causes a reaction in which air is inhaled but not exhaled. The alveoli then overdistend and may possibly rupture. The result is a series of complications such as pulmonary interstitial emphysema, pneumomediastinum, and pneumothorax. This leads to further involvement of the central nervous system, kidneys, and metabolism.
Meconium initiates an inflammatory response in the airways known a chemical pneumonitis. The inflammation results in thickening of the alveolar walls which further reduces oxygen diffusion.
Because of the inflammation caused by meconium, secondary pneumonia is common. Meconium is a good breading ground for bacteria.
Symptoms of MAS
The severity of the symptoms depend on the extent of the aspiration of meconium. Often these infants need resuscitation at birth and may need ventilation afterwards. Babies with thick particulate meconium should need careful management. They may appear fine at birth. However if meconium is present in their lungs they may rapidly decline. Medical attention should be sought for these babies.
Symptoms of meconium aspiration syndrome include:
1) Fetal hypoxia in utero prior to birth that is diagnosed by:
* a sudden increase in activity followed by decreased activity, a
* decrease in heart rate
* a weak and irregular heartbeat
* loss of beat to beat variability
* meconium staining of the amniotic fluid.
2) Signs of distress at birth:
* tachypnea greater than 60 per minute
* low heartbeat
* low Apgar scores.
* sternal and intercostal recession
* An overdistended barrel-shaped chest is common.
* Upon auscultation, rales and rhonchi are heard with diminished air movement.
* The baby's liver may be displaced because of overexpansion of the lungs.
* Chest x-ray may show hyperinflation and coarse patchy densities. A pulmonary air leak may be present.
* Biochemical reactions include:
- metabolic acidosis is caused by cardiopulmonary shunting andhypoperfusion
- extreme respiratory acidosis
- extreme hypoxia, even when being ventilated with 100% oxygen.
This can lead to persistent pulmonary hypertension.
MAS can be prevented or minimized through appropriate management. Meconium aspiration is life-threatening, therefore active management is essential. After the birth of the head, the mother is instructed not to push and the baby's oropharynx and nasopharynx are suctioned with a DeLee suction catheter. It is important for this to be done before the birth of the shoulders and chest. If not adequately suctioned on the perineum, the baby's first breath will draw meconium into the lungs. Stimulation of the newborn should be avoided until the baby has been adequately suctioned to minimize respiratory movements.
If the baby is doing well and the amniotic fluid was lightly stained management would be the same as for the normal newborn. If the baby is depressed, the vocal cords should be visualized by laryngoscope. If the cords are stained the trachea should be suctioned. Care should be taken to avoid touching the vocal cords because this may induce laryngospasm, apnea, and bradycardia.
If there was thick meconium in the amniotic fluid it may need to be lavaged out of the trachea with normal saline through an endotracheal tube. Positive pressure ventilation should be avoided until the infant is suctioned because ventilation will push the meconium further into the lungs.
Controlled ventilation with high concentrations of oxygen may be necessary. The minimum amount of pressure necessary should be used to prevent pnuemothorax. Unfortunately, high pressures are often necessary. Blood pressure and pulmonary blood flow must be maintained. Medications such as tolazoline (Priscoline) or isoproterenol (Isuprel) may be used medically to increase pulmonary blood flow.
Treatment may also include chest percussion and drainage. This can be done by placing the baby face down. A slapping motion is used on the baby's back with a cupped hand or with the fingertips in each quadrant. The baby's head is placed downward to facilitate drainage. Steam can also help to loosen and release any particulate matter.
Antibiotics are usually given prophylactically. These baby's are very difficult to oxygenate which causes a high mortality rate.
In summary, babies with meconium staining usually do fine as long as the birth is managed properly. Meconium staining is fairly common and it is not a reason for alarm. Complications usually don't occur unless there is thick particulate meconium.
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