What Is Meconium Aspiration Syndrome?

Meconium aspiration syndrome (MAS) refers to the fetal inhalation of amniotic fluid contaminated with meconium during intrauterine or delivery, which results in airway obstruction, pulmonary inflammation and a series of systemic symptoms. Respiratory distress predominantly occurs after birth. A group of syndromes with other organ injuries are more common in term and postpartum infants.

Basic Information

English name: meconium aspiration syndrome; MAS
Visiting department: Pediatrics
Multiple groups: Term and expired

Common causes: Meconium excretion and inhalation, uneven airway ventilation, chemical inflammation, etc.
Common symptoms: Respiratory distress

Causes of Meconium Aspiration Syndrome

1. Meconium excretion and suction

Hypoxia occurs in the fetus during intrauterine or childbirth. Mesenteric vasospasm of the fetus causes intestinal motility and relaxation of the anal sphincter to excrete meconium. At the same time, hypoxia makes the fetus appear wheezing, and the amniotic fluid mixed with meconium is sucked into the trachea and lungs. The initial breathing after birth further aggravates the obstructive effect of meconium.

2. uneven airway ventilation

The main changes in lung morphology in children with MAS in the early stage were atelectasis, emphysema, and normal alveoli.

3. chemical inflammation

Mostly occurs in the 24-48 hours after birth. Meconium (mainly bile salts in it) can stimulate local bronchi and alveolar epithelium to cause chemical inflammation, leading to diffuse and ventilatory dysfunction, thereby aggravating hypoxemia and hypercapnia. .

4. Pulmonary hypertension

That is, persistent pulmonary hypertension (PPHN) in newborns. Severe cases due to severe hypoxia and mixed acidosis lead to pulmonary vasospasm or pulmonary vascular myocardial hyperplasia (long-term hypoxemia), which increases pulmonary vascular resistance and increases pressure on the right heart, allowing blood to pass through the oval hole that has not been dissected and closed. Heyu artery catheter, right to left shunt occurs at the heart level, further worsening hypoxemia and mixed acidosis, forming a vicious cycle.

In addition, severe cases due to hypoxemia and mixed acidosis are often associated with brain, heart, kidney and other organ damage.

Meconium Aspiration Syndrome Examination

Laboratory inspection

Blood routine, blood glucose, blood calcium and corresponding blood biochemical examination, tracheal aspirate culture and blood culture; blood gas analysis may show PaO2 decrease, PaCO2 increase, and acidosis.

2.X-ray inspection

The enhanced translucency of the two lungs is accompanied by segmental or lobular atelectasis, and there may also be diffuse infiltrates or concurrent mediastinal emphysema and pneumothorax.

3.Color Doppler Ultrasound

The shunt level and direction can be determined, which is helpful for the diagnosis of PGHN.

Clinical manifestations of meconium aspiration syndrome

1. Meconium mixed in amniotic fluid is a prerequisite for the diagnosis of MAS

Including: amniotic fluid mixed meconium can be seen during delivery. Meconium remains on the skin, umbilical fossa, fingernails and toenails. Meconium is contained in the mouth and nasal suction. Meconium can be confirmed in the organ attractant.

2. Respiratory system performance

The severity of the symptoms is related to the physical properties of the inhaled amniotic fluid (suspension or massive meconium, etc.) and the amount. Those who inhale a small amount and evenly mix amniotic fluid may have no symptoms or mild symptoms, and those who inhale a large amount of mixed amniotic fluid may cause stillbirth or death shortly after birth. Respiratory distress manifestations such as shortness of breath (> 60 beats / min), cyanosis, nasal fan movement, and inspiratory tricuspid signs are usually present within a few hours of life. A small number of children may also exhale moaning, increase the anterior and posterior diameter of the thorax, early snoring sounds or thick wet snoring sounds in the two lungs, and medium and fine wet snoring sounds later. If respiratory distress suddenly worsens and the breathing sound on one side is significantly reduced, pneumothorax should be suspected.

3.PPHN performance

Severe MAS is often accompanied by PPHN, which is mainly manifested as severe cyanosis. Its characteristics are: oxygen concentration is greater than 60%, cyanosis is still not relieved, cyanosis worsens when crying, worse or agitated, and the degree of cyanosis is unbalanced with abdominal signs (severe cyanosis, Pulmonary signs are light.) Contraction noise can be heard in the second intercostal space of the left margin of the sternum, and in severe cases, shock and heart failure can occur.

Hair cyanosis is also the main manifestation of severe lung disease and bruising-type congenital heart disease, which can be identified clinically by the following tests. High oxygen test: 15 minutes of inhalation of pure oxygen, such as PaO2 or transcutaneous oxygen saturation (TcSO2), increased significantly compared with the previous, suggesting that it is caused by lung disease. Nitrox-hyperventilation test: pure oxygen holding the ball through the trachea intubation, ventilation at a frequency of 60 to 80 times per minute for 10 to 15 minutes, if PaO2 increased> 30mmHg (4.0kPa) or TcSO2 increased before ventilation> 8%, indicating the presence of PPHN. The difference in blood oxygen pressure before and after the arterial catheter: determine the PaO2 or TcSO2 before the arterial catheter (right radial or temporal artery) and after the arterial catheter (umbilical or lower extremity arteries), such as PaO2 difference> 15mmHg (2.0kPa) or TcSO2 The difference is> 10%, indicating that there is a PGHN shunt in the arterial duct, but there is no significant difference in the PPHN shunt in the oval foramen.

Severe MAS can be complicated by HIE, erythrocytosis, hypoglycemia, hypocalcemia, multiple organ dysfunction, and pulmonary hemorrhage.

Meconium Aspiration Syndrome Diagnosis

According to evidence of contamination with amniotic fluid and meconium in term or expired infants, the fingernails, umbilical cord, and skin of newborns are contaminated with meconium, and breathing difficulties occur early after birth. Meconium is drawn from the trachea and there are typical chest X-rays. A diagnosis can be made during performance. If the gestational age of the child is less than 34 weeks, or when the amniotic fluid is clear, meconium aspiration is unlikely.

Meconium Aspiration Syndrome Treatment

1. Obstetric management and prevention of MAS

For mothers with placental insufficiency, preeclampsia, chronic cardiopulmonary disease, and overdue births, they should be closely monitored. When meconium is contaminated with amniotic fluid during delivery, ear wash or DeLee tube should be used to clean the nose before delivery of fetal shoulders and chest. And oropharyngeal meconium; if the newborn is viable, it can be observed without tracheal intubation, such as "non-vigor" should be tracheal intubation to clear meconium; for children with MAS who are severely affected and within a few hours after birth Both should use routine tracheal intubation to suck up meconium. If the meconium is viscous, it can be washed out with normal saline and aspirated. This method can significantly reduce the severity of MAS and prevent PPHN.

2. Neonatal treatment

(1) Oxygen therapy uses nasal cannula, mask or hood to wash and inhale oxygen according to the degree of hypoxia to maintain PoO260 80mmHg (7.9 10.6kPa) or TcSO290% 95%.

(2) Correction of acidosis. When the airway is usually maintained and oxygen therapy is provided, when the negative value of residual base (BE) is greater than 6, an alkaline medicine is required. The dose can be calculated according to the formula: 5% sodium bicarbonate = BE × weight × 0.5, when the negative value of BE is less than 6, it can be corrected by improving the cycle.

(3) Those who have hypothermia, paleness, hypotension and other shock manifestations while maintaining normal circulation should use plasma, whole blood, 5% albumin or normal saline for dilatation, and intravenous drip of dopamine and / or dobutamine. .

(4) Those who have indications for mechanical ventilation should perform mechanical ventilation, but the air supply pressure and the expiratory membrane pressure should not be too high, so as not to cause lung air leakage. Nor is the use of continuous positive airway pressure.

(5) Restriction of fluid intake is often accompanied by cerebral edema, and a few are accompanied by pulmonary edema or heart failure, so fluid intake should be appropriately restricted.

(6) Antibiotics For patients with secondary bacterial infections, antibiotics should be applied based on blood and tracheal aspirate bacteria culture and drug sensitivity results.

(7) The exact clinical efficacy of pulmonary surfactant in treating MAS has yet to be confirmed.

(8) Pneumothorax treatment should be performed with emergency thoracentesis, and then the amount of gas in the thorax is used to determine the thoracentesis or closed chest drainage.

(9) Others pay attention to heat preservation, meet the requirements of heat cards, and maintain normal blood glucose and blood calcium.

3.PPHN treatment

(1) Etiology treatment.

(2) Alkaline blood is applied at a rapid frequency (> 60 beats / min) for mechanical ventilation, maintaining a pH of 7.45 to 7.55, PaCO230 to 35mmHg (4.0 to 4.7kPa), PaO280 to 100mmHg (10.6 to 13.3kPa), or TcSO295% to 98. %, Increasing blood pH can reduce pulmonary arterial pressure, which is a classic and effective treatment in clinical practice. Intravenous application of sodium bicarbonate may be effective in reducing pulmonary arterial pressure.

(3) Although vasodilator intravenous torasulin can reduce pulmonary arterial pressure, it also causes systemic blood pressure to decrease correspondingly or more severely. The pressure difference is strong without significant change or even increase, which may increase right to left release. Sixthly, it has rarely been used clinically.

(4) Carbon monoxide inhalation (iNO) reduces the pulmonary artery pressure due to the local effect of iNO, but the arterial blood pressure is not affected. Clinical trials in recent years have shown good efficacy in some cases. In addition, high-frequency oscillations in the treatment of PPHN Ventilation and extracorporeal membrane lung (ECMO) have also achieved good results.

Meconium aspiration syndrome prevention

Actively prevent and treat fetal distress and try to avoid overdue delivery. If you find that amniotic fluid is mixed with meconium at birth, you should intubate the meconium before the child begins to breathe.