Managing meconium aspiration

Managing meconium aspiration

By Mamta Fuloria, MD, and Thomas E. Wiswell, MD

The baby born through meconium-stained amniotic fluid is at risk for acute respiratory distress, long-term sequelae, or even death. Early identification and treatment of sick infants limits complications.

Pediatricians attending a newborn who is born through meconium-stained amniotic fluid (MSAF) may well be confronted with an infant in distress. All general pediatricians, especially those who do not have immediate access to a neonatologist, need to be able to identify meconium aspiration syndrome—a potentially fatal condition—and know how to manage it. Familiarity with strategies to prevent or minimize illness also is important.

Dangers of meconium aspiration

MSAF is present in 8% to 19% of all term deliveries, and the risk increases with advancing gestational age.^1­4 MSAF is one-and-one-half times as likely in births to African-American women than in those to Caucasian women.⁵ The presence of MSAF may be associated with adverse fetal and neonatal outcomes, including acute respiratory distress, long-term pulmonary and neurologic sequelae, and death. Meconium aspiration syndrome (MAS) develops in 5% to 33% of all infants born through MSAF, and more than a third of these infants require mechanical ventilation.³ Approximately one third of infants with MAS have persistent pulmonary hypertension of the newborn (PPHN).⁶ A large proportion of the disturbingly high mortality associated with MAS (4% to 19%) is probably attributable to PPHN. Though most infants with MAS survive, some have abnormal pulmonary function both during infancy and in later childhood, including increased functional residual capacity and airway reactivity.^7,8

Infants born through MSAF of a moderate to thick consistency are seven times more likely than other infants to have neonatal seizures, presumably secondary to hypoxic events, and are five times more likely to have hypotonia.⁹ Babies born through MSAF who have a low Apgar score also appear to be at increased risk of cerebral palsy.¹⁰

The accompanying box, "How MAS develops," describes how meconium moves from the fetal gastrointestinal tract to the mother's uterus and then is aspirated into the infant's lungs.

How MAS develops

Meconium is primarily water (72% to 80%) and also contains gastrointestinal secretions, bile, mucus, pancreatic juice, cellular debris, swallowed amniotic fluid, vernix caseosa, and lanugo. In the infant with MAS, meconium moves from the infant's gastrointestinal tract to the mother's uterus, where it is aspirated into the infant's lungs. Although meconium may be present in the gastrointestinal tract as early as 10 to 16 weeks' gestation, the substance rarely is passed into the mother's uterus before 37 weeks' gestation. Passage initially is inhibited by the fetus's lack of strong intestinal peristalsis, along with the presence of a tonically contracted anal sphincter and a terminal cap of viscous meconium. Levels of motilin, a promotility hormone, are lower in preterm neonates than in those who are born at or after term, suggesting that maturation of the gastrointestinal tract plays a role in the passage of meconium.¹¹ Other research indicates that the amniotic fluid swallowed in early gestation contains small amounts of surfactant and is therefore very viscous. This may help inhibit meconium passage in utero.¹²

Meconium passage may also be associated with antenatal or intrapartum stresses such as hypoxemia or acidemia.⁴ Some observers believe that fetal hypoxia may cause the anal sphincter to relax, allowing passage of meconium. A distressed, gasping fetus is most apt to aspirate meconium, and this phenomenon is more likely in term and postterm neonates than in those who are preterm. Term and postterm neonates are more likely to gasp than preterm babies because of their maturity. In addition, preterm infants are not likely to pass meconium in utero.

Respiratory symptoms can be caused by aspiration of meconium in utero or at the time of birth, or by alterations in the pulmonary vasculature secondary to asphyxia or the meconium itself. Aspiration of meconium can lead to partial or complete airway obstruction, followed by atelectasis with ventilation-perfusion mismatch, air trapping with increased functional residual capacity, and air leaks.^13,14 Meconium in the airways induces an inflammatory response characterized by microvascular endothelial damage, which results in pulmonary edema, cellular necrosis, and chemical pneumonitis.¹³ In vitro studies show dysfunction of both alveolar macrophages and cord blood neutrophils.

Meconium can directly inhibit surfactant's ability to reduce the surface tension of pulmonary fluids, a dose dependent effect.^{15, 16} One study shows that meconium mixed with a cow-derived surfactant alters the surfactant's structure, so that spherical lamellar and folded linear structures replace the loosely stacked layers. When this happens, the surface tension of the mixture increases significantly.¹⁶ High concentrations of meconium are also toxic to rat type II alveolar cells, and studies with rats suggest that the presence of meconium either decreases production of surfactant proteins or increases their degradation.¹⁷

As many as one third of infants with MAS develop pulmonary hypertension, which may be related to acute or chronic hypoxia in utero or to acidosis, hypoxia, or hypercapnia associated with meconium aspiration. The figure below depicts the pathophysiology of meconium aspiration and MAS.

Identifying MAS

Whether respiratory complications develop in an infant born through MSAF depends on the degree of hypoxia in utero and on the consistency of the meconium. Infants born through thick MSAF, as well as those whose respiration is depressed at birth, are at greatest risk for developing MAS. Neonates who are apparently vigorous at birth, particularly if born through thin MSAF, are at low risk for developing MAS. In a large multicenter clinical trial, 3% of apparently vigorous infants born through MSAF developed MAS.¹⁸ When the MSAF was thin, incidence of MAS was about 1% compared with 7% in infants born through thick MSAF. Incidence of all respiratory complications in the trial, including transient tachypnea of the newborn, sepsis and pneumonia, pulmonary edema, and pneumothorax, was significantly higher in infants born through thick MSAF (15%) than in those born through thin MSAF (2%). About half of the infants with MAS required mechanical ventilation.¹⁸

Infants with MAS are often postterm. In addition to showing signs of respiratory and neurologic depression at birth, they may have respiratory distress with tachypnea, grunting, flaring, retractions, and cyanosis. Infants with MAS may develop pulmonary hypertension, with significant hypoxemia, hypercapnia, and right-to-left shunting of blood at either the ductus arteriosus or the foramen ovale. Typical findings on chest radiography include diffuse patchy infiltrates, areas of consolidation, and hyperinflation (Figure 1). Other radiographic findings may include pneumothorax and cardiomegaly, if perinatal hypoxia was significant.

Based on clinical presentation and chest radiography, it may be almost impossible to differentiate MAS from sepsis or pneumonia. Table 1 lists the differential diagnosis.

Management and monitoring

The management of infants with severe MAS can be extremely difficult. Since avoiding morbidity and mortality depends in part on alleviation of hypoxemia, the rapid identification of sick infants and instigation of treatment—which is largely supportive—are crucial.

Umbilical arterial and venous catheter placement helps monitor arterial blood gases and cardiovascular function and provides central venous access for administration of drugs and nutrition. Serum electrolytes should be closely monitored and attention paid to fluid intake and urine output.

Antibiotic administration. Given possible confusion of MAS with sepsis, all infants diagnosed with MAS should probably initially be treated for possible sepsis, at least until the blood culture is negative for two to three days. Ampicillin and gentamicin are adequate to treat most gram positive and gram negative bacteria that may be present in the immediate neonatal period.

Ventilatory support. Depending on the severity of respiratory distress, required ventilatory support ranges from supplemental oxygen, using an oxygen hood or nasal prongs, to mechanical ventilation, either conventional or high frequency.

Surfactant therapy is not yet approved by the Food and Drug Administration (FDA) specifically for use in MAS, but exogenous surfactant (beractant) 6 mL/kg, may help improve oxygenation.¹⁹

Echocardiography is often necessary to evaluate cardiac contractility and ventricular filling, as well as to look for evidence of pulmonary hypertension. Treatment of pulmonary hypertension can be frustrating because infants with MAS can be severely hypoxemic and hemodynamically unstable. Management includes correction of acidosis and maintenance of adequate blood pressure using ionotropic or chronotropic drugs. Reduced intravascular volume, as evidenced by hypotension and decreased ventricular filling on echocardiography, needs to be corrected. In some instances, inhaled nitric oxide (iNO), a experimental selective pulmonary vasodilator, improves oxygenation. The FDA recently approved this therapy for treatment of term infants with hypoxic respiratory failure associated with clinical and echocardiographic evidence of pulmonary hypertension. Infants with severe hypoxemia not responsive to supportive management and iNO therapy, and infants who are hemodynamically unstable, may benefit from extracorporeal membrane oxygenation (ECMO).

Table 2 lists suggested studies and monitoring techniques in infants with MAS.

Preventing aspiration: A worthy goal

Several strategies for preventing meconium aspiration or lessening its effects have been proposed, but only a few have undergone sufficient study to be proven effective.

Prenatal strategies. Several studies have evaluated amnioinfusion (AI) for preventing MAS. AI is based on the principle that increasing the volume of amniotic fluid will dilute meconium, decreasing its toxicity. Furthermore, if the level of amniotic fluid is low (oligohydramnios), an increase in fluid volume might relieve cord compression, which could prevent fetal hypoxia and acidosis, further meconium passage, gasping, and meconium aspiration. In the largest retrospective study evaluating the efficacy of AI, however, investigators were unable to show any difference in neonatal outcomes between infants born to women who had received AI for MSAF and those born to women who did not. Moreover, investigators noted a higher incidence of fetal heart rate abnormalities, instrument-assisted deliveries, and endometritis in women who received AI.²⁰ Given these and other findings so far, AI must be considered an experimental therapy for preventing MAS.

Intrapartum strategies include several options with demonstrated efficacy.

•Oropharyngeal suctioning at the perineum when MSAF is present has been shown to decrease the incidence of MAS.²¹ This technique, which is recommended by the American Academy of Pediatrics and American Heart Association (AAP/AHA), is associated with improved Apgar scores at one and five minutes, fewer abnormalities on chest radiography, and less need for mechanical ventilation than in infants born through MSAF who were not suctioned.^22,23 A bulb syringe and a suction catheter give similar results.

•Some authors recommend cesarean sections for infants surrounded by MSAF. They speculate that preventing the stresses of vaginal delivery may decrease the risk of meconium aspiration. No data support this strategy.

•Other authors recommend administering strong sedatives, such as morphine or paralytics, to the mother. They suggest that since these agents cross the placenta, they prevent the fetus from gasping and inhaling MSAF. No clinical investigations have shown this strategy to be beneficial, and we do not recommend it.

Postnatal strategies in the infant born through MSAF include several that appear promising but require more study as well as some that are dangerous to the newborn and should not be used.

•Tracheal suctioning has generated considerable controversy. Based on existing data, 1992 AAP/ AHA guidelines recommended tracheal suctioning of infants with depressed respiration and those born through thick, particulate meconium.²³ A recent multicenter clinical trial was unable to demonstrate any difference in the incidence of MAS or other respiratory complications in apparently vigorous infants who underwent intubation and tracheal suctioning compared with those who received routine delivery room care (expectant management). Only a few children (3.8%) experienced complications of intubation, and all the complications were transient.¹⁸ Based on these findings, routine tracheal suctioning seems to be unnecessary. We do recommend intubation and tracheal suctioning of infants born through MSAF whose respiration is depressed and who require positive pressure ventilation soon after birth, or who develop respiratory distress after the initial assessment. The intubation should be performed rapidly since prolonged attempts could be damaging.

•Surfactant replacement therapy with high concentrations of surfactant overcomes the inhibition of surfactant activity associated with meconium.^15,16 In a randomized clinical trial, infants with MAS treated with 6 mL/kg of beractant within six hours of birth showed improved oxygenation, decreased incidence of air leaks, and reduced likelihood of requiring ECMO. Of particular significance, respiratory status improved only after the second dose of surfactant, suggesting inadequate initial dosing.¹⁹ Further research is needed to establish the optimal dose(s), timing of administration, type of surfactant, and mode of delivery. Based on results of the trial cited above, we currently recommend administering beractant at 6 mL/kg every six hours for four doses. A newer synthetic surfactant (Surfaxin) contains a mixture of phospholipids and KL4 peptide, a synthetic peptide with physicochemical properties of surfactant protein B (SP-B). Since increasing concentrations of SP-B in surfactant have been shown to confer enhanced resistance to inactivation by protein, this new product may prove to be beneficial.

•Lung lavage with exogenous surfactant replacement is a promising therapy for infants who have aspirated meconium. Ideally, the surfactant dose for infants with MAS should be based on the amount of meconium aspirated and the degree of surfactant inactivation. This is possible only with a bronchoalveolar lavage, followed by evaluation of surfactant function in the lavage fluid.

Bronchoalveolar lavage is thought to remove meconium and inflammatory mediators, decreasing pulmonary symptoms caused by airway obstruction and inflammation. Improved aeration of the lungs also permits delivery of more homogenous surfactant and decreases ventilation-perfusion mismatch. In animal models, lavage of the airways with surfactant improves oxygenation.²⁴ A pilot clinical trial demonstrated improved oxygenation and decreased duration of mechanical ventilation after lung lavage with a synthetic surfactant.²⁵ Until other randomized clinical trials are completed, surfactant lavage for the management of infants with MAS should be considered an experimental therapy.

•Glucocorticoids might decrease the inflammation in the lungs associated with meconium aspiration. A randomized clinical trial showed improved oxygenation and ventilation and decreased duration of mechanical ventilation in infants with MAS who were treated early with dexamethasone.²⁶ Though these results are promising, the role of steroids in the treatment of MAS needs to be investigated further.

•iNO, a selective pulmonary vasodilator, was recently approved by the FDA for treatment of term infants with hypoxic respiratory failure associated with clinical or echocardiographic evidence of pulmonary hypertension. It is believed that NO decreases extrapulmonary right-to-left shunting and ventilation-perfusion mismatch by dilating the blood vessels in well-aerated lung fields. Its efficacy depends on adequate delivery of the agent to the target resistance vessels within the lung. Some clinical trials have indicated that the response to NO is disease-specific, with more consistent improvement in oxygenation in infants with extrapulmonary right-to-left shunting than in other infants.

•Liquid ventilation with perfluorocarbons appears promising. Meconium- stained lambs with respiratory distress improve rapidly after treatment with partial or total liquid ventilation.²⁷ It may be that the unique ability of perflubron to dissolve large quantities of oxygen and carbon dioxide at atmospheric pressure can be useful for ventilating neonates with MAS, providing much-needed time for the lungs to recuperate from the insult.²⁸

•Three therapies are potentially dangerous and should not be used: (1) thoracic compression performed by encircling the child's chest with one's hands and applying pressure so as to prevent meconium aspiration before tracheal suctioning, (2) application of cricoid pressure to block the passage of MSAF into the thorax, and (3) manual epiglottal pressure in which one inserts one to three fingers into the hypopharynx and pushes the epiglottis over the glottis to prevent fluid entering the trachea. None of these maneuvers has undergone the scrutiny of clinical trials.

Figure 2 suggests a management plan for minimizing meconium aspiration and its effects.

Perinatal asphyxia—a dangerous complication

MSAF and MAS are often associated with suspected perinatal asphyxia, whose effects include central nervous system dysfunction, congestive heart failure and cardiogenic shock, and persistent pulmonary hypertension. The neurologic status of neonates suspected of having perinatal asphyxia should be closely monitored. According to the 1997 AAP/American College of Obstetricians and Gynecologists Guidelines for Perinatal Care, the term asphyxia applies only to an infant who meets all these criteria:

• Profound metabolic or mixed acidemia (pH <7.00) on an umbilical cord artery blood sample, if obtained

• Apgar score between 0 and 3 for longer than five minutes

• Neonatal neurologic abnormalities, such as seizures, coma, or hypotonia

• Multisystem organ dysfunction (cardiovascular, gastrointestinal, hematologic, pulmonary, or renal).²⁹

A pathologist should examine the placenta and umbilical cord of any infant with perinatal asphyxia to identify any placental or cord-related cause for fetal hypoxemia. Urine output as well as serial blood urea nitrogen and creatinine should be closely followed. In addition, serial liver function tests and complete blood counts (platelet, total lymphocyte, and absolute nucleated red cell count) should be performed. Asphyxiated infants are at risk for early hypoglycemia and hypocalcemia. Finally, if neurologic findings are abnormal, neuroimaging (computed tomography or magnetic resonance imaging) should be performed within two to seven days of birth.

A complex disorder

MAS is a complex disorder resulting in a spectrum of clinical findings from mild respiratory distress to pulmonary hypertension and death. Although our limited understanding of the syndrome's pathophysiology hampers development of new treatment strategies, oropharyngeal suctioning at the perineum helps prevent problems in infants born through MSAF. Intubation and tracheal suctioning are required only if the infant is not vigorous or develops respiratory distress after the initial evaluation.

Management of infants with MAS is mainly supportive. Exogenous surfactant improves oxygenation, though optimal dose, timing, and route of administration need further evaluation. The role of other therapies, including surfactant lavage, steroid use, NO inhalation, and liquid ventilation also merits more investigation.

References

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25. Wiswell TE, Surfaxin lavage for MAS trial group: Bronchoalveolar lavage with dilute Surfaxin (KL4-Surfactant) for the management of the meconium aspiration syndrome. Pediatr Res 1999;45(4):326A

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27. Foust R III, Tran NN, Cox C, et al: Liquid assisted ventilation: An alternative ventilatory strategy for acute meconium aspiration injury. Pediatr Pulmonol 1996;21:316

28. Fuloria M, Ying W, Brandt ML, et al: Effect of meconium- saline suspensions on the surface properties of perflubron. J Appl Physiol, in review 1999

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DR. FULORIA is Assistant Professor, Department of Pediatrics, Wake Forest University School of Medicine, Winston-Salem, NC.

DR. WISWELL is Professor of Pediatrics, Thomas Jefferson University, Philadelphia, PA. He is working with Discovery Laboratories, Inc., Doylestown, PA, on development of a surfactant (Surfaxin) for meconium aspiration syndrome.

 

Mamta Fuloria,Thomas Wiswell. Managing meconium aspiration. Contemporary Pediatrics 2000;4:125.