Definition :
Meconium aspiration syndrome is defined as respiratory distress associated with the passage of meconium around the time of birth, with characteristic radiological changes and without an alternative etiology for the respiratory symptoms. The radiological features consist of areas of atelectasis and consolidation, along with regions of hyperexpansion.
Clinical features :
As mentioned in the definition above, respiratory distress in the presence of evidence of in-utero meconium passage leads to suspicion of MAS in a newborn. The neonate may have tachypnea intercostals retractions, nasal flaring, end-expiratory grunting, cyanosis and a barrel shaped chest suggestive of air trapping. Classically, babies with MAS are post-term, IUGR, and they have yellow-stained nails and umbilical cords. Meconium pigments can be absorbed by the lung and excreted in urine making the colour of urine green within 24 hours after birth. There could also be features suggestive of Hypoxic ischemic insult to other organs especially brain. One should also look for the features of causative and or comorbidities.
Investigations :
The investigations should be aimed at establishing the diagnosis and management of respiratory distress and the complications that arise during the course of the illness.
- Hemogram & Sepsis Screen: Hemoglobin and hematocrit levels must be sufficient to ensure adequate oxygen-carrying capacity. Neutropenia or neutrophilia with left shift of the differential may indicate perinatal bacterial infection.
- Chest X-ray: The severity of chest radiographic abnormalities may not correlate with the severity of clinical disease, as PPHN may not show any specific radiologic characteristics but could be clinically significant.
- Arterial blood gases: Pulse oximetry for SpO2 monitoring and arterial blood gases are necessary for appropriate respiratory and ventilatory management.
- Serum electrolytes:
Differential diagnosis:
The other conditions with respiratory distress with good lung volumes should be excluded. Some of the conditions are other
Aspiration Syndromes, CDH, PPHN, TTNB, Pneumonia, Pulmonary Hemorrhage, Transposition of great vessels etc.
Prevention:
- Antepartum surveillance: To detect episodes of fetal hypoxia and appropriate intervention. Also one should watch for a less frequent but possible maternal listeriosis, which could produce MAS even in a preterm neonate.
- Amnioinfusion: This involves infusing normal saline into the amniotic space, which dilutes the meconium reducing the likelihood of airway blockage (occurrence of MAS increases with increasing consistency of meconium in amniotic fluid) and/or decompresses the umbilical cord. As per the Cochrane systematic review, under standard perinatal surveillance, amnioinfusion was associated with a reduction in the following: heavy meconium staining of the liquor, variable fetal heart rate deceleration and reduced caesarean section rate overall. No perinatal deaths were reported. Under limited perinatal surveillance, amnioinfusion was associated with a reduction in the following: meconium aspiration syndrome, neonatal hypoxic ischemic encephalopathy and neonatal ventilation or intensive care unit admission; there was a trend towards reduced perinatal mortality.
- Intrapartum management: Appropriate intrapartum management is aimed to reduce the likelihood of meconium aspiration in to the airways.
- When the amniotic fluid is meconium-stained, suction the mouth, pharynx, and nose as soon as the head is delivered (intrapartum suctioning) regardless of whether the meconium is thin or thick. Either a large-bore suction catheter (12F to 14F) or bulb syringe can be used. Thorough suctioning of the nose, mouth, and posterior pharynx before delivery of the body appears to decrease the risk of MAS.
- If the infant has absent or depressed respirations, decreased muscle tone, or heart rate less than 100 bpm, perform direct laryngoscopy immediately after birth for suctioning of residual meconium from the hypopharynx (under direct vision) and intubation/suction of the trachea. Accomplish tracheal suctioning by applying suction directly to a tracheal tube as it is withdrawn from the airway.
- Repeat intubation and suctioning until little additional meconium is recovered or until the heart rate indicates that resuscitation must proceed without delay.
- It may be a good practice to give free flow or oxygen on the nostrils while ET suction is being done. Delay gastric suctioning to prevent aspiration of swallowed meconium until initial resuscitation is complete.
- There is evidence that tracheal suctioning of the vigorous infant with meconium-stained fluid does not improve outcome and may cause complications.
- Meconium-stained infants who develop apnea or respiratory distress should receive tracheal suctioning before positive-pressure ventilation, even if they are initially vigorous.
- Maneuvers to prevent meconium from descending in to infant's airway such as cricoid pressure, epiglottal blockage with fingers and thoracic compression are potentially dangerous and should not be used.
- For endotracheal suctioning, one could even directly pass the suction catheter in the trachea with ET tube, if one is experienced, however, one should keep sterility of the device, degree of suction pressure and risk of Hypoxia in mind. There are several innovative ET suction devices. (For details of innovation refer http://neonatologywithvision.com under innovation link.)
Treatment:
Treatment of MAS essentially includes the conventional management, management of complications and maintenance of a normal homeostasis.
Conventional Management:
Postnatal therapy for MAS begins with continuous observation and monitoring of infants at risk. Pulmonary vasoconstriction is associated with MAS and rapid correction of hypoxemia and acidosis is essential.
Routine care:
Therapy of infant with MAS includes careful observation and vigorous treatment of other sequelae of neonatal asphyxia, including thermal instability, hypoglycemia, hypocalcemia, hypotension and decreased cardiac function. Attention needs to be given to specific therapy directed to sequelae of multiorgan hypoxemia and ischemia, including reduced renal function, reduced liver production of clotting factors, hypoalbuminemia, cerebral edema and seizures. In view of likelihood of pulmonary hypertension, tactile stimulation should be minimized. Sedation and muscle relaxation may be used in ventilated babies, though the scientific evidence of their benefit is lacking.
Oxygen:Oxygen is a pulmonary vasodilator. As pulmonary vasoconstriction usually accompanies MAS, there is often an attempt to maintain higher than usual oxygenation levels early in the course of illness. The benefits of this practice have not been scientifically validated so far. Because of the potential for air trapping and air leaks, some advocate increasing FiO2 to 1.0 before assisting ventilation. It may be better to assist ventilation once FiO2 requirements exceed 0.60. Oxygen administered should have high degree of humidification and warm. One should use warmidifiers for safe and effective use of oxygen. One should deliver oxygen by head box with care to give higher FiO2.
Continuous positive airway pressure (CPAP): CPAP may be begun once FiO2 requirements exceed 0.60. Studies have shown improvement in oxygenation in MAS at CPAP of 4-7 cms H20. Major potential complications of CPAP are air trapping leading to air leaks and compromise of venous return to the heart. Some clinicians therefore prefer to move directly to mechanical ventilation without a trial of CPAP.
Conventional mechanical ventilation: Presence of pulmonary hypertension influences the strategy of mechanical ventilation in MAS. These vary from hyperventilation to achieve respiratory alkalosis and pulmonary vasodilation to “Gentle” ventilation allowing higher PaCO2 levels (keeping pH in the normal range) in an attempt to prevent injury from barotrauma and volutrauma. There have been no prospective randomized trials comparing various mechanical ventilator strategies in MAS. Keeping the pathophysiology of MAS in view, we must allow sufficient Te (expiratory time) so as to minimize air trapping. PIP and PEEP should be kept at levels, which permit adequate oxygenation without hyperinflation. Air leaks occur in 10-20% of patients with MAS, being more frequent in ventilated babies. A high index of suspicion for air leaks with availability of equipment for drainage of a pneumothorax is essential. However, as a general rule one should start positive pressure ventilation only when it is essential, as lungs are already hyperinflated in MAS and ready to develop air leaks.
Other Therapies:
Antibiotics: Board spectrum antibiotics are routinely used in the therapy of MAS in infants with respiratory distress and abnormal radiological findings. Their efficacy in MAS remains unproven.
However, it is desired that one of the antibiotics selected is Ampicillin, as it is also effective against Listeria infection, which could be associated with MAS.
Role of antibiotics in management of non-ventilated cases of meconium aspiration syndrome without risk factors for infection is doubtful. Therefore, it is probably worth withholding antibiotics in such cases and simultaneously maintaining close observation of the baby for signs and laboratory parameters of early sepsis, whenever if at all they appear one may immediately exhibit broad spectrum antibiotics.
Surfactant: As per the systematic review, in infants with meconium aspiration syndrome leading to moderate severe respiratory failure, surfactant administration decreases the number of infants treated with extracorporeal membrane oxygenation. Surfactant lung lavage (SLL) followed by surfactant administration: These data suggest that SLL is associated with a rapid and significant improvement in pulmonary mechanics, together with an improvement in oxygenation, in newborns with severe MAS. It is found that the beneficial effects of SLL on pulmonary mechanics persisted for at least 48 h after introduction of the procedure.
High frequency ventilation and inhaled nitric oxide (INO): High frequency oscillatory ventilation (HFOV) has been studied alone and in combination with INO as treatment for MAS. Response rates of infants with MAS to HFOV plus INO were greater than the response rate to either HFOV or INO with conventional ventilation alone.
