1. Meconium Aspiration Syndrome (MAS)
By: Nicole Stevens

2. Overview
Meconium is the first substance discharged from the GI tract in the perinatal period.
Rarely found in amniotic fluid before 34wks gestation, likely because its passage may require neural stimulation of a mature GI tract.
Meconium aspiration syndrome (MAS), where the newborn aspirates the meconium containing liquor, therefore, mainly affects term or post term newborns.

3. Overview
In MAS, the meconium is aspirated through the trachea into the bronchial tree, and sometimes into the alveoli.
Sequence of events that may follow:
Obstruction of airways
Gas trapping
Chemical pneumonitis (possibly caused by the bile and liver enzymes in the mec)
PPHN

4. Risk factors
Hypoxia is thought to be the catalyst that leads to passage of meconium into amniotic fluid. Hypoxic episodes may be due to:
Head or cord compression
Maternal risk factors leading to uteroplacental insufficiency (such as, maternal HT, PE, chronic respiratory disease, chronic cardiovascular disease, maternal diabetes, post term, heavy smoking)
MAS is seen most frequently in the term, post term and growth restricted infant.

5. Perinatal events leading to MAS
Physiological passage of meconium, or a stress induced passage (hypoxic event)
This meconium stained liquor can be aspirated during gasping inutero, or by aspiration after birth (in mouth/nose)
Downwards in the respiratory system you can get proximal airway obstruction, peripheral airway obstruction, alveolitis and epithelial damage, then disruption of alveolo-capillary barrier

6. Events after birth with MAS
Airway obstruction can cause air-trapping, and atelectasis, and decreased lung compliance
Meconium in the lung and the increase in exudate from alveolar that it causes can inhibit surfactant production
An hypoxic incident causing the passage of meconium may be ongoing, leading to perinatal asphyxia, increasing likelihood of PPHN and HIE

7. Main effects of MAS There are 3 main effects of MAS
Chemical pnuemonitis
Surfactant dysfunction
Airway obstruction

8. Chemical pnuemonitis
An inflammatory response that occurs when enzymes, bile salts, and fats in meconium irritate the airways and lung parenchyma
Creates a diffuse pneumonia shortly after delivery
Also creates a cascade of events that leads to vascular leakage and injury

9. Surfactant dysfunction
Occurs when free fatty acids in the meconium strip surfactant from the alveolar surface and produce diffuse atelectasis
The role of surfactant is to reduce the surface tension in the alveoli, without it the alveoli walls will collapse in and want to stick together.
Surfactant has a dispersing and spreading effect (like a drop of detergent in oil)

10. Airway obstruction
Can be partial or complete, results from the meconium being aspirated into small airways, can result in atelectasis or overdistention of the alveoli
Overdistention occurs when there is gas trapping behind meconium particles (ball valve effect); causes uneven ventilation with hyperinflation in some areas and atelectasis in others
Leads to ventilation/perfusion mismatch which contributes to hypoxaemia, hypercapnea and acidosis; which in turn leads to pulmonary arteriolar vasoconstriction and pulmonary hypertenison

11. Differential diagnosis
TTN
Surfactant deficiency
Sepsis/pnuemonia
Pneumothorax
CDH
Birth asphyxia with PPHN and/or haemorrhagic pulmonary oedema

12. Characteristics of MAS
Early onset of respiratory distress and hypoxaemia
Most babies with MAS will only need some oxygen therapy and general supportive care
Ventilator support should be commenced if there is ongoing hypoxaemia or respiratory acidosis
Local incidence is around 1.5 per 1000 births.

13. Clinical features of MAS
Early onset of respiratory distress in a baby born through meconium stained liquor
Tachypnoea, cyanosis, variable hyperinflation
Auscultation reveals widespread ‘wet’ inspiratory crackles, occasionally with expiratory noises suggesting ball-valve airway obstruction
Blood gases: hypoxaemia and hypercarbia
Radiologically: typically starts with global atelectasis in early X-rays to widespread patchy opacification accompanied by areas of hyperinflation and/or actelectasis

14. Management of MAS
Administration of O2 is critically important, and in many neonates is all that will be needed for respiratory therapy. The target saturation range is 91 – 95%. The target PaO2 is 60 – 90mmHg.
Consider CPAP if there is moderate respiratory distress and hypoxaemia
Intubate if there is persistent hypoxaemia in 100% oxygen; of if there is respiratory acidosis with pH <7.20

15. Ventilation
If requiring intubation in the birth room it is usually done without drugs
If done after transfer to a nursery, preferable to sedate and muscle relax prior to intubation
May require high PIPs to achieve gas exchange (30 -35cm H2O)
Should be aiming for ↑oxygenation whilst minimizing barotrauma that can lead to air leaks

16. Ventilation cont…
Most evidence favours the use of high PEEP (6-8cm H2O) and a long expiratory time
Ventilation rates of 40 – 60 breaths/min (the lower the rate, the longer the expiratory time)
Inspiratory times of 0.5 – 0.6 secs
If gas trapping occurs, expiratory time can be increased and PEEP decreased
Particularly if there is also PPHN, sedation and muscle relaxation should be maintained if the disease is severe

17. Ventilation cont…
Mode of ventilation will be case dependent; could range from conventional ventilation, to HFOV, to HFJV and in infants with severe MAS, if available, ECMO may be used.
In Melbourne ECMO and HFJV only available at RCH
MAS has been the leading diagnosis for neonates requiring ECMO
Use has decreased with the advent of new therapies
Survival rate 95%, risk of severe disability low, risk of any disability about 17% in survivors (treated with ECMO)

18. Surfactant therapy
Current evidence not showing a consistent benefit, some babies acutely deteriorate when administered
Lung lavage with surfactant is currently being investigated, but not yet able to be recommended
Might be used as a supportive therapy for PPHN

19. Suctioning (birth room and beyond)
Routine intrapartum suction not recommended anymore
Newborns with depressed respiratory effort, poor muscle tone and HR <100 born through mec-liquor are recommended to have direct ET suctioning prior to stimulation
Stomach contents should be evacuated, and a gastric tube should be inserted
In intubated infants the large airways can be lavaged with a small amount of normal saline and suctioned PRN.

20. General care
Cardiovascular support – need IV fluids for volume/hydration, +/- inotropic support
Fluid restriction
Antibiotic therapy until primary bacterial infection can be excluded
IV therapy (10% dextrose, then 10% dextrose with additives, then parenteral nutrition if unable to feed by days) and NBM until the respiratory distress is resolving

21. Complications
PPHN
Air leak (pneumomediastinum, pneumothorax, Cystic lung disease)
Pulmonary haemorrhage
Complications of asphyxia (seizures, oliguria, coagulopathy, thrombocytopenia)

22. If there is PPHN
Correct potentiating factors: hypoglycaemia, hypocalcaemia, hypomagnesaemia, polycythaemia, hypothermia, pain
Increase systemic blood pressure to reduce right to left ductal shunt (by the use of volume expansion and pressor agents)
Improve right ventricular function (by the use of inotropes)
Use of pulmonary vasodilators (inhaled nitric)

23. MAS (day 1)
Meconium aspiration. Serial radiographs in a newborn with uncomplicated meconium aspiration. Radiograph obtained shortly after birth shows ill-defined, predominantly perihilar opacities in the lungs; these are more severe on the right than on the left. The lungs are hyperexpanded. The neonate's heart size is within normal limits. The abnormalities on the initial chest radiograph, aside from the presence of an endotracheal tube and an umbilical artery catheter, are identical to those seen in severe cases of transient tachypnea of the newborn

24. MAS (2 days later)
Meconium aspiration. Radiograph obtained 2 days after that of the previous image shows almost complete resolution of the pulmonary opacities

25. MAS (more severe case)
Meconium aspiration. Radiographic findings in a more severe case of meconium aspiration. This initial radiograph obtained shortly after birth shows patchy, coarse parenchymal opacities and severe hyperexpansion. In addition, pneumomediastinum is present on the right (arrows), outlining the right lobe of the thymus (T).

26. MAS (same baby)
Meconium aspiration. Follow-up radiograph of the patient in the previous image. This image was obtained after placement of bilateral thoracostomy tubes for pneumothoraces and shows pneumopericardium (arrows) and extensive imaging lucencies in the lungs. These findings indicate pulmonary interstitial emphysema.

27. ECMO

28. Management Best defense is good offense
Amnioinfusion??? (infusion of warmed normal saline to relieve oligohydramnios and fetal distress, and to decrease meconium staining)
Birth room management. Have personnel available who have intubation capabilities; suction with size 12F catheter, or through and ET tube/mec aspirator
Assessment for respiratory distress and/or hypoxia after birth and escalate care as required.

29. Nursing care Observations and documentation Blood taking as required
Assistance with procedures as required
Cluster cares to reduce handling
Minimise noxious stimulus (lights, noise, voices)
Parent support, education
Preparation for transfer if requiring tertiary care (consider accommodation needs of parents, handover to transport nurse, paperwork for transfer)
Clean up and preparation for next admission!

30. Interesting facts
Meconium staining of the cord can appear with as little as 1 – 3 hours of exposure
Staining of the fingers and nails occurs within 4 – 6 hours of exposure
Staining of the vernix occurs within 12 – 14 hours

31. REFERENCES http://emedicine.medscape.com/article/410756-overview
International Journal of PediatricsVolume 2012 (2012), Respiratory Support in Meconium Aspiration Syndrome: A Practical Guide. Peter A. Dargaville