Meconium-stained amniotic fluid (MSAF)

Posted by e-Medical PPT Thursday, July 5, 2012
Meconium aspiration syndrome (MAS) is a respiratory disorder in an infant born through Meconium stained amniotic fluid  whose symptoms cannot be otherwise explained.
Cleary&Wiswell proposed severity criteria to define MAS:
Mild: requires <40%O2 for <48hrs
Moderate: >40%O2 for >48hrs, no air leak.
Severe: assisted ventilation for >48hrs often with PPH.

MSAF observed in 13% of all live births.
MAS occurs in 5% of newborns delivered through MSAF.
25,000 to 30,000 cases and 1,000 deaths related to MAS annually in US.
More frequently in infants who are postmature and small for gestational age.
Decline from 5.8% to 1.5% (1990–1997), attributed to a 33% reduction in the incidence of births >41 weeks gestation.

Physiology
The passage of meconium from the fetus into amnion is prevented by lack of peristalsis (low motilin level), tonic contraction of the anal sphincter, terminal cap of viscous meconium.
MSAF may be a natural phenomenon that doesn’t indicate fetal distress but mature GI tract in post term fetus with increased motilin level.
Vagal stimulation by cord or head compression may be associated with passage of meconium in the absence of fetal distress.

Risk factors for MSAF
Maternal HT
Maternal DM
Maternal heavy cigarette smoking
Maternal chronic respiratory or CV Dx
Post term pregnancy
Pre-eclampsia/eclampsia
Oligohydramnios
IUGR
Poor biophysical profile
Abnormal fetal HR pattern

Pathophysiology
The pathophysiology of MAS is complex.
Intrauterine fetal gasping, mechanical airway obstruction, pneumonitis, surfactant inactivation, and damage of umbilical vessels: all play roles in the pathophysiology of meconium aspiration.
There is also a strong association between MAS and persistent pulmonary hypertension of the newborn (PPHN).
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