CHICAGO -- Meconium aspiration syndrome may just as likely be the result of nonhypoxic mechanisms and preexisting injury as it is asphyxia at delivery, contrary to widespread medical assumptions, according to Dr. Sean C. Blackwell.
In a retrospective study of the most severe cases of meconium aspiration syndrome (MAS) that occurred during a 4-year period at Wayne State University, Detroit, Dr. Blackwell and his associates found virtually no differences between 29 newborns who had normal umbilical pH levels (7.2 or greater) and 19 newborns who had abnormal pH values (less than 7.2).
There is an inverse relationship between the risk of MAS and umbilical pH values at delivery, Dr. Blackwell noted during the annual meeting of the Central Association of Obstetricians and Gynecologists.
But at the same time studies have shown that about half of all newborns with MAS are born with a normal pH at delivery, he said.
In fact, one of the largest series, which came from the University of Texas Southwestern Medical Center at Dallas, showed that 55% of all meconium aspiration cases had a pH value of greater than 7.2, he said.
There are also multiple cases in the literature that describe severe MAS following elective cesarean delivery in which there were normal heart rate patterns and no evidence of fetal compromise.
In his retrospective study, Dr. Blackwell homed in on the most severe cases of MAS with and without acidemia at delivery in an effort to better understand its parthophysiology and clinical manifestations when umbilical pH values are normal.
All babies in the study required mechanical ventilation for a period of at least 48 hours.
"We chose [the 7.2] cutoff value to keep consistent with the literature and in order for us to be able to compare our findings with [those of] other studies of meconium stain and aspiration," Dr. Blackwell explained.
The only difference between the groups: Newborns with abnormal pH levels had significantly higher cesarean delivery rates than did those with normal pH levels (84% vs. 41%), and the majority of these C-sections were done because of fetal indications.
There were no significant differences between the two patient groups in rates of clinical chorioamnionitis or in the incidence of thick meconium vs. light-stain meconium.
Based on neonatal records, 71% of babies in the normal pH group and 56% of those in the abnormal pH group had meconium below the vocal cords at intubation, a difference that was nor statistically significant.
There were no significant differences among the two groups of babies in length of stay in the neonatal intensive care unit, days on the ventilator, or incidence of seizures.
There were no neonatal deaths or cases of intraventricular hemorrhage that were reported.
Transcervical amnioinfusion was performed on all patients who had meconium stains; De Lee suction was done after delivery of the head, and tracheal suctioning, aspiration, or intubation was performed at the discretion of the neonatologist, he said.
One explanation for how MAS could present in the setting of normal pH values is that a hypoxic insult that occurs well before labor leaves enough time for umbilical acid-base levels to normalize while continuing to confer a higher risk of meconium aspiration, Dr. Blackwell proposed.
Or nonhypoxic factors such as inflammation, infection, and thrombosis may be involved.
Clinical and experimental evidence suggests that all of these mechanisms may play a role, he said.
In the case of hypoxia and ischemia, Dr. Blackwell commented at the meeting that it's likely that a combination of pathways are involved.
"There is clear evidence linking intrauterine infection to meconium staining in the amniotic fluid," he said. It is also well known that bile acids are inflammatory, and in autopsy reports of death from MAS, inflammation of the airways is common.
The findings of this latest study raise important issues for obstetricians concerning the "documentation of all labor courses and placental pathology and its interpretation," Dr. Robert J. Carpenter, who is in private practice in Houston, said in a formal review of the study presented at the meeting.
The results drive home the importance of acquiring cord gasses and ensuring placental sufficiency in cases in which an adverse outcome occurs.
"Frequently, in reviewing medical-legal cases, that was not done," Dr. Carpenter commented.
And since many newborns with MAS will not be sick at birth, protocols should be in place to refrigerate the placenta and cord segments for up to 7 days to ensure reliable posthoc evaluation of placental sufficiency and the acid-base status of the fetus.
Meconium is detected in the amniotic fluid of approximately 5%-25% of all deliveries.
Among those, 5%-15% of the babies develop MAS, the severity of which ranges from a transient need for oxygen supplementation during the first few hours after birth to severe and persistent pulmonary hypertension.
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