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Although
inflammation is widely accepted as a major contributing factor in the
development of BPD, the clinical studies are confusing. For example, earlier
studies reported an increased association of BPD with chorioamnionitis,5,6 but
more recent articles do not show a clear association.7,8 These
discrepancies may partly be due to the different patient populations in
the different studies. Previous studies had documented the presence of
bacterial products in the chorioamnion of a majority of preterm infants,9,10,11 but
less than 2% of these infants had positive blood cultures soon after birth.12 However,
Goldenberg et al (reviewed in this issue) reported the astounding observation
that nearly 25% of preterm infants, less than 32 weeks gestation, were
cord blood culture-positive for Ureaplasma or Mycoplasma species.13 Interestingly,
in the same series of neonates from Alabama, while histologic chorioamnionitis
did not correlate with BPD, infants with a positive cord blood culture
for Ureaplasma or Mycoplasma had an increased incidence
of BPD. Therefore, there may be subsets of patients exposed to chorioamnionitis
at a greater risk for BPD than others.
Other variables in the development of injury
responses in the preterm neonate are the differences in the immune responses
to inflammatory stimuli. Using a preterm sheep model of lipopolysaccharide
(LPS) induced chorioamnionitis, Kallapur et al (reviewed in this issue)
report that: a) chorioamnionitis “matured” the normally immature fetal
monocytes, making them responsive to LPS; and b) while a single exposure
to LPS caused lung and systemic inflammation, repeated exposures to intra-amniotic
LPS blunted inflammatory responses, consistent with tolerance to endotoxin
(LPS). While endotoxin tolerance may blunt injury responses, it may also
increase the risk for nosocomial sepsis and death, as has been reported
in adult patients in an intensive care setting.14 Therefore,
both positive and negative modulation of innate immunity can result from
exposure to inflammatory stimuli, and immune responses can, in turn, influence
inflammation and injury responses. This interaction is visualized below:
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In an elegant study in preterm baboons developing
BPD, Rosen et al (reviewed in this issue) documented precocious maturation
of thymic cells and development of auto-reactive T-cells. Interestingly,
treatment with an antibody against bombesin-like peptide reversed these
T-cell changes and decreased BPD.15,16 Clinical
studies aimed at understanding the role of immune alterations in preterm
infants at risk for BPD are sorely needed.
Mechanical ventilation and resuscitation
injury are important factors in the pathogenesis of lung inflammation and
BPD.7,17 Centers
with liberal use of nasal continuous positive airway pressure (nCPAP) rather
than mechanical ventilation have a lower incidence of BPD.18 Therefore,
the results of the first large randomized study evaluating these modalities
was keenly awaited. Morley et al19 (reviewed
in this issue), report that preterm infants randomized to nCPAP or mechanical
ventilation had an equivalent incidence of BPD or death. While the infants
randomized to nCPAP had increased incidence of pneumothorax, they had less
oxygen use.
Despite many studies, so far only vitamin
A20 and
caffeine21 have
been shown to reduce BPD in large clinical trials. The role for routine
use of iNO to reduce BPD is being evaluated. Presently, using either nasal
CPAP or intubation followed by early extubation are both acceptable treatment
choices for the initial management of the preterm infant. The COIN trial
demonstrated that use of nasal CPAP is safe and feasible.
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