++
Birth involves changes in numerous organ systems in the infant,
but only a few require relatively rapid postnatal adjustment. The
most important of these birth-related changes involve transitional
function in the respiratory, cardiovascular, thermoregulatory, and
metabolic systems.
++
Within minutes after birth, regular breathing efforts are sustained,
lung compliance improves, airway resistance diminishes, and a functional
residual capacity is established. With these changes, gas tensions
of both oxygen and carbon dioxide in the blood approach those expected
in the mature postnatal infant. The physiological triggers for establishing regular
breathing efforts are probably the loss of the umbilical circulation
and the increase in systemic oxygen content.1-3
++
A critical element necessary for pulmonary adaptation after birth
is surfactant. Surfactant derives its importance from its ability
to lower the surface tension of the alveolar lining layer, the shallow
pool of liquid that overlies the cells of the distal airspaces.
Without a very low surface tension at end-expiration, the airspaces
become atelectatic with exhalation. It is a deficiency of surfactant
that underlies the pathophysiology of the respiratory distress observed
after premature birth.4
++
Surfactant is synthesized in the type II cells that line the
distal airspaces. It is composed primarily of lipids, including
the disaturated phospholipids, which are responsible for the relevant
biophysical properties. Associated with these lipids are a small
but critical collection of proteins, the surfactant-associated proteins
A, B, C, and D. Proteins B and C are water-insoluble, hydrophobic
proteins that are closely associated with the lipid component of surfactant.
The pivotal role of these latter 2 proteins is exemplified by respiratory
distress syndromes that result from their genetic alteration. Specifically,
the absence of surfactant protein B results in severe respiratory
failure unresponsive to routine supportive care.5
++
During the latter stages of intrauterine development, the enzymes
important in surfactant production increase and result in an increase
in intracellular surfactant content. At the time of birth, much
of this stored surfactant is released into the alveolar space. Surfactant
release is stimulated by lung inflation and the increase in circulating
catecholamine concentration that accompanies birth. The premature
infant has less surfactant available for extrusion into the airspace
at the time of birth than does a term infant.6
++
In addition to an adequate concentration of surfactant, postnatal
lung adaptation also depends on clearance of fluid from the lumen
of the lung. Before birth, the potential airspaces are filled with
liquid, and failure to remove this liquid after birth results in
respiratory difficulty and hypoxemia. Fetal lung liquid is produced
by a process that is dependent on the secretion of Cl ions across
the respiratory epithelium into the lung lumen. The importance of
fetal lung liquid derives from its ability to act as a dynamic template
around which the lung develops in utero. If the fetal airspaces are
inadequately distended with liquid, lung growth is stunted and lung
cell differentiation is disturbed.7
++