Neonatal respiratory distress syndrome is the most common cause
of respiratory failure in the first days after birth, occurring
in 1% to 2% of newborn infants. Until about 25
years ago, approximately 50% of infants with this condition
died.1 Over the last three decades, improved methods
of treatment have markedly reduced mortality, and the majority now survive.
Hyaline membrane disease (HMD) occurs mainly in prematurely born
infants and is more common in whites than in blacks. It develops
when an infant attempts to ventilate an immature lung with small
respiratory units that inflate with difficulty and do not remain gas-filled
between respiratory efforts. This behavior is due to rudimentary
alveolarization of the lung and an inadequate amount of pulmonary
surface-active material or surfactant. When surfactant is absent,
the surface tension at the interface between alveolar gas and the alveolar
wall is high, and the lung tends to become progressively atelectatic.
This causes increasingly labored breathing and cyanosis as the volume
of the lung decreases and the infant hypoventilates.
It appears to take only 1 to 2 days following birth for an immature
lung to mature as it responds to the surge of glucocorticoids and β-adrenergic
compounds released by the stress of delivery. Glucocorticoids increase
surfactant synthesis, and β-adrenergic stimulation promotes
its secretion. Also, in 1 to 2 days, structural changes occur in
the lung. Thin-walled respiratory units develop, and the number
of capillaries increases. With these changes, the signs and symptoms
of respiratory distress subside. However, during this brief interval,
lung damage may occur from the combined effects of pulmonary edema,
ischemia, pulmonary air leaks, oxygen toxicity, and injury from barotrauma
if high pressures are used to mechanically assist ventilation. As
a consequence, HMD can result in chronic lung disease that may persist
for weeks or months.
At postmortem examination, the lungs from infants with neonatal
respiratory distress are firm and airless. Atelectasis is striking
on gross inspection; when the lungs are fixed in inflation, only
the airways and a few alveolar ducts are air filled (Fig.
54-1). Diffuse atelectasis and dilated terminal bronchioles
and alveolar ducts lined with a homogenous hyaline-staining material
characterize the microscopic picture (Fig. 54-2).
The hyaline membranes are plasma clots containing fibrin, other
plasma constituents, and cellular debris. The small pulmonary arterioles
appear constricted. There is congestion of pulmonary capillaries
and veins and an increase in pulmonary water with dilation of the
A. Longitudinal section of the
left lung of a 1560-g infant, born at 30 weeks’ gestation,
who died at 2.5 days of age from neonatal respiratory distress syndrome. The
lung was expanded with air to a pressure of 40 cm H2O,
then deflated to 10 cm H2O and fixed with the bronchus
clamped. The airways are distended, and a few of the respiratory bronchioles
are overinflated. ...