RESPIRATORY DISTRESS SYNDROME
Respiratory distress syndrome (RDS) presents within 4 four hours of birth, usually in prematurely born infants. It used to be called hyaline membrane disease because hyaline membranes line the terminal airways of infants who are surfactant deficient. The hyaline membranes are formed by coagulation of plasma proteins that have leaked onto the lung surface through damaged capillaries and epithelial cells. The term hyaline membrane disease should only be used if there is histologic confirmation; therefore, the term RDS is now widely used.
EPIDEMIOLOGY
The risk of RDS is inversely proportional to the
gestational age. It is more common in white than black infants and nearly twice
as common in boys as girls. There is also the likelihood of familial recurrence
in a subsequent prematurely born infant. Surfactant protein B deficiency results
in lethal respiratory failure; it has an autosomal recessive inheritance.
Delivery by cesarean section in the absence of previous labor also poses a risk,
particularly if the birth occurs before 37 weeks of gestation. Precipitous
delivery after maternal hemorrhage, asphyxia, or maternal diabetes is
associated with a greater likelihood of RDS, and a second-born twin is at
greater risk than the firstborn. Maternal conditions that are thought to have a
sparing effect on the development of the disease are conditions associated with
chronic intrauterine distress that lead to growth-retarded infants. There is no consensus
on the impact of pro- longed rupture of the membranes; an apparent sparing
effect may be explained by greater use of antenatal corticosteroids. Antenatal
administration of dexamethasone or betamethasone to women in preterm labor
significantly reduces the risk of RDS and neonatal death.
PATHOLOGY
On gross examination, the lungs are found to be liver-
like, and they generally sink in water or formalin. Under the microscope, much
of the lung appears solid because of the tight apposition of most of the
alveolar walls. Scattered throughout are dilated airspaces, respiratory
bronchioles, alveolar ducts, and a few alveoli, some of whose walls are lined
with pink-staining “hyaline” material containing fibrin and cellular debris. The
capillaries are strikingly congested, and pulmonary edema and lymphatic
distension may be present.
Epithelial necrosis in the terminal bronchioles at
sites underlying the hyaline membranes suggests that a reaction to injury has
taken place. Hypersecretion of tracheobronchial mucus is evident, and
reparative proliferation of type II cells is seen in infants who die on the
second or third day of life.
These changes are now rarely seen because prematurely
born infants have usually received prophylactic surfactant (see below).
PATHOGENESIS
RDS is caused by immaturity of the lung with respect
to surfactant synthesis or suppression of synthesis adequate to meet postnatal
demands as, for example, by asphyxia. Surfactant deficiency results in failure
of stabilization of small airways at end-expiration with consequent reduction
of functional residual capacity. Each new inspiration requires the application of sufficient
transpulmonary pressure to reinflate atelectatic air-spaces. A high respiratory
frequency and large applied pressures have to be used to maintain effective
ventilation. Uneven distribution of inspired air and perfusion of nonventilated
alveoli result in poor gas exchange characterized chiefly by hypoxemia. The
infant grunts in an attempt to prolong end-inspiration, a pattern of breathing
that can be shown experimentally to improve alveolar ventilation.
Pulmonary vascular resistance is increased by
vasoconstriction caused by hypoxia, with a resulting increase in right-to-left
shunts through the persistent fetal vascular pathways, ductus arteriosus, and
foramen ovale. The hypoxemia is further aggravated because as much as 80% of the cardiac output
may be shunted past airless lungs.
Wasted ventilation and ineffective perfusion initiate
a train of events that accounts for most of the findings in RDS. Reduced oxygenation
of the myocardium impairs cardiac output and perfusion of the kidneys, whose
ability to maintain acid-base homeostasis is com- promised. Poor perfusion of
peripheral tissues contributes to lactic acidemia and a profound metabolic
acidosis.
DIAGNOSIS
The onset of symptoms is within minutes of birth and
always within hours of birth. Tachypnea, grunting, and indrawing of the sternum,
intercostal spaces, and lower ribs during inspiration are characteristic.
Increasing cyanosis is a notable feature of the disease. In the absence of
treatment with exogenous surfactant, the dyspnea worsens over the next 36 to 48
hours, and the infant becomes edematous. Surfactant synthesis then commences,
and this is associated with spontaneous diuresis.
RADIOLOGIC FINDINGS
The earliest radiographic finding is a fine miliary
mottling of the lungs. The air-filled tracheobronchial tree stands out in relief
against the opacified lung roots, which often obscure the cardiothymic
silhouette. The appearance may change depending on the lung volume at which the
radiograph is taken. A good cry can aerate both lungs, and a deep inspiratory
effort may produce a radiographic picture suggesting minimal disease. The
miliary reticulogranularity of the lung parenchyma is usually present within
minutes of birth.
During the course of the disease, chest radiographs
may show a number of changes, including pulmonary interstitial emphysema,
pneumomediastinum, and pneumothorax. In some infants, recovery is slow, with
infants remaining ventilator and oxygen dependent for weeks and even months.
TREATMENT
Exogenous surfactant therapy is usually given. In many
centers, this is administered within the first few minutes after birth
(prophylactic surfactant). Both synthetic and natural surfactants have been used.
Meta-analyses of the results of randomized trials have demonstrated that
prophylactic surfactant reduces mortality and pneumothoraces. The results of
other trials have demonstrated that it is better to give surfactant
prophylactically rather than selectively (i.e., when RDS has developed) and
early rather than late.
Of primary importance is the need to correct any blood
gas abnormalities. Some babies may only require supplementary oxygen to keep
arterial oxygen tensions at 50 to 70 mm Hg. Others, however, have an associated
respiratory acidosis and need more respiratory support. Some centers prefer to
use continuous positive airway pressure delivered by nasal cannulae, but others
intubate and ventilate. Numerous forms of mechanical ventilation are available,
including positive-pressure ventilation, patient-triggered ventilation,
high-frequency jet ventilation, and high-frequency oscillation. Randomized
trials have been undertaken, but to date, the form of respiratory support with
the least chronic respiratory morbidity has not been identified.
COMPLICATIONS OF RESPIRATORY THERAPY
Infants may develop pulmonary interstitial emphysema
and pneumothorax during the course of ventilatory therapy, although these
complications are less common in infants who have received surfactant therapy.
Infants who survive the first week or so of illness may become respirator and
oxygen dependent. Typically, their lungs undergo a series of changes that are
characterized by air trapping, atelectasis, fibrosis, cyst formation, and basilar emphysema. This condition
was described by
Northway and Rosan in 1967 and called bronchopulmonary
dysplasia (BPD). Nowadays, infants who remain oxygen dependent for more
than 28 days after birth are described as having BPD. The course is chronic,
sometimes lasting months or years. Complete recovery is possible, but death
from intercurrent illness is a continuing threat. At autopsy, the lungs are
found to be heavy, hypercellular, and fibrotic, with squamous metaplasia of even
the small airways. Because the cilia are gone, it is not surprising that
secretions pool; either atelectasis or lobular emphysema is common. BPD has a
multifactorial cause and may occur in very prematurely born infants exposed to
high inspired oxygen concentrations and high airway pressures.
Increasingly, however, it is now appreciated that BPD
can occur in infants who initially had minimal or even no respiratory distress.
Antenatal infection and inflammation contribute to the development of BPD, and
there appears to be a genetic predisposition. Affected infants may experience
chronic respiratory morbidity with lung function abnormalities and exercise
intolerance even as adolescents.
PROGNOSIS
When antenatal corticosteroids and prophylactic
surfactant are used, the overall mortality rate rom RDS has been reduced to between 5% and
10%.
