Although the exact
pathogenesis of the syndrome is not clear, the evidence
indicates that it is related to or caused by immaturity of the lung and
inadequate release or storage of surfactant by type II pneumocytes.
At term the
fetal lungs are anatomically mature and functionally prepared to adapt for
the transition from an intrauterine to extrauterine environment.
alveoli are fully formed by the 25th week, and by the 31st
week the type II pneumocytes already produce adequate amounts of
surfactant, which still differs from that of the term newborn.
bronchial and pulmonary circulations are characterized in utero by
and low flow.
This situation leads to a high pulmonary resistance, which exceeds the
Most of the blood circulates through the placenta,
and only 5% to 10% perfuses the lungs.
control of respiration is effectively established long before birth.
External compression of the thorax during passage through the vaginal
canal ejects some of the amniotic fluid from the lungs and also leads to
subsequent recoil of the chest wall.
These actions facilitate the first
active inspiratory movement and the entry of air into the lungs, which
inflates the alveoli.
The amniotic fluid is exhaled and resorbed, leaving
the alveoli coated with surfactant.
The sequence of events decreases the
external compression of the capillaries in the alveolar walls, and the
pulmonary blood flow suddenly rises.
Increased alveolar oxygen tension
further expands the precapillary vascular space through dilatation of
arteries and arterioles.
Decreased vascular pressure in the entire
pulmonary circulation, accompanied by an enormous influx of blood from the
right ventricle, further facilitates the absorption of alveolar fluid and
results in increased lymph flow and increased flow of oxygenated blood
into the left atrium.
These events, followed in early infancy by the
closure of the ductus arteriosus, lead to the transformation of the fetal
high-resistance, low-flow pulmonary
circulation into the adult
low-resistance, high-flow circulation.
in lung maturation and in the various transitions from fetal to adult
pulmonary circulation, mechanical factors, and disturbances in the neural
control of respiration all cause respiratory distress in newborn.
the identifiable causes are incidents affecting the respiratory center in
the central nervous system.
For example, oversedation of the mother during
the delivery may affect the fetal brain.
Traumatic brain injury of the
fetus at birth, with bleeding into or ischemic necrosis of the respiratory
centers, prevents normal respiration.
Asphyxia may be due to mechanical
factors, birth trauma, or umbilical cord strangulation.
disorders in the mother- for instance, diabetes mellitus-may impair
Blockage of the air passages due to particulate matter, blood
clots, or aspirated meconium from the amniotic fluid can also cause
However, more common than all other forms of
respiratory distress is the idiopathic syndrome, presumed to be due to functional and anatomical immaturity
of the fetal lungs.
idiopathic respiratory distress syndrome of neonates
is usually a
disease of preterm, appropriate for gestational age (AGA) babies.
Most infants appear normal and have a good Apgar score.
However, some were born in protracted labour, showed intrapartum asphyxia, and needed resuscitation.
respiratory effort is the first symptom and is easily recognizable 30 to
60 minutes after birth.
The infants show forceful intercostals retraction
and use accessory neck muscles.
A loud expiratory grunt is produced by the
forceful passage of air through the partially closed glottis.
closure of the glottis represents a Valsalva maneuver to maintain the
positive end-respiratory pressure and keep the alveoli open.
However, as the
compliance of the lungs diminishes, this maneuver becomes less efficient
and the baby cannot compensate for the respiratory insufficiency by
closing the glottis.
To meet for the demand for oxygen, the
respiratory center increases the number of respirations to more than 100
per minute, and the accessory thoracic respiratory muscles are used to
maintain the terminal airways open by forcing the expansion of lungs
through negative intrathoracic pressure.
A characteristic seesaw respiration
reflects the diminished compliance of the lungs , which forces the
protrusion of the diaphragm into the abdominal cavity and, thus, the
bulging of the abdomen with each inspiration.
Finally, the sternum and the
anterior portion of the ribs collapse under the negative intrathoracic
The skin is pale because of peripheral
vasoconstriction, but the internal organs are congested with unoxygenated
Edema, most prominent on the face, palms, and soles, becomes
The chest radiograph shows a characteristic
“ground glass” granularity, with prominent bronchi extending into
In terminal stages the fluid-filled alveoli
contribute to the complete “whiteout” of the chest.
The infant becomes
progressively obtunded and flaccid and experiences long periods of apnea
interspersed with periods of irregular breathing.
Many infants are saved
by intensive care and assisted ventilation, but the mortality is still
Type II pneumocytes need 3 to 4 days to become fully functional.
If the child survives these first few days,
it often recovers. Recovery, unless there are major complications, is
usually quite satisfactory.
After protracted respiratory distress and
prolonged treatment with a respirator, a few babies develop chronic
A basic defect causing the idiopathic
respiratory distress syndrome in preterm babies is immaturity of the
lungs, particularly type II penumocytes.
Qualitatively and quantitatively,
fetal surfactant is less efficient than adult surfactant in lowering the
alveolar surface tension and keeping the alveoli open.
compliance is low, the critical negative pressure needed to allow influx
of air into the lungs cannot be attained.
The collapse of alveoli (atelectasis)
not adequately coated with surfactant reduces the pulmonary surface,
allowing exchange of gases only through the walls of alveolar ducts and
terminal bronchioles- structures that are not suitable for that purpose.
Anoxia and hypercapnia cause acidosis, leading to peripheral vasodilatation
and pulmonary vasoconstriction.
This situation, in turn, leads to the
reestablishment of a partial fetal circulatory pattern.
shunting of unoxygenated blood through the ductus arteriosus and foramen
ovale further contributes to hypoperfusion of the lungs, jeopardizing the
respiratory oxygen supply even more.
Hypoxia adversely affects pulmonary cells,
and necrosis of endothelial, alveolar, and bronchial cells takes place.
Vascular disruption causes transudation of plasma into the alveolar
spaces and layering of fibrin (”hyaline membranes”) along the surface of
alveolar ducts and respiratory bronchioles partially denuded of their
normal cell lining.
This in turn further impedes the passage of oxygen
from the alveolar spaces across the respiratory surface into the pulmonary
Moreover, extravasation of blood into the
respiratory passages, combined with the collapse of the alveoli, further
contributes to the consolidation of the lungs.
In terminal stages air is found only in
bronchi and dilated bronchioles, and the rest of the lung is consolidated
RDS is associated with a deficiency of
pulmonary surfactant, which is synthesized by type-II pneumocytes.
This is most abundant after 35 weeks’
Decreased surfactant results in
increased alveolar surface tension with progressive atelectasis of
A higher inspiratory pressure is
required to expand the alveoli (hence respiratory distress).
Hypoxemia results in acidosis, pulmonary
vasoconstriction, pulmonary hypoperfusion, capillary endothelial and
alveolar epithelial damage.
Plasma leak into the alveolus,
which combines with fibrin and necrotic alveolar pneumocytes to form
Corticosteroids help to prevent RDS,
they induce the formation of surfactant lipids and apoprotein in fetal
are solid, airless, and reddish purple.
Alveoli are poorly developed and frequently collapsed and pink
hyaline membranes line respiratory bronchioles, alveolar ducts and
On histologic examination the lungs have
a characteristic appearance. The alveoli are collapsed and the alveolar
ducts and respiratory bronchioli are dilated.
Within these spaces cellular
debris, proteinaceous edema fluid and some red blood cells accumulate.
lining of the alveolar ducts is covered with fibrin-rich hyaline
The walls of the collapsed alveoli are thick, the capillaries
are congested, and the lymphatics are filled with proteinaceous material.
infant with respiratory distress syndrome is preterm but appropriate for gestational age.
The condition is associated with
maternal diabetes (surfactant synthesis may be suppressed by high levels
of insulin) and cesarean section delivery.
Before delivery, assessment of
amniotic fluid phospholipids (lecithin/sphingomyelin ratio) is often
performed in preterm infants as an indicator of the fetal level of
When the surfactant level is
low, glucocorticoides may be administered in an attempt to induce
At birth, the infant may need to
be resuscitated but quickly establishes spontaneous rhythmic breathing and
normal color for a short period of time.
Shortly thereafter respiratory
distress ensues, the infant becomes cyanotic.
Radiographically, the lungs show
diffuse reticulogranular densities (ground-glass appearance).
The outcome of the idiopathic respiratory
distress syndrome of the neonate depends on the severity of the disease,
the gestational age of the infant at birth, and the presence of
complications or aggravating conditions.
The overall mortality is still about 30%,
and in infants born before 30 weeks of pregnancy it is over 50%.
Factors such as maternal
diabetes, anoxia during delivery, or extensive blood loss, also adversely
influence the outcome.
Rapid development of profound respiratory
insufficiency unresponsive to standard treatment is a bad sign and
is associated with high mortality.
of Neonatal Respiratory Distress Syndrome:
Oxygen therapy may alleviate the
symptoms in mild cases, but in some serious cases respiratory distress persists, cyanosis
increases and the infant becomes flaccid, unresponsive and apnoetic.
intensive care to correct acidosis, sustain
the failing circulation, and assist the respiration.
Surfactant replacement is often
administered while ventilatory assistance is provided.
In uncomplicated cases, recovery begins
in 3 to 4 days.