Respiratory Distress Syndrom e (RDS)
n the 1950s, approximately 10,000 premature infants per year
died of respiratory distress syndrome (RDS). This disease is
caused by very high surface tension in the alveoli. This surface
tension is caused mostly by the thin film of water that coats the
epithelial surface of the alveolis and helps prevent the tissues
from drying out. Water contains numerous hydrogen bonds be-
tween the molecules that tend to hold the water molecules close
together and resist separation. This is what causes water to bead
up on a surface rather than spread out. During breathing, the
lung tissue is stretched by the wall of the thorax. The lung tissue
resists the stretching because it makes water molecules separate
from one another. This is part of the mechanism that provides
elasticity to the lung tissue and allows lungs to recoil when the
muscles stop their contraction.
This high surface tension also requires the expenditure of
a great deal of energy just to inflate the lungs. To overcome this
situation, Type II alveolar cells produce surfactant, a phospho-
lipid that mixes with the water layer in the alveolus, reduces the
surface tension, and makes the lungs more flexible (or
c o m p li-
a n t ) .
Surfactant also prevents the alveoli from collapsing, reduc-
ing the need to completely reinflate each alveolus with every
breath and ensuring that the lungs remain in contact with the
walls of the thorax.
Unfortunately, a fetus does not begin to secrete surfactant
until 20 weeks gestation and does not secrete sufficient quanti-
ties to survive outside the womb until 26-28 weeks. Even babies
born before the eighth month of gestation need to use more
energy to ventilate than babies born closer to the normal end
of pregnancy because of their lower levels of surfactant. RDS is
more prominent in infants born to European American parents
than those in other ethnic groups.
Symptoms of RDS include labored or irregular breathing,
flaring of the nostrils upon inhalation, grunting during exhala-
tion, and perhaps even a blue skin color (cyanosis). RDS can
be detected in chest X-rays and with a blood test. Treatment
depends on the severity of the symptoms and may include ad-
ministration of supplemental oxygen and/or surfactant aerosols.
The baby may also be put on a ventilator temporarily to help me-
chanically assist breathing until he or she starts to manufacture
enough surfactant to make the lungs flexible and easy to expand.
If the baby has to expend a tremendous amount of energy just to
breathe, there will be little energy left for growth.
Pregnant women at risk for premature delivery may be given
steroids to promote surfactant production by the fetus prior to
delivery. Such treatments have lowered the RDS death rate dra-
matically, from 10,000 per year to about 1,000 per year.
. W hat does a blue color indi-
cate about external respiration in a b aby with RDS?
Explain the benefits of surfactant adm inistration
to a b aby with RD S.
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