Neonatology
Hyaline Membrane Disease (HMD)
The Alveoli and Surfactant
The lung is constructed of numerous branching
airways, the smallest of which are microscopic
and end in small air sacs called alveoli. With
each breath, the alveoli expand with air, and
oxygen and carbon dioxide are exchanged with the
blood which flows in tiny blood vessels surrounding
the alveoli. Alveoli are coated inside with a
substance called surfactant
which prevents the alveoli from collapsing completely
between breaths.
HMD
Premature infants can have both immature lung
tissue and a deficiency of surfactant. The immature
lung tissue may have difficulty getting oxygen
into the blood and removing carbon dioxide from
the blood. The inadequate amount of surfactant
causes alveoli to collapse when the baby breathes
out. Collapsed alveoli are hard to re-expand when
the next breath is taken. The repetitive collapsing
and expanding of the surfactant-deficient alveoli
causes inflammation and swelling of the walls
of the alveoli, and this makes breathing even
more difficult. The condition of surfactant deficiency
and resulting inflammation is called hyaline
membrane disease (HMD), also referred to as
respiratory
distress syndrome (RDS). HMD is the most common
lung disease of prematurity.
Treatment
HMD can range from very mild to severe and is
diagnosed by examination of the infant and chest
X-ray. If very mild, HMD can be managed by administering
extra oxygen to the infant via a nasal cannula.
Mild-moderate cases may be managed with CPAP,
the use of short tubes placed into the nose to
give air and oxygen under mild pressure. If severe,
a tube may be placed into the airway and a ventilator
used to administer breaths to the baby and maintain
sufficient pressure in the lungs to keep them
expanded. The use of the ventilator takes over
the work of breathing for the infant, and the
infant becomes more comfortable.
If a ventilator is used, surfactant may be administered
as a medication through the tube in the airway
to help the alveoli expand more easily. Surfactant
therapy has resulted in a dramatic improvement
in the care of premature infants. Over the course
of hours to days, the infant's own production
of surfactant will increase, resulting in an improvement
in lung function that allows support therapies
to be weaned.
Pregnant women considered at high risk of delivering
prematurely may be given steroid injections to
accelerate maturity of the fetal lungs, and increase
production of surfactant. Prenatal steroids have
reduced the incidence and severity of HMD.
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