C h a p t e r 2 2
Respiratory Tract Infections, Neoplasms, and Childhood Disorders
555
often is used in treating lung cancer. Combination chemo-
therapy, which uses a regimen of several drugs, is often
employed for lung cancer treatment. New targeted treat-
ments are under development with the goal of increasing
survival and ultimately providing a cure for this type of
cancer.
Therapy for SCLC is based on chemotherapy and
radiation therapy.
50,51
Advances in the use of combina-
tion chemotherapy, along with thoracic irradiation, have
improved the outlook for persons with SCLC. Because
SCLC may metastasize to the brain, prophylactic cra-
nial irradiation is often indicated. In most persons who
achieve a complete remission from SCLC, the brain is
the most frequent site of relapse. About half of such per-
sons develop clinical metastasis within 3 years. Newer
combination chemotherapy regimens and targeted ther-
apies are being developed in hopes of providing treat-
ment alternatives that increase survival and produce
fewer treatment liabilities.
Respiratory Disorders
in Children
Acute respiratory diseases are the most common cause
of illness in infancy and childhood. This section focuses
on (1) lung development, with an emphasis on the
developmental basis for lung disorders in children; (2)
respiratory disorders in the neonate; and (3) respiratory
infections in children. A discussion of bronchial asthma
in children and cystic fibrosis is included in Chapter 23.
Lung Development
Although other body systems are physiologically ready
for extrauterine life as early as 25 weeks of gestation,
the lungs take much longer to mature. Immaturity of the
respiratory system is a major cause of morbidity and mor-
tality in infants born prematurely. Even in infants born
at term, the lungs are not fully mature, and additional
growth and maturation continue well into childhood.
Lung development may be divided into four charac-
teristic stages: the embryonic, pseudoglandular, cana-
licular, saccular, and alveolar stages. It is generally
accepted that weeks 0 to 6 of gestation comprise the
embryonic stage
; weeks 6 to 16, the
pseudoglandular
stage
; weeks 16 to 26, the
canalicular
stage; weeks 24 to
birth, the
terminal sac (saccular)
stage; and 32 weeks to
8 years, the alveolar stage.
52,53
The first three stages are
devoted to development of the conducting airways and
the last two stages to development of the gas exchange
portion of the lung. By the 25th to 28th weeks, sufficient
terminal sacs are present to permit survival. Before this
time, the premature lungs are incapable of adequate gas
exchange. Development of the pulmonary circulation
occurs in parallel with lung development. The vessels
increase in length and diameter. By the 20th week of
gestation, the full number of pre-acinar vessels is present
in each segment.
53
During the terminal saccular stage of
development, the saccular epithelium becomes very thin
and the pulmonary capillaries begin to bulge into these
sacs to form the alveoli of the lung.
By 28 weeks, the terminal sacs are lined with squa-
mous epithelial cells or type I alveolar cells, across which
gas exchange takes place. Scattered among the squamous
epithelial cells are rounded secretory epithelial cells–type
II alveolar cells. Type II alveolar cells begin to develop at
approximately 24 weeks. These cells produce surfactant,
a substance capable of lowering the surface tension of
the air–alveoli interface (see Chapter 21). By the 26th to
30th weeks, sufficient amounts of surfactant are avail-
able to prevent alveolar collapse when breathing begins.
Although transformation of the lungs from glandlike
structures to highly vascular, alveoli-like organs occurs
during the late fetal period, mature alveoli do not form
for some time after birth. The growth of the lung during
infancy and early childhood involves an increase in the
number rather than the size of the alveoli. Only about
one sixth of the adult number of alveoli is present in the
lungs of a full-term infant. By the 8th year of life, the
adult complement of alveoli is present.
52
Ventilation in the Neonate
Effective ventilation requires coordinated interaction
between the muscles of the upper airways, including
those of the pharynx and larynx, the diaphragm, and the
SUMMARY CONCEPTS
■■
Cancer of the lung is a leading cause of death
worldwide, with cigarette smoking being
implicated in the majority of cases. Environmental
hazards, such as exposure to asbestos, increase
the risk for development of lung cancer. Because
the disease develops insidiously, it often is far
advanced before it is diagnosed, a fact that
explains the poor 5-year survival rate.
■■
For purposes of staging and treatment, lung
cancer is divided into nonsmall cell and small cell
carcinoma.The main reason for this is that almost
all small cell lung cancers have metastasized at
the time of diagnosis and are not amenable to
surgical resection.
■■
The manifestations of lung cancer can be
attributed to the involvement of the lung and
adjacent structures, the effects of local spread
and metastasis, and paraneoplastic syndromes
involving endocrine, neurologic, and hematologic
disorders. As with other cancers, lung cancer
causes nonspecific symptoms such as anorexia
and weight loss.Treatment methods for
lung cancer include surgery, irradiation, and
chemotherapy.
