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Respiratory Tract Infections, Neoplasms, and Childhood Disorders

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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. ■■ 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. SUMMARY CONCEPTS

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

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