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Pulmonary & Critical Care Bulletin
Vol. VII, No. 3, July 15, 2001
In this issue :
From Editor's Desk
Basics of Pediatric Pulmonology for Physicians (PartI)
Respiratory problems in children are managed by pediatricians who are well trained in pediatric medicine and its specialities. Both internists and specialist chest physicians however, are frequently consulted, especially at places when pediatricians or padiatric pulmonologists are not available. The purpose of this article is to acquaint the physicians with growth and development of the lung (Part I) and introduce a few important lung diseases (Part II).
GROWTH AND DEVELOPMENT OF THE LUNG
There are seven stages of lung development:
The Embryonic Period is characterized by the appearance of a lung bud from the esophagus at about the 26th day of gestation. This branches into right and left lung sacs which subsequently form the segmental and subsegmental bronchopulmonary branches by 7 weeks.
The bronchial tree is developed by the 16th week of intrauterine life (Pseudoglandular Period) including all axial generations as well as the beginning of airway cartilage and mucous glands. At this stage the lung has a glandular appearance. During the Canalicular Period acinar formation begins accompanied by the proliferation of mesenchyme and vascularization. The epithelium becomes thinned and differentiates. Towards the end of this period, type I and type II alveolar epithelial cells can be identified. Capillaries protrude into the epithelium and begin to bulge into the lumen (future air spaces).
Over the last 14 weeks of gestation (Terminal Sac Period), clusters of thin walled saccules develop and are lined with flattened epithelium. Capillaries come in closer approximation to the airway surface. In the human, some alveoli are present at birth although the majority of alveolar development occurs after birth. During the Terminal Sac Period, the ultrastructural and biochemical maturation of the lung determines viability in the event of a premature birth. Surface active material is produced as the type II alveolar epithelial cells become more numerous and develop cytoplasmic osmiophillic lamellar inclusions. Disaturated phosphatidylcholine, an important component of pulmonary surfactant, accompanied by increased lecithin biosynthesis, begins to increase in the fetal lung by about 60-70% of gestation.
After birth (Postnatal Period), alveolar development occurs by the formation of alveolar septae within the terminal saccules. As the lung continues to grow, new alveoli continue to form over the first 6 to 8 years of life. This alveolar development occurs in a centripetal fashion with lateral diverticulation through the terminal bronchiolar wall. Alveoli number about 24 million at birth, 120 million at 1 year, 220 million at 3 years and about 300 million in the adult.
It is fascinating to contemplate the physiological consequences of miniaturizing the adult lung in proportion to differences in body weight or size. If all lung components at birth were 1/20th of their eventual size, serious impairment of pulmonary mechanics and gas exchange would occur.
Fortunately, the lung grows in a geometric fashion so that lung volume increase approximately 27 fold; the number of alveoli 10 fold; the air tissue interface 21 fold; alveolar diameter 5 fold; tracheal diameter 3 fold; and the terminal bronchiole less than 2 fold.
CHANGES IN THE LUNG WITH AGE
Lung growth and development therefore have considerable influence on the types and severity of pulmonary 'abnormalities' of infants and children, many of which are unique to particular age groups.
Acute and chronic respiratory diseases account for the major portion of illnesses and deaths occurring in childhood. Acute respiratory infections cause about 75% of all illnesses up to 18 years of age. A normal child has an average of five or six respiratory infections per year, and these are the major cause of his school absenteeism. Lower respiratory infection (Laryngotracheobronchitis, bronchiolitis or pneumonia) occurs in one of every three infants in the first year of life; the incidence decreasing to about one in 100 by adolescence. Chronic lung disease (asthma, recurrent respiratory infections, cystic fibrosis) affect about 10% of the population less than 18 years of age.
The change in lung structure and function with growth influences to some extent the type of pathological process occurring in children of different ages. The younger infant is more susceptible to particular types of respiratory disease. This susceptibility may be related to anatomical differences such as small caliber of airways and partially developed immune or defense mechanisms. It is interesting to note that bronchiolar diameter in the newborn infant is already half its adult size. If the airway diameters were in proportion to lung weight and size, the small infant would have an airway resistance incompatible with efficient gas exchange. Since resistance to flow of air through the airways is a function of the airway radius, slight narrowing of the airways in the infant will greatly increase the work of breathing.
In the newborn, who is an obligate nose breather, nasal obstruction due to upper respiratory infection can result in marked respiratory distress. Similarly, in the relatively narrow larynx of the infant, edema and inflammation can produce extremely high resistance, and partial or complete airway obstruction. The viral and bacterial etiology of pneumonia changes with age and the establishment of active resistance. Bronchiolitis is usually limited to infants under two years of age and acute laryngotracheobronchitis (croup) usually occurs in children one to four years of age. No doubt, environmental factors and exposure to infectious disease contribute to the propensity of certain diseases to occur at certain ages. The assessment of lung function and disability is essential to a rational approach to therapy. The history and physical examination will usually provide the most important diagnostic information.
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