Porth's Essentials of Pathophysiology, 4e

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Respiratory Function

U N I T 6

The contraction and relaxation of the smooth muscle layer, which is innervated by the autonomic nervous sys- tem, controls the diameter of the bronchial airways and consequent resistance to airflow. Parasympathetic stimu- lation through the vagus nerve and cholinergic receptors produces bronchoconstriction, whereas sympathetic stimulation, through β 2 -adrenergic receptors, produces bronchodilation. At rest, a slight vagal-mediated bron- choconstrictor tone predominates. When there is need for increased airflow, as during exercise, the broncho- dilator effects of the sympathetic nervous system are stimulated and the bronchoconstrictor effects of the parasympathetic nervous system are inhibited. Bronchial smooth muscle also responds to inflammatory media- tors, such as histamine, that act directly on bronchial smooth muscle cells to produce bronchoconstriction. Bronchial Asthma Bronchial asthma is a common obstructive airway disease that affects adults and children and occurs in all popula- tions and locations throughout the world. It has been estimated that 25.7 million people in the United States suffer from asthma, 7.0 million of them children under 18 years of age. 17,18 The disease continues to be costly, both in terms of emergency room visits and lost work days. Close to 2.1 million emergency room visits were attributed to asthma in 2009, and in 2008 it accounted for an estimated 14.2 million lost work days for adults. 18 Asthma is a chronic inflammatorydisease of the airways involving recurring symptoms of airflow obstruction and bronchial hyper-responsiveness. 19–22 Airway obstruction is characterized by episodic wheezing, difficulty breath- ing, feeling of chest tightness, and a cough that often is worse at night and in the early morning. These episodes, which usually are reversible either spontaneously or with treatment, also cause an associated increase in bronchial responsiveness to a variety of stimuli. Etiology and Pathogenesis Asthma is commonly categorized into two types: extrin- sic or allergic, due to a type I hypersensitivity reaction, and intrinsic or non-atopic, that occurs without an aller- gic component. In either type, episodes of bronchospasm can be triggered by diverse nonimmune mechanisms, including respiratory tract infections, exercise, ingestion of aspirin, emotional upset, and exposure to bronchial irritants such as cigarette smoke. 15,16 Asthma may also be classified according to the agents or events that trig- ger an attack. These include seasonal, exercise-induced, drug-induced (e.g., aspirin), and occupational asthma. The common denominator underlying all forms of asthma is an exaggerated hypersensitivity response to a variety of stimuli. After exposure to an inciting factor (allergens, drugs, cold, or exercise), inflammatory media- tors released by activated macrophages, eosinophils, mast cells, and basophils induce bronchoconstriction, increased

SUMMARY CONCEPTS

Obstructive Airway Disorders Obstructive airway disorders are caused by conditions that limit expiratory airflow. Bronchial asthma repre- sents an acute and reversible form of airway disease caused by narrowing of the airways due to broncho- spasm, inflammation, and increased airway secretions. Chronic obstructive disorders include a variety of airway diseases, such as bronchial asthma, chronic obstructive pulmonary disease, bronchiectasis, and cys- tic fibrosis. Physiology of Airway Disease Air moves through the upper airways (i.e., trachea and major bronchi) into the lower or pulmonary air- ways (i.e., bronchi and alveoli), which are located in the lung. 1,2 In the pulmonary airways, the cartilaginous layer that provides support for the trachea and major bronchi gradually disappears and is replaced with crisscrossing strips of smooth muscle (see Chapter 21). ■■ Disorders of the pleura include pleuritis, pleural effusion, and pneumothorax. Pleuritis, or inflammation of the pleura, characteristically causes unilateral pain that is abrupt in onset and exaggerated by respiratory movements. Pleural effusion refers to the abnormal accumulation of fluid in the pleural cavity.The fluid may be a transudate (i.e., hydrothorax), exudate (i.e., empyema), chyle (i.e., chylothorax), or blood (hemothorax). ■■ Pneumothorax refers to an accumulation of air in the pleural cavity that causes partial or complete collapse of the lung. Pneumothorax can result from rupture of an air-filled bleb on the lung surface or from penetrating or nonpenetrating injuries. A tension pneumothorax is a life- threatening event in which air accumulates in the thorax, collapsing the lung on the injured side and progressively shifting the mediastinum to the opposite side of the thorax, producing severe cardiac and respiratory impairment. ■■ Atelectasis refers to an incomplete expansion of the lung. Primary atelectasis occurs most often in premature and high-risk infants. Acquired atelectasis occurs mainly in adults and is caused most commonly by a mucus plug in the airway or by external compression by fluid, tumor mass, exudate, or other matter in the area surrounding the airway.