Porth's Essentials of Pathophysiology, 4e

581

Disorders of Ventilation and Gas Exchange

C h a p t e r 2 3

A

B

FIGURE 23-10. Characteristics of normal chest wall and chest wall in emphysema.The normal chest wall and its cross-section are illustrated on the left (A). The barrel-shaped chest of emphysema and its cross-section are illustrated on the right (B). (From Smeltzer SC, Bare BG. Medical-Surgical Nursing. 10th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004:572.)

labeled blue bloaters, a reference to cyanosis and fluid retention associated with right-sided heart failure. In practice, differentiation between the two types of COPD is often difficult. This is because persons with COPD often have some degree of both emphysema and chronic bronchitis. The manifestations of COPD are associated with episodes of moderate to severe respiratory impairment due to obstruction of airflow, which is greater on expi- ration than inspiration, resulting in increased work of breathing but decreased effectiveness. The development of exertional dyspnea, often described as increased effort to breathe, heaviness, air hunger, or gasping, can be insidious. Activities involving significant arm work, particularly above the shoulders, are usually difficult for persons with COPD. The breathing becomes increas- ingly more labored, even at rest; the expiratory phase of respiration is prolonged; and expiratory wheezes and crackles can be heard on auscultation. Persons with severe airflow obstruction may also exhibit use of the accessory muscles, often sitting in the characteris- tic “tripod” position in which the arms are braced to facilitate use of the sternocleidomastoid, scalene, and intercostal muscles. Pursed-lip breathing enhances air- flow because it increases the resistance to the outflow of air and helps to prevent airway collapse by increas- ing airway pressure. Eventually, persons with COPD are unable to maintain normal blood gases by increasing their breathing effort. Hypoxemia, hypercapnia, and cyanosis develop, reflecting an imbalance between ven- tilation and perfusion.

Exacerbations, which are characterized by increased cough, sputum, dyspnea, and fatigue, are increasingly frequent as the disease progresses. 44,45 They are often difficult to distinguish from other causes of respira- tory deterioration, such as pneumonia, congestive heart failure, pulmonary emboli, and pneumothorax with radiologic or laboratory tests. Persons with frequent exacerbations exhibit a faster decline in lung function and have a lower quality of life, an increased need for hospitalization, and a higher mortality rate. Severe hypoxemia, in which arterial PO 2 levels fall below 55 mm Hg, causes reflex vasoconstriction of the pulmonary vessels and further impairment of gas exchange in the lung. It is more common in persons with the chronic bronchitis form of COPD. Hypoxemia also stimulates red blood cell production, causing poly- cythemia. The increase in pulmonary vasoconstriction and subsequent elevation in pulmonary artery pres- sure further increase the work of the right ventricle. As a result, persons with COPD may develop right-sided heart failure with peripheral edema (i.e., cor pulmo- nale). However, signs of overt right-sided heart failure are seen less frequently since the advent of supplemental oxygen therapy (to be discussed). Diagnosis andTreatment The diagnosis of COPD is based on a careful history and physical examination, pulmonary function studies, chest radiographs, and laboratory tests. 10,42,44 Airway obstruc- tion prolongs the expiratory phase of respiration and