Which type of pulmonary disease requires more force to expire a volume of air?

FUNCTIONAL ABNORMALITIES IN AIRWAY DISEASE

In the larger airways (the bronchi), an increase in the mucus-secreting apparatus and the amount of mucus produced results in the symptoms of excessive cough and sputum production characteristic of chronic bronchitis. A decrease in the size of the large airways as a result of secretions, an increase in the mucus-secreting apparatus, and inflammation might be expected to correlate well with the degree of airflow obstruction, but this does not necessarily appear to be the case. Some patients with typical symptoms of chronic bronchitis do not exhibit abnormally high resistance or changes in other measurements of airflow. When airflow obstruction exists, in general additional pathologic factors, either in the small airways (inflammation and fibrosis) or the pulmonary parenchyma (emphysema), are critical for the presence of obstruction. In relatively mild airflow obstruction associated with chronic bronchitis, disease in the small airways often makes an important contribution to airflow obstruction. When airflow obstruction is more marked, coexisting emphysema is often the primary reason for the obstruction.

Coexistent small airways disease, emphysema, or both contribute significantly to decreased expiratory flow rates in chronic bronchitis.

In patients who have a component of airway hyperreactivity contributing to their disease, the clinical expression often is more like asthmatic bronchitis. Airway smooth muscle constriction adds more reversible airflow obstruction than is typically seen in the patient without airway hyperreactivity.

The common problem produced by the processes affecting airways is a decrease in the overall cross-sectional area of the airways. Airways resistance (Raw) is potentially increased by anything that compromises the lumen of the airways, such as intraluminal secretions; bronchospasm; or thickening of the airway wall caused by edema, inflammatory cells, fibrosis, or enlargement of the mucus-secreting apparatus. When disease is located primarily in the peripheral airways and is mild, the functional consequences may be relatively subtle. Because the peripheral airways contribute only approximately 10% to 20% of overall airways resistance, total resistance is preserved unless the small airways disease is considerable or additional disease affects the larger airways.

As another consequence of airways disease, expiratory flow rates, including forced expiratory volume in 1 second (FEV1), FEV1/forced vital capacity (FVC) ratio, and maximal midexpiratory flow rate (MMFR), are generally decreased. Use of inhaled bronchodilators may or may not result in significantly improved flow rates. Patients with asthmatic bronchitis and greater airways reactivity generally have the most striking improvement in flow rates after receiving an inhaled bronchodilator.

Before a discussion of how lung volumes change in patients having the airway disease associated with COPD, it is useful to review the factors that determine the major lung volumes, namely, total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV). TLC is the point at which the force of the inspiratory muscles acting to expand the lungs is equaled by the elastic recoil of the respiratory system (primarily lung recoil) resisting expansion (see Chapter 1). At FRC, the resting point of the respiratory system, there is a balance between the elastic recoil of the lungs and the elastic recoil of the chest wall, which are acting in opposite directions—the lungs inward and the chest wall outward. The determinants of RV depend to some extent on age. In a normal young person, RV is the point at which the relatively stiff chest wall can be compressed no further by the expiratory muscles. With increasing age, a sufficient number of airways close at low lung volumes to limit further expiration, and airway closure is an important determinant of RV. In disease states in which airways are likely to close at low lung volumes, airway closure is associated with an elevated RV, even in young patients.

In patients with pure airway disease, TLC theoretically remains relatively close to normal because neither the elastic recoil of the lung nor inspiratory muscle strength is altered. Similarly, FRC should remain normal because the recoil of the lung and the recoil of the chest wall are unchanged. However, if expiration is prolonged and the respiratory rate is high, then the patient may not have sufficient time during expiration to reach the normal resting end-expiratory point. In this case, FRC is increased. RV is generally also increased with these processes that involve airways because the narrowing and occlusion of small airways by secretions and inflammation result in air trapping during expiration.

FUNCTIONAL ABNORMALITIES IN EMPHYSEMA

Although emphysema (i.e., destruction of alveolar walls) leads to decreased expiratory flow rates, the pathophysiology is different from the situation in pure airway disease. The primary problem in emphysema is loss of elastic recoil (i.e., loss of the lung's natural tendency to resist expansion). One consequence of decreased elastic recoil is a decreased driving pressure that expels air from the alveoli during expiration. A simple analogy is a balloon filled with air, in which the elastic recoil is the “stiffness” of the balloon. With a given volume of air inside an unsealed balloon, a stiffer balloon will expel air more rapidly than will a less stiff balloon. An emphysematous lung is like a less stiff balloon: a smaller than normal force drives air out of the lungs during expiration.

In emphysema, decreased expiratory flow rates are largely due to loss of elastic recoil of the lung, resulting in the following:

Lower driving pressure for expiratory airflow

Loss of radial traction on the airways provided by supporting alveolar walls, thus promoting airway collapse during expiration

Loss of driving pressure is not the only consequence of emphysema. There is also an indirect effect on the collapsibility of airways. Normally, outward traction is exerted on the walls of airways by a supporting structure of tissue from the lung parenchyma. When the alveolar tissue is disrupted, as in emphysema, the supporting structure for the airways is diminished, and less radial traction is exerted to prevent airway collapse (Fig. 6-6). During a forced expiration, the strongly positive pleural pressure promotes collapse. Airways lacking an adequate supporting structure are more likely to collapse (and have diminished flow rates and air trapping) than are normally supported airways.

The decrease in elastic recoil in emphysema also alters the compliance curve of the lung and the measured lung volumes. The compliance curve relates transpulmonary pressure and the associated volume of gas within the lung (see Chapter 1). Because an emphysematous lung has less elastic recoil (i.e., is less stiff), it resists expansion less than does its normal counterpart. Therefore, the compliance curve is shifted upward and to the left, and the lung has more volume at any particular transpulmonary pressure (Fig. 6-7). TLC is increased because loss of elastic recoil results in a smaller force opposing the action of the inspiratory musculature. FRC also is increased because the balance between the outward recoil of the chest wall and the inward recoil of the lung is shifted in favor of the chest wall. As in bronchitis, RV is substantially increased in emphysema because poorly supported airways are more susceptible to closure during a maximal expiration.

MECHANISMS OF ABNORMAL GAS-EXCHANGE

In obstructive lung disease, many of the observed pathologic changes affecting airflow are not uniformly distributed. For example, in chronic bronchitis some airways are extensively affected by secretions and plugging, whereas others remain relatively uninvolved. Therefore, ventilation is not uniformly distributed throughout the lung. Regions of the lung supplied by more diseased airways receive diminished ventilation in comparison with regions supplied by less diseased airways. Although there may be a compensatory decrease in blood flow to underventilated alveoli, the compensation is not totally effective, and inequalities and mismatching of ventilation and perfusion result. This type of ventilation-perfusion disturbance, with some areas of lung having low ventilation-perfusion ratios and contributing desaturated blood, leads to arterial hypoxemia.

In obstructive lung disease, nonuniformity of the disease process results in

Mechanisms that contribute to alveolar hypoventilation and CO2 retention in obstructive lung disease are the following:

Increased work of breathing

Abnormalities of ventilatory drive

Decreased effectiveness of the diaphragm

Carbon dioxide elimination is impaired in some patients with obstructive lung disease. The mechanism of alveolar hypoventilation and CO2 retention is less clear than the mechanism of hypoxemia. Several factors probably contribute, including increased work of breathing (resulting from impaired airflow), abnormalities of central ventilatory drive, and ventilation-perfusion mismatch creating some areas with high ventilation-perfusion ratios that effectively act as dead space.

An additional problem, fatigue of inspiratory muscles, has received attention as a factor contributing to acute CO2 retention when affected patients are in respiratory failure (see Chapter 19). The importance of diaphragmatic fatigue in the stable patient with chronic hypercapnia is less certain. However, it is clear that contraction of the diaphragm, the major muscle of inspiration, is less efficient and less effective in patients with obstructive lung disease. When FRC is increased, the diaphragm is lower and flatter, and its fibers are shortened even before the initiation of inspiration. A shortened, flattened diaphragm is at a mechanical disadvantage compared with a longer, curved diaphragm, and it is less effective as an inspiratory muscle.

PULMONARY HYPERTENSION

A potential complication of COPD is the development of pulmonary hypertension (i.e., high pressures within the pulmonary arterial system). Long-standing pulmonary hypertension places an added workload onto the right ventricle, which hypertrophies and eventually may fail. The term cor pulmonale is used to describe disease of the right ventricle secondary to lung disease (either COPD or other forms of lung disease); this topic is discussed in Chapter 14. The primary feature of COPD that leads to pulmonary hypertension and eventually to cor pulmonale is hypoxia. A decrease in Po2 is a strong stimulus to constriction of pulmonary arterioles (see Chapter 12). If the hypoxia is corrected, the element of pulmonary vasoconstriction may be reversible, although vascular remodeling from chronic hypoxia may not fully reverse.

The major cause of pulmonary hypertension in COPD is hypoxia. Additional factors include hypercapnia, polycythemia, and destruction of the pulmonary vascular bed.

Several other but less important factors that may contribute to elevated pulmonary artery pressure are hypercapnia, polycythemia, and reduction in area of the pulmonary vascular bed. Hypercapnia, like hypoxia, is capable of causing pulmonary vasoconstriction. To a large extent, this effect may be mediated by the change in pH resulting from an increase in Pco2. An elevation in hematocrit (i.e., polycythemia) is often found in the chronically hypoxemic patient, producing increased blood viscosity and contributing to elevated pulmonary artery pressure. Finally, in emphysema, the destruction of alveoli is accompanied by a loss of pulmonary capillaries. Therefore, in extensive disease, the limited pulmonary vascular bed may result in a high resistance to blood flow and consequently an increase in pulmonary artery pressure.

TWO TYPES OF PRESENTATION

In practice, clinicians have often distinguished two pathophysiologic types of COPD, termed type A and type B, or, more colloquially, pink puffer and blue bloater, respectively. Originally, type A (pink puffer) physiology was associated with underlying emphysema, and type B (blue bloater) physiology was equated with chronic bronchitis. Although the association with a particular pathologic process appears to be an oversimplification, the pathophysiologic types of presentation can provide a helpful conceptual framework and are sometimes useful clinically. In most cases, a patient does not fall clearly into one or the other category but has some features suggestive of both.

Two presentations of obstructive lung disease are pink puffer (type A) and blue bloater (type B), but a clear distinction between their underlying processes is an oversimplification.

The patient with type A disease is referred to as a pink puffer because (1) arterial Po2 tends to be reasonably well preserved so that the patient is “pink,” that is, the patient is not cyanotic, and (2) dyspnea and high minute ventilation are prominent features, with the patient appearing to be working hard to get air, that is, the patient is “puffing.” Not only is Po2 not markedly decreased, but Pco2 is not abnormally high. On the basis of the general (although oversimplified) concept that emphysema is the primary process in these patients, relative preservation of Po2 may be related to a simultaneous and matched loss of ventilation and perfusion when alveolar walls are destroyed. Because gas-exchange abnormalities are not a striking feature of patients with type A disease, significant hypoxia is absent and therefore does not provide a prominent stimulus for significant pulmonary hypertension. In addition, an elevated hematocrit value, often a result of hypoxemia, is not seen.

The patient with type B disease, on the other hand, is characterized by major problems with gas exchange, namely, hypoxemia and hypercapnia. This patient is termed a blue bloater because (1) cyanosis can result from significant hypoxemia and (2) the patient frequently is obese and can have peripheral edema resulting from right ventricular failure. Again, on the basis of the oversimplified concept that patients with type B disease have primarily chronic bronchitis, it is reasonable to attribute hypoxemia to ventilation-perfusion mismatch. Presumably, regions of lung supplied by diseased airways are underventilated, while perfusion is relatively preserved. Ventilation-perfusion mismatch results in arterial hypoxemia because of desaturated blood coming from areas with a low ventilation-perfusion ratio. As discussed earlier, several mechanisms may contribute to the development of CO2 retention, although the primary differences explaining why type A patients do not retain CO2 and type B patients often do are not entirely clear. As a consequence of the gas-exchange abnormalities (particularly a decrease in Po2) in type B patients, pulmonary hypertension, cor pulmonale, and elevated hematocrit values (secondary polycythemia) commonly accompany the clinical picture.

Despite the common association of type B pathophysiology with the symptoms of chronic bronchitis, patients frequently also have pathologic evidence of emphysema, particularly of the centrilobular variety. How much of the clinical picture is secondary to bronchitis and how much is secondary to coexisting centrilobular emphysema are difficult to determine.