What are pulmonary diseases?
Disorders of the pulmonary system are among the most common diseases. Because it acts as an interface between the external and internal environments, the pulmonary system is subject to continual attacks on its health and integrity. A wide variety of disease-causing agents reach the lung with each breath. Infectious organisms (such as bacteria, viruses, and molds), environmental toxins (such as tobacco smoke and air pollutants), and various airborne allergens are the primary causes of lung disease.
The pulmonary system consists of an intricate bronchial tree terminating in very delicate, thin-walled sacs known as alveoli, each of which is surrounded by blood vessels. The entire network is contained within the supporting tissue of the lungs. These individual parts are perfectly suited to efficiently carry out their two life-sustaining functions: air conduction and the gas exchange between oxygen in the air and carbon dioxide (a waste product) in the bloodstream. Disruption of either function renders a person vulnerable to potentially fatal consequences.
All pulmonary diseases can be categorized in two ways. The first is based on the cause, such as a virus, asbestos, or cigarette smoke; the second is based on the result, the specific loss of a structure and its function. Infectious diseases are the most common causes of respiratory problems. Infection usually occurs through inhalation, although it can come from another source within the body as well. A vast number of microorganisms are trapped by the hairs, mucus, and immune system cells that line the respiratory tract. Those that are not repelled generally infect the upper tract, namely the nose and throat, but it is the few that reach the bronchi and lungs that cause the most serious illnesses—bronchitis, pneumonia, and tuberculosis. Bronchitis, an inflammation of the bronchial tree, is the result of viruses or bacteria that invade the airways and infect the bronchial cells. In a counterattack, the body responds by sending large numbers of immune system cells (white blood cells), which destroy the invaders both by direct contact and by releasing chemical substances. The inflamed bronchi begin to leak significant amounts of fluids, producing the most obvious symptom of bronchitis: a frequent cough that yields initially clear white and later yellow or green phlegm. Rarely does bronchitis progress to serious disability; more often it resolves, although recurrence is common.
Unlike bronchitis, pneumonia is extremely serious. It can develop from bronchitis or can occur as a primary infection. Pneumonia is an infection that goes beyond the airways into the alveoli and supporting lung tissue. While the process of the disease is the same as that of bronchitis, fluid accumulates not only in the bronchi but also in the alveolar sacs, which cannot be efficiently cleared by coughing. The normally air-filled sacs, now filled with fluid, cannot perform their vital function of gas exchange. If the fluid continues to accumulate, larger and larger areas of lung become unable to function, and the person literally drowns.
In the case of tuberculosis, one specific bacterium (Mycobacterium tuberculus) is inhaled, generally from the spray of coughs or sneezes of infected persons. The bacterium settles in the bronchus, where it begins to invade and multiply. Unlike the organisms that cause bronchitis and pneumonia, it passes through the airways into the substance of the lung. Again, the body reacts in an attempt to confine the organisms’ spread by forming walled-off circular areas (cavities) around the destruction. Up to this point, the person may have been only minimally ill. However, while the cavities are successful at containing the spread, some bacteria within them may not have been killed and remain dormant for many years. Later, when the person’s immune system is weakened by disease, alcoholism, drug abuse, or another disorder, the bacteria reawaken and invade the lung, producing massive destruction and the loss of both structure and function. Left untreated, death results.
In ways different from infectious diseases, toxic substances such as tobacco smoke cause severe disability and death either through permanent structural damage (emphysema) or by transforming respiratory cells into abnormal ones (lung cancer). Many toxic chemicals are released when tobacco is burned, and these substances affect the entire lining of the respiratory tract both in the short term and in the long term. In the immediate period, the small hairs that line the upper tract no longer function to filter the air, and large amounts of fluid enter the airways because they are constantly inflamed, producing the familiar smokers’ cough. As the irritation continues for years, permanent damage ensues. Emphysema, which is present in nearly all smokers to some degree, is characterized by widespread destruction of the walls of individual alveolar sacs. As adjacent walls break, the alveoli coalesce into very large, balloonlike structures. The supporting lung tissue, which is normally soft and spongy, becomes stiff and hard, making breathing very difficult. Although the lungs become overinflated, the air is stale as it is unable to move in and out with each breath. The picture of a patient with severe emphysema is dramatic: the patient labors forcefully with an open mouth, trying unsuccessfully to draw air in and out. Both air conduction and gas exchange are seriously affected. If lung function falls below a critical minimum, death occurs.
Lung cancer is a major health problem, claiming tens of thousands of lives each year in the United States alone, more than any other cancer. The mechanism by which toxic substances transform normal cells into cancer cells is complex, involving damage to the cells’ genetic material. Many factors interact to allow cancer cells to grow into tumors, including the failure of the immune system to destroy these abnormal cells. Tumors may form in either the bronchial tree or the substance of the lung itself. In either case, the end result is the same: the tumor destroys normal structure by compression and invasion, replacing large areas of lung. Cancer cells also enter the bloodstream and travel to distant sites in the body, where they can grow into equally destructive tumors.
A pulmonary disease that affects millions of adults as well as children is asthma. The trachea and bronchial tree of an asthmatic are highly sensitive to a variety of stimuli as diverse as cold air, dust, exercise, and emotional stress. The bronchial muscles respond to the agent by spasming, producing narrow, constricted airways. Thick secretions are released that plug the bronchial tree and add to the serious decline in air conduction. An asthma attack may range from mild bronchial contractions to life-threatening closure. Many asthmatic patients have multiple allergies to foods, animal dander, plant pollen, dust, and so on, implying that their respiratory systems respond abnormally to otherwise harmless substances. Asthma is usually a lifelong problem, and while most attacks subside, death can occur.
The most common symptoms associated with pulmonary disease are coughing, chest pain, and shortness of breath. Because each of these symptoms is present in such a wide variety of pulmonary diseases, it often is necessary to use other tools to determine the specific illness present. The most important of these diagnostic tools is the chest x-ray, in which nonspecific symptoms can be correlated with structural and functional abnormalities. A critical advancement in the use of x-rays is the computed tomography (CT) scan. Using a computer, a large number of detailed x-rays are combined to create a very detailed picture, allowing an ambiguous abnormality on a chest x-ray to be visualized with much greater accuracy. If further information is needed in order to determine the exact nature of an abnormality revealed by the chest x-ray and the CT scan, a sample of lung tissue must be obtained. The bronchoscope, a flexible or rigid fiber-optic tube, is passed through the mouth into the bronchial tree, allowing direct inspection of the pulmonary system. Performed using anesthesia in the hospital operating room, bronchoscopy can be used to remove a small amount of tissue for biopsy. While the procedure has a higher risk than either the chest x-ray or the CT scan, it also has a high yield of information.
Once a specific diagnosis is made, treatment is begun that addresses the particular cause or resulting dysfunction. Infectious agents such as those causing bronchitis, pneumonia, and tuberculosis have the most direct treatment, antibiotics. These drugs, first discovered in the early part of the twentieth century, revolutionized modern medicine. Penicillin, sulfa drugs, erythromycin, and tetracycline are among the most useful antibiotics for pulmonary infections. The particular microorganisms that are destroyed are specific to each drug, although significant overlap exists. Diseases that once claimed millions of lives can now be successfully cured.
Patients with asthma, lung cancer, and emphysema are not as fortunate. All these conditions are progressive pulmonary diseases: asthma can remain stable for years but causes significant disability, emphysema slowly worsens, and lung cancer is sometimes curable but is frequently fatal. No cure exists for asthma; treatment is directed at alleviating the symptoms. The drugs that are used fall into three categories: those that reverse the bronchial constriction and open the airways (epinephrine, methylxanthines), those that reduce the inflammation and hence the thick mucus secretions (steroids), and those that attempt to stabilize respiratory cells, decreasing their abnormal response to stimuli (cromolyn sodium). During an asthma attack, epinephrine and similar-acting compounds are administered through inhalation or as injections in order to relieve the spasms that dangerously narrow bronchial airways. Between attacks, patients may use nasal sprays that contain mild doses of epinephrine-like drugs, as well as steroids that reduce the inflammation associated with asthma. Two other commonly used medications are caffeinelike drugs known as methylxanthines, which also serve to open narrowed airways, and cromolyn sodium, an interesting substance that appears to stabilize the bronchial cells and prevent their hypersensitive reactions to various allergens. The reality of all these drugs is that although they reduce the severity of attacks, they do not prevent their occurrence.
Emphysema is more difficult than asthma to treat. The enlarged alveoli and stiffened surrounding lung tissue are permanent structural changes. Progression of the disease can be significantly reduced if, in the early stages, environmental insults, particularly smoking, cease. Patients with emphysema have frequent serious pulmonary infections because the defense mechanisms of the bronchial tree are severely impaired as well. Such repeated infections hasten the decline in respiratory function. Both air conduction and gas exchange are affected. Supplementing oxygen is the mainstay of treatment, both during sudden deterioration and in later stages. Eventually, when the emphysemic’s lungs no longer function, mechanical ventilators (artificial respirators) are needed. Need for this technology generally heralds a fatal outcome.
Lung cancer has the most dismal prospects of all the pulmonary diseases. Treatment has met with limited success because lung cancer becomes symptomatic relatively late in its course and because it is such an aggressive disease, spreading to other parts of the body. Three main modalities exist in attempting to cure lung cancer: surgical removal of the tumor and surrounding lung tissue, radiation therapy, and chemotherapy. Surgery and radiation are localized treatments, while chemotherapy is systemic, reaching the whole body via the bloodstream. Very often, the latter two are used to alleviate symptoms when attempts at a cure fail. When lung cancer is discovered early, all three procedures may be used. Bronchoscopy allows a sample of the tumor to be analyzed, and based on various other findings, a treatment plan may be instituted that begins with surgically removing the mass. Radiation is then used in very controlled ways to destroy any remaining cancer cells in the surrounding lung tissue. If it is found that cancer cells have already spread to other regions—such as the bone, brain, or liver—then chemotherapy consisting of highly toxic drugs is given directly into the bloodstream in order to reach migrating cancer cells. Unfortunately, because lung cancer is an extremely destructive disease extending beyond its local site of inception to distant, unrelated organ systems, treatment has been disappointing and fatality rates are high.
A treatment modality that plays a very important role for many pulmonary diseases, and indeed has supported countless lives, is the respirator. This mechanical device, essentially an artificial lung, delivers a preset volume of air rich in oxygen into the lungs through a conducting tube that lies in the trachea, the largest airway, from which the right and left main bronchi divide. Although fraught with ethical issues about unnecessary prolongation of death and suffering, the artificial respirator is clearly indicated when the person will most likely fully recover from a sudden illness. In these cases, mechanical breathing can provide adequate oxygenation to the body as it repairs itself.
Death has long been defined as the cessation of respiration. Artificial respirators have forced a rethinking of that definition, which now requires cessation of brain activity. Many pulmonary diseases in their final stages lead to dependence on these mechanical ventilators. Many of these same diseases, and those of other systems that affect the lungs, can also cause sudden respiratory failure. Cardiopulmonary resuscitation (CPR) is a highly effective emergency procedure that essentially substitutes a rescuer for a machine. Through delivering exhaled air into the unconscious person and simultaneously compressing the chest, the critical functions of breathing and circulation are maintained. CPR is a simple procedure to learn, and one that has saved innumerable victims.
Pulmonary diseases have caused an extraordinary number of deaths throughout human history. Whereas lung cancer claims the most lives today, infectious diseases, especially pneumonia, claimed many more lives in the thousands of years before the introduction of antibiotics in the early twentieth century. Many potentially fatal illnesses, particularly those that are viral in origin, are transmitted through the respiratory route. Because of the ease with which they can be spread—person to person, through coughs and sneezes—epidemics often occur. Rubella, measles, chickenpox, smallpox, mumps, diphtheria, and pertussis (whooping cough) are among such illnesses. Many of these kill by secondary pneumonias that overwhelm the body’s defense mechanisms. The well-known rashes that occur in several of these illnesses are simply manifestations of viremia, the passage of viruses through the lungs into the bloodstream. Most of the victims of these diseases were children; indeed, these illnesses were among the principal reasons for the high child mortality rates. While antibiotics are ineffective in treating viral diseases (as opposed to bacterial or fungal diseases), vaccinations have proven very successful, reducing or even eliminating them.
Two epidemic diseases that have killed millions of people throughout recorded history have been the pneumonic plague and influenza. Both have been somewhat controlled by improved sanitation (in the case of the plague) and improved vaccine programs (as with influenza). General sophistication in caring for the victims of these diseases has minimized mortality in those cases that do occur.
The plague has been feared since ancient times, and at least three major epidemics are known in which large portions of populations were destroyed. The first of these was recorded in Europe and Asia Minor during the sixth century, the second (known as the Black Death) was in the fourteenth century, and the last began in China in 1894, an epidemic that eventually spread to all continents, including North America, by 1900. The plague is an infectious disease caused by a bacterium that lives in the bodies of rodent fleas. It is transmitted to humans through bites of rat fleas in particular and enters the bloodstream. High fever, very enlarged and painful lymph glands, and severe weakness characterize the illness, which occurs a few days after the flea bite. In this stage of the disease, known as bubonic plague, the fatality rate ranged from 50 to 90 percent, but the disease was not contagious. As the infection spread from the bloodstream to lung tissue, a highly contagious pneumonia resulted that allowed person-to-person transmission through infectious droplets expelled by coughing. This form of the disease, pneumonic plague, was almost invariably fatal, with nearly 100 percent mortality within a few days of infection. Approximately one-half of the population of Europe died during the Black Death. Improved sanitation methods that separated the rat population from human habitations have played the most important role in stemming the outbreak of new epidemics. Such problems continue to exist in much of the developing world, however, and plagues still occur sporadically.
While influenza was not recorded well historically, the disastrous epidemic of 1918 proved just as deadly as the plague, killing 35 million people worldwide in a few short months. Because of immigration, the disease spread rapidly throughout Europe and North America within a few months. Influenza is caused by a virus and spread solely by the respiratory route, through inhalation. High fever, muscle and joint pain, coughing, chest pain, and weakness are common symptoms. The pneumonia that may develop within a few days of the onset of the illness can rapidly progress to death. Early twentieth-century medicine was completely overwhelmed by the number of cases and the severe pneumonia that followed. Vaccinations with killed virus particles have become routine preventive medicine for those most at risk: the elderly, the sick, and infants. The mortality associated with influenza in the past has been reduced but definitely not eliminated.
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