Asthma

Asthma is a lung disease that affects approximately four million people in the United States. In people with asthma, the airways of the lungs are hypersensitive to irritants such as cigarette smoke or allergens. When these irritants are inhaled, the airways react by constricting, or narrowing. Some people with asthma have only mild, intermittent symptoms that can be controlled without drugs. In others, the symptoms are chronic, severe, and sometimes life threatening. Although researchers have learned more about the underlying causes of asthma in recent years, a definitive treatment is still unavailable. In fact, deaths from asthma are on the rise. In the last decade, asthma deaths worldwide rose 31%. The reasons for this increase are not clear; however, many experts believe that the lack of standard treatments and the inconsistent monitoring of asthma patients have contributed to the increased mortality rate .

What is asthma?

Asthma is sometimes referred to as a disease of "twitchy lungs," which means that the airways are extremely sensitive to irritants. The airways are the tubes that bring air from the windpipe, or trachea, to the lungs. These tubes are called the bronchi. Each bronchus, in turn, branches into smaller tubes called bronchioles. At the end of the bronchioles are small, balloon-like structures called alveoli. The alveoli are tiny sacs that allow oxygen to diffuse into the blood and carbon dioxide to diffuse from body tissues into the lungs to be exhaled.

During an asthma attack, the bronchi and bronchioles constrict and obstruct the passage of air into the alveoli. Besides constricting, the airways may secrete copious amounts of mucus in an effort to clear the irritation from the lungs. The airway walls also swell, causing inflammation and further obstruction. As the airways become increasingly obstructed, oxygen cannot reach the small airsacs; blood oxygen levels drop and the body's tissues and organs become oxygen-deprived. At the same time, carbon dioxide cannot escape the small airsacs for exhalation; blood levels of carbon dioxide increase, and exert a toxic effect on the tissues and organs of the body.

Underlying the bronchial inflammation is an immune response in which white blood cells known as type 2 helper T (Th2) cells (a type of CD4 helper T cell ) are prominent. Th2 cells secrete chemicals known as interleukins that promote allergic inflammation and stimulate another set of cells known as B cells to produce IgE and other antibodies. In contrast, type 1 helper T (Th1) cells, another class of CD4 T cells , produce interferon-g and interleukin-2, which initiate the killing of viruses and other intracellular organisms by activating macrophages and cytotoxic T cells. These two subgroups of helper T cells arise in response to different immunogenic stimuli and cytokines, and they constitute an immunoregulatory loop: cytokines from Th1 cells inhibit Th2 cells, and vice versa. An imbalance in this reciprocal arrangement may be the key to asthma and there is credible evidence that, when freed from the restraining influence of interferong, Th2 cells can provoke airway inflammation. Recent experiments in support of this concept have focused on a newly discovered transcription factor, T-bet, which is necessary to induce helper T cells to differentiate into Th1 cells and for Th1 cells to produce interferon-g. For these reasons, T-bet is thought to be central to the feedback loops that regulate Th1 and Th2 cells, and in this way it could be important in asthma.

One of the hallmarks of asthma is that the airway obstruction is reversible. This reversibility of the airway swelling is used to definitively diagnose asthma. If the swelling and inflammation can be brought under control with asthma drugs, the person has asthma and not some other upper respiratory tract disease.

In addition to cigarette smoke and various allergens, other triggers can cause asthma attacks. A cold or other upper respiratory infection may bring on an asthma attack. Strong emotions, such as excitement, tension, or anxiety , may trigger asthma symptoms. Exercise can cause symptoms of asthma. Weather conditions, such as extreme cold, heat , or humidity can cause an asthma attack. Pollution and increasing ozone levels are also associated with episodes of asthma. Other environmental factors include occupational exposure to certain substances like animal dander, wood particles, dusts, various industrial chemicals, and metal salts.

The characteristic sign of asthma is wheezing, the noisy, whistling breathing that a person makes as he or she tries to push air in and out of narrowed airways. Other symptoms of asthma include a tight chest, shortness of breath, and a cough.

Treatment of asthma

Currently, several drugs are used to treat asthma. Not all of the asthma drugs, however, should be used by every asthma patient. Some patients with mild asthma only need to use medication intermittently to control wheezing, while patients with more serious asthma need to take medication at regular intervals to avoid life-threatening attacks. It is important for asthma patients to see their doctors if the frequency or severity of their symptoms change. It has been suggested that many of the life-threatening asthma attacks are in people who once had mild asthma—with symptoms that could be treated as they occurred—which then progressed to a more severe case of the disease.

Bronchodilators dilate constricted lung airways by relaxing the muscles that line the bronchial tubes. Oral bronchodilators include theophylline; theophylline's counterpart, aminophylline, is used through a needle in the vein (intravenous or IV) for severe episodes of asthma. During severe, acute attacks of asthma, injections of epinephrine are given just under the patient's skin. Epinephrine has a quick, but short-lasting effect of bronchodilatation.

Most asthma patients are given bronchodilators such as albuterol that are used in a mist form that is inhaled from either a special inhaler device or an aerosol machine. Some patients are instructed to use their bronchodilator at regular intervals, while others may just be told to use the inhaler if they notice the beginning of an asthma attack. The inhaled medications are quick-acting because they are directly applied to the constricted airways.

In the 1990s, some controversy about inhaled bronchodilators arose in the medical field. In a study published in 1993, doctors found an increased risk of death or near-death from asthma when patients used a type of inhaled bronchodilator commonly prescribed to control asthma. Although more information is still needed regarding the reasons behind the increase in deaths and near-deaths and their association with inhaled bronchodilators, some experts think that the association can be explained by several factors:

1. More people who use inhaled bronchodilators die because their asthma suddenly becomes more severe and they do not see their doctors. These patients are treating severe asthma with a drug usually prescribed for milder forms.
2. Bronchodilators may have long-term effects on organ systems.
3. Bronchodilators may, over time, increase airway hyper-responsiveness.
4. Physicians are not adequately monitoring their patients for progression from mild to severe asthma.

These factors are currently being investigated. Asthma experts stress, however, that people with asthma who use inhaled bronchodilators should continue to do so, but under medical supervision. They should also immediately contact their physicians if they notice a change in the severity and frequency of their symptoms.

A newer inhaled bronchodilator, called salmeterol, combines the direct effects of inhaled bronchodilators with the long-lasting protection afforded by oral bonchodilators. Salmeterol offers a new, effective drug choice for mild and severe asthma.

Anti-inflammatory drugs

Anti-inflammatory drugs reduce the swelling and inflammation of the airways. These drugs can be inhaled or taken in pill form. Two types of anti-inflammatory drugs are prescribed for asthma patients: chromolyn sodium and corticosteroids. Chromolyn sodium is also prescribed for people with allergies, and it has few side-effects. Oral corticosteroids are very effective in treating asthma, but should be reserved for severe cases, due to their serious side-effects. Short-term side effects include increased appetite, weight gain, hypertension , and fluid retention. Over the long-term, corticosteroids may cause osteoporosis , cataracts, and impaired immune response. These side-effects usually preclude the use of corticosteroids for long periods of time. In fact, short-courses of steroids are preferred. These "steroid bursts" are given over about a week's time and then discontinued, as a treatment for a sudden severe asthma attack, perhaps brought on by exposure to an allergen or a viral infection.

Inhaled corticosteroids have few side effects. These medications are also prescribed for allergy patients. Unlike their oral counterparts, these drugs can be taken for much longer periods of time. They are especially useful in controlling moderate asthma.

Another newer class of asthma medications are called leukotriene receptor antagonists (LTRAs). These drugs, including an oral medication called zafirlukast, interfere with the actions of a class of chemicals called leukotrienes. Leukotrienes help produce the symptoms of asthma. Interference with their actions decrease asthma symptomatology. LTRAs are thought to greatly reduce asthma severity, when taken daily. LTRAs seem to work especially well in conjunction with inhaled steroids and salmeterol inhalers. This regimen (zafirlukast + inhaled steroids + inhaled salmeterol) seems to improving daily living for many asthmatics. Other inhaled bronchodilators are then reserved for exacerbations, such as may occur during a viral infection.

Can asthma attacks be prevented?

Asthma experts are currently working to dispel many myths about asthma. For instance, it was once thought that increasing fluid intake would lessen mucus production and therefore lessen the frequency and severity of asthma attacks. But drinking lots of water does not affect asthma (although drinking a lot of water is a good idea for maintaining general health).

Breathing into a paper bag is also a bad idea for someone with an asthma attack. Persons with asthma should avoid over-the-counter medications advertised to treat asthma.

Depending on the triggers associated with asthma attacks, patients can sometimes avoid attacks by taking certain preventive steps. If allergens such as dust and pollen trigger an attack, asthma can be avoided by doing the following:

1. Avoid being outside during the early morning and late afternoon hours, when pollen levels are highest.
2. Since dust has been associated with asthma attacks, thoroughly and frequently clean the indoor environment. Dust and vacuum every day. Wash bed linens in hot, soapy water every few days. Replace air filters in air conditioners and furnaces regularly.
3. During hot weather, use air conditioning.

Eliminating the irritant is the key. If asthma is brought on by cigarette smoke, the patient must avoid this irritant. If asthma is brought on by exercise, the person should try to find a level of exertion that is comfortable. Using an inhaled bronchodilator before exercising may also control asthma symptoms.

For all persons with asthma, communication with and regular visits to their physicians are essential components of treatment. Without periodic check-ups, the physician cannot monitor progress or potential worsening of symptoms. Thus, the most important aspect of prevention and treatment for asthma patients is the regular physician visit.

See also Antibody and antigen; Respiratory diseases; Respiratory system.

Resources

books

Adams, Francis V., MD. The Asthma Sourcebook: EverythingYou Need to Know. 2nd ed. New York: McGraw-Hill, 1998.

Lavy, M., Hilton, S., Barnes, G. Asthma at Your Fingertips, 3rd ed. London: Class Publishing, 2000.

Periodocals

Spilner, Maggie. "Get Off the Asthma Tightrope." Prevention 46, no. 9 (September 1994): 88.

Other

American Lung Association. 61 Broadway, 6th Floor New York, New York 10006. 1–800–LUNG–USA (1–800–586–4872).) Asthma Website [cited October 16, 2002]. <www.lungusa.org/asthma>.

National Asthma Education Program, Expert Panel. ExecutiveSummary: Guidelines for the Diagnosis and Management of Asthma. Washington, DC: U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health, 1997.

Kathleen Scogna

KEY TERMS

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Bronchiole

—The smallest diameter air tubes, branching off of the bronchi, and ending in the alveoli (air sacs).

Bronchodilator

—A drug, either inhaled or taken orally, that dilates the lung airways by relaxing the chest muscles.

Bronchus (plural, bronchi)

—One of the two main airway tubes that branch off from the windpipe and lead to each lung.

Wheezing

—The characteristic sound of asthma; it results when a person tries to push air in and out of narrowed airways.