Asthma is a disorder of the respiratory system in which the passages that enable air to pass into and out of the lungs periodically narrow, causing coughing, wheezing, and shortness of breath. This narrowing is typically temporary and reversible, but in severe attacks, asthma may result in death. Asthma most commonly refers to bronchial asthma, an inflammation of the airways, but the term is also used to refer to cardiac asthma, which develops when fluid builds up in the lungs as a complication of heart failure. This article focuses on bronchial asthma.
More than 17 million Americans suffer from asthma, with nearly 5 million cases occurring in children under age 18. In the United States, asthma causes nearly 5,500 deaths each year. Asthma occurs in males and females of all ages, ethnic groups, and socioeconomic levels. For reasons not completely understood, asthma is generally more common in poor urban neighborhoods, in cold climates, and in industrialized countries.
Among all Americans, the prevalence of asthma increased more than 60 percent between 1982 and 1994, especially among children. Deaths from asthma increased more than 55 percent from 1979 to 1992. Scientists suspect that increased exposure to second-hand cigarette smoke, growing populations in polluted city centers, and new housing that is poorly ventilated contribute to the increase in asthma cases.
Every cell in the human body requires oxygen to function, and the lungs make that oxygen available. With every breath we take, air travels to the lungs through a series of tubes and airways. After passing through the mouth and throat, air moves through the larynx, commonly known as the voice box, and then through the trachea, or windpipe. The trachea divides into two branches, called the right bronchus and the left bronchus, that connect directly to the lungs. Air continues through the bronchi, which divide into smaller and smaller air passages in the lungs, called bronchioles. The bronchioles end in clusters of tiny air sacs, called alveoli, which are surrounded by tiny, thin-walled blood vessels called capillaries.
Here, deep in the lungs, oxygen diffuses through the alveoli walls and into the blood in the capillaries and gaseous waste products in the bloodmainly carbon dioxidediffuse through the capillary walls and into the alveoli. But if something prevents the oxygen from reaching the alveoli, the body’s cells do not receive a constant supply of vital oxygen, and carbon dioxide builds up to harmful levels in the blood.
Asthma attacks occur when the bronchi and bronchioles become inflamed, reducing the space through which air can travel through the lungs. This causes the asthmatic to work harder to move air in and out of the lungs. Asthma attacks usually begin with mild chest pressure and a dry cough. As an attack intensifies, wheezing develops and increases in pitch; breathing becomes difficult; and coughing produces thick, stringy mucus. As the airway inflammation prevents some of the oxygen-rich air from reaching the alveoli, the cells of the body start to burn oxygen at a higher rate, actually increasing the body’s demand for oxygen. The frequency of asthma attacks varies considerably among asthma suffers. Some people have daily attacks, while others can go months or even years without having an attack.
Inflammation of the airway occurs when an irritantsuch as pet hair or cigarette smokecomes into contact with the airway walls. Upon detecting the irritant as a harmful invader, the body’s immune system sends special cells known as mast cells to the site of irritation, in this case the airway walls. The mast cells release histamine, a chemical that causes swelling and redness in a process called the inflammatory response. Histamine also causes bronchospasms, in which the muscles lining the airway walls contract repeatedly, causing the airways to narrow even more. In addition, cells that lubricate the airways with mucuscalled goblet cellsoverreact to the inflammatory response by secreting too much mucus. This mucus clogs the bronchioles, resulting in wheezing and coughing.
Asthma attacks are caused by airway hyperresponsivenessthat is, an overreaction of the bronchi and bronchioles to various environmental and physiological stimuli, known as triggers. The most common causes of asthma attacks are extremely small and lightweight particles transported through the air and inhaled into the lungs. When they enter the airways, these particles, known as environmental triggers, cause an inflammatory response in the airway walls, resulting in an asthma attack.
For some people the environmental triggers are allergens. Allergens are usually natural substances, such as plant pollen and mold spores, animal dander fecal material from dust mites and cockroaches. Allergens produce an exaggerated response of the immune system in which a specific antibody, immunoglobulin E, initiates the inflammatory response. These same allergens may cause little or no reaction in nonallergic people.
Asthma also occurs in people who do not have allergies. In these people, chemical irritants trigger an inflammatory response that is initiated in a different way than in allergen-triggered asthma. For example, some people are sensitive to certain common chemical irritants, such as perfume, hairspray, cosmetics, and household cleaners. Other chemical irritants include industrial chemicals and plastics, as well as many forms of air pollution, such as exposure to high levels of ozone, car exhaust, wood smoke, and sulfur dioxide. Current research seeks to determine whether indoor pollutants also contribute to the development of asthma.
Not all triggers are environmental. Aggravations from within the body are known as physiological triggers and include exercise and infections, such as the common cold. Sometimes substances that asthmatics eat or drink bring on attacks. Chemicals found in food or medicinesuch as food sulfites found in beer and wineand medications such as aspirin and ibuprofen are especially problematic for many asthma sufferers. Intense emotion, such as crying, shouting, or laughing, may provoke hyperventilation, a rapid inhalation of oxygen that causes the airway to narrow. In asthmatics, hyperventilation often results in an attack. Many asthmatics are especially sensitive to physical exercise in cold weather.
Research suggests that genetic factors may increase the risk of developing the disorder. Children with a family history of asthma are more likely to develop asthma than other children. Despite this apparent genetic link, many people without a family history of asthma develop the disorder, and scientists continue to investigate additional causes.
Physicians typically diagnose asthma by looking for the classic symptoms: episodic problems with breathing that include wheezing, coughing, and shortness of breath. When symptoms alone fail to establish a diagnosis of asthma, doctors may use spirometry, a test that measures airflow. By comparing a patient’s normal airflow, airflow during an attack, and airflow after the application of asthma medication, doctors determine whether the medicine improves the patient’s breathing problems. If asthma medication helps, doctors usually diagnose the condition as asthma.