Asthma, 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.
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.
Inflammation of the airway occurs when an irritant—such as pet hair or cigarette smoke—comes 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 mucus—called goblet cells—overreact to the inflammatory response by secreting too much mucus. This mucus clogs the bronchioles, resulting in wheezing and coughing.
In many people, an asthma attack may be brought on by exposure to
allergens—substances that can trigger allergic reactions in susceptible people. Pollen grains, house dust, and dust mites are some of the most common and pervasive allergens.
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.
Although there is no cure for asthma, effective treatment is available for preventing attacks and for controlling and ending attacks soon after they have begun. Asthma medications are taken orally or inhaled in vapor form using a metered-dose inhaler, a hand-held pump that delivers medication directly to the airways. There are two kinds of asthma medications:
bronchodilators, which reduce bronchospasm; and anti-inflammatory medications, which reduce airway inflammation.
Bronchodilators are the most widely used medications for controlling sudden asthma attacks and for preventing attacks brought on by physical activity or exercise. They work directly on sites called beta-receptors that are attached to small muscle bands encircling the airways. When these drugs attach to the beta-receptors, the muscles relax and the airway dilates.
Theophylline is a bronchodilator that works by relaxing the muscles surrounding the airways.
Anti-inflammatory medications work mainly by interfering with the activity and chemistry of immune cells, such as mast cells, that cause inflammation in the airway walls. Anti-inflammatory medications also help relax the airway muscles that constrict during bronchospasm.
Immunotherapy is a treatment option for asthma caused by
allergens. This form of therapy modifies a person’s allergic response by repeated exposure to small amounts of allergens. The asthmatic is injected periodically with known allergens, a procedure that trains the asthmatic’s body to react to the allergens differently. Immunotherapy is especially effective in reducing allergic reactions to dust mites, animal dander, pollen, and
fungi.
