Why are different kinds of drugs needed to manage asthma?
Asthma medications are directed toward treating the specific cause of the airway narrowing. Although there are many different medications, most can be grouped into 2 categories:
- bronchodilators that treat the bronchospasm caused by a muscle contraction
- anti-inflammatory drugs that prevent or treat inflammation in the airways
Bronchodilators are often inhaled but can also be taken as a pill or liquid. They usually are very fast-acting, especially if inhaled, and last from a few hours to 10 or more hours.
The most widely used bronchodilators are called beta2-agonists. These drugs are chemical modifications of a natural compound in the body called adrenalin. Beta2-agonists cause the bronchial muscle to relax, thus widening the diameter of the airways. A similar group of drugs, the selective beta2-agonists, are further modified to target their action more specifically on the bronchial muscles. Other types of bronchodilators include adrenergics, anticholinergics, and xanthenes.
When should bronchodilators be used?
Bronchodilators should be used as soon as wheezing or shortness of breath appears. Doctors call this use "as needed" or "PRN" (which is short for the Latin for "as needed"). Bronchodilators used in these circumstances are usually short-acting beta2-agonists (SABA), such as salbutamol or terbutaline. Sometimes, if the asthma is moderate or severe, and requires the use of bronchodilators many times a day or night, the doctor will prescribe a long-acting form of bronchodilator. The most commonly used are the long-acting beta2-agonists (LABAs), such as formoterol or salmeterol. Xanthenes, such as oral theophylline and oxtriphylline, are also sometimes used. Warning: these drugs must be used ONLY AS DIRECTED, as overuse can have very serious effects on the heart and cardiovascular system. In addition, overuse of LABAs can lead to a decrease in the body’s ability to respond to these medications over time.
In certain situations, bronchodilators are used preventively. For example, they will often prevent exercise-induced asthma - the type of asthma caused by jogging, running, and other sports where vigourous exercise is required.
Anti-inflammatory agents act by preventing or reducing the inflammation that causes asthma symptoms. The inflammation is thought to start when the person with asthma is exposed to an allergen such as some pollen to which the person with asthma is allergic. The allergen triggers a series of reactions that cause the mucosa to swell up and secrete mucus. Because the inflammation narrows the bronchial tube, it makes the bronchi very susceptible to any added bronchial narrowing from muscle bronchospasm. Examples of anti-inflammatory drugs include corticosteroids, prophylactic (preventive) agents, and leukotriene receptor antagonists (LTRAs). The preventive agents are used before symptoms develop, for example before being exposed to a cat or dog (visiting a friend or relative) or before a pollen season (i.e., in springtime when the trees pollenate).
Corticosteroids are the most commonly used anti-inflammatory agents. They are effective at controlling asthma symptoms, but they also have the greatest risk of side effects. Because the side effects are most prominent when the drug is taken in pill form or intravenously, the safest way to minimize them is to take the drug by inhalation. These agents are generally very safe, and are the main treatment for moderately severe to severe asthma. They are most effective at controlling asthma symptoms when used on a regular basis.
Corticosteroids are not sex hormones and they do not cause muscle size to increase. Over many years of use, small amounts of the drugs are absorbed into the blood stream, and may cause some side effects such as cataracts and decreased bone mass. However, these are rare and do not usually occur in low to medium doses of inhaled corticosteroids.
Prophylactics include sodium cromoglycate, ketotifen and nedocromil. These drugs have a more specific action than corticosteroids, and work by blocking certain chemicals that cause the inflammation.
LTRAs are taken as a pill, and are effective for about one-third of people who have asthma. They may be used alone or in addition to using corticosteroids. They are also used to prevent symptoms in exercise-induced asthma, and by people who are sensitive to acetylsalicylic acid (ASA).
The main advantage of prophylactics and LTRAs is that they are not associated with the long-term side effects of the corticosteroids. They are, unfortunately, not as effective as the corticosteroids.
Omalizumab an injection that works by blocking the body’s response to allergens. It is used for people with asthma whose symptoms are caused by allergies and continue to have asthma symptoms despite the use of inhaled corticosteroids.
Monitoring asthma at home with a peak-flow meter
Since asthma can change in severity over time due to a number of factors, one way to check whether you're taking the right drug for your asthma is to monitor your condition at home with a peak-flow meter. Peak-flow meters are a reliable, relatively inexpensive, and accurate way of assessing the state of the asthma. They work by measuring airflow in the bronchial tubes, which tells you the amount of narrowing that is occurring.
Peak-flow meters are very useful for determining a plan of action when new drugs have been added, for detecting a worsening of the condition, or for helping an individual assess the severity of their airway obstruction.
Your doctor can tell you what your "predicted flow rate" should be. This value depends on a number of factors, including your age, gender, and height. However, your own "best flow rate" may be less than this value, depending on the state of your asthma. This value will indicate what type of action needs to occur.
For example, at less than 80% of their predicted value, the person with asthma should follow their asthma action plan as recommended by their doctor. The action plan has specific steps a person can take, such as doubling the dose of inhaled corticosteroids.
Other drug use depends on the severity of the asthma
For example, some people have very mild asthma, such as exercise-induced asthma. However, others experience symptoms more frequently, including cough, coughing up of mucus, wheezing, and shortness of breath. If these symptoms are not relieved by an inhaled bronchodilator, they may have moderate asthma. These individuals should be taking an inhaled anti-inflammatory agent. Those with severe asthma often take a number of different agents to help control their symptoms.
Anyone with moderate or severe asthma should be evaluated by a pulmonary (respiratory) or allergy specialist.
The symptoms that a person with asthma suffers from can vary with circumstances. For example, a person's mild asthma may become moderate or severe after catching a respiratory viral infection (such as the cold or flu). During that time, their medication may need to be adjusted. Similarly, sometimes a person with moderately severe asthma becomes better if a source of allergy, for example the family cat, is removed from their environment.
David Ostrow, MD
in association with the MediResource Clinical Team