Pathophysiology of Asthma

Prehospital Pathophysiology is a new monthly column that provides an opportunity for EMS providers of all levels to either refresh their knowledge related to the etiology of a certain disease or expand their knowledge base regarding common and not-so-common disease processes. This column is for both basic- and advanced-level prehospital care providers. The authors hope that through this column, EMS providers will gain a more thorough understanding of disease processes. If you would like to see a specific topic addressed in this column, send your request via e-mail to emseditor@aol.com.

About 15 million people have asthma, and as many as two million asthma-related ED visits and 15,000 deaths occur annually. In a disease that is widely treatable, these numbers are staggering.

While asthma is a relatively common complaint in both the field and emergency departments, prehospital providers are not always familiar with the pathophysiology of the disease. Asthma often develops early in childhood and is more common in children than adults. Thirty percent of asthmatic children do not experience asthma in adulthood. In youth, males can be twice as likely to have asthma, but this changes in adulthood, where the prevalence of asthma is equal or slightly more common in females. However, it is not uncommon for adults to develop asthma up to age 40. A significant number of adult-onset cases occur in urban areas and may be the result of environmental factors.

Pathophysiology

In EMT class, we are taught that there are chronic obstructive pulmonary diseases, such as chronic bronchitis and emphysema, and, in a separate category, is asthma. This is actually the basis for some of the misunderstanding of the disease. Asthma is, in fact, a chronic obstructive lung condition that has an episodic presentation. That is, asthma patients are usually asymptomatic between attacks, whereas patients with emphysema and chronic bronchitis almost always are affected in some fashion by their disease.

Asthma has many known "triggers," including physical exertion, allergens, medications, occupational infection, emotions and stress. In response to contact with a triggering substance or mechanism, mast cells of the immune system, which are found in loose connective tissue, are responsible for releasing vasoactive (action on vessels) chemical mediators, including histamine, bradykinin, leukotrienes, cytokines and prostaglandins. Chemotactic (produces specific cell movement) chemical mediators released from the mast cells cause neutrophils, lymphocytes and eosinophils to infiltrate the cells of the bronchial lining. These target the respiratory system and cause bronchoconstriction, vascular congestion, vasodilation, increases in capillary permeability, mucosal edema, impaired mucociliary action (removal of mucus and contaminants within the bronchial tree by movement of the cilia inside the bronchioles), and increased mucus production, which leads to an increase in airway resistance. Mucus plugging may also occur in the smaller bronchioles. These pathophysiologic factors produce the typical clinical presentation of asthma, including wheezing and respiratory distress.

The Asthma Attack

Patients who experience an asthma attack have some level of ongoing airway inflammation, which sets a foundation for the attack. When exposed to a trigger (although some forms of asthma are idiopathic and without known triggers), the body initiates an immune response.

In response to the chemical mediators, the airway reacts by constricting, which creates the classic wheezing seen in asthma. Due to bronchial constriction, air becomes trapped in the distal airway, which causes the patient to become increasingly short of breath. The use of accessory muscles is noticeable, as the narrowing airways make it more difficult to move air out of the lungs. It should be noted that a dry, nonproductive cough is also frequently seen in asthma. You may see a hyperinflated chest with an increased anterior/posterior diameter. Wheezing becomes worse and increases in pitch.

In severe attacks, wheezing will diminish or disappear. While this may appear to be good, it comes at a time when the patient is not able to move enough air to make the wheezing noise any longer and signals impending respiratory arrest. Fortunately, most attacks do not reach this point.

An ongoing and chronic issue, however, even in non-life-threatening attacks, is the cyclical nature of asthma. If the patient has frequent attacks and/or increased inflammation, the cycle continues with mucus plugs and increasing edema in the airways. This, in turn, causes more frequent and severe attacks.

Treatment Based on Pathophysiology

The pathophysiology of asthma indicates that treatment must be done on two fronts: reducing chronic inflammation (and the resulting hyperreactivity) and treating acute bronchoconstriction.

Inhaled steroids (e.g., Azmacort) are used daily to reduce inflammation of the airways. In severe cases and after serious attacks, oral steroids, such as prednisone or methylprednisolone, may be used. Bronchodilators such as albuterol, Xopenex and Combivent will reverse bronchoconstriction, but will not deal with the chronic inflammation.

One indicator of a potentially serious attack and a higher risk of a fatal attack is the patient's frequent need to use the bronchodilating inhaler. Generally, if a patient has to use the inhaler more than two or three times per week, he/she also needs concurrent inhaled corticosteroids to address the inflammation issue.

Asthma is more than respiratory attacks and narrowing airways. It is a complex series of reactions involving inflammation, the immune system, constricted airways and, in severe cases, mucus plugging and airway edema. Understanding the pathophysiology of asthma allows you to obtain an accurate and relevant history, understand the medications taken by the patient and, perhaps most important, it helps you provide better patient care.

Bibliography

• Asthma (Emergency Medicine) accessed online at www.emedicine.com, February 20, 2004.
• Braunwald, E, et al (eds). Harrison's Principles of Internal Medicine, 15th edition. New York, NY: McGraw-Hill, 2001.
• Marx, J, et al (eds). Rosen's Emergency Medicine, 2nd edition. St. Louis, MO: Mosby, 2002.