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Tracking the cause of asthma's wheeze.

Until recently, scientists believed that asthma resulted when muscles surrounding the lung's airways went into spasm, restricting the flow of air. But a computer study augments recent evidence that inflammation and thickening of the airway tubes is the chief cause of an asthmatic's wheezy breathing.

Mathematician Barry R. Wiggs and his colleagues at the University of British Columbia in Vancouver turned to a computer model of the human lung in their search for the underlying cause of asthma. First, the team obtained data on the thickness of airway walls by studying tissue removed at autopsy from the lungs of people with severe asthma. The researchers gathered further data on airway walls by looking at lung tissue removed during surgery from people without asthma. Compared to the controls, the asthmatics' airway tubes were severely thickened by the effects of chronic inflammation.

Next, the Canadians plugged the data into the computer model. When the team simulated the constriction of the smooth muscles surrounding the airway tubes, they found some airflow resistance. That's to be expected, says Wiggs, who notes that a person with healthy lungs experiences some resistance to airflow when bronchial muscles constrict. Most people might notice a little more difficulty breathing but wouldn't experience any discomfort, he adds.

When the team looked at a model of an asthmatic lung, with its thickened and inflamed airway walls, a dramatic increase in airflow resistance occurred with the same constriction of smooth muscles, he says.

"The smallest airways had constricted so far down that they had collapsed," Wiggs says. The collapse of the very smallest bronchia tubes corresponds to the severe symptoms of asthma, he notes.

Healthy lungs can handle periodic muscle constrictions, which often occur in order to expel pollutants, Wiggs says. However, the effect of muscle spasm on airways already swollen with chronic inflammation are magnified, he notes.

The Canadian team's findings fit in with a growing body of scientific evidence that shifts the focus in asthma research away from the muscles surrounding the airway tubes.

"We still think that muscle constriction is important," comments asthma researcher Jeffrey M. Drazen of Harvard Medical School in Boston. Yet, Drazen and other now believe that muscle spasm alone doesn't explain the asthmatic's wheeze. "If you sit down and work out the fluid dynamics, you find that a little wall thickening has a big effect" on breathing ability, he says.

The new study, published in the June AMERICAN REVIEW OF RESPIRATORY DISEASE, underscores a recommendation made last year by a federally appointed panel. That group urged U.S. doctors to rely on anti-inflammatory drugs such as inhaled steroids as their first line of defense against asthma (SN: 2/9/91, p. 86). However, Drazen says many doctors still rely on bronchodilators, drugs that temporarily improve breathing but do nothing to ease the underlying inflammation.
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Author:Fackelmann, Kathy A.
Publication:Science News
Article Type:Brief Article
Date:Jun 20, 1992
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