NEW YORK (Reuters Health) - In asthma patients, dynamic hyperinflation, following a test measure airway hypersensitivity, is greater in obese individuals than in their nonobese counterparts, which helps explain why asthma is perceived to be more severe in patients with a higher body mass index (BMI), investigators in New Zealand report.
A BMI is the ratio between height and weight, and is used to classify people as underweight, overweight or normal weight.
"The greater dynamic hyperinflation means that obese individuals lose the ability to inhale as deeply or exhale as fully as normal weight individuals," Dr. D. Robin Taylor explains in an American Thoracic Society statement.
Taylor's team at the University of Otago in Dunedin studied the changes in airway expansion and lung volume that occur with acute constriction of the bronchial tubes in a group of 30 adult women with asthma. Ten women each were classified as normal weight, overweight, or obese, and lung volumes were measured.
The degree of bronchial constriction following the airway sensitivity test did not vary by group, the team reports in the American Journal of Respiratory and Critical Care Medicine. The only significant difference among the subjects was a decrease in vital capacity that significantly corresponded with increasing BMI.
Lung volume measurements also showed that the volume of air retained in the lung following exhalation was significantly higher as BMI increased, while the amount of air that could be inhaled was lower.
These variations "were significantly greater in relation to BMI" after taking into account the effects of "airway hyperresponsiveness, the severity of airflow obstruction, and lung volume measurements, indicating the effect of BMI was an independent one," the investigators note, meaning that BMI alone was associated with the severity of asthma.
This puts obese individuals at greater disadvantage due to enhanced gas trapping, they point out, a significant contributor to difficulty in breathing.
Taylor and his associates also noted that conventional lung function tests "may have limitations when used to evaluate symptoms in obese patients with asthma, with the potential for misinterpretation" in the absence of lung volume measurements.
SOURCE: American Journal of Respiratory and Critical Care Medicine, May 1, 2008.
