An experimental asthma treatment that uses heat to reduce airway constriction provided some relief from severe asthma that is poorly controlled with medications, U.S. researchers said on Monday.

They said the Alair device, made by privately held Asthmatx Inc of Sunnyvale, California, cut the rates of extreme asthma attacks by 32 percent and reduced trips to the emergency room by 84 percent in patients with severe asthma.

Patients missed fewer days of work or school because of asthma symptoms and had more symptom-free days compared with people who received a placebo, according to results of the late-stage clinical trial, which was presented at a meeting of the American Thoracic Society in San Diego.

The Alair device uses a thin tube to gently heat the walls of the lung's air passages, killing off some of the muscle tissue to reduce narrowing of the airways.

"In asthma, what happens is these patients develop enlarged smooth muscles surrounding their bronchial tubes. That contributes to asthma attacks. The idea is to decrease that," Dr. Mario Castro of Washington University in St. Louis, who led the study, said in a telephone interview.

Castro and colleagues tested the device in 297 patients with severe asthma in six countries.

Researchers split the patients into two groups. Two-thirds got three treatments with the Alair device, and the rest received a placebo treatment, in which the heat was not applied.

The patients were followed closely for a year.

Overall, 79 percent of patients who got the experimental treatment improved.

Castro said the other group also improved, but the treatment group showed a statistically significant improvement.

"I think it's a meaningful advance," Castro said. "We have very limited options for these patients, who are very disabled."

He said all of the patients were taking inhaled drugs combining a corticosteroid and a long-acting beta-agonist, such as in GlaxoSmithKline Plc's best-selling drug Advair. But they were still not getting adequate relief.

The only other option for these patients is taking Xolair or omalizumab, Novartis AG's drug for treating allergic asthma, but not everyone has asthma caused by allergies, Castro said.

Asthmatx Inc is seeking U.S. Food and Drug Administration approval for the device, Castro said, and a decision is expected this fall. The treatment has been approved in Europe.