CHICAGO - Inhaler drugs used by millions of people with emphysema and bronchitis may slightly raise the risk for heart attacks and even death, a study suggests.

The results aren't conclusive and inhalers provide significant relief for these patients struggling to breathe. But the study authors urged doctors to closely monitor patients who use the inhalers.

Most affected patients have both emphysema and chronic bronchitis. The condition's formal name, chronic obstructive pulmonary disease, COPD, is the nation's fourth-leading cause of death.

The study's increased risks were small, and the drugs' marketer said both medicines were safe. Outside experts called the study compelling but said it had limitations that made it hard to know whether the drugs or something else was at fault.

The drugs are tiotropium, sold as Spiriva Handihaler by Boehringer Ingelheim Pharmaceuticals Inc., and ipratropium, available generically and sold by Boehringer under the brand name Atrovent.

Spiriva, approved in 2004, and the decade-old Atrovent are used once or more daily to relax muscles and open lung airways. They have been used by eight million patients worldwide.

A Veterans Affairs study published last week linked ipratropium with an increased risk for heart-related deaths in men. The new study is in today's Journal of the American Medical Association.

COPD affects as many as 24 million Americans and kills more than 100,000 each year. It involves thickened and narrowed lung airways and excess mucous. Symptoms include persistent coughing and severe shortness of breath. Smoking is a leading cause.

Patients describe COPD breathing problems as feeling like they're "living the entire day under water, unable to come to the surface," said Aaron Milstone of Vanderbilt University medical school.

The new study pooled results from 17 randomized studies comparing mostly older patients on either of the drugs with those on different medicine or dummy drugs. Randomized studies are the most rigorous kind of medical research, but Milstone said drawing conclusions from a pooled analysis could be problematic because of differences in patients' characteristics.

It found that using either drug for more than one month appeared to increase chances for fatal and nonfatal heart problems including heart attacks by more than 50 percent.

Among about 7,400 patients on either inhaled drug, 1.8 percent or 135 people developed fatal or nonfatal heart problems over a period of several weeks to several years. By contrast, among about 7,300 patients on other drugs or dummy medicine, 1.2 percent or 86 had those problems.

In absolute terms, out of 40 patients using either drug for one year, there would be one extra drug-linked death, said study author Sonal Singh of Wake Forest University's medical school.