A review of prior research suggests that up to one in four patients who are hospitalized for worsening chronic obstructive pulmonary disease (COPD) will have a blood clot in their lungs, a potentially life-threatening problem called pulmonary embolism.

COPD, a progressive disease that makes it hard to breathe, can cause coughing that produces large amounts of mucus, wheezing, shortness of breath, chest tightness, and other symptoms.

Cigarette smoking is the leading cause of COPD — most people who have COPD smoke or used to smoke, but it is also associated with long-term exposure to other lung irritants, such as air pollution and chemical fumes.

The two main components are emphysema and chronic obstructive bronchitis, which both damage the walls of the lung, making breathing difficult.

"A diagnosis of pulmonary embolism should be considered in patients with (COPD flare-ups) severe enough to warrant hospitalization," Dr. Don D. Sin, from the University of British Columbia, Vancouver, and colleagues advise.

In roughly 30 percent of COPD flare-ups, the cause is unclear, according to the report in the journal Chest. Although COPD patients are known to be at risk for pulmonary embolism, the exact frequency of pulmonary embolism during flare-ups was not known.

The current review featured five studies drawn from 2,407 published papers identified through a medical database search. All told, data from 550 patients were included in the analysis.

The findings suggest that one in five patients with a COPD flare-up had a pulmonary embolism. In COPD patients hospitalized for their flare-up, however, the rate of pulmonary embolism was even higher, about one in four.

No differences in the initial signs or symptoms were seen between patients who did and did not have a pulmonary embolism, the report shows.

The authors call for a large, multicenter study to examine this topic, noting that although reviews, such as the present one, "are useful tools to synthesize the existing body of evidence, they cannot substitute for the findings of large, well-conducted clinical trials."