The U.S. Food and Drug Administration warned physicians on Wednesday that two chemicals used to conduct cardiovascular stress tests can cause heart attacks and death, and it suggested resuscitation equipment and trained staff be available when the tests are conducted.

The injectable products, Lexiscan and Adenoscan, are marketed by Astellas Pharma U.S. They work by stressing the heart, allowing physicians to take images that can show areas of low blood flow and damaged heart muscle. The tests are given to patients who are physically unable to exercise.

The FDA said heart attacks may be triggered by the tests because the chemicals dilate the heart's arteries and increase blood flow to help identify obstructions. Blood flows more easily to unblocked arteries, which can reduce blood flow to an obstructed artery. In some cases, that can lead to a heart attack, the agency said.

The FDA advised physicians not to use the products in patients with signs of acute myocardial ischemia, which occurs when blood flow to the heart muscle is decreased. Some cases of heart attack and death have occurred in patients with unstable angina, or chest pain, and other heart problems.

Astellas said it takes its safety reporting obligations very seriously and has been working with the FDA to ensure healthcare practitioners are aware of the latest safety information, which will be included in the drug's prescribing information.

"Changes to product labels are not uncommon," the company said in a statement. "Lexiscan and Adenoscan have been determined by the FDA to be safe and effective when use is consistent with the approved indications."

The FDA said it analyzed its safety database for Lexiscan for side effects reported between June 24, 2008, and April 10, 2013, and for Adenoscan from May 18, 1995, to April 10, 2013, correlating with the start of marketing for each drug.

The database contained 26 heart attacks and 29 deaths following administration with Lexiscan, and six heart attacks and 27 deaths following administration with Adenoscan. The reports did not always specify when the events occurred, but when the timing was specified, they tended to occur within six hours of a stress test, the FDA said.

The FDA said that due to limitations in the data, it was unable to discern a difference in risk between the two chemicals.