Chicago ? New research has overturned one of the most fundamental beliefs among doctors treating heart attacks: that opening a blocked artery is always a good idea, even days or weeks later.

Instead, the study revealed that doing this too late may not help, and there were disturbing hints that it might even be harmful. People who had balloon angioplasty to open an artery three to 28 days after their heart attacks fared no better than those given standard medicines to prevent a second attack.

The results don’t apply to most Americans suffering a heart attack but suggest that 100,000 of them a year might be able to skip the expense and risk of angioplasty and take medications instead, doctors said.

“These findings were really a surprise,” said Dr. Elizabeth Nabel, director of the National Heart, Lung and Blood Institute, which funded the large international study. “For a long time we thought that opening up the artery any time after a heart attack was better than leaving it closed. My guess is you may see some guideline recommendations” on what to do in such situations, she said.

Opening arteries quickly is crucial to surviving heart attacks, and the study’s findings do not change the need for urgent action or the evidence that angioplasty saves lives when done soon after an attack.