"It is disappointing that bypass surgery o­n the beating heart does not translate to better long-term cognitive outcomes when compared to conventional surgery with a heart-lung machine," said study author Dr. Diederik van Dijk, a professor of medicine at University Medical Center Utrecht.

Loss of cognitive thinking function is common after bypass surgery, ranging from less than 5 percent to more than 30 percent in the year after an operation, according to the researchers. The hope of the study was that doing the procedure "off-pump" would preserve mental function.

However, there was some good news: "Long-term cardiac outcomes after off-pump surgery are as good as after conventional surgery," van Dijk reported.

The study included 281 coronary bypass patients; 139 were put o­n a heart-lung machine and 142 had the operation with their hearts still beating. After five years, 130 patients were still alive in each group.

Tests of mental function done six months after surgery showed a slight edge for the off-pump patients. But after five years, half the patients in each group showed some cognitive decline, defined as a 20 percent reduction in 20 percent of the functions tested.

"The results are just what I would have suspected," said Dr. Frank M. Sellke, chief of cardiothoracic surgery research at Beth Israel Deaconess Hospital and vice chairman of the American Heart Association's council o­n cardiovascular surgery and anesthesia.

Sellke was a lead author of a 2005 report that looked at more than 50 studies comparing o­n-pump and off-pump bypass operations. That study also concluded that the results of each method were about the same, as long as the surgeon and the hospital had the required expertise.

Before these studies, some experts had hoped that off-pump surgery might reduce the number of microemboli -- tiny fragments of blood clots that can travel to the brain and block small arteries, reducing mental function.

"But whether you do o­n-pump or off-pump procedures, in the vast majority of cases you are still putting a clamp o­n the aorta," Sellke said. That clamp o­n the heart's main artery, "disrupts the inside of the aorta, and those microscopic emboli still go to the brain and other organs," he explained.

Sellke said he favors o­n-pump bypass in his own practice. His advice for people facing bypass surgery: "I would go to the best surgeon and leave it up to the discretion of that surgeon. I would rather have an o­n-pump bypass and have the best surgeon."

But van Dijk had a different view.

"Because it is conceivable that off-pump surgery decreases the risk of perioperative stroke -- although definite evidence for this is not available yet -- I would personally prefer to undergo an off-pump procedure, and I would recommend the same to patients who have an increased risk of this terrible perioperative complication," he said.