NEW YORK — It seemed like a great idea: Do bypass surgery while the heart is still beating, sparing patients the complications that can come from going on a heart-lung machine. Now the first big test of this method has produced a surprise: Bypass has fewer problems and is more successful done the old way.

There were no signs of mental decline in those on the machines. Avoiding this problem was thought be one of the benefits of "off-pump" surgery without a machine.

Heart bypass is believed to be the most common surgery in the world — an estimated 253,000 Americans have the operation each year. Traditionally, the surgery is done while the patient is hooked up to a heart-lung machine, which takes over the job of circulating blood while the beating heart is stopped. Today, about one in five bypasses are done off-pump, and it's been hotly debated which is better.

The research reported in today's New England Journal of Medicine is the largest to date to compare the two techniques in a rigorous manner. The study involved 2,203 patients at 18 Veterans Affairs medical centers.