A novel kind of pacemaker can substantially boost the pumping power of victims of congestive heart failure, offering an entirely new way of treating perhaps 1 million Americans with bad hearts.

Doctors tested the device on people with especially severe heart failure, all of whom were already receiving standard heart medicines. Two-thirds improved significantly.

"The data are clear and impressive, but what impresses me most is the experiences of individual patients, who have improved to a degree we never would have expected," said Dr. William Abraham of the University of Kentucky, who presented the results Tuesday at a medical conference in Orlando.

Earlier research has suggested that this approach, called cardiac resynchronization therapy, can energize sluggish hearts, but the new work is the first study to test it in a rigorous, scientific way.

Heart failure is the only form of heart disease that is actually on the increase in the United States. This is because it is largely a disease of the elderly, and many of those who once would have died of heart attacks now survive long enough to have this disorder.

Heart failure is not, as its name implies, the sudden failure of the heart to stop beating. Instead, it is an inability of the heart to pump powerfully enough to circulate the blood adequately. Symptoms include shortness of breath, fatigue and fluid accumulation.

Between 2 million and 3 million Americans have heart failure, and 400,000 new cases are diagnosed each year. It causes 39,000 deaths annually and contributes to 225,000 more.

Pacemakers are widely used to speed up hearts that beat too slowly. But using them to make weak hearts beat more powerfully is a new development. At least three companies are testing them in the United States.

In one-quarter to one-half of all heart failure victims, the heart fails to beat in a uniform way. The walls of their left and right ventricles _ the main pumping chambers _ do not squeeze at precisely the same moment. This sloppy motion robs the heart of its pumping power.

The new pacemakers deliver an electrical impulse that synchronizes the ventricles so their walls pump in unison.

Abraham and colleagues tested a device called InSync, developed by Medtronic Inc. of Minneapolis, which paid for the study. Doctors implanted the quarter-size devices in 266 patients. However, they left the devices switched off in half of the patients and followed their progress for six months.

Overall, 63 percent of the patients getting working pacemakers improved substantially, as did 38 percent in the comparison group. (At the end of the study, doctors turned on those patients' pacemakers, too.)

Abraham estimated that between 750,000 and 1.25 million U.S. heart failure patients might be good candidates for the devices. He said the devices are likely to cost $10,000 to $12,000 apiece, about the same as a high-end standard pacemaker.

Dr. Barry Massie of the University of California at San Francisco said it is still unclear whether the devices will help those with less severe heart failure or change the long-term course of the disease.

Nevertheless, he said, "The results are pretty exciting. It can only add to what drugs do. The two together could be remarkable."

Most of the patients studied were taking diuretics, ACE inhibitors and beta blockers, the standard medicines. Doctors classify heart failure on a scale of 1 to 4, and all of the patients were in the worst two categories.

After six months on the pacemaker, two-thirds of the patients had improved by at least one category. They also scored substantially higher on a scale that measured quality of life, and they were able to walk an average of 128 feet farther in six minutes.

The first patient in the study was a former World War II fighter pilot from the Cincinnati area. His heart was so weak that he had been given six months to live when he received his pacemaker in 1998. Now, at age 82, he can climb stairs, walk several blocks without getting winded and recently took a trip to New York City to see Broadway shows.