The number of Americans getting pacemakers implanted has risen in the past two decades - and the recipients are increasingly older and sicker, a new study finds.

In 1993, there were about 121,300 pacemaker implantations in the U.S. By 2009, that number was 188,700 - a 56-percent increase.

Most of the increase happened in the 1990s, researchers report in the Journal of the American College of Cardiology. Pacemaker implantations hit a peak in 2001, and have since leveled off.

But recipients' typical age has continued to creep up - as have hospital charges, the study found.

Pacemakers are small battery-powered devices that are implanted to treat certain problems in the heart's electrical activity - most often, a too-slow heart rate caused by dysfunction in the heart's natural pacemaker.

By normalizing the heartbeat, pacemakers can ease symptoms like fatigue and fainting, and help people be more physically active.

For the new study, researchers looked at a national database on U.S. hospital discharges. They found that between 1993 and 2009, nearly three million Americans had a pacemaker implanted. And over time, those patients became somewhat older and sicker.

Among those given "dual-chamber" pacemakers - the most common type used in the U.S. - the average age rose from 73 to 75 years.

And pacemaker recipients in 2009 tended to be in worse shape, being more likely to have co-existing conditions like diabetes or kidney disease.

The reasons aren't clear, since the findings are based only on hospital discharge records, according to lead researcher Dr. Arnold J. Greenspon, of Thomas Jefferson University Hospital in Philadelphia.

But it likely reflects the fact that more people are becoming candidates for a pacemaker, he said.

"Patients are living longer because we're doing a better job with medical therapy" for heart disease, Greenspon said in an interview.

The study also found that devices known as dual-chamber pacemakers, which work on two heart chambers rather than one, became increasingly dominant. By 2009, they accounted for 82 percent of all pacemaker implants.

"It's the technology of choice in the U.S.," Greenspon said.

Dual-chamber pacemakers are more expensive than single-chamber ones. But Greenspon said he doesn't think this is a matter of "doctors wanting to put in the most expensive device."

Guidelines from the American College of Cardiology and other groups recommend dual-chamber pacemakers for most patients. That's based on studies showing those pacemakers tend to be more effective at preventing certain heart rhythm problems and strokes, and for boosting quality of life.

But there's no way of telling from the current data, Greenspon said, whether doctors were sticking to guidelines with all of these patients.

What is clear is that hospital charges for pacemaker implantations have climbed. On average, charges increased by 45 percent - from $53,700 in 1993 to $78,000 in 2009, based on 2011 dollars, Greenspon's team found.

The question is whether those costs will keep going up, the researchers write - or, if they do, whether the "healthcare system can withstand this financial burden."

Some of the risks of having a pacemaker implanted may be inching up as well. In a study last year, Greenspon found that the rate of infection among people receiving either a pacemaker or an implantable defibrillator rose between 1993 and 2008.

There were 2,660 infections in 1993 and 8,230 in 2008. Still, less than two percent of all patients over the study period developed an infection.

Greenspon said the fact that today's patients are older and have more medical conditions may help explain the rising infection rate.

He and some of his colleagues on the study have received speakers' fees or research funds from companies that make pacemakers and implantable defibrillators.