New mini pacemakers from Medtronic and St. Jude Medical are expected to be more reliable and make the implant surgery less invasive.

About 3 million Americans have a pacemaker to prod an abnormally slow heart, which otherwise can cause dizziness, fainting, fatigue, and shortness of breath.

Traditionally, the devices are about the size of a pocket watch. But the pencil-thin pacemakers being developed by Medtronic and St. Jude Medical are shorter than a AAA battery.

"We've spent the last probably four years and invested in the $50 million range to really totally change how a pacemaker is going to be used in the future," said Pat Mackin, president of Medtronic's Cardiac Rhythm Disease Management division.

"This technology is revolutionary in that it is implanted directly inside the heart," he said. "There's no generator underneath the collarbone and there's no longer a wire that goes into the heart. It's all kind of self-contained in one unit. You'll get roughly the same longevity, the same performance from an outcome standpoint."

Snaked into a the heart through a blood vessel in the leg, the devices from both companies require no chest incision for the implant. Instead of connecting to heart tissue with wires that can fail or wear out, the mini-pacemakers attach directly to the organ's inner wall.

The Medtronic device resembles a marriage between a battery and a grappling hook, but it's only about as tall as a nickel.

Medtronic last month announced the first human implant of its mini-pacemaker, as part of a worldwide clinical trial. Initial results from the first 60 patients are expected in the second half of this year 2014.

St. Jude's tiny pacemaker, nanostim, was approved for use in Europe last year. The company expects to begin testing on human patients in the United States this year, as part of an effort to win regulatory approval here.

Mark Carlson, St. Jude's chief medical officer, said the smaller pacemaker will become the preferred choice of patients and their doctors. He said the devices should enable patients to recover faster from surgery, with much less chance of infection.

"With the standard device, you'll have a scar on your chest, a can underneath your skin that may be visible," Carlson said.

"With the new device, you won't have either one of those. With the standard device, you won't be able to move your arms; you won't be able to play golf for a few weeks. I may not even want you to drive for a few days. With the new device, you won't have those limitations."

So far, the new devices from St. Jude and Medtronic are for patients needing a single- chamber pacemaker. But the companies are working on dual-chamber devices, the type of device that accounts for the majority of pacemakers implanted.

Medtech companies also have been developing leadless defibrillators, which can deliver not only electronic pulses to maintain rhythm but also shocks to keep a heart beating.

The smaller devices address the weak links of pacing technology, said Dr. William Katsiyiannis, a cardiologist at Abbott Northwestern Hospital.

He said leads for pacemakers have tended to perform much better than leads on defibrillators, which have had some high-profile deadly lead failures.

"Most of the problems with leads have occurred in defibrillator technology," he said. "Because those leads have to carry high-voltage current and do multiple tasks, they're more complex and more subject to failure. Pacemaker leads tend to have much better performance records."

Katsiyiannis said its advantageous to forego using leads on pacemakers because they can wear out and need to be replaced, especially in patients who might get a pacemaker relatively early in life.

"If someone gets pacemaker in their 50s or 60s, which is not uncommon, we're talking about decades of performance of a lead that will in most circumstances need to be replaced," he said.

"Leads that go into the heart are under the stress of a beating heart over a hundred thousand times a day. They're bent and twisted by the heartbeat. That lead over time breaks down and the insulation or the wires themselves become subject to failure."

Studies will be done on how well the devices perform not only in pacing but how doctors manage them long-term.

"Do they stay in the heart and another one is placed next to them when the battery depletes or are they removed from the heart?" Katsiyiannis asked. "And if they're removed, what's that process like and how difficult might that be? There are some management issues that need to be thought through on any new technology like this."

Between 1993 and 2009, 2.9 million Americans received permanent pacemakers in the United States, according to a 2012 article in the Journal of the American College of Cardiology.

The rate of implantations rose from 46.7 per 100,000 persons in 1993 to about 61.6 per 100,000 persons in 2009.

As members of the Baby Boom generation age in coming years, the number of pacemakers implanted -- and sales -- should increase, Edward Jones analyst Jeff Windau said.

People who need pacemakers generally receive them when they are in their 70s, an age that the oldest members of that generation are entering.

"We definitely think the market is growing, with the baby boomers," he said. "We know there's an aging population."

Meanwhile, demand is expected to grow at a double-digit annual rate in some developing countries, including China, India and some Latin American nations.

"Those will all be areas that will support future growth for pacemakers in general, as health care not only gets expanded to more people but also the depth of coverage continues to increase," said Debbie Wang, a medtech analyst for Morningstar.

Wang said Medtronic and St. Jude likely will be rewarded with higher prices if the new devices prove reliable and meet patients' needs while lowering surgical and other medical costs.