Medtronic says patients using its new battery-sized pacemaker had nearly half the rate of serious complications as people who get larger traditional devices, pointing to study results published Monday in the New England Journal of Medicine.

Minnesota-run Medtronic PLC is trying to get the U.S. Food and Drug Administration to approve its experimental Micra pacemaker, which is so small that it fits entirely in the heart's right ventricle and can be delivered through an incision in the femoral vein. Pacemakers, including a similarly small experimental one made by St. Jude Medical, monitor the heart and send tiny electric shocks if it beats too slowly.

The Medtronic-sponsored study found that 4 percent of the 725 patients who received a Micra pacemaker had serious complications that led to hospitalization and new surgery after six months. The study did not have a control group, but Medtronic estimated the serious complication rate for patients with a traditional pacemaker would have been 7.4 percent in that time.

"We wound up with about 50 percent [fewer] of the complications that traditional systems have," said Dr. Dwight Reynolds, lead author of the NEJM paper and principal investigator of the non-randomized single-arm study, called the Micra Transcatheter Pacing Study.

The Micra weighs 2 grams and is just over an inch long. It was designed by Medtronic staff mainly in Minnesota to fit a $700 million niche for small, single-chamber pacemakers that can be implanted with a catheter, avoiding the need for open surgery. The market size is expected to grow as the device companies figure out how to make the small devices effective for more complex heart-rhythm problems.

St. Jude Medical's experimental device is called the Nanostim, which has not fared as well during the testing phase, though it was approved for commercial use in Europe much earlier. Little Canada-based St. Jude got European approval in 2013; Medtronic did this year.

In September, research sponsored by St. Jude led to a paper published in the NEJM that found Nanostim patients had a 6.7 percent rate of major device-related complications at six months, compared with a 3.2 percent complication rate it estimated for traditional pacemakers, not including incidents of lead-fracture.

"These two studies demonstrate that leadless pacing is feasible and relatively safe, at least in the short term," Boston electrophysiologist Dr. Mark S. Link wrote in an editorial accompanying Monday's Micra study results in the NEJM.

Larger traditional pacemakers sit in the body within a "pocket" of tissue attached to the chest wall and deliver their electric energy to the heart via long wires called leads. Small pacemakers like Micra and Nanostim eliminate infections in the pocket and problems associated with leads by attaching to the tissue inside the heart and stimulating it directly.

But the devices are susceptible to their own risks.

The Micra study documented 11 traumatic cardiac injuries, including perforation or fluid buildup around the heart. That translated to a 1.6 percent rate of cardiac injuries among 725 patients, compared with an expected rate of 1.1 percent in patients who had traditional pacemakers implanted. Medtronic said that was not a statistically significant difference. None of the Micra patients had devices that became dislodged.

St. Jude's Nanostim study had a cardiac-injury rate of 1.3 percent, and six dislodgements, among 526 patients.

St. Jude officials highlighted the retrievability of their device, which was safely removed from seven patients, not including the dislodgements. "However, most of the devices … were explanted within 1 year after implantation, and there are few data on the feasibility of the removal of leadless cardiac pacemakers beyond this point," the article authors wrote in the NEJM in September.

Medtronic's Micra is not designed to be removable after a few months, when it becomes embedded in the body. But it can be switched off if a second Micra with a new fresh battery is implanted.

Last year, BMO Capital Markets analyst Joanne Wuensch noted that the worldwide market potential for the devices is about $700 million as currently designed. However, a much larger market may be coming down the road, once engineers understand how to make the devices applicable to more heart conditions.

The Micra and Nanostim are intended for patients who can get by with a device that stimulates only one chamber of the heart — the right ventricle. That includes people with pacing for atrial fibrillation or a slow heart beat called bradycardia, as well as patients who don't need frequent pacing.

A larger group of patients get heart-rhythm devices to manage more complicated problems like sinus-node dysfunction or heart block, or heart failure that requires stimulation to the left ventricle.

"Importantly, the results of these studies [on Micra and Nanostim] suggest the potential value of the next generation of leadless devices, which will include atrial-based systems and left-ventricular resynchronization systems that are more widely applicable," Link wrote in Monday's editorial.

Twitter: @_JoeCarlson