Fifty-seven years ago, when Medtronic co-founder Earl Bakken invented the world’s first battery-powered transistor pacemaker, it was the size of a small book.

Medtronic’s latest is about as big as a vitamin.

The world’s biggest medical device company just grabbed the mantle for having the ­smallest pacemaker cleared for sale by a government regulator. That approval came in Europe, Medtronic said Tuesday. Now, Medtronic joins crosstown device rival St. Jude Medical Inc. in speeding toward full commercial approval from the Food and Drug Administration for the sale in the United States of tiny pacemakers that fit entirely inside the heart.

Both devices are so small that they’re not implanted through traditional surgery, but by using a tube fed to the heart through an incision in the femoral artery in leg. Both companies are enrolling hundreds of patients with slow heartbeats in clinical trials that are needed to get U.S. approval.

“We view this technology as disruptive,” Medtronic’s cardiac rhythm medical director Dr. David Steinhaus said of the Micra Transcatheter Pacing System.

“It may be that someday all pacemakers use similar technology.”

Like the Bakken pacemaker, the Micra was invented in Minnesota. But unlike the Bakken, the Micra is not the first to market — St. Jude Medical in Little Canada nabbed European approval for a similarly small pacemaker called the Nanostim in 2013. It is slightly larger than the Micra.

Pacemakers are designed to electrically stimulate the heartbeats of patients whose heart rhythm is too slow because the chambers don’t communicate with each other properly, a multitiered condition known as heart block or bradycardia.

1 million pacemakers

More than a million pacemakers a year are implanted in patients worldwide. Pacers and a related group of cardiac rhythm devices called implantable defibrillators have long served as work horses for such large diversified devicemakers as St. Jude and Medtronic, which is based in Ireland but has the bulk of its operations in the Twin Cities.

But sales have grown stagnant in the past five years, and the question of whether implantable heart-rhythm devices are returning to sustained growth is a perennial topic in quarterly earnings calls with investors.

The new devices have design features that let them operate at much lower power, which allows their tiny batteries to last about as long as traditional pacers. But the devices also have some important limitations. For one, patients who lives long enough may end up having more than one implanted inside their hearts.

Dr. Charles Gornick, the cardiac arrhythmia director at Abbott Northwestern Hospital and the Micra principal investigator at the Minneapolis Heart Institute Foundation, said that after a couple of years the heart is likely to form tissue around the device that will make it impossible to remove.

Traditional pacemakers can be swapped out when the batteries die after 10 years or so.

Gornick said the Micra is expected to last eight years in the body — Steinhaus said 10 years — so eligible patients will have to think about their life expectancy and how many Micras they can safely have embedded.

“There is enough room for two or three of them in the heart,” Gornick said.

“In the younger population, you might say that’s a clear drawback because you don’t have a way to get it out.”

But he said the patients he’s implanted with the devices so far have loved the minimally invasive procedure, particularly the fact that they don’t have to have a special “pocket” created under the skin to hold a traditional pacemaker, which can leave a small bulge visible in some people.

The Micra also delivers its low-power current to the heart through an electrode on the device itself, eliminating the need for long wires that run from the pocket to the heart.

Tens of thousands of traditional heart leads have been recalled by device companies for unexpected failures in recent years. “In some ways, leads are the weak link of pacemaker systems,” Steinhaus said.

Finally, the Micra is a single-chamber device, meaning that it is intended only to stimulate electrical activity in the right ventricle. But at least 70 percent of patients need a pacemaker that can stimulate more than one chamber — which Medtronic sees as yet another business opportunity.

“Expect over time that we will have a dual chamber device, and perhaps even a triple chamber device with technology like this,” ­Steinhaus said.