The actual size of a Boston Scientific defibrillator battery made in Arden Hills. Pictured is the back cover of an unfinished battery. (The Minnesota Star Tribune)

When your watch battery dies, you slip in a fresh one and continue with your day.

When your implantable defibrillator battery gives out, you have to go under the knife. The surgery creates risks of infection, death or other complications, plus medical bills that can quickly climb into the thousands of dollars. Better batteries could spare patients pain and money, yet there's little economic incentive for manufacturers to make them.

At least a third of the 200,000 defibrillators implanted in the U.S. this year will need a replacement at some point because the battery reaches its expected life or wears out early. Some heart doctors say medical device companies could make batteries that last 25 years, instead of the current five to 11 years of expected life.

But manufacturers are focused on making smaller devices with ever-smaller batteries. And Minnesota's big heart-device companies don't get paid more if they produce devices with longer-lived power cells.

"Increasing longevity would reduce profits for manufacturers, implanting physicians, and their institutions," two U.K. cardiologists wrote earlier this year in an editorial in the British medical journal, the BMJ, echoing concerns in other journals.

Some of the world's longest-lasting defibrillator batteries are made on a whirring robotic assembly line in Arden Hills. Boston Scientific Corp. makes implantable defibrillator batteries at its factory there that are labeled to last more than 11 years, well beyond the five-year life spans of earlier defibrillators, which use powerful electric shocks to avert death from sudden cardiac arrest.

During a plant tour Thursday, Boston Scientific's battery factory manager agreed that it would be technically feasible to make a 25-year defibrillator battery today. Doing so would noticeably increase the size of the devices, which are smaller than a deck of cards.

"Honestly, if you had said, 'Let's make a 25-year device' — we probably could do it. I think we could do it. And I think we could do it via making a much larger device," Production Manager Steve Young said, regarding a 25-year battery. "Size is an important dimension. Size, and how efficiently you pack your space."

Defibrillators had list prices between $12,000 to $18,000 last year, according to industry data. Boston Scientific said its prices are not more expensive than competitors, even though their Minnesota-made batteries may last years longer.

Using unpublished company data, Boston Scientific estimates its line of EnduraLife batteries for implantable defibrillators last year saved the U.S. health care system $300 million in avoided replacement surgeries, compared to its older batteries. The company saw none of that money.

Instead, much of the economic benefit went to the health insurers that help defray the medical bills of people like Emily Herman.

The Shoreview woman was diagnosed as a teenager with hypertrophic cardiomyopathy, putting her at high risk for sudden cardiac arrest. Today, at age 33, Herman has already had two defibrillators replaced. The first time, in 2008, left her with a feeling of dread as she realized it needed replacing after five years — the expected life of the unit at the time.

"In 2008, I heard a little beeping going on inside of me, and my stomach dropped, because I knew exactly what it meant," Herman said of the implanted device's battery-life alarm. Her second defibrillator needed replacing just two years later for reasons unrelated to the battery. Her third device is now six years old, but seems to be going strong, she said during a panel talk sponsored by Boston Scientific in Arden Hills.

As engineers at many med-tech companies focus on making smaller devices, not larger ones, younger patients like Herman are left to wonder how many replacement surgeries they will need over the course of a lifetime. "I'm hoping to get close to 40 before I need to have that next one replaced," Herman said.

Economists and policymakers in Washington and beyond are designing and studying payment systems that could reward better-performing devices.

In health care, the phrase "value-based payment" is usually code for finding ways to pay more for higher quality or less for worse quality. With medical technology, where list prices tend not to rise, value-based purchasing should drive market demand toward technologies that avoid future costs, like expensive hospitalizations that can run double or triple the list price of the implanted device.

"Today, knowing where we are heading, all of a sudden battery life becomes a much more important aspect of the technology," said Philip Martin, executive director of the CentraCare Heart & Vascular Center at St. Cloud Hospital. "I think that all the companies are adopting strategies to increase battery life, knowing that this is where the entire health care world is headed."

Three med-tech companies active in Minnesota account for the lion's share of U.S. pacemaker and defibrillator sales — Boston Scientific Corp., Medtronic PLC, and St. Jude Medical.

It's difficult to compare their battery longevities side-by-side because they use different measurement standards and product specifications, and battery life can be affected by settings programmed by the physician and the level of impedance, or electric resistance, in the wires that send pulses from the generator to the heart tissue.

Companies typically shy away from sponsoring studies that pit competitors' performance head-to-head. And the technology evolves fast enough that it may not be fair to compare one company's current offerings with a device made by a competitor years earlier through a long-term study.

A 2012 study of 980 consecutive patients who got implantable defibrillators at a hospital in Bonn, Germany over a 21-year span concluded that Medtronic devices had the longest average life, at 5.8 years, followed by St. Jude at 5 years and Boston Scientific/Guidant at 4.7 years, according to the study report in the Journal of Cardiovascular Electrophysiology.

A 2015 study of 3,436 consecutive patients who got implantable defibrillators at hospitals in the Netherlands and Switzerland over a 20-year span also found Medtronic had the longest-lasting devices before 2006. But for defibrillators implanted in 2006 or later, 98 percent of Boston Scientific's devices were still in use at six years, compared to 73 percent for Medtronic and 61 percent for St. Jude, the study report in Europace said.

Boston Scientific opened its new robotic production line for batteries in 2007.

"In my opinion, Boston is leading that at the moment, but industry competition is going to continue to drive excellence as it relates to battery life," Martin said. "Whether that is Boston Scientific, or Medtronic, or St. Jude Medical, I'm sure engineers are rapidly working toward all aspects of the technology related to those devices."

Joe Carlson • 612-673-4779