The first unexpected power failure in a Medtronic InSync III pacemaker didn't set off the alarm bells. But by the fourth time, doctors at the Minneapolis Heart Institute knew they had to act quickly.

They took their concerns about the discontinued-but-popular pacemaker to Medtronic in late September, after a fourth device among the 448 InSync IIIs implanted at the institute had a power failure.

By Thanksgiving, Minnesota-run Medtronic had issued a recall for nearly 100,000 of the devices worldwide, including all 22,000 that still rest in patients' chests. As many as 162 of the devices — fewer than 1 percent — may fail because of the power trouble, a Medtronic spokeswoman confirmed.

The recall affects three models of the InSync III — 8042, 8042B and 8042U.

In letters posted online last week by regulators in the United States and overseas, Medtronic acknowledged that its InSync III pacemakers have a higher-than-expected rate of power output failures. But a relatively low failure rate made the problem difficult to detect.

"Sometimes you could have random failure, and you don't know if this is an isolated event. … When we started to see two, three, and then four patients affected by potentially the same problem, that's when we took it straight to [Medtronic]," said Dr. Jay Sengupta, a heart doctor with in the Minneapolis Heart Institute's Pacemaker and ICD Follow Up Clinic Program.

Medtronic's cardiac device division in Mounds View is handling the worldwide recall, including about 9,300 implanted units in the United States.

"Due to the unpredictable nature of this issue, it is not possible to identify which devices might fail or when they might fail. The issue cannot be mitigated by programming changes or increasing patient follow-up frequency," Medtronic wrote to doctors, according to a copy of its "urgent field safety notice" published Nov. 26 in Germany.

Medtronic stopped distributing InSync III CRT pacemakers in 2011 because new models were approved at the time. Current devices have a modified battery design that is not susceptible to the battery issue, the company spokeswoman said via e-mail.

Cardiac resynchronization therapy pacemakers (often called CRT-P devices) can apply small electric pulses to both sides of the heart to keep the left and right chambers beating in proper sync. They can treat heart failure by improving the body's ability to pump blood and distribute oxygen throughout the body.

CRT-P devices use small computers to monitor heart rhythm and insulated wires called leads to deliver electric pulse to heart tissue.

The defective pacemakers have a glitch that limits how the power flows out of the battery, preventing the device from getting the power as it needs to function properly.

One death

"Medtronic has received one report of a patient death, where it is possible, but unconfirmed, that this issue was a contributing factor," the field safety notice says.

Medtronic confirmed that at least 30 devices have been affected by the issue as of Oct. 27, including some units that are just more than 4 years old. A dozen of the devices failed to correctly pace the heart, while the others showed "erratic" symptoms like significant fluctuations in longevity, early elective replacement warnings, and inaccurate power-flow readings from the leads.

The devices have an expected battery life of 5.8 to 7.5 years, depending on how the doctor programs the device settings.

The field safety warning says the devices have an estimated failure rate between 0.16 percent and 0.6 percent. That would translate into a worldwide total of 65 to 162 devices being affected by the battery problem, Medtronic spokeswoman Tracy McNulty said in an e-mail.

The FDA's Nov. 27 alert says the company will offer a supplemental device warranty.

"We regret any difficulties this may cause you and your patients," Medtronic's statement in the FDA recall says.

The recall was classified as Class 2, which means the FDA judged that the defect may cause only temporary or reversible adverse health effects and the probability of serious health consequences is remote.

Medtronic's letter to doctors about the problem notes that patients who need a pacemaker to keep their heart rates up should carefully weigh the risks of getting a replacement device vs. the surgical risks from an early device replacement, which are comparable.

Sengupta said the Minneapolis Heart Institute has already reached out to its InSync III patients to talk about options, including early replacement. A letter to patients noted that symptoms of lightheadedness or fainting may be signs that the pacemaker is not working properly.

Stock in Medtronic, which has corporate headquarters in Dublin and executive offices in Fridley, dropped 1.5 percent on Monday to close at $75.34.

Twitter: @_JoeCarlson