A new study of malfunction reports says hundreds of thousands of special wires called “leads” for heart defibrillators made by St. Jude Medical are vulnerable to a problem that can cause unneeded high-voltage shocks or a sudden loss of lifesaving therapy in unsuspecting patients. 

The company’s Durata leads are susceptible to internal insulation breaches that “may result in serious adverse events without forewarning,” concludes the paper, which was underwritten by the Minneapolis Heart Institute Foundation. The paper also says standard tests may not uncover these insulation defects.

Losing defibrillator therapy in this way could be fatal because patients may not realize their device won’t correct potentially lethal heart problems like ventricular tachycardia. Conversely, receiving unneeded defibrillator shocks can be psychologically traumatic for some patients.

The devices’ maker stands behind Durata leads’ long-term safety and durability, and the devices remain on the market. Nearly 300,000 people in the United States have St. Jude’s Durata heart-device leads, which deliver current from an implanted defibrillator to restore a normal heart beat or shock the heart back to life.

The study in the peer-reviewed journal HeartRhythm examined more than 1,000 reports of lead failures and found 137 reports of Durata failing due to internal insulation breaches, including one case with a patient death. It doesn’t report an overall failure rate, instead quoting a company report that says 97 percent of the leads were free from mechanical failures 10 years after implant.

“The fact is, over time, all leads that we put into patients will begin to break down and could fail. To some extent, that is expected over time,” said Minneapolis Heart Institute cardiology chairman Dr. William Katsiyiannis, who did not work on the study. “What this identifies is a failure mechanism that potentially creates a situation where therapy can’t be delivered, or inappropriate therapy is delivered in the form of a shock. Because of this specific mechanism of failure, we are taking notice.”

St. Jude Medical, which has offices just north of St. Paul, no longer exists as a stand-alone medical device company after it was acquired by Chicago-based Abbott Laboratories in 2017.

An Abbott spokesman said Friday that comprehensive real-world study data published by the company support the Durata lead’s safety.

“Every year, our real-world performance data consistently show our cardiac leads are some of the industry’s most durable and reliable leads. They have a strong history of performance and safety and one of the industry’s lowest rates of lead insulation breach,” Abbott spokesman Justin Paquette said in an e-mail.

Durata leads found wider use after insulation defects prompted mass recalls of similar St. Jude devices, known as Riata or Riata ST leads. But the study published Feb. 26 in HeartRhythm found that Durata leads are also susceptible to insulation breaches.

“These [Durata] defects appear to be similar to those found in Riata and Riata ST leads, and they occurred despite the addition of Optim [insulation] to the Durata lead,” the report in HeartRhythm says. The lead author is retired Minnesota cardiologist Dr. Robert Hauser, who has a history of blowing the whistle on heart devices.

The HeartRhythm report was based on information gleaned from public adverse event reports filed in the Food and Drug Administration’s MAUDE database — a source of information that medical device companies routinely criticize as unreliable, incomplete, subject to reporting bias and not representative of the wider population.

Indeed, an FDA disclaimer says the MAUDE reports are not intended to be used to evaluate rates of adverse events, and the filing of a report does not mean the manufacturer believes its device caused or contributed to the health problem.

However, MAUDE is the only public source of narrative information about individual malfunctions and injuries, and federal law says device makers must report any instance in which anyone at the firm learns that a device may have caused or contributed to a death or serious injury.

Like other device makers, Abbott regularly publishes product performance reports that track overall rates of problems. The public-facing versions of the reports typically consist of statistical graphs and summaries, but no details on specific patients.

“Durata lead performance continues to meet expectations by all measures,” Abbott wrote in a 2018 product performance report. The report noted a “very low rate of abrasion failures that have been identified on Optim-insulated defibrillation leads.”

Rates of insulation breaches varied from 0.08 percent to 0.38 percent, depending on the exact model of Durata used, according to company-published data encompassing 291,000 total implanted leads.

By searching the FDA’s MAUDE database, Hauser and his team identified 1,044 reports of lead failures between 2008 and 2018 involving three different devices: St. Jude’s Durata leads, Medtronic’s Sprint Quattro Secure leads, and Boston Scientific’s Endotak Reliance leads.

Those reports included 293 reported cases in which Durata leads had insulation breaches, including 137 in which the insulation was breached from the inside out. All told, insulation breaches accounted for 93 percent of the Durata failure reports located during those 11 years. About 16 percent of the Medtronic lead failures were from insulation breaches, and 38 percent of the Boston Scientific lead failures were.

The adverse event reports included 11 confirmed cases of St. Jude Durata leads breaching their internal insulation and failing to terminate dangerous heart rhythms, and another 51 cases of internal insulation breaches associated with inappropriate high-voltage shocks or low-voltage pacing.

In the report’s only documented fatality, a patient with a Durata lead fell into a vegetative state and later died after his device failed to give him a high-voltage shock that he needed.

According to the MAUDE report, the device passed in-office testing before the patient’s death. But a postmortem analysis found the lead was badly degraded, with the external and internal insulation worn away, and one of the internal cables, known as a conductor, showed “slight melting.”

A defibrillator lead is not a simple device. From the outside it may look like a basic wire with a plug on one end for the pulse generator and a sharp hook or screw on the other to permanently attach to the heart. But inside, leads contain several paths for energy to travel to and from the heart. Some carry sensing signals, while others send voltage to the heart.

A lead must be thin enough to fit inside a blood vessel without blocking it and durable enough to reliably survive thousands of flexions inside the hostile environment of the body.

The HeartRhythm report speculates that the Durata’s unique vulnerability to erode its insulation from the inside out stems from a design in which two conductor cables are contained inside a single channel that is separated from a rigid “shocking coil” by insulation. Under constant movement, the cables may eventually break through to the coil, triggering either signal noise or a short-circuit, depending on which cable is exposed.

The study recommends that doctors make sure all alerts and notifiers are activated for patients, and that patients understand them.

“I’d like to see doctors take active steps to detect these failures and mitigate potential failure with appropriate device programming,” said study co-author Dr. Edward J. Schloss, division chief of cardiac electrophysiology at the Christ Hospital in Cincinnati. “The information we report should be used to inform physician lead implant choices and industry design choices.”