SCOTTSDALE, Arizona – The X26 is in a class by itself.

The electrified dart gun is sleek, lightweight and compact. Yet Taser International says its capacity to stop suspects is second to none.

Introduced in 2003, the X26 was Taser’s third model and quickly became its “gold standard.” Taser’s No. 1 moneymaker for a decade, the X26 contributed more than half of all revenues for much of that time.

But its takedown power came at a price: higher cardiac risk, a Reuters examination of scientific literature and corporate, court and patent records found.

When its darts impale flesh, the X26 delivers more electricity than any other Taser and almost twice that of newer “smart” weapons.

Taser stopped selling the X26 in the United States and Canada in 2014, telling police customers its analog workhorse was approaching obsolescence. That was five years after Taser launched the first of its “smart” digital weapons – all less likely to endanger the heart.

After signs of the outsize risk of the X26 surfaced, Taser neither alerted police nor recalled its best-selling weapon. It remains on the hips of police officers to this day. The company attempted to retrofit the X26 to reduce its maximum electrical output. In the end, Taser abandoned the effort.

Its engineers were unable to rein in the one attribute that, scientists told Reuters, is most responsible for a Taser’s capacity to stop suspects, as well as potentially endanger their hearts: its charge, the amount of electricity in each of its rapid-fire electrical pulses.

The greater the electrical output, the greater the risk to the human heart, said Dr Zian Tseng, a professor at the University of California at San Francisco and a cardiac electrophysiologist at UCSF Health.

“If there’s more energy, it’s able to capture the heart muscle easier,” said Tseng, who has published studies on Tasers funded by UCSF and the U.S. National Institutes of Health. “Therefore, it’s a higher cardiac risk.”

Taser says there has never been a reason to recall the X26. The risk of all of its weapons is low and outweighed by their benefits, company spokesman Steve Tuttle said.

Taser disputes the idea that the X26 poses special risks. Instead, it emphasizes that its newer models are safer. Its view, Tuttle told Reuters, is that because the charge of the new Tasers is “well below” the maximum charge of the X26, “it further increases cardiac safety margins.”

A weapon’s charge isn’t the only factor affecting safety, Tuttle said. Other considerations include the shape of the pulses of electricity emitted by conducted electrical weapons, or CEWs, as Taser calls them.

“By the proper definition, all Taser CEWs are extremely safe,” he said.

The company’s leadership does not believe any Taser has ever triggered a fatal cardiac arrest, Tuttle said.

Yet across the United States, medical examiners and coroners have cited the manufacturer’s stun guns as a cause or contributor in more than 150 deaths, Reuters reported in August.

Taser’s development of its “smart” weapons came at an inflection point driven in part by the very success of the X26. Taser had sold 425,000 weapons, most of them X26s, and counted 15,000 police agencies as customers. It was time to sell old customers new products, the company said.

“There’s a $600 million upgrade opportunity, and we’re going to be aggressively going after them,” founder and chief executive Rick Smith said on a 2009 call introducing the X3, the first of its “smart” weapons, to stock analysts. “These cops we’ve found out are like kids in a candy store.”

Taser encouraged cash-strapped cities to apply for federal stimulus funds to pay for the upgrades, and many did.

The plan coincided with peak litigation: 46 wrongful death lawsuits were filed over police use of Tasers in 2009.

The company sold its “smart” models as upgrades, touting improvements, such as laser sights and backup shots. Taser’s announcement for the X3 said its ability to adjust its electrical output “enhanced safety.” A footnote reported that its “output pulse” was less than the maximum delivered by the X26.

Taser offered millions in discounts, including hundreds of dollars off the price of a new weapon for every “legacy” trade-in.

Now that the company has persuaded many law enforcement agencies to upgrade, it has heard another complaint: The new Tasers don’t work as well as the old ones – posing a risk to its own customers.

A lawsuit filed by a Houston police officer challenges the game-changing promise Taser made about its stun guns – that the weapons operate in an electrical sweet spot, emitting just enough juice to take down even the most determined suspect but not enough to endanger the heart. Taser, which recently changed its name to Axon Enterprise Inc, disputes the allegations. (See related story).

CHARGING UP

There are many ways to characterize electricity: Volts, amps, joules, watts. Media coverage of the stun guns has tended to highlight the 50,000 volts that propel the Taser’s electricity.

But that voltage never reaches the targets. And it’s not what matters most when it comes to cardiac hazard. The key, as CEO Smith told a court in 2008, is the delivered charge, measured in microcoulombs.

Tasers send pulses of electricity along copper wires into targets through barbed darts. The charge is the number of electrons in each pulse.

Smith’s first electrified dart gun, the Air Taser introduced in 1995, emitted a charge of 70 microcoulombs per pulse. As Smith later described, that wasn’t enough to stop determined suspects.

In a demonstration in Prague, Smith told police in a letter, a “pumped up” volunteer “managed to fight his way through” the Air Taser.

Smith got to work developing a better Taser. He told the patent office it was a matter of public safety: If police officers relied on a too-weak stun gun to stop suspects, there could be “dire consequences.”

“The muscles must lock up,” he warned.

In 1999, Smith unveiled the M26, the first electroshock weapon designed to lock up muscles. The M26 packed a bigger charge, with each pulse delivering 85 microcoulombs of electricity, according to early electrical specifications published by Taser.

In 2003, Taser pushed the charge higher with the X26, in part by increasing the duration of each pulse. Early specifications said the X26 delivered 100 microcoulombs per pulse.

The X26 offers “the highest degree of takedown power ever available with the same level of safety,” Taser said in information distributed to police instructors in January 2004.

The X26 captured the market. Taser’s 2004 revenues, the model’s first full year of sales, surged 177 percent.

It was not only more effective but also lighter and more compact. The bulkier M26s tended to be left in squad cars. The X26 was easier to holster, to have at the ready.

There was an early complaint from cops: After the X26 knocked suspects down, some were able to get back up and break the Taser wires toward the end of the 5-second shock.

Initially, the X26 had been designed to deliver 19 pulses per second for the first two seconds, dropping to a slower rate of 15 pulses for the last three seconds. The idea had been that it would take more juice to knock suspects down than to keep them down, Taser explained in a 2004 bulletin to police. The slower rate would extend battery life.

Before the first year of sales was out, the company reconfigured the X26 – sending a software update out with a new power pack. With that, the X26 was retrofitted to deliver a reliable 19 pulses per second for the full length of a trigger squeeze.

Two years later, in 2006, a medical journal article carried some troubling news about the X26.

Canadian doctors published a study showing that both the M26 and the X26 captured pig hearts and sent them racing. The study also showed that the X26 captured them nearly every time – about double the rate of the M26.

Capture is not necessarily dangerous. But a heart captured by a rapidly moving outside source of electricity can lead to potentially lethal ventricular fibrillation.

The authors theorized the longer duration of the X26 pulse could explain its greater potency. Taser criticized the study, saying its use of synthetic adrenalin to mimic the effects of stress made the pigs’ hearts more vulnerable.

A VARIABLE POWER OUTPUT

In 2008, more than four years after Taser launched the X26, it learned that the output of the weapon could be as much as a third higher than it had disclosed in its early spec sheets. Taser updated its specifications after a company-funded study showed that when the darts impale flesh, the X26 can deliver a charge of up to 135 microcoulombs.

In 2009, Taser got another surprise: As Reuters reported Wednesday, a Taser “captured” a human heart.

The episode occurred during the company’s own test of the forthcoming model X3. The weapon’s electronic pulses grabbed the heart of a police volunteer identified only as Subject No. 8 and held it in thrall for 10 seconds. It raced at 240 beats per minute – four times his resting heart rate.

The researchers halted the test and notified company officials. Taser immediately lowered the charge and took the X3 to market with a target output of 63 microcoulombs.

“We had a reasonable scientific basis for the device being safer based on the lower charge,” Dr Donald Dawes, a research consultant on the study, told Reuters.

Two weeks after the capture episode, Smith introduced the dialed-down X3 to stock analysts. On a July 29, 2009 call, he said the X3’s safety margin was “about double” that of the X26. He compared the two weapons to different generations of motor vehicles.

The older stun gun “is like a 1999 minivan, very safe with the crumple zones and airbags and all that,” Smith said. “Is a 2009 minivan even safer? Sure, it's got side airbags now and there's been more technology advances.”

A training course Taser issued the next month explicitly linked safety to electrical output. According to the instructor’s notes for Slide 44, “63 charge units is about the optimal level to achieve incapacitation while maximizing safety.”

Taser considered retrofitting the X26 but abandoned the idea when engineers were unable to work it out, Magne Nerheim, Taser’s vice president for research, told a court earlier this year. In an email, Tuttle said the X26 lacked enough space and memory to add “charge metering technology.”

In September 2009, two months after No. 8’s episode, Taser issued its first cardiac hazard warning and advised police to avoid chest shots.