Easy as 1-2-3: The initial Heartstream Forerunner, launched in 1996, was developed with the intention that anyone could use it by next uncomplicated prompts on the display screen.
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Sudden cardiac arrest is among the leading causes of death. Unlike a heart attack, in which blood flow to the heart becomes blocked, cardiac arrest occurs when the heart stops beating. The onset is sudden and comes without warning. Unless there’s immediate treatment, the chances of surviving cardiac arrest drop 10 percent with every passing minute. It takes paramedics an average of 8 to 12 minutes to respond, according to the National Safety Council. Do the math. It’s not encouraging for people in arrest.
Heartstream’s Forerunner radically improved the survival odds from sudden cardiac arrest by virtue of being the first automated external defibrillator (AED) that could be used by anyone, not just certified medical personnel, who are almost guaranteed to arrive too late to provide effective treatment. Now a common sight in airports, schools, and shopping malls, lunchbox-size AEDs are easy and safe to use and highly effective at administering a precise electrical jolt to restart the heart. Since 1996, when the Forerunner was introduced, AEDs have helped save tens of thousands of lives worldwide. The NIH recently estimated that AEDs save 1,700 people every year in the United States alone.
Heartstream engineers spent more a decade transforming bulky traditional defibrillators into a reliable automated device that anyone could use. The design of the Forerunner, introduced in 1996, was so innovative that it prompted the American Heart Association to conduct an intensive review revising what qualified as a safe and effective external defibrillator.
The team that designed the Forerunner got its start at a company based in Tacoma, Wash., called Physio-Control. In the mid-1980s, Physio-Control began developing a home defibrillator for use on people at risk for cardiac arrest. The unit the company came up with was certainly a solution, but implanted defibrillators proved to be a better one. Physio-Control’s device flopped on the market, according to Carl Morgan, who helped develop it.
Photo: Charlie Bennett/AP
How to Save a Life: By 1999, external defibrillators had started to become common in public places, such as Chicago’s O’Hare airport, where this Forerunner was installed.
Morgan and a few colleagues wanted to refine the product, but for a variety of reasons Physio-Control executives weren’t interested. In 1992 the engineers left the company to form Heartstream. Eager to get started, they didn’t know how to secure venture capital, so they pooled their personal credit cards, which they maxed out one after another. They were almost out of credit when they finally lined up some outside investors. “We did some stupid stuff,” Morgan recalls in an interview.
Heartstream had learned Physio-Control’s lesson: One AED per person wasn’t going to work, according to Morgan. Instead, the design team wanted the AED to be more like a fire extinguisher: a safety device that would be widely available, small enough to be hung on a wall (even smaller than the suitcase-size portable AEDs used by emergency medical technicians), and usable by anybody in an emergency to prevent things from getting worse—or maybe even solve the problem.
Then the engineers had to design a product to fit that vision. The traditional defibrillators used in operating rooms are large machines, capable of delivering enormous shocks—up to 350 joules and perhaps 20 kilowatts using waveforms associated with an RLC (resistance, inductance, capacitance) circuit. A wall-mountable and battery-operated AED would necessarily have to operate at lower energy. To be usable by someone with no medical training, it had to be automated. And it had to be safe enough so that users, certain to be under stress, didn’t end up injuring or killing either themselves or the person they were trying to help. In particular, the AED had to not work on anyone who wasn’t actually in cardiac arrest.
There was already a precedent for low-energy defibrillation. Implantable defibrillators used tiny batteries and delivered significantly smaller jolts than did traditional defibrillators. They managed to do that by using biphasic waveforms, which were much lower energy and worked just as well.
An RLC waveform in a traditional defibrillator has a single phase—in essence, one big surge of power. A biphasic defibrillator waveform, on the other hand, discharges energy in two phases, as the term suggests. The shock applied by a biphasic defibrillator uses 30 to 40 percent less peak current than a monophasic (RLC) defibrillator does at the same applied energy level.
Taking their cue from the implant manufacturers, Heartstream engineers experimented with biphasic waveforms, something that had not been previously attempted with external defibrillators. The engineers expected that the nature of the electric fields and waveforms would have to be different, given that an implant is placed next to the heart, whereas the fields from an AED would have to go through the chest wall. That turned out to be true, and the good news was that they also found that biphasic waveforms would work with an external defibrillator.
The Heartstream engineers also discovered that they could tailor the waveform to the individual patient. In fact, the results were better when they did so, because a personalized waveform accounts for variations in resistance due to a person’s size. To measure the resistance, the defibrillator’s two electrodes were placed on either side of the chest. The engineers devised a method that would measure the impedance between the two electrodes almost instantly. The range was roughly 50 to 100 ohms, according to Kent Leyde, another Heartstream engineer.
The Forerunner would then take an electrocardiogram (ECG), recording the electrical signals coming from the person’s heart. The developers spent months compiling ECG waveforms from heart patients, which they used to develop a sophisticated algorithm. The algorithm would first determine whether the person was in cardiac arrest based on the ECG and then figure out the appropriate waveform to deliver, taking into account the impedance measurement (which indicated the person’s size) and the characteristics of the ECG.
Meanwhile, the Forerunner’s high-energy capacitor was charging up to deliver the life-saving jolt. Measuring weak signals from the heart while simultaneously charging up the device presented a design challenge, Leyde recalls. “You want a noise floor down around, say, a couple of microvolts, and you’re charging a high-energy capacitor on the order of 2,000 volts. So you’ve got nine orders of magnitude separated by a couple of inches, which is a really interesting signal isolation problem,” he explains. “We used to joke that the front end was sort of like tuning a piano next to a rolling hand grenade.” The Forerunner used a single microprocessor, Leyde says, probably a Motorola 68HC16, which was a popular 16-bit embedded microcontroller at the time. The AED also included a couple of ASICs, high-voltage semiconductors, and high-voltage power supplies. Much of the other circuitry was laid out by hand because the company couldn’t afford sophisticated electronic design automation (EDA) tools, which are used to automate the design of electronic circuitry.
Photo: David Zalubowski/AP
Good Job: Michael Tighe—who as an employee of Boston’s Public Health Commission had supported the installation of portable defibrillators in the city—became the first person to be saved by one on an airplane during a flight from Boston to Los Angeles in November 1998.
To keep the Forerunner compact, Heartstream needed small but powerful capacitors, which weren’t widely available at the time. They decided to contact Maxwell Technologies, a manufacturer of advanced capacitors that primarily worked with large defense contractors such as Boeing and Northrop.
Heartstream cofounder Tom Harris was the guy in charge of the stack of credit cards, so he made the call, Morgan says. Harris explained what Heartstream was doing and asked Maxwell to create a prototype capacitor. “And they said, ‘Yeah, we can cook you up a prototype capacitor,’ ” Morgan recalls. “So Tom says, ‘Good! Do you take Mastercard?’ There was a long pause, and the guy from Maxwell finally said, ‘Well, we do now.’ So many people that we dealt with stepped up like that. It was wonderful.”
Heartstream anticipated that customers wouldn’t want to spend much time, if any, maintaining their AEDs, and yet the devices had to be ready to deliver a full pulse even after months of disuse. The company opted for disposable lithium manganese dioxide (Li/MnO2) batteries rather than rechargeable batteries, which had an unpredictable shelf life. The company also built in extensive self-testing, which included daily low-power CPU checks and power-supply/capacitor measurements.
Each month, the CPU would fully charge the defibrillator twice, “once to calibrate the capacitor value and once to stress-test the waveform-delivery system,” Heartstream project manager Dan Powers told the electronics industry magazine EDN in 1998. “A fail-safe circuit displays a red X on the LCD and chirps a piezo beeper if the CPU stops updating the LCD. This approach indicates the status of the defibrillator, even with extremely low battery power.”
Heartstream began selling the Forerunner in 1996. Not quite two years later, flight attendants on an American Airlines flight used a Forerunner to revive a passenger, making national news and demonstrating the value of having an AED on hand. Within a few years, the U.S. Federal Aviation Administration mandated AEDs in most commercial aircraft. Various jurisdictions passed “good Samaritan” laws to limit the liability of those who used AEDs, while other laws provided funding to distribute AEDs in public places.
For a time, Heartstream claimed to have more than 50 percent of the AED market. Hewlett-Packard bought the company in 1998 and folded it into its Agilent subsidiary, which then sold it to Philips in 2000. Philips rebranded the line as Philips HeartStart AED. The company claims it is still the leading provider of AEDs.
Morgan points out that the Smithsonian Institution has a Heartstream AED in its collection. “If you’re an American, you’re proud of that,” he said. He’s even prouder of the number of lives saved by AEDs, which he said number in the tens of thousands. “We paid our rent for our spot on Earth with the Forerunner,” he says.