Electronic Design

A Prescription-less Portable Defibrillator

The HeartStart low-cost automated external defibrillator for home use breaks new ground in consumer health electronics.

Clever design, close attention to human safety issues, and a top man-machine interface make the Philips HeartStart portable home defibrillator one of the hottest medical electronics devices on the market. Although other companies make portable defibrillators (Agilent Technologies, Cardiac Science, Welch Allyn, HeartSine, Defibtech, Medtronic, and Zoll Medical, to name a few), the HeartStart is the first to be available over the counter without a medical prescription.

The device is low enough in cost (which keeps decreasing) to make it affordable for mass-market consumer applications. Though the most recent price for the HeartStart has hovered under $1300, it's expected to drop below $1000 in the near term as more mass retail outlets like Walgreen's, Staples, and Sam's Club, as well as Web sites like amazon.com and drugstore.com, start offering it for sale. The defibrillator can even be purchased through the company's own Web site at heartstart.com.

The HeartStart is part of a new generation of automated external defibrillators (AEDs), a class of life-saving devices made possible thanks to advances in medical electronics technology. These devices allow lay persons to intervene in instances of sudden cardiac arrest (SCA), saving millions of lives (see "A Chronology Of The AED's Development," p. 42).

Interest in portable AEDs is mounting, spurred on by government legislation and decreasing end-user costs. Two driving forces have been the Government's Cardiac Arrest Survival Act (CASA) for federal public buildings and the Rural Access to Emergency Devices Act (RAEDA), which enables rural communities to purchase AEDs and provides local community training.

But until now, three factors have limited widespread acceptance of AEDs in the consumer market: high cost, the general fear of hurting someone while using an AED, and a lack of public awareness and education about the functionality of AEDs. The Heart-Start AED has answered all three factors (Fig. 1).

Over 6000 HeartStart AEDs have been deployed in homes since the U.S. Food and Drug Administration (FDA) approved it for over-the-counter sale last Sept. 16. That action follows the recommendations of the American Heart Association (AHA) and the FDA's System Devices Panel.

The most recent information furnished by Philips shows more than 125,000 HeartStart AEDs deployed, including homes. Other locales include airplanes, airports, stadiums, workplace offices, plants, shopping malls, schools, and casinos. ( According to Philips, it has the largest worldwide market share of AED revenues at 45%. It also claims the largest U.S. market revenue share at 42%.)

Market analyst Frost & Sullivan sees a huge market for home AEDs, also known as public-access defibrillators or PADs. In fact, with 121 million homes in the U.S., that segment will become the largest in the overall market (see "A Glossary Of Terms" at www.elecdesign.com, Drill Deeper 11660). The firm reported $392 million in total AED revenues for U.S., European, and Japanese markets for 1999, and that will grow to over $1 billion by next year.

THE NEED FOR AN AED
According to the AHA, SCA is one of the leading causes of death in the U.S., claiming more than 340,000 lives a year—more than breast cancer, prostate cancer, AIDS, house fires, handguns, and traffic accidents combined.

Studies also show that despite the widespread deployment of AEDs in public places, 70% of all cardiac arrests occur in the home, where an AED can potentially save thousands of lives. The same studies show that when an SCA occurs in a home, the event is witnessed in more than half the cases.

Digging into the numbers further,-another study shows that the longer it takes to treat SCA victims, the smaller the chances of survival. Every minute counts. Calling for emergency medical services or looking for the nearest PAD may take too much time to save a life. In fact, chances of survival in such cases, according to the AHA, drop to a dismal 7% to 10%. But according to the AHA, the quickness of home AEDs can save 40,000 lives.

The whole idea behind the HeartStart AED was to give the victim enough time before calling emergency medical services. Shocking the heart back into action, on the spot, provides more time for other help to arrive to stabilize a patient and increases the victim's odds of survival.

SIMPLICITY SUPPRESSES COST
A primary goal for Philips' HeartStart team was to keep the device's design as simple as possible and reduce the parts count, yet still retain the highest levels of reliability and user-interface friendliness. "To bring the defibrillator down to the $1500 range required a very careful design with a minimum number of hardware components," explains David Snyder, senior research scientists at Philips.

Many standard off-the-shelf parts were used, save for a standard-cell ASIC made by Fextronic (Fig. 2). The main components of the HeartStart's circuit include a Motorola 68HC16Z1 13.1-MHz 16-bit microcontroller, a Motorola 68HC908KX8 8-bit supervisory processor, an Analog Devices AD8606 very low-noise CMOS op amp, 4 Mbytes of flash memory, 128 kbytes of RAM, an 8-bit analog-to-digital converter (ADC) that samples at 10 MHz, power-supply components, and the ASIC. Code-named "sarapo" (electric eel in Spanish), the ASIC includes a very large hardware decimate filter, voice generation and decompression circuitry, therapy-engine control, and miscellaneous glue logic parts.

NON-INTIMIDATING TO USE
The HeartStart includes a strong user interface that was designed not to intimidate the average person (Fig. 3). It's available in a 3.3-lb zippered Velcro red carrying case (including pads and battery). The case even has fabric scissors in a top flap, allowing anyone to neatly cut away a patient's clothing before using the defibrillator. The AED also can be used while it's in the carrying case.

The user interface was one of the most important aspects of the HeartStart's design. It makes the HeartStart very easy to use, and it's nearly foolproof in the HeartStart's application. "A little over one-half the flash memory we use is taken up by voice prompts," explains Tim Hanson, R&D project manager.

According to David Snyder, "We performed extensive human-factors testing to ensure that the HeartStart would be a very viable end-user product."

"Our user interface is the primary reason why our AED can be obtained without a medical prescription, while AEDs from other manufacturers require prescriptions," Hanson adds.

"One of the biggest challenges was to ensure that the user correctly and definitively places the HeartStart's pads in the correct locations on the patient," Hanson continues. "We developed some very good techniques for detecting patient motion to minimize damage to the patient. This is to ensure that the AED isn't fooled. We made sure that prompting by the HeartStart is such that the next command doesn't occur unless certain procedures are performed by the user."

Pads that come with the HeartStart use digital technology to immediately analyze the heart's rhythm, allowing the user to determine whether or not the patient needs a shock from the AED. Everything is clearly explained by audible commands during each step of the process, including instructions during and after applying an electrical shock if needed.

I examined a HeartStart AED courtesy of Philips, and all of the aforementioned features are indeed there and easy to use. Fortunately, there was no patient to try it on. But Philips supplied the unit with a set of training pads as well as a practice plastic mat that substitutes for a patient. The mat has a drawing on it with an outline of a person's chest.

The unit delivers a truncated exponential biphasic waveform, consisting of 150 J in a 50-½ load, with a shock-to-shock cycle time of about 20 seconds. The level of the shock signal is 1869 V. According to the HeartStart's designers, this necessitated placing all of the computing circuitry in a metal shield enclosure about 2 in. away from the high-voltage circuitry.

The AED comes with a 9-V, 4.2-Ah disposable lithium-manganese-dioxide battery with a lifetime of four years after it's first used. It offers a capacity of 200 shocks minimum or four hours of operating time (see "Battery Design Issues For An AED," p. 40).

NEED MORE INFORMATION?

Agilent Technologies Inc.
www.agilent.com

Cardiac Science Inc.
www.cardiacscience.com

Defibtech Inc.
www.defibtech.com

Frost & Sullivan Inc
www.frost.com

HeartSine Technologies Inc.
www.heartsine.com

Medtronic Inc.
www.medtronic.com

Welch Allyn
www.welchallyn.com

Zoll Medical Corp.
www.zoll.com

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