[Success Story]
A Prescription-less Portable Defibrillator
The HeartStart low-cost automated external defibrillator for home use breaks new ground in consumer health electronics.
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).
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