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Defibrillator saves lives

Defibrillator saves lives

You've all seen the dramatic scenes on TV or in movies: An emergency medical technician or emergency-room specialist gets a cardiac-arrest case, and time is precious. The specialist quickly readies the victim, fires up the defibrillator unit, slaps on the paddles, and yells "clear." A few commands of "clear" and some jolts later, the patient comes back to life or, after a longer wait and more applied shocks, a doctor declares the patient dead.

The key factors in the outcome of this familiar scene are the defibrillating shock, along with the time period that elapses before applying it. The prospects of a successful outcome for the patient drop dramatically with every minute after the cardiac arrest occurs. Yet, defibrillator units are relatively costly and require considerable training to use. These units do more than administer a fixed, high-voltage zap. Instead, they must deliver a controlled, complex waveform into the variable impedance across the patient. This waveform differs from individual to individual, and the defibrillators must deliver these jolts while monitoring the faint electrocardiogram signals on the patient's skin. The sequence and system settings must be just right to be effective: too few joules or the incorrect waveform give an ineffective jolt; too many joules produce an overwhelming jolt that may seriously harm the patient.

To address these problems, Royal Philips Electronics began to sell its HeartStart home defibrillator to consumers in November 2002 after receiving Food and Drug Administration approval. Consumers must have a doctor's prescription to purchase the device. The company designed this unit, which took about two years from project initiation to engineering release to complete, with the key goals of adherence to FDA and mandatory medical protocols; ease of use by untrained users, or "lay responders"; and low cost. The fully self-contained, battery-powered unit measures 6.6321.8321.8 cm, weighs 2.1 kg with batteries, and sells for a retail price of $1,995.

A Shock to the System

Many U.S. patents cover the system design embedded in the HeartStart unit. One patent discusses the waveform it uses and the algorithm that controls this waveform; the other focuses on the circuitry design and the self-test algorithms. Although you might assume that the Philips unit uses a high-end processor, perhaps adding a DSP, the company chose a different approach. Instead, the unit uses a Motorola HC16 processor and an HC08.

The operational sequence begins before a user applies a jolt. The unit has a voice that tells the probably panicked lay responder what to do. The unit has no written instructions and displays no front-panel messages. The responder applies special pads to the patient, the unit senses that they are applied, and an automated sequence takes over. The system first reads the patient's ECG (electrocardiogram) to determine if a shock is appropriate treatment, and it simultaneously determines the impedance of the chest by injecting a small ac current and measuring the resulting voltage drop. With this information and its determination that the patient requires a defibrillation shock, the unit is ready to go.

The defibrillation waveform itself has total energy of about 150J, delivering about 1,800-V peak value and 20-A peak current; nominal load of the patient is 90-V. (Note that the voltage and current levels, energy, and protocols differ widely between internal and external defibrillators and pacemakers; these numbers apply only to external units).

Power for the unit comes entirely from a series-connected set of nine standard CR123 lithium batteries, such as those that cameras use, supplying a nominal 9V with 4.2-Ahr capacity. For ease of replacement, they are available in a custom battery pack rather than individual batteries, which users could insert incorrectly. The battery pack has enough reserve to deliver 90 typical shocks and provides standby power for four years.

The unit delivers the shock waveform once, and the defibrillator monitors the sourced waveform and then measures the patient's ECG to determine whether the jolt was successful. The American Heart Association protocol recommends that a defibrillator provide as many as three consecutive shocks, and the operator should then perform CPR (cardiopulmonary resuscitation) if the patient does not respond. Users of this unit, however, may not know or may be unskilled at CPR, so the unit uses a calm voice to coach the user through the CPR sequence. It also keeps prompting the responder to administer jolts until the patient is revived or the battery runs out. The defibrillator delivers a waveform that must meet stringent requirements and is critical to increasing the likelihood of success.

The design uses standard, off-the-shelf components, including a custom standard-cell ASIC that provides various circuit functions and supervision of disparate parts of the system. The designers of the unit simulated the performance of the ASIC simulating the critical 1.6 seconds of its actual operating time. The simulation took about 11 days on a high-end PC. Other than the simulation tool, the designers used no advanced EDA tools. The primary tools were standard board-layout software and numerous physical prototypes, which tested many packaging- and cost-related concepts. A key to minimizing cost was to reduce the number of internal pc boards, despite the mix of conventional and high-voltage signals.

Despite the waveform's complexity, the designers chose to use neither digital-waveform-synthesis techniques nor a supercapacitor, sometimes chosen for high-energy, high-rate-discharge applications. Instead, they chose a 100-mF film capacitor. The circuit controls the discharge through SCRs (silicon-controlled rectifiers) and an IGBT (insulated-gate bipolar transistor) with special attention to the di/dt parameter. The discharge-control circuitry generates the positive-going waveform, turns off, and then generates the negative-going waveform.

The designers also did not digitally synthesize the soothing voice that guides the untrained user. They instead used the voice of Peter Thomas, the reassuring narrator of the Public Broadcasting System Nova series. The defibrillator stores the voice and re-creates his speech using standard ADPCM (adaptive-differential-pulse-code-modulation).

In addition, testing the unit was critical to the design team, which faced the issue of how to know whether the design works to the mandated medical protocols and specifics. The designers needed to ensure that the unit, which can sit unused for long periods, will still perform properly.

To address these issues, the team used industry-standard patient simulators from Symbio and Dynatech-Nevada, now Fluke Biomedical. For the ongoing system check, the unit performs a daily self-test, as well as a series of tests when a user replaces the battery pack. These tests check battery condition, internal circuitry, the overall waveform-generation and -delivery subsystem, and calibration of key components and signals.

The self-contained HeartStart also has some communications capability. It records the first 15 minutes of the ECG and key parameters of the incident, and can transmit to a PC. Philips provides event-review software for viewing, analyzing, and managing the data from the defibrillator.

In Control: The architecture of the unit relies on one custom ASIC (a) to implement a low-level data-acquisition and analysis system working with a high-voltage, carefully controlled signal source (b).
Web Resources
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Royal Philips Electronics:
HeartStart home defibrillator:
TAGS: Medical
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