May 7, 1998

Defibrillator technology: Help in a Heartbeat

Warren Webb, Technical Editor

Advances in medicine normally come from the laboratories of white-coated scientists and physicians. This time, designers can take the credit for an evolutionary step in automatic-defibrillator technology that could save as many as 100,000 lives a year.   

On February 18, Robert Giggey, a 52-year-old North Carolina executive rushed to catch his American Airlines flight from Dallas-Fort Worth International. As he boarded the plane, his heart went into full cardiac arrest, and he lost consciousness. Within minutes, flight attendants and an off-duty paramedic were at his side following the audio instructions given by a small onboard medical device. A new generation of automatic defibrillators saved Giggey's life and promises to put lifesaving medical therapy within minutes of the onset of cardiac arrest.

Cardiac arrest is the nation's leading single cause of sudden death, striking more than 350,000 victims annually--almost 1000 per day. The victim's heart electrically malfunctions and stops pumping blood. Almost immediately, breathing stops, and the victim falls unconscious. Most victims die within 10 minutes. Yet, there is a medical treatment: A controlled electric shock applied to the heart by a defibrillator during the first 8 to 10 minutes can restore electrical activity and give the victim a good chance of survival. The problem is that most defibrillators sold over the past decade are large, expensive devices available only in hospitals and selected emergency-response vehicles. Currently, there's little probability of getting the required shock therapy, and most cardiac arrest victims die. The American Heart Association (www.americanheart.org) estimates that readily available automatic defibrillators could save 100,000 lives annually.

Cardiac arrest results from an electrical abnormality called "ventricular fibrillation." Chaotic electrical impulses stop the coordinated contractions of the heart, leaving a ventricular quiver and loss of blood pressure. Physicians are unsure of what causes cardiac arrest, and there are plenty of cases in which the victim has no history of heart problems. The average age of victims is 65, but cardiac arrest strikes many in their 30s and 40s. Even well-conditioned athletes, such as basketball star Hank Gathers of Loyola Marymount University (Los Angeles) have succumbed to sudden cardiac arrest (SCA). Although people often use the terms "SCA" and "heart attack" interchangeably, the two conditions differ (see box "Heart attack or cardiac arrest?").

As early as the 1950s, physicians recognized that an electrical current applied through the heart could stop ventricular fibrillation and restore a normal heartbeat. A basic defibrillator contains a high-voltage power supply, a storage capacitor, an optional inductor, and patient electrodes (Figure 1). A highly damped sinusoid waveform results when you discharge the capacitor through the inductor and the chest-cavity resistance of the patient. Most of the energy dissipates within a few milliseconds. Without the inductor, the circuit delivers a truncated exponential waveform. Both waveforms are common in today's defibrillators.

In early defibrillators, a physician read the patient's electrocardiogram (ECG) to assess the heart's rhythm before administering any shock treatment. The ECG patterns representing ventricular fibrillation are well-defined, and physicians have identified shockable rhythms requiring immediate defibrillation. Applying a shock to a patient with a normal ECG may disrupt the heartbeat, so it is important to verify that the heart is in ventricular fibrillation. To eliminate the need for an attending physician, manufacturers in the 1980s developed automatic defibrillators with circuitry to capture ECG signals from the therapy electrodes. All current automatic defibrillators contain processors to run an ECG-analysis algorithm that identifies shockable rhythms. ECG analysis takes about 5 seconds, and the CPU then advises the operator about whether defibrillation is necessary.

Low survival rate

These automatic defibrillators speed up ECG diagnosis and improve patients' safety; however, less than 5% of SCA victims survive. Patients sometimes die before the defibrillator arrives because emergency workers or physicians have only 8 to 10 minutes from the start of SCA to deliver the shock. In congested New York, where the average emergency-response time is 12 minutes, only 1% of SCA victims survive. Seattle's average response time is 4 to 6 minutes, and the survival rate is 30%. Today, only 50% of ambulances, less than 15% of fire-department first-response vehicles, and less than 1% of police vehicles carry defibrillators.

The low survival rate prompted the American Heart Association to develop its "chain-of-survival" program for cardiac arrest. This program includes a 911 call for help, cardiopulmonary resuscitation (CPR), early defibrillation, and advanced cardiac care. The program recommends that the first people to respond to emergencies--police, firefighters, security, and flight attendants--have access to automatic external defibrillators (AEDs).

Vendors recognized that there were problems with the widespread deployment of defibrillators. A defibrillator may be suited to a hospital emergency room or an ambulance, yet maintenance and training problems arise in low-usage situations, such as airlines, stadiums, and office buildings. Re-chargeable batteries require constant attention to ensure that defibrillators are always ready to produce a 50-kW pulse on command. In a study sponsored by the Food and Drug Administration (www.fda.org), the major cause of defibrillator failure was improper care of rechargeable batteries (see box "Defibrillators and regulatory standards"). This study prompted most vendors to recommend rechargeable-battery tests during each shift--a major headache at any low-usage site. A new trend in defibrillators is to employ single-use batteries. Heartstream, Physio-Control, and SurVivaLink now offer disposable lithium-manganese-dioxide (LiMnO₂) batteries in their AEDs. Lithium batteries provide a shelf life of five years and a standby life of more than one year. All AEDs contain battery-test circuitry to indicate a low-battery condition.

Automatic instructions

Operator training is also a problem in low-usage situations. The security guard that receives a defibrillator training session may feel uneasy in a high-stress cardiac-arrest emergency six months later. Manufacturers have responded with audio instructions and visual prompts to guide the operator through the defibrillation procedure. In a typical defibrillation sequence (Figure 2), the AED instructs the user to attach the patient electrodes and starts acquiring ECG data. If the AED analyzes the patient's ECG and detects a shockable rhythm, the capacitor is charged. Then, following the instructions, the operator presses the shock button to deliver the high-voltage pulse. Many jurisdictions and medical directors also require that the AED record the audio from the scene of a cardiac arrest for postevent analysis. Manufacturers take different approaches to scene audio. SurVivaLink's FirstSave contains a slot for a 4-Mbyte compact flash card, and the Heartstart 911 from Laerdal has an optional microcassette recorder.

All AEDs include a means to store and retrieve patient ECG patterns; however, Physio-Control's LifePak 500 extends this capability. After a defibrillation event, an operator can download LifePak's audio and ECG data through its RS-232C port to PC-based data-management software for analysis. The LifePak 500 stores events such as when the operator connected the electrodes, pushed the analyze button, and pushed the shock button, so that LifePak can display these occurrences in relation to the patient's ECG. Physio-Control's Code-Stat software accepts data from the LifePak 500 to create an integrated patient-information database for statistical reports or to review cases.

The ideal defibrillator for widespread deployment may sit on the shelf for a year between uses. Users want zero maintenance and a visual indicator of operational status. These requirements call for an extensive self-test schedule designed around failure probabilities and battery conservation. According to Heartstream Project Manager Dan Powers, daily low-power CPU self-checks, ROM CRC, power-supply/capacitor measurements, and partial waveform-delivery-circuit tests conserve power. Weekly tests add magnitude and phase response of the ECG-acquisition circuitry. Each month, the CPU twice charges the defibrillator to full energy--once to calibrate the capacitor value and once to stress-test the waveform-delivery system. The CPU actively drives a front-panel LCD to indicate that the defibrillator is operating. 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.

Lower voltage waveforms

One clever technique to reduce the size and weight of defibrillators is to reduce the amount of energy they deliver to patients. Defibrillators from Heartstream use a biphase, truncated-exponential waveform. Clinical studies show that reversing the current direction during the application of the waveform reduces the amount of energy necessary to stop ventricular fibrillation. An H-bridge circuit switches the current direction and provides waveform timing (Figure 3). Heartstream also compensates for patient chest resistance by evaluating the slope of the discharging capacitor voltage and adjusting the length of each waveform phase. The lower energy waveform creates many opportunities to reduce the size of the product. Lower voltage levels reduce the size of and clearance distances between high-voltage components. The largest benefit of size and weight reduction is a smaller battery system. In addition, each vendor developed its own operating software to minimize the processing system drain on the battery.

"We had to keep this thing very inexpensive and low-power," says Heartstream's Powers. The defibrillator runs on a battery that must last for a long time to give customers value, he says. The company's programmers hand-coded the time-critical portions of the algorithm and wrote everything else in C. For code safety, the company placed an independent watchdog on the processor and program-flow monitoring into the software. To spot software errors, the company designed user-interface- and algorithm-state-machine transitions around Hamming distances--the number of bit differences between two equally long code words.

Even with all the technology problems solved, cost will be a major factor in putting defibrillators in the hands of those likely to be first on the scene of a cardiac arrest. Companies must deploy defibrillators in thousands of police, security, and fire vehicles. Even at their current prices of $3000 to $4000, widespread defibrillator deployment will be expensive. At higher production rates, you can expect the prices to decrease somewhat, but an FDA-regulated medical instrument will remain costly.

Defibrillator vendors have produced instruments suitable for widespread deployment. The units are small, lightweight, easy-to-use and have zero-maintenance standby times. Thanks to the electronic designer, the technology is now in place to meet the goal of public-access defibrillation.

Reference

1. Poole, Jeanne E, Roger D White, et al, "Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest," Journal of Cardiovascular Electrophysiology, December 1997.

Acknowledgments

Thanks to Carl Morgan and Dan Powers of Heartstream, Paul Tamura of Physio-Control, and Gerry Heslin of Laerdal Medical for their assistance.

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