Put on your private paramedic

Pittsburgh-Defibrillators are medical devices that deliver life-saving shocks to help jolt failing hearts back to normal function. According to a study sponsored by medical device manufacturer Guidant (Indianapolis, IN), implantable cardioverter defibrillators (ICDs) offer a 30% reduction in fatality rates. Consequently, an estimated 60,000 new implanted defibrillators are installed each year in the U.S. at a cost of about $40,000 per procedure. However, for patients identified as candidates for an ICD or heart transplant, the heart attack may strike while they're on the waiting list for the procedure.

External defibrillators available to the public through EMT services aren't much help for the 70% of all heart attacks that occur at home or where no ICD is available, because the chance for survival may decrease 10% for every minute that the patient has to wait.

To help improve patient survival rates, the FDA has recently approved the WCD 2000, a wearable cardiac defibrillator from Lifecor. The system is the first defibrillator cleared in the U.S. as a device to be worn externally. For those at temporary but high risk for sudden cardiac arrest, or who are not indicated for an implanted defibrillator, it's like having a private paramedic that they can wear.

The FDA approved the system based on a review of laboratory and animal tests, and the company's clinical trials. It reported 289 patients fitted with the defibrillator vest at 16 medical centers in the U.S. and Europe. In the trials, the defibrillator successfully treated sudden cardiac arrest episodes 71% of the time, vs. a 25% success rate for patients who called 911 reporting a heart attack. All were either awaiting heart transplants or had recently had a heart attack or a coronary bypass operation. Average time of wear was about three months, for 20 hours each day.

Like implanted defibrillators, the device uses monitoring sensors to detect dangerous arrythmia (irregular heart rhythms), and then delivers a life-saving electrical pulse via patented dry-to-wet electrodes. Intended for short-term use, it promises cost advantages through rental or leasing programs, according to VP of engineering Marshal Linder.

The battery-powered, monitor/defibrillator, which is worn around the patient's waist on a belt, contains two subsystems: a computer/analog board that constantly monitors and analyzes the patient's heart rhythms, and the defibrillator itself, which produces the high energy (2,000V) monophasic pulse for delivery to the patient. Before initiating and controlling the treatment, it sets off an alarm unit that warns the patient and bystanders.

The monitor/defibrillator connects via cables to four sensing electrodes and three treatment electrodes held in place by a strap-like vest garment. Sensing electrodes pick up ECG signals from the heart through the skin, operating in pairs to create two independent channels. One treatment electrode located in front over the apex of the heart under the patient's left breast, and two electrodes on the back, create an electrical path to deliver the shock directly through the heart.

Comfort is key. A major issue for any device designed to be worn continuously, except while bathing, is comfort. Engineers came up with a design for the shocking electrode that uses gel capsules to discharge electrolytic gel only when needed. It consists of a dry foil plate separated from the skin with a loose mesh cloth material. Because each capsule has a seal that retains the gel prior to activation, the electrodes are normally dry.

Each electrode is a laminate constructed of multiple layers of preformed and unformed plastic film, foam, and metallic foil. An outer shell protects an array of formed blisters containing a small quantity of electrolytic gel. Spaced to ensure uniform distribution over the entire interface area, the blister pack delivers gel by means of a self-contained nitrogen gas generator assembly.

The assembly consists of a pyrotechnic device and porous metal filters inside the housing. "A small electric igniter, similar in operation to what's used in automotive airbags, activates the gas generator, and the gas is filtered and cooled as it flows," Linder explains.

When gas fills a pocket adjacent to the gel-filled blister pack, the pressure between adjacent layers provides the necessary mechanical force to squeeze the gel from the blisters. Within a predetermined pressure range, the seals rupture and the gel passes through holes in the metallic surface into the absorptive cloth interface on the side of the single-use electrode next to the patient's skin.

Even for short periods, adhesive electrodes commonly irritate the skin. So for the sensing electrodes, Lifecor engineers use capacitive instead of conductive sensing technology because it eliminates the need for adhesive. "These capacitive sensors only have to touch the skin. They don't have to be attached to it," Linder says. The sensors also minimize signal noise that normally accompanies conductive metal electrode movement on the skin. "So we reduce signal noise that can block the ECG signal," he adds.

Miniaturization boosts comfort. Lifecor is awaiting approval of a third-generation device called the WCD 3000 system or LifeVest. The device is smaller, lighter, and delivers a 150-J biphasic electrical pulse instead of a monophasic, 300-J shock. It also uses lithium ion batteries, instead of nickel cadmium.

"At approximately 1.7 lbs, our 3000 system is about half the weight and size of the 2000 system," Linder says. While the 2000 system charges up a capacitor bank and discharges a monophasic waveform through the patient, the biphasic system reverses the polarity of the pulse part way through the discharge cycle.

"Biphasic defibrillation is more effective at lower energies, which means we can use smaller batteries and capacitors," Linder says. "It also means lower currents and less risk of damaging the heart muscle."