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Shock Treatment
Defibrillator engineers design out the false positives
By Peggy Allbright, Contributing Editor -- Design News, February 3, 2003
Pittsburgh, PA — Product design is not without irony. For example, engineers who design a product to do something at just the right time often can spend as much time making sure the product doesn't do that something at the wrong time.
That was the case with a new, wearable defibrillator, called LifeVest, by Lifecor (www.Lifecor.com), which received FDA approval last year. Like implantable defibrillators, it is designed to administer an electric shock when arrhythmia is detected. But while defibrillation therapy is highly effective and has saved thousands of lives, unnecessary shocks can put unwarranted stress on an already unhealthy heart.
"It's important that no one gets shocked without need," says Steven Szymkiewicz, MD, vice president of medical affairs at Lifecor. Eliminating false positives is particularly important with a wearable defibrillator, since external defibrillators used by emergency personnel are not deployed at all unless clinical indications confirm the need.
During clinical tests for the FDA on 289 actual patients representing 901 months of use, the LifeVest caused just 6 false shocks, a rate of 0.67%. About halfway through that study, engineers adjusted the algorithms that analyze the heart's electrical signals and control the timing of the shock to further reduce that false shock rate.
Clinical testing has now reached 1,300 months of use. As a result, there have been no false shocks for the last 700 of those months. In contrast, the rate for implanted defibrillators is equivalent to 1 to 2% per month, says Steven Szymkiewicz, MD, vice president of medical affairs at Lifecor.
Built-in Redundancy
So how did design engineers pull it off? Integral to their success was the development of a redundant sensing system, which employs two leads to carry two independent channels of sensing electrodes to pick up ECG signals from the heart through the skin. Conventional ICDs, on the other hand, use a single lead to carry just one pair of electrodes to the heart.
The two-lead system provides a back-up in case one electrode slips out of place. The additional information gathered also presents a more sophisticated presentation of heart activity. A detection algorithm compares readings to a template of the patient's normal heart behavior to determine if treatment should be administered.
Because the LifeVest is a wearable device, engineers were also able to take advantage of the fact that patients themselves play a role in ensuring its appropriate use. Once the device detects an arrhythmia, it sets off a 30-second alarm sequence, beginning with a vibrating alarm that the patient receives on a small pager-like device. After five seconds, the alarm becomes audible, gradually increasing in volume and culminating in a verbal warning.The patient can prevent a shock by holding down two buttons, although the device itself will abort the sequence if it determines that the heart rate has returned to normal.
One of the biggest challenges, Szyymkiewicz says, was designing the halter-like vest system: It needs to keep the electrodes snugly in place and yet be as comfortable as underwear so that patients are willing to wear it for long periods of time. Engineers came up with an elastic belt that holds the plastic-encased electrodes snug to the skin while providing several inches of forgiveness for weight change that heart patients can experience.
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