Redmond, WA--Millions of Americans might someday be able to live fuller, healthier lives thanks to the first implantable atrial defibrillator. Called MetrixTM, the device addresses atrial fibrillation (AF), an uncoordinated quivering of the atrium that leads to a chaotic heartbeat and ineffective pumping. Developing a safe, reliable, economical, implantable atrial defibrillator was widely believed to be impossible. But after more than five years of work that resulted in 49 U.S. patents, researchers and engineers at InControl have the Metrix system in clinical trials worldwide.
A cousin to the usually fatal ventricular fibrillation, AF can cause problems such as shortness of breath, palpitations, fainting, fatigue, and an inability to carry out many normal activities for the duration of the episode--which can last from just minutes to years.
In addition, AF can lead to strokes. "During AF, blood pools in parts of the heart and can clot," says John Adams, executive vice president of engineering. "It's possible for these clots to break off and go to the brain." In the United States, the American Heart Association estimates that 75,000 people suffer AF-related strokes each year.
The Metrix device works by automatically diagnosing AF and delivering a low-energy (six joules maximum) shock to convert the heart to normal rhythm. By comparison, an external defibrillator may use as much as 360 joules to cardiovert the heart. Such a high-energy input usually requires anesthetizing the patient to minimize pain.
The real challenge was developing technology to reliably detect AF and then provide therapy without doing harm. Some of the key issues engineers and researchers grappled with and believe they've overcome include:
Defibrillating with low energy: a large device with limited life would never be successful in the market.
Reliable AF detection: researchers had to determine how to best gather ECG data and analyze it to recognize arrhythmia.
Perfectly timing therapeutic shocks: doing so is necessary to avoid inducing potentially fatal ventricular fibrillation.
Designing a high-speed telemetry system: this system provides real-time communication with external diagnostic and programming equipment.
Finally: developing a robust shock- and sensing-lead system.
Metrix consists of several primary components. Implanted in the patient are the atrial defibrillator itself--it weighs about 80 grams and takes up some 53 cubic centimeters--and three sensing and shocking leads. An external PC-based programming/analyzing system communicates with the Metrix unit via high-speed telemetry.
Power for the Metrix unit is supplied by a lithium-silver-vanadium-oxide battery. To produce the cardioversion shock, a maximum 300-volt, 6-joule burst, the battery charges a 160-microfarad, aluminum-electrolytic capacitor of the type used in camera flashes. The capacitor passes the current to several FETs that deliver to the heart, on cue from the system, a bi-phasic waveform--6 msec on one polarity and 6 msec on the other.
Three transvenous leads connected to Metrix are implanted in the heart, one each in the coronary sinus, right atrium, and right ventricular apex. They provide data for three separate ECG monitors--a first for an implantable device--all used to help the system make decisions. "The idea is to enclose as much tissue as possible to be able to defibrillate with low energies," says Clifton Alferness, vice president of research.
The coronary sinus (CS) and right atrium (RA) leads are innovative single-electrode designs that both sense and transmit shock. The CS lead has a patented tip with a helical twist that fixes it in place and prevents blood flow from pushing it back out. During insertion, a center wire keeps the lead straight. Once positioned, the wire is removed and the lead curls into its final shape. "This design solved an industry problem of making a lead stay pointed upstream in a vein," says Adams. The third (ventricular) lead is a standard pacemaker lead that supplies signals for shock synchronization and post-shock heart pacing, if needed.
A custom circuit board holds two ICs developed in-house, the microprocessor, and static RAM. One of the units developed in-house, a digital IC, manages operations such as the high-voltage control logic and telemetry interface. The second, an analog IC, handles such items as the three ECG channels and the 8-bit D/A converter. Supplied by Philips Components-Signetics (Sunnyvale, CA), the microprocessor is a variation of the 8051 and runs at 2 MHz on just 1.8 volts. One megabyte of SRAM (8 × 128k) stores the unit's program as well as ECG, performance, and therapy data.
Engineers took some unique approaches to reduce Metrix's power drain and extend battery life. "The challenge was to put all this intelligence in the device and yet have it last four years and draw typically less than 20 microamps," says Phillip Foshee, design engineering director.
Metrix can be operated in automatic, manual, and physician-activated modes. But even in automatic mode, it is almost completely off more than 95% of the time. At programmed intervals--say 20 minutes--a timer turns on the CPU so the unit can collect ECG data and analyze it. If the heart is in normal rhythm, Metrix goes back to sleep. To allow the unit to be awakened on demand, the telemetry system continuously looks for a signal from the InControl programmer unit by powering up for 60 µsec every 125 msec.
The telemetry system communicates with the external InControl Programmer via a 2-MHz carrier wave that can simultaneously transmit digital data for the three ECGs as well as status and command information. "We can telemeter the contents of the 1-meg static RAM in four seconds," says Foshee. This performance represents as much as a factor of ten speed improvement over other implantable devices that often take minutes to transmit far less data. Engineers achieved the high transmission speed by removing the telemetry coil from inside the metal case and placing it in the plastic header.
Metrix is appropriately methodical in its approach to AF detection and shock delivery. It always first checks to see if the heart is in normal rhythm, and if so, goes back to sleep. If not, it begins a second algorithm, specific to detecting AF, and begins charging the capacitor. It then checks again for both normal and AF rhythm and invokes a patented dual-channel synchronizer algorithm to determine when it is safe to apply the shock.
The synchronizer monitors two different channels of ECG simultaneously--one taken at the right ventricle, the other measured from the right ventricle to left atrium. It looks for correlation between them as well as for an acceptablylong(>500 msec) heartbeat cycle at which to apply the shocks. The cycle timing is essential, since animal studies revealed that on rare occasions, shocks could trigger ventricular fibrillation. Thanks to the dual ECGs and proprietary interval timer, "we've never recorded a mistimed shock," Adams says.
Additional details…Contact Emily Tidball, Product Manager, InControl, 6675 185th Ave. NE, Redmond, WA 98052-6734, (206) 861-9800.
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