When Earl Bakken started Medtronic back in 1949, little did he realize that his pioneering work in electrical heart pacemakers would spawn a vast industry whose devices would save millions of lives and improve the quality of life of millions more (see "A Short History of The Pacemaker," p. 54). Today, Medtronic enjoys worldwide leadership in a variety of medical products and services, many of which trace their roots to the cardiac pacemaker. Its revenues in the year ended April 25, 2003 were $7.665 billion, nearly half of which are derived from pacemakers and defibrillators (which also contain pacemakers) to treat a variety of ailments.
Cardiac pacemakers provide electric signals to the heart to make it beat properly when the heart's own natural pacemaker fails. Failure can occur due to blockage of the heart's natural electric signals, an ischemia (decreased flow of oxygenated blood to an organ due to obstruction in an artery), or a myocardial infarction (destruction of heart tissue resulting from obstruction of the blood supply to the heart muscle). The electronic pacemaker senses the heart's own rhythm and sends a correcting signal when needed. The pacemaker is implanted under the skin, most often just under the collarbone (Fig. 1).
After the pacemaker was developed, implantable defibrillators came along. Unlike pacemakers, which generally put out a train of 0.5- to 8-V (4 V nominally) pulses, defibrillators "shock" a failing heart into action with 750-V pulses. Over time, the defibrillator was combined with a pacemaker, and it's now called an implantable cardioverter defibrillator (ICD). All defibrillators made today contain a pacemaker (Fig. 2). Heart disease is the leading cause of death in the U.S., and many of these deaths can be attributed to irregular heartbeats, which pacemakers and defibrillators are designed to alleviate.
GOING FORWARD
Mention pacemakers and most people think of cardiac applications. Although heart stimulation was the original application, pacing technology has gone far beyond cardiac applications. The newest uses include pain management; the control of essential tremors and Parkinson's disease; the treatment of seizure disorders, sleep apnea, and epilepsy; and bladder control. Pacemakers now interface with the spinal column, the brain, and the abdomen, in addition to the pectoral region of the chest, where they're typically implanted for cardiac applications.
One advanced medical device spawned by the pacemaker is Medtronic's Chronicle, an implantable cardiac monitor that may revolutionize the advanced detection and treatment of heart problems. It provides physicians with readings of the heart's workings, enabling them to modify cardiac therapy by adjusting programmable pacemaker or defibrillator operation or by altering medications.
Chronicle is currently being tested in a clinical study of about 300 patients. If all goes well, it could be on the market in 2005. This device will add to the company's expanding monitoring device business, launched in 1998 with the release of the Reveal syncope monitor (syncope is a spontaneous loss of consciousness caused by insufficient blood to the brain).
"One of the most promising and exciting new areas for the cardiac device industry is implantable monitors," says Ed Duffin, director of Medtronic's Heart Failure Research Group. "These devices provide objective information that should help the physicians to be more successful in managing therapy."
Pacemakers are no longer standalone devices. The momentum for remote patient care has brought about the development of Internet-based systems that cost-effectively expand the scope of patient-care coverage. Early last year, for example, Medtronic introduced its CareLink network, which lets physicians manage more patients with implanted devices via the Internet.
"The physician gives the patient a small remote monitoring device to use at home. That unit interrogates the implantable device and sends, via the Internet, information to the physician for examination," explains Duffin.
WHAT'S INSIDE?
All modern implantable pacemakers contain an input sense amplifier, a microprocessor, a sensor, some memory to store programming code, a transceiver circuit to allow monitoring and programming via an external telemetry loop, a pulse generator, and a power supply, all powered from a single small battery. Reliability, extremely low power consumption, and small size are critical design issues for any implantable pacemaker and defibrillator. The sensing element in the pacemaker is usually a microelectromechanical system (MEMS) accelerometer, which monitors the patient's physical activity.
Because an implantable device and the thin wires (leads) connecting it to the heart are placed in the human body in an environment that's very hostile, and since 10 years of reliability are mandated, the device's behavior must be well understood. Such a task requires sophisticated mathematical modeling and simulation. Medtronic's large and active Materials and Modeling Group specifically works on these issues. It does so for all of its medical products, including pacemakers, using the latest modeling and simulation techniques.
In their continuing quest to reduce pacemaker and defibrillator power levels, designers at Medtronic's Bakken Research Center in Maastricht, the Netherlands, working with designers at Delft University, also in the Netherlands, recently developed an ultra-low-power sense amplifier that operates from just 2 V and dissipates a mere 240 nW. Operating on a dynamic-translinear (DTL) circuit, it comprises a voltage-to-current converter, a bandpass filter, and absolute-value rms-dc converter and comparator circuits (Fig. 3).
The designers are proposing analog usage of the wavelet transform, via the DTL circuit, to further reduce power requirements. They point out that a fully integrated analog QRS complex detection circuit can be built to operate from a 2-V supply and dissipate only 55 nW/scale. The QRS complex comprises the deflections in an electrocardiographic tracing, representing ventricular activity of the heart.