Implanted Defibrillators and Pacemakers:
A Gentler Jolt and Tickle for Trembling Hearts
by Dixie Farley

     On a Boston subway train in December 1990, John Thomas'
heart stopped its normal steady beating. Instead, it began to
quiver ineffectively in a type of cardiac arrest called
fibrillation.
     Automatically, built-in protection came to the rescue. A
medical device implanted in Thomas' abdomen delivered an electric
shock to his malfunctioning heart, jolting it back to regular
rhythm.
     "I lost consciousness for just a few seconds," says Thomas,
36, a social worker in Boston. "Nobody on the subway knew. I
wasn't even sure what had happened." That same day, his doctor
confirmed the device had indeed responded.
     The device is a cardioverter defibrillator, one of the
newest heart-rhythm regulators in pacemaker-defibrillator
evolution. ("Cardioverter" indicates the capability to deliver
low-energy shocks.) Thomas needs the device to treat primary
ventricular fibrillation, a condition in which the lower heart
chambers (ventricles) periodically have disorganized electrical
activity and are unable to effectively pump blood to the body. He
received his defibrillator after collapsing with cardiac arrest
at Cape Cod the previous Fourth of July.
     Thomas' defibrillator gave his heart three other jump-starts
that fall and winter--his last one occurring in January 1991. To
reduce the frequency of these episodes, his doctor prescribed
drug therapy with Tenormin (atenolol), and Thomas has been free
of the episodes ever since.

The Stats
     While most cardiac arrests result from rapid heartbeat
(tachycardia), some are due to slowed heartbeat (bradycardia),
which is often treated with an implanted pacemaker. The Food and
Drug Administration estimates that doctors implant about 15,000
defibrillators and 110,000 pacemakers each year.
     Clinical studies submitted to FDA show the newest heart-
regulating device--a pacemaker-cardioverter-defibrillator--
corrected nearly 98 percent of patients' abnormal heart rhythm or
cardiac arrest episodes. Some 400,000 Americans die annually from
abnormally fast or irregular heart rhythm, FDA said in announcing
approval of the device in February 1993.
     The Antiarrhythmics vs. Implantable Defibrillators (or AVID)
pilot study at the National Heart, Lung, and Blood Institute is
examining whether defibrillators or anti-arrhythmia drugs are
more effective in reducing deaths.

Pacemaking, Naturally
     Responsibility for pacing heartbeats, which circulate blood,
belongs to the sinus node atop the heart's right atrium (one of
two upper chambers). This natural pacemaker's specialized cells
fire electrical impulses that cause the atria and their
respective ventricles to contract to move the blood in perfect 
timing.
     The impulses travel down the atria, which receive blood
through the veins from the body and lungs, and cause their
contraction to "top off" the amount of blood in the ventricles.
The impulses continue through a conductive pathway into the
ventricles and cause their contraction, resulting in the pumping
of blood through the arteries to the body and lungs. The right
chambers circulate oxygen-depleted blood from the body to the
lungs, while the left chambers circulate oxygen-rich blood from
the lungs to the body.
     Aside from speeding up in situations such as physical
activity and slowing during rest or sleep, the normal heart
typically completes 72 of these cycles a minute.
     Some hearts, however, beat less than 60 times a minute
(bradycardia) or race at over 100 a minute (tachycardia). These
"arrhythmias" may have any number of causes, such as a birth
defect, injury, chemical imbalance, even anti-arrhythmia
medication. But the main predisposing factor, according to the
American Heart Association, is acquired heart disease. Some
arrhythmias aren't serious enough to warrant treatment.
     To treat serious arrhythmias, doctors can turn to a
permanently implanted artificial pacemaker or defibrillator.
(Temporary emergency pacing with an external pacemaker is
possible by threading lead wires through a vein to the patient's
heart. Emergency defibrillation is possible with external
defibrillator paddles.) Alternative treatments are drug therapy
and surgical correction.

Artificial Pacemakers
     The pulse generator of the artificial pacemaker corrects for
a defective sinus node or conduction pathway by emitting rhythmic
electrical impulses similar to those of the sinus node. Usually,
it's made with a titanium metal case and other materials
compatible with the body and powered by a lithium battery system.
     The doctor implants the generator under the skin in the
upper left part of the chest, attaching it to lead wires threaded
to the heart.
     Traveling along the wires, the impulses "tickle" the heart,
stimulating it to beat at a normal pace, says Donald Dahms, chief
of the pacing and electrophysiology devices branch at FDA's
Center for Devices and Radiological Health.
     The first pacemaker was implanted in 1958. These early
devices had only one wire and paced the ventricles at regular
intervals. They paced at a single rate prescribed by the doctor--
usually 70 beats a minute.
     The first advance, around 1976, was a design change that
allowed "demand" pacing: A pacemaker would only pace if the
patient's heart didn't beat within a given time period.
     Then, early in the 1980s, programming capability was
introduced. The subsequent development of transmitting by
telemetry, a system for sending and receiving electromagnetic
signals as radiofrequency, enables the doctor to check and adjust
the pacemaker. In addition, simple monitoring can be done over
the telephone.
     Placing a transmitting wand (called a programming head) on 
the patient's skin overlying the pacemaker, the doctor can
determine voltage, pacing rate, and the status of the wiring and
electrodes and can make adjustments to fit the changing needs of
the patient. For instance, an increase in voltage output might be
needed if the connection to the heart gets bad, Dahms says.
     "The problem with the early pacemakers' having only a
constant rate," he says, "is when you need a faster rate--if you
run, for instance--they can't give it to you. So your heart has
to strain to increase its blood output to make up the difference.
And if your heart beats on its own, the pacer continues beating,
competing with your natural heartbeat." A constant rate also can
cause adverse effects, such as dizziness, he says.
     By 1983, the first dual-chamber pacemakers entered the
market. Using two lead wires, these devices pace the atrium and
then the ventricle, when necessary.
     "Dual pacing lets you synchronize pacing of both chambers,
more like the heart's natural functioning," Dahms says. "If you
have a wire pacing away just on the ventricle, your atrium might
get out of synch, which reduces your blood output. Without the
'atrial kick' you have less efficient circulation."
     Within the next couple of years, firms developed techniques
to make single-chamber pacemakers "rate responsive."
     These pacemakers contain a sensor to detect the need for
increased rate. The simplest ones, Dahms says, gauge response by
body movement. Some, however, respond to the person's breathing.
Still others base their response on changes in blood temperature.
Thus, if a person starts to walk or run, the device senses the
activity and increases the heart rate.
     Rate-responsive, two-wire, dual-chamber devices became
commercially available in 1987.
     Many pacemakers automatically provide for a slower heart
rate at night or when the person rests. A very few devices are
clock-timed to slow at night.
     Despite such sophisticated capabilities, miniaturization
techniques have allowed pacemakers to become quite tiny in size,
some as small as a quarter and less than an ounce in weight. The
many companies making pacemakers today offer more than a hundred
models.
     Pacemakers last four to 12 years. Factors that shorten a
pacemaker s life include the requirement to pace every beat, the
size of the batteries, dual-chamber pacing, and the tightness of
the electrical connection to the heart, which is not completely
controllable.
     Another use for pacemakers has been to treat heart rates
that are too fast, slowing them back to a normal rhythm. But
anti-tachycardia pacing can also pose a problem. If pacing
doesn't stop a rapid heartbeat, Dahms says, it may accelerate it,
possibly leading to fibrillation.

Defibrillators
     Newer than the pacemaker in regulating heartbeats is the
implantable defibrillator, made of the same materials but powered
by a special higher energy battery.
     Implanted under the skin in the upper abdomen, the
defibrillator is connected with lead wires to two defibrillation
electrodes placed surgically in or around the heart. There must 
always be two, Dahms says, to provide the electric field needed
across the heart.
     The defibrillator interrupts the abnormal rhythm, allowing
the normal rhythm to resume. The abnormal beating is so rapid and
uncoordinated that the heart can only quiver ineffectively.
     Sensors inside the generator monitor the heart. If a sensor
detects an irregularity, such as fibrillation, the generator is
programmed to deliver a strong electric shock directly to the
heart. Being kicked in the chest by a horse is how some people
describe this jolt.
     Others, like Thomas, lose consciousness before they
experience the shock.
     "I've never felt it," Thomas says. "I'm just a little
lightheaded, like when you stand up too quickly. Then I'm
unconscious for 5 to 10 seconds. When it's happened at home, my
wife has observed my body jerk as though I'm having a seizure."
     Although a patient may sometimes be unaware the
defibrillator has been activated, this information is stored in
the device's memory. During periodic checkups, the doctor can
therefore tell how frequently it has gone off and how much shock
it has delivered.
     FDA approved the first defibrillator in 1985, after clinical
trials with the device produced dramatic results. In the trials,
the agency announced, the cardiac arrest death rate was under 5
percent a year, down from a 27 to 66 percent annual rate
previously reported (before the use of implanted defibrillators)
for patients not helped by medication.
     These early devices were limited in detection and
programming capability and delivered only one level of therapy--a
painful, jolting shock. Still, says Dahms, they provided almost
certain protection against death from cardiac arrest.
     In 1988 and 1989, FDA approved the first cardioverter
defibrillators, which offer the programmability of a two-stage
shock treatment. The life-saving jolts of these defibrillators
continued to be painful, causing significant psychological
suffering in some patients with frequent episodes.
     The newer pacemaker-cardioverter-defibrillator monitors the
heart and refrains from delivering a shock if the heart rhythm
has returned to normal on its own. Using staged therapy, this
first "third-generation" defibrillator measures varying degrees
of rapid heartbeat so that it can deliver gentle pacing impulses
to slow the heart, low-energy cardioversion shocks to restore
normal rhythm when the pacing is inadequate, or high-energy
defibrillation shocks to restart the quivering heart.
     Some devices also use "biphasic" pulses, which holds promise
for extending the life of the device, as well as other possible
benefits. (The biphasic wave-form pulse derives its name from the
fact it delivers a pulse with a positive and negative polarity.) 
     Last August, FDA approved a new lead system that can be
implanted through a vein, eliminating the need for open-chest
surgery when implanting a defibrillator in certain patients. In
some of these patients, generators with biphasic pulses are used
with the leads.
     A defibrillator usually lasts two or three years, depending 
on how often it has to defibrillate.
     Thomas' defibrillator was replaced last September. His
concern about being off work for the surgery and uncertainty
about whether the replacement, like the first device, would go
off several times were soon alleviated. Unlike his first surgery,
which required opening the chest to attach the lead system to his
heart, this implantation was a minor outpatient procedure under
local anesthetic because the leads were already attached to his
heart. "It was much easier the second time," he says.
     Although today's defibrillators are the size of a deck of
cards and weigh about 8 ounces, Dahms says future devices, now in
clinical studies, will be small enough for implanting in the
upper chest, like pacemakers.

Risks and Precautions
     As with anything electronic, glitches can occur in a
pacemaker or defibrillator. Among possible causes of malfunction,
says Dahms, are circuitry failure, lead breakage, electrode
displacement, and scarring around the electrodes. Regular
checkups by the doctor every two or three months are crucial so
that any problems, including battery depletion, are detected as
early as possible.
     There's no question that defibrillator failure due to
battery depletion could be life-threatening. However, death is
extremely rare following pacemaker failure. In almost all
pacemaker-battery failures, the underlying natural heartbeat
takes over (albeit at a slowed rate) until a new device is
implanted or an external pacemaker applied.
     Nevertheless, as a precaution for pacemakers as well as
defibrillators, FDA requires special labeling specifying a
warning period when a battery is about to wear out, to allow for
timely replacement. (The warning is detected during medical
checkups.)
     Extreme electromagnetic interference can cause some devices
to malfunction. Someone implanted with a defibrillator, for
instance, should avoid airport security scans, because the
interference can turn off a device, in which case some devices
start beeping. It is inadvisable for patients with pacemakers or
defibrillators to undergo magnetic resonance imaging, a
diagnostic technique that pictures the body's internal
structures.
     Some older pacemakers were susceptible to interference from
microwave ovens. More recent models are shielded. Patients in
doubt about any interference should check with the doctor.
     Under FDA regulations required by the tracking provisions of
the Safe Medical Devices Act of 1990, manufacturers must track
pacemakers and defibrillators from production through
distribution to patients. This will speed patient notification if
a problem should arise. These regulations went into effect on
Aug. 28, 1993.
     In addition, a few state laws prohibit driving for a period
of time after arrhythmia-induced unconsciousness. While
Massachusetts doesn't have such a law, Thomas was under his
doctor's orders to not drive for six months following an episode.
     Not driving that first year was hard, Thomas says, along 
with constant anxiety about having an episode. As for today:
     "It does tend to give me more perspective, so that little
things don't bother me as much. Once in a while it's on my mind.
And it impresses upon me how fragile life can be." 

Dixie Farley is a staff writer for FDA Consumer.
     FDA encourages people implanted with a pacemaker or
defibrillator to join the free Medic Alert International Implant
Registry by calling (1-800) 344-3226.
     Those who enroll will receive a free Medic Alert bracelet or
necklace to notify health-care staff of the implant in the event
of a medical emergency. 
Language of the Heart
-    arrhythmia--abnormal heart rhythm
-    artificial pacemaker--corrects for a defective sinus node
(see "sinus node" entry) or conduction pathway by emitting a
series of rhythmic electrical discharges to control the heartbeat
-    atria--the two upper heart chambers
-    bradycardia--slowed heartbeat, less than 60 beats a minute
-    cardiac arrest--stopped heart activity
-    defibrillator--electronic device that helps reestablish
normal heartbeat in a malfunctioning heart
-    fibrillation--rapid, uncoordinated heart muscle
contractions; the chamber involved can't pump effectively, so
blood flow is compromised
-    sinus node--the natural pacemaker, whose cells in the top of
the right atrium produce electrical impulses that travel to the
ventricular muscle, causing the heart to contract
-    tachycardia--fast heart rate, over 100 beats a minute
-    ventricles--two lower chambers of the heart.