29 September 2009
A small device using transistor circuitry that regulates
the heartbeat of the patient in whom it is surgically emplaced.
The people behind the invention:
Ake Senning (1915- ), a Swedish physician
Rune Elmquist, co-inventor of the first pacemaker
Paul Maurice Zoll (1911- ), an American cardiologist
The fundamentals of cardiac electrophysiology (the electrical activity
of the heart) were determined during the eighteenth century;
the first successful cardiac resuscitation by electrical stimulation occurred
in 1774. The use of artificial pacemakers for resuscitation was
demonstrated in 1929 by Mark Lidwell. Lidwell and his coworkers
developed a portable apparatus that could be connected to a power
source. The pacemaker was used successfully on several stillborn
infants after other methods of resuscitation failed. Nevertheless,
these early machines were unreliable.
Ake Senning’s first experience with the effect of electrical stimulation
on cardiac physiology was memorable; grasping a radio
ground wire, Senning felt a brief episode of ventricular arrhythmia
(irregular heartbeat). Later, he was able to apply a similar electrical
stimulation to control a heartbeat during surgery.
The principle of electrical regulation of the heart was valid. It was
shown that pacemakers introduced intravenously into the sinus
node area of a dog’s heart could be used to control the heartbeat
rate. Although Paul Maurice Zoll utilized a similar apparatus in
several patients with cardiac arrhythmia, it was not appropriate for
extensive clinical use; it was large and often caused unpleasant sensations
or burns. In 1957, however, Ake Senning observed that attaching
stainless steel electrodes to a child’s heart made it possible
to regulate the heart’s rate of contraction. Senning considered this to
represent the beginning of the era of clinical pacing.
Development of Cardiac Pacemakers
Senning’s observations of the successful use of the cardiac pacemaker
had allowed him to identify the problems inherent in the device.
He realized that the attachment of the device to the lower, ventricular
region of the heart made possible more reliable control, but
other problems remained unsolved. It was inconvenient, for example,
to carry the machine externally; a cord was wrapped around the
patient that allowed the pacemaker to be recharged, which had to be
done frequently. Also, for unknown reasons, heart resistance would
increase with use of the pacemaker, which meant that increasingly
large voltages had to be used to stimulate the heart. Levels as high
as 20 volts could cause quite a “start” in the patient. Furthermore,
there was a continuous threat of infection.
In 1957, Senning and his colleague Rune Elmquist developed a
pacemaker that was powered by rechargeable nickel-cadmium batteries,
which had to be recharged once a month. Although Senning
and Elmquist did not yet consider the pacemaker ready for human
testing, fate intervened.Aforty-three-year-old man was admitted to
the hospital suffering from an atrioventricular block, an inability of
the electrical stimulus to travel along the conductive fibers of the
“bundle of His” (a band of cardiac muscle fibers). As a result of this
condition, the patient required repeated cardiac resuscitation. Similar
types of heart block were associated with a mortality rate higher
than 50 percent per year and nearly 95 percent over five years.
Senning implanted two pacemakers (one failed) into the myocardium
of the patient’s heart, one of which provided a regulatory
rate of 64 beats per minute. Although the pacemakers required periodic
replacement, the patient remained alive and active for twenty
years. (He later became president of the Swedish Association for
Heart and Lung Disease.)
During the next five years, the development of more reliable and
more complex pacemakers continued, and implanting the pacemaker
through the vein rather than through the thorax made it simpler
to use the procedure. The first pacemakers were of the “asynchronous”
type, which generated a regular charge that overrode the
natural pacemaker in the heart. The rate could be set by the physician
but could not be altered if the need arose. In 1963, an atrial-
triggered synchronous pacemaker was installed by a Swedish team.
The advantage of this apparatus lay in its ability to trigger a heart
contraction only when the normal heart rhythm was interrupted.
Most of these pacemakers contained a sensing device that detected
the atrial impulse and generated an electrical discharge only when
the heart rate fell below 68 to 72 beats per minute.
The biggest problems during this period lay in the size of the
pacemaker and the short life of the battery. The expiration of the
electrical impulse sometimes caused the death of the patient. In addition,
the most reliable method of checking the energy level of the
battery was to watch for a decreased pulse rate. As improvements
were made in electronics, the pacemaker became smaller, and in
1972, the more reliable lithium-iodine batteries were introduced.
These batteries made it possible to store more energy and to monitor
the energy level more effectively. The use of this type of power
source essentially eliminated the battery as the limiting factor in the
longevity of the pacemaker. The period of time that a pacemaker
could operate continuously in the body increased from a period of
days in 1958 to five to ten years by the 1970’s.
The development of electronic heart pacemakers revolutionized
cardiology. Although the initial machines were used primarily to
control cardiac bradycardia, the often life-threatening slowing of
the heartbeat, a wide variety of arrhythmias and problems with cardiac
output can now be controlled through the use of these devices.
The success associated with the surgical implantation of pacemakers
is attested by the frequency of its use. Prior to 1960, only three
pacemakers had been implanted. During the 1990’s, however, some
300,000 were implanted each year throughout the world. In the
United States, the prevalence of implants is on the order of 1 per
1,000 persons in the population.
Pacemaker technology continues to improve. Newer models can
sense pH and oxygen levels in the blood, as well as respiratory rate.
They have become further sensitized to minor electrical disturbances
and can adjust accordingly. The use of easily sterilized circuitry
has eliminated the danger of infection. Once the pacemaker
has been installed in the patient, the basic electronics require no additional
attention.With the use of modern pacemakers, many forms
of electrical arrhythmias need no longer be life-threatening.
See also : Artificial heart; Contact lenses; Coronary artery bypass
surgery; Electrocardiogram; Hearing aid; Heart-lung machine; Heart .