Pacemaker

The invention: 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

Cardiac Pacing

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 atrialtriggered 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.

Consequences

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.