The invention: A small device using transistor circuitry that regulates the heartbeat of the patient in whom it is surgically em-placed.
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. A forty-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 a trial 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.
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.
See also Artificial heart; Contact lenses; Coronary artery bypass surgery; Electrocardiogram; Hearing aid; Heart-lung machine.
