Indications and Procedures The first human-made pacemaker, which used electronic pulses to stimulate a regular heart rhythm, was built in the 1950s. Since then, the device has evolved into a sophisticated and reliable instrument. It was miniaturized so that it could be implanted under the skin of the patient. Tiny batteries that would last from five to fifteen years were developed. A microprocessor that can sense the need for different heart rates during sleep or strenuous exercise has become a standard component. Most recently, a small automatic defibrillator has been incorporated into some pacemakers to supply several large jolts of electrical energy in case of heart stoppage or other emergencies.
The normal rhythm of a healthy heart is regulated by natural pacemaker cells. These unique cells are located at the
sinoatrial (S-A) node near the top interior of the heart, where blood empties from the veins into the right atrium. Electrical impulses originating at the S-A node travel to the atrioventricular (A-V) node, which is located where the four chambers of the heart come together. From there, the signal is relayed to the ventricles, causing the muscle fibers to contract. This pumping action forces blood to flow from the two ventricles to the lungs and the body arteries.
If the natural pacemaker cells or the nerve pathways in the heart do not function properly, the heart may beat too rapidly, too slowly, irregularly, or not at all. For example, the condition called
heart block interrupts or delays the electrical signal at the A-V node. It can happen that only every second or third pacemaker signal triggers a contraction. Sometimes, the ventricles will start a contraction on their own, but it will not be synchronized with the blood flow from the atrium. An artificial electronic pacemaker can be used to overcome heart block.
The electrical activity of the heart is observed in an electrocardiogram (ECG or EKG). Metal electrodes are placed in contact with a patient’s left arm, right arm, left leg, and sometimes chest. After suitable amplification, the signal can be displayed on a video screen or recorded by an ink pen on moving paper.
For a healthy heart, the normal ECG pattern starts with a small pulse (the P wave), which is followed by a group of three closely spaced pulses (the QRS complex) and a final small pulse (the T wave). This pattern is repeated approximately seventy-two times per minute for a person sitting at rest.
In brief, the P wave indicates contraction of the atrium, the QRS complex shows contraction of the ventricles, and the T wave represents the muscles’ return to the resting state. If the heart “skips a beat” because of a heart block at the A-V node, the ECG will show a missing or delayed pulsation in the otherwise regular pattern. If this happens in a sustained fashion, electronic stimulation is needed.
Two other serious malfunctions of the heart’s electrical system are flutter and fibrillation. Flutter is a very rapid but still constant rhythm that may produce 200 to 300 beats per minute. Fibrillations are much more serious, causing chaotic, random contractions that can occur as often as 500 times per minute. There is insufficient time between contractions for blood to fill the ventricles. Pumping action becomes very inefficient, and death is likely to occur if the fibrillations continue.
To restore normal heart rhythm, a defibrillator is used to send a strong electric shock through the ventricular muscle fibers, which deactivates the heart’s electrical system for several seconds. An electronic pacemaker may then replace the natural pacemaker cells to prevent the recurrence of fibrillations.
The cause of flutter and fibrillation is a process called “circus movement.” Suppose the electrical impulses are diverted from their normal pathway by thickened or dead heart tissue. In such a case, the timing may be thrown off so that the ventricles are restimulated to contract again without waiting for the pacemaker’s signal. Therefore, the heart is unable to reach its resting state.
In the ECG pattern, flutter shows up as a rapid pulsation with an indistinct QRS complex. Fibrillation is indicated by irregularly spaced pulses of random size that have no pattern at all. It is something like electrical noise coming from the heart, with no synchronization. Heart cells at many locations fire at random, producing ripples similar to those made by a handful of pebbles thrown into a lake.
The first artificial pacemaker was developed by Paul Zoll in 1952. When a patient suffering from heart block went into heart failure during surgery, Zoll inserted a needle electrode into the man’s chest and applied regular voltage pulses from an external circuit. After two days, the man’s heart resumed beating on its own, and the circuit was disconnected.
A portable artificial pacemaker was developed in 1957 by C. W. Lillehei and Earl Bakken. The electrode was inserted directly against the outer surface of the heart, and a battery pack and timer circuit were worn around the patient’s waist. Three years later, the pacemaker was miniaturized sufficiently to be implanted under the skin of the patient’s chest. This had the advantage of reducing the risk of infection.
The next major improvement was to redesign the fixed-rate pacemaker so that it could respond to variable demand during exercise or sleep. The demand pacemaker has a built-in sensor that monitors the heart’s electrical system. An electronic microprocessor is programmed to recognize abnormal ECG pulses. Generally, the demand pacemaker is set to deliver a trigger pulse only when the heart rate falls below a certain point.
For people with a potential for unpredictable heart stoppage or fibrillation, a device called an implantable cardioverter defibrillator (ICD) has been developed. This unit, which is comparable to the external defibrillators used by emergency medical technicians but much smaller, can deliver several large jolts directly to the heart. Since implanted batteries are quite small, the circuit requires some time to recharge between shocks. The circuit is quite similar to the flash attachment of a camera, with its “slow charge, fast discharge” process.
Uses and Complications The implantation of a pacemaker may become necessary as a result of a coronary artery disease, in which a buildup of plaque leads to irregularities in the heart’s rhythm. Coronary artery disease is the main form of heart disease, which is the leading cause of death in the United States. Heart disease claimed nearly 600,000 American lives in 2010; it afflicts 11 percent of the US population. It is primarily a disease of modern, industrial society and is less frequently found in more rural, underdeveloped countries. In the United States, the death rate from heart attacks increased sharply after 1920, reached a peak in the mid-1960s, and has declined substantially since then.
A
heart attack is usually caused by an oxygen deficiency in the heart muscle. The attack may come suddenly and without warning, but most often there is previous tissue damage that has weakened the heart over a period of time. A buildup of plaque in the arteries, called atherosclerosis, can reduce the rate of blood flow to a dangerously low level. The heart muscle tries to compensate for its reduced pumping power and may develop rhythmic irregularities, or arrhythmia. Eventually, heart block or ventricular fibrillations can ensue, leading to heart failure and death.
The famous Framingham Heart Study, initiated in 1948 in Framingham, Massachusetts, has been following the medical histories of approximately 5,000 men and women in order to identify the most important risk factors for heart disease. For example, the rate of heart disease among male smokers in this study was three times as high as that among nonsmokers. (This result is in addition to the much higher rate of lung cancer among smokers.) Other risk factors are excessive alcohol consumption, lack of exercise, high blood cholesterol, emotional or physical stress, and excess weight. Some unalterable risk factors are age, sex, and a family history of heart disease. The decline in heart attack deaths in recent years has been attributed to widespread changes of lifestyle to reduce the risk factors, as well as to improvements in medical diagnosis and treatment.
Modern pacemakers are remarkably reliable and safe. One of the few precautions for pacemaker wearers is to avoid standing near high-level microwave sources (although household microwave ovens are harmless). The problem is that the metal wire going into the heart acts like an antenna; it can pick up stray microwave radiation, which can disrupt the electronics in the sensitive pacemaker circuit. Also, the battery in a pacemaker must be changed at five- to ten-year intervals to ensure proper operation.
Thousands of people receive implanted pacemakers each year. The procedure has become so routine that even small community hospitals are equipped to handle it. Many patients with heart block and irregular rhythm, especially elderly patients, have benefited greatly from this technological development.
Perspective and Prospects The creation of effective electronic pacemakers depends on an understanding of the structure and function of the human heart. Also, instruments such as x-ray machines and electrocardiographs are indispensable for monitoring an individual patient’s response. This section will review the progress of the medical ideas and instruments that were the essential prerequisites for modern pacemakers. Good starting points are the pioneering studies of human anatomy made by Leonardo da Vinci (1452–1519) and Andreas Vesalius (1514–1564).
Leonardo dissected and studied the human body and made anatomical sketches in his notebooks. He recognized that the heart had four chambers, and he also drew the heart valves in detail. His interest in anatomy was that of an artist rather than that of a physician.
Vesalius was a professor of medicine at the University of Padua, in Italy. He taught anatomy and wrote a famous seven-volume treatise on the structure of the human body that had many excellent illustrations. His knowledge of anatomy came from the dissection of animals and of human cadavers obtained at night from paupers’ graves. Some of his anatomical investigations contradicted traditional medical doctrine and brought him into conflict with the Catholic Church. Like Galileo, he believed that experimental information was superior to ancient textbooks.
William Harvey, a British physician, received his medical degree from the University of Padua in 1602. He is known for formulating the first accurate description of the circulation of the blood through the body. He showed that the volume of blood is fairly constant, so the function of the heart is to act as a recirculating pump. He had a clear understanding of the way in which the right ventricle pushes blood through the lungs and the left one circulates it to the rest of the body. There was, however, one missing link in Harvey’s theory: How did the blood get from the arteries to the veins for its return flow? The invention of the microscope in the 1670s made it possible to see the tiny, previously invisible capillaries, thus providing final confirmation of the circulation process.
The scientific investigation of electricity began in the eighteenth century. Benjamin Franklin studied lightning rods, and scientists learned how to build a friction machine that produced electricity in the laboratory. Taking an electric shock became an amusing, although somewhat dangerous, entertainment at parties.
About 1790, the Italian anatomist Luigi Galvani made an important, though accidental, discovery. A metal scalpel lying near an electrostatic machine came into contact with the leg of a recently dissected frog, causing a sudden twitching of the muscle. Evidently, there was a connection between the electric shock and the muscle contraction.
The modern pacemaker that stimulates the heart muscle works in the same way that Galvani’s scalpel worked; however, a major evolution in physiological knowledge and medical practice had to take place before the pacemaker could be developed.
Wilhelm Conrad Röntgen was experimenting with high voltages in his laboratory in 1896 when he observed a mysterious new type of radiation, which he called x-rays. Unlike light, x-rays were able to pass through black paper, wood, and even thin metal sheets. They could cause certain paints to glow in the dark and could expose photographic film that was still in its light-tight box. For the medical profession, the discovery of x-rays was a major breakthrough.
X-ray technology has been improved in recent years. Electronic image intensifiers were developed in the 1950s in order to brighten the dim pictures on a fluorescent screen. A major breakthrough in the 1970s was the invention of computed tomography (CT) scanning. Instead of using film or a fluoroscope, a computer generates images of the heart and other internal organs on a video screen. For pacemaker implantation, x-ray apparatus is indispensable in order to observe the electrode’s precise placement into the interior of the heart.
The electrodes of most pacemakers are installed with a catheter that is inserted through a vein, through the right atrium, through the valve, and finally touches the inside of the right ventricle. The first human heart catheterization is credited to Werner Forssmann in 1929, when he was a young intern at a hospital in Berlin, Germany. He requested permission to try the procedure on a patient, but his supervisor refused. Forssmann then decided to try it on himself. He anesthetized his left elbow, opened a vein, and inserted the catheter tube. As he pushed it up the arm, he watched its progress on an x-ray fluoroscope, which he had to view by reflection in a mirror held by a nurse. When the catheter had gone in 65 centimeters, Forssmann asked an x-ray technician to record it on film to prove that it had entered his heart. During the next two years, he repeated the procedure several times, but criticism by his medical colleagues forced him to discontinue it. He became a small-town doctor and was amazed to learn in 1956 that he had been awarded the Nobel Prize for Medicine.
Accumulated knowledge about the structure of the heart, improvements in surgery, the development of new drugs, and the availability of modern instrumentation have all contributed to a substantial improvement in the medical treatment of heart ailments in modern times. The development of artificial heart valves, the heart-lung machine, the success of heart bypass surgery, the use of laser beams for surgery, and the use of drugs to control high blood pressure are recent developments.
An important contribution from the field of electronics was the development of the transistor in the early 1950s. It made possible the whole technology of miniaturized electronics, replacing the bulky vacuum tubes that were used in old radio circuits. Implantable pacemakers and microprocessor sensors would not have been possible without transistors.
Human ingenuity no doubt will continue to develop new instruments for cardiac diagnosis and rehabilitation, building on the accomplishments of the innovators of the past.
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