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Pacemaker

Pacemaker

The pacemaker is an electronic biomedical device that can regulate the human heartbeat when its natural regulating mechanisms break down. It is a small box surgically implanted in the chest cavity and has electrodes that are in direct contact with the heart. First developed in the 1950s, the pacemaker has undergone various design changes and has found new applications since its invention. Today, pacemakers are widely used, implanted in tens of thousands of patients annually.

Background

The heart is composed of four chambers, which make up two pumps. The right pump receives the blood returning from the body and pumps it to the lungs. The left pump gets blood from the lungs and pumps it out to the rest of the body. Each pump is made up of two chambers, an atrium and a ventricle. The atrium collects the incoming blood. When it contracts, it transfers the blood to the ventricle. When the ventricle contracts, the blood is pumped away from the heart.

In a normal functioning heart, the pumping action is synchronized by the pacemaker region of the heart, or sinoatrial node, which is located in the right atrium. This is a natural pacemaker that has the ability to create electrical energy. The electrical impulse is created by the diffusion of calcium ions, sodium ions, and potassium ions across the membrane of cells in the pacemaker region. The impulse created by the motion of these ions is first transferred to the atria, causing them to contract and push blood into the ventricles. After about 150 milliseconds, the impulse moves to the ventricles, causing them to contract and pump blood away from the heart. As the impulse moves away from each chamber of the heart, that section relaxes.

Unfortunately, the natural pacemaker can malfunction, leading to abnormal heartbeats. These arrhythmias can be very serious, causing blackouts, heart attacks, and even death. Electronic pacemakers are designed to supplement the heart's own natural controls and to regulate the beating heart when these break down. It is able to do this because it is equipped with sensors that constantly monitor the patient's heart, and a battery that sends electricity, when needed, through lead wires to the heart itself to stimulate the heart to beat.

In addition to outer units, artificial pacemakers can be permanently implanted in a patient's chest. This is done by first guiding the lead through a vein and into a chamber of the heart, where the lead is secured. Fluoroscopic imaging helps facilitate this process. The pacemaker itself is next placed in a pocket, which is formed by surgery just above the upper abdominal quadrant. The lead wire is then connected to the pacemaker, and the pocket is sewn shut. This is a vast improvement over early methods, which required opening the chest cavity and attaching the leads directly to the outer surface of the heart.

History

The idea of using an electronic device to provide consistent regulation of the beating heart was not initially obvious to the early developers of the pacemaker. The first pacemaker, developed by Paul Zoll in 1952, was a portable version of a cardiac resuscitator. It had two lead wires that could be attached to a belt worn by the patient. It was plugged into the nearest wall socket and delivered an electric shock that stimulated the heart of a patient having an attack. This stimulation would usually be enough to cause the heart to resume its normal function. While moderately effective, this early pacemaker was primarily used in emergency situations.

Through 1957 and 1960 significant improvements were made to Zoll's original invention. In an attempt to reduce the amount of voltage needed to restart the heart and increase the length of time electronic pacing could be accomplished, C. Walton Lillehei made a pacemaker that had leads attached directly to the outer wall of the heart. Later, in 1958, a battery was added as the power source, making the pacemaker truly portable, which allowed patients to be mobile. This also enabled patients to use the pacemaker continuously instead of only for emergencies. Lillehei's pacemaker was external. William Chardack and Wilson Greatbatch invented the first implantable pacemaker. It was implanted in a living patient in 1960.

The modern technique for putting a pacemaker into a patient's heart was developed by Seymour Furman. Instead of cutting open the chest cavity, he used a method of inserting the leads into a vein and threading them up into the ventricles. With the leads inside the heart, even lower voltages were needed to regulate the heartbeat. This increased the length of time a pacemaker could be inside a person. Although his method was not widely used initially, by the late 1960s most cardiac specialists had switched to Furman's endocardial pacemakers. Since then improvements have been made in their design, including smaller pacemaker devices, longer lasting batteries, and computer controls.

Raw Materials

The materials used to construct pacemakers must be pharnacologically inert, nontoxic, sterilizable, and able to function in the environmental conditions of the body. The various parts of the pacemaker, including the casing, microelectronics, and the leads, are all made with biocompatible materials. Typically, the casing is made of titanium or a titanium alloy. The lead is also made of a metal alloy, but it is insulated by a polymer such as polyurethane. Only the metal tip of the lead is exposed. The circuitry is usually made of modified silicon semiconductors.

Design

Many types of pacemakers are available. The North American Society of Pacing and Electrophysiology (NASPE) has classified them by which heart chamber is paced, which chamber is sensed, how the pacemaker responds to a sensed beat, and whether it is programmable. Despite this vast array of models, all pacemakers are essentially composed of a battery, lead wires, and circuitry.

The primary function of a pacemaker battery is to store enough energy to stimulate the heart with a jolt of electricity. Additionally, it also provides power to the sensors and timing devices. Since these batteries are implanted into the body, they are designed to meet specific characteristics. First, they must be able to generate about five volts of power, a level that is slightly higher than the amount required to stimulate the heart. Second, they must retain their power over many years. A minimum time frame is four years. Third, they must have a predictable life cycle, allowing the doctor to know when a replacement is required. Finally, they must be able to function when hermetically (airtight) sealed. Batteries have two metals that form the anode and cathode. These are the battery components through which charge is transferred. Some examples include lithium/iodide, cadmium/nickel oxide, and nuclear batteries.

Pacemaker leads are thin, insulated wires that are designed to carry electricity between the battery and the heart. Depending on the type of pacemaker, it will contain either a single lead, for single chamber pacemakers, or two leads, for dual chamber pacemakers. With the constant beating of the heart, these wires are chronically flexed and must be resistant to fracture. There are many styles of leads available, with primary design differences found at the exposed end. Many of the leads have a screw-in tip, which helps anchor them to the inner wall of the heart.

The circuitry is the control center of the pacemaker. Located here are heart monitoring sensors, voltage regulators, timing circuits, and externally programmable controls. The circuitry is composed primarily of resistors, capacitors, diodes, and semiconductors. Modern pacemaker circuitry is a vast improvement over earlier models. With the application of semiconductors, circuit boards have become much smaller. They also require less energy, produce less heat, and are highly reliable.

The Manufacturing
Process

Pacemakers are sophisticated electronic devices. Therefore, some manufacturers rely on outside suppliers to provide many of the component parts. The construction of a pacemaker is not a linear process but an integrated one. Component parts such as the battery, leads, and the circuitry are constructed individually, then pieced together to form the final product.

Making the battery

  • 1 The primary type of battery used in pacemakers is a lithium/iodine cell. One method used by manufacturers to make these batteries involves first mixing together the iodine and a polymer such as poly2-vinylpyridine (PVP). They are heated together, forming a molten charge-transfer complex. This liquid is then poured into a half moon-shaped, preformed cell which contains the other components of the battery, including the lithium anode (positive charge) and a cathode collector screen. The iodine/polymer blend solidifies as it cools to form the cathode. After the cathode is formed, the battery is hermetically sealed to prevent moisture from entering.

Making the leads

  • 2 The leads are typically composed of a metal alloy. The wire is made by an extrusion process in which the metal is heated until it is molten, then pushed through an appropriately sized opening. It is cut, then bundled with many other wires and treated with a polymeric insulator such as polyurethane. One end of the lead wires is fashioned with a shaped tip, and the other is fitted with a pacemaker connector.

Making the motherboard

  • 3 The motherboard contains all the electrical circuitry of the pacemaker, including the semiconductor chips, resistors, capacitors, and other devices. Using a complex method known as hybridization, these components are combined to form a single complex circuit. Construction begins with a small board (less than 0.32 sq in [2 sq cm]) which has the electronic configuration mapped out. The appropriate components are put in place on the board. They are then affixed using a minimum number of soldering welds.

Final assembly and packaging

  • 4 When all of the component pieces are available, final assembly takes place. The circuitry is connected to the battery, and both are inserted into the metal casing. The casing used for a pacemaker is typically formed using titanium or a titanium alloy. It is constructed in multiple pieces that are sealed together after the other pacemaker components are introduced. A fitting is also affixed to the casing, providing a connecting point for the leads.
  • 5 The finished devices are then put into final packaging along with accessories. After being exhaustively tested, they are then sent out to distributors and finally to doctors.

Quality Control

The quality of each pacemaker is ensured by making visual and electrical inspections throughout the entire production process. These tests will detect most flaws. Since the batteries must be absolutely reliable, they are specially manufactured and exhaustively tested, thereby increasing the associated costs tremendously. The functionality of each finished pacemaker is also tested before it is sent out for sale. Many of these tests are done under varying environmental conditions, such as excessive humidity and stress.

Manufacturers set their own quality standards for the pacemakers that they produce. However, standards and performance recommendations are required by various medical organizations and governmental agencies. In the United States, pacemakers are classified as Class III biomedical devices, which means they require pre-market approval from the United States Food and Drug Administration (FDA).

The Future

With the increasing numbers of senior citizens in the United States, it is anticipated that a greater percentage of the population will require pacemakers. As research efforts continue, future devices promise to be longer lasting, more reliable, and more versatile. Advances in battery technology, such as using radioactive isotopes for power, will undoubtedly improve the longevity of implanted pacemakers. Developments in microelectronics should provide even smaller devices which are less prone to environmental interferences. A late-breaking development in the field is the application of cardiac pacemaking technology to the brain. In this system, scientists connect the lead wires to a specific site on the brain and stimulate it as needed to regulate heartbeat. This device has been shown to be particularly effective in calming the tremors associated with Parkinson's disease.

Where to Learn More

Books

Banbury, Catherine. Surviving Technological Innovation in the Pacemaker Industry, 1959-1990. Garland Pub., 1997.

Ellenbogen, Kenneth, ed. Clinical Cardiac Pacing. Saunders, 1995.

Fox, Stuart. Human Physiology. WCB Publishers, 1990.

Moses, H., J. Schneider, B. Miller, and G. Taylor. A Practical Guide to Cardiac Pacing. Little, Brown and Co., 1991.

Periodicals

Jeffrey, Kirk. "Many Paths to the Pacemaker." Invention & Technology, Spring 1997, pp. 28-39.

PerryRomanowski

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Pacemakers

Pacemakers

Definition

A pacemaker is a surgically implanted electronic device that regulates a cardiac arrhythmia.

Pacemakers are most frequently prescribed when the heartbeat decreases under 60 beats per minute at rest (severe symptomatic bradycardia). They are also used in some cases to slow a fast heart rate over 120 beats per minute at rest (tachycardia).


Demographics

The population for pacemaker implant is not limited by age, sex, or race. Over 100,000 pacemakers are implanted per year in the United States. The occurrence is more frequent in the elderly with over 85% of implants received by those over age 65. A history of myocardial infarction (heart attack), congenital defect, or cardiac transplant also increases the likelihood of pacemaker implant.


Description

Approximately 500,000 Americans have an implantable permanent pacemaker device. A pacemaker implantation is performed under local anesthesia in a hospital by a surgeon assisted by a cardiologist. An insulated wire called a lead is inserted into an incision above the collarbone and guided through a large vein into the chambers of the heart. Depending on the configuration of the pacemaker and the clinical needs of the patient, as many as three leads may be used in a pacing system. Current pacemakers have a double, or bipolar, electrode attached to the end of each lead. The electrodes deliver an electrical charge to the heart to regulate heartbeat. They are positioned on the areas of the heart that require stimulation. The leads are then attached to the pacemaker device, which is implanted under the skin of the patient's chest.

Patients undergoing surgical pacemaker implantation usually stay in the hospital overnight. Once the procedure is complete, the patient's vital signs are monitored and a chest x ray is taken to ensure that the pacemaker and leads are properly positioned.

Modern pacemakers have sophisticated programming capabilities and are extremely compact. The smallest weigh less than 13 grams (under half an ounce) and are the size of two stacked silver dollars. The actual pacing device contains a pulse generator, circuitry programmed to monitor heart rate and deliver stimulation, and a lithium iodide battery. Battery life typically ranges from seven to 15 years, depending on the number of leads the pacemaker is configured with and how much energy the pacemaker uses. When a new battery is required, the unit can be exchanged in a simple outpatient procedure.

A temporary pacing system is sometimes recommended for patients who are experiencing irregular heartbeats as a result of a recent heart attack or other acute medical condition. The implantation procedure for the pacemaker leads is similar to that for a permanent pacing system, but the actual pacemaker unit housing the pulse generator remains outside the patient's body. Temporary pacing systems may be replaced with a permanent device at a later date.


Diagnosis/Preparation

Patients being considered for pacemaker implantation will undergo a full battery of cardiac tests, including an electrocardiogram (ECG) or an electrophysiological study or both, to fully evaluate the bradycardia or tachycardia.

The symptoms of fatigue and lightheadedness that are characteristic of bradycardia can also be caused by a number of other medical conditions, including anemia. Certain prescription medications can also slow the heart rate. A doctor should take a complete medical history and perform a full physical work-up to rule out all non-cardiac causes of bradycardia.

Patients are advised to abstain from eating six to eight hours before the surgical procedure. The patient is usually given a sedative to help him or her relax for the procedure. An intravenous (IV) line will also be inserted into a vein in the patient's arm before the procedure begins in case medication or blood products are required during the insertion.


Aftercare

After an implant without complications the patient can expect a hospital stay of one to five post-procedure days. Pacemaker patients should schedule a follow-up visit with their cardiologist approximately six weeks after the surgery. During this visit, the doctor will make any necessary adjustments to the settings of the pacemaker. Pacemakers are programmed externally with a handheld electromagnetic device. Pacemaker batteries must be checked regularly. Some pacing systems allow patients to monitor battery life through a special telephone monitoring service that can read pacemaker signals.

Patients with cardiac pacemakers should not undergo a magnetic resonance imaging (MRI) procedure. Devices that emit electromagnetic waves (including magnets) may alter pacemaker programming or functioning. A 1997 study found that cellular phones often interfere with pacemaker programming and cause irregular heart rhythm. However, advances in pacemaker design and materials have greatly reduced the risk of pacemaker interference from electromagnetic fields.


Risks

Because pacemaker implantation is an invasive surgical procedure, internal bleeding, infection, hemorrhage, and embolism are all possible complications. Infection is more common in patients with temporary pacing systems. Antibiotic therapy given as a precautionary measure can reduce the risk of pacemaker infection. If infection does occur, the entire pacing system may have to be removed.

The placing of the leads and electrodes during the implantation procedure also presents certain risks for the patient. The lead or electrode could perforate the heart or cause scarring or other damage. The electrodes can also cause involuntary stimulation of nearby skeletal muscles.

A complication known as pacemaker syndrome develops in approximately 7% of pacemaker patients with single-chamber pacing systems. The syndrome is characterized by the low blood pressure and dizziness that are symptomatic of bradycardia. It can usually be corrected by the implantation of a dual-chamber pacing system.

Normal results

Pacemakers that are properly implanted and programmed can correct a patient's arrhythmia and resolve related symptoms.



Morbidity and mortality rates

In the United States, patients experience complications in 3.3% and 3.8% of cases, with those over 65 years of age demonstrating a slightly higher complication rate of 6.1%. The most common complications include lead dislodgement, pneumothorax (collapsed lung), and cardiac perforation. The risk of death is less then 0.5% throughout the course of the hospital stay.


Resources

books

DeBakey, Michael E. and Antonio Gotto Jr. The New Living Heart. Holbrook, MA: Adams Media Corporation, 1997.

periodicals

Gregoratas, Gabriel, et al. "ACC/AHA Guidelines for Implantation of Pacemakers and Antiarrhythmia Devices." Journal of the American College of Cardiology 31 (April 1998): 1175209.

Link, Mark S, et al. "Complications of Dual Chamber Pacemaker Implantation in the Elderly." Journal of Interventional Cardiac Electrophysiology 2 (1998): 175179.

organizations

American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-6300. <http://www.americanheart.org>.


Paula Anne Ford-Martin Allison J. Spiwak, MSBME

WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?



Pacemaker implants are performed by a cardiologist who has completed medical school and an additional internship and residency program. Additional training as an electrophysiologist may be acquired by the physician during the residency program. Specific training by the pacemaker manufacturer may also be acquired. Hospitals performing these procedures have access to cardiac catheterization facilities or operating rooms equipped with portable fluoroscopy units.

QUESTIONS TO ASK THE DOCTOR



  • How many pacemaker implants has the physician performed?
  • What type of pacemaker will be implanted, univentricular or biventricular, and how many of the specific procedure has the physician performed?
  • How long will the expected hospital stay be?
  • What precautions should be taken in the weeks following discharge from the hospital?
  • What precautions will need to taken in day to day activities following pacemaker implant?
  • When can normal daily, such as driving, exercise and work, activities be initiated?
  • What will indicate that the pacemaker is failing and when should emergency care be sought?
  • How long will the battery function and when should treatment to replace the device be sought?
  • Is there special documentation I will need for air travel during security screenings?
  • Will there be notification of manufacturer recalls?

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Pacemakers

Pacemakers

Definition

A pacemaker is a surgically-implanted electronic device that regulates a slow or erratic heartbeat.

Purpose

Pacemakers are implanted to regulate irregular contractions of the heart (arrhythmia). They are most frequently prescribed to speed the heartbeat of patients who have a heart rate well under 60 beats per minute (severe symptomatic bradycardia). They are also used in some cases to slow a fast heart rate (tachycardia).

Precautions

The symptoms of fatigue and lightheadedness that are characteristic of bradycardia can also be caused by a number of other medical conditions, including anemia. Certain prescription medications can also slow the heart rate. A doctor should take a complete medical history and perform a full physical work-up to rule out all non-cardiac causes of bradycardia.

Patients with cardiac pacemakers should not undergo a magnetic resonance imaging (MRI) procedure. Devices that emit electromagnetic waves (including magnets) may alter pacemaker programming or functioning. A 1997 study found that cellular phones often interfere with pacemaker programming and cause irregular heart rhythm. However, advances in pacemaker design and materials have greatly reduced the risk of pacemaker interference from electromagnetic fields.

Description

Approximately 500,000 Americans have an implantable permanent pacemaker device. A pacemaker implantation is performed under local anesthesia in a hospital by a surgeon assisted by a cardiologist. An insulated wire called a lead is inserted into an incision above the collarbone and guided through a large vein into the chambers of the heart. Depending on the configuration of the pacemaker and the clinical needs of the patient, as many as three leads may be used in a pacing system. Current pacemakers have a double, or bipolar, electrode attached to the end of each lead. The electrodes deliver an electrical charge to the heart to regulate heartbeat. They are positioned on the areas of the heart that require stimulation. The leads are then attached to the pacemaker device, which is implanted under the skin of the patient's chest.

Patients undergoing surgical pacemaker implantation usually stay in the hospital overnight. Once the procedure is complete, the patient's vital signs are monitored and a chest x ray is taken to ensure that the pacemaker and leads are properly positioned.

Modern pacemakers have sophisticated programming capabilities and are extremely compact. The smallest weigh less than 13 grams (under half an ounce) and are the size of two stacked silver dollars. The actual pacing device contains a pulse generator, circuitry programmed to monitor heart rate and deliver stimulation, and a lithiumiodide battery. Battery life typically ranges from seven to 15 years, depending on the number of leads the pacemaker is configured with and how much energy the pacemaker uses. When a new battery is required, the unit can be exchanged in a simple outpatient procedure.

A temporary pacing system is sometimes recommended for patients who are experiencing irregular heartbeats as a result of a recent heart attack or other acute medical condition. The implantation procedure for the pacemaker leads is similar to that for a permanent pacing system, but the actual pacemaker unit housing the pulse generator remains outside the patient's body. Temporary pacing systems may be replaced with a permanent device at a later date.

Preparation

Patients being considered for pacemaker implantation will undergo a full battery of cardiac tests, including an electrocardiogram (ECG) or an electrophysiological study or both to fully evaluate the bradycardia or tachycardia.

Patients are advised to abstain from eating 6-8 hours before the surgical procedure. The patient is usually given a sedative to help him or her relax for the procedure. An intravenous (IV) line will also be inserted into a vein in the patient's arm before the procedure begins in case medication or blood products are required during the insertion.

Aftercare

Pacemaker patients should schedule a follow-up visit with their cardiologist approximately six weeks after the surgery. During this visit, the doctor will make any necessary adjustments to the settings of the pacemaker. Pacemakers are programmed externally with a handheld electromagnetic device. Pacemaker batteries must be checked regularly. Some pacing systems allow patients to monitor battery life through a special telephone monitoring service that can read pacemaker signals.

Risks

Because pacemaker implantation is an invasive surgical procedure, internal bleeding, infection, hemorrhage, and embolism are all possible complications. Infection is more common in patients with temporary pacing systems. Antibiotic therapy given as a precautionary measure can reduce the risk of pacemaker infection. If infection does occur, the entire pacing system may have to be removed.

The placing of the leads and electrodes during the implantation procedure also presents certain risks for the patient. The lead or electrode could perforate the heart or cause scarring or other damage. The electrodes can also cause involuntary stimulation of nearby skeletal muscles.

KEY TERMS

Electrocardiogram (ECG) A recording of the electrical activity of the heart. An ECG uses externally attached electrodes to detect the electrical signals of the heart.

Electrophysiological study A test that monitors the electrical activity of the heart in order to diagnose arrhythmia. An electrophysiological study measures electrical signals through a cardiac catheter that is inserted into an artery in the leg and guided up into the atrium and ventricle of the heart.

Embolism A blood clot, air bubble, or clot of foreign material that blocks the flow of blood in an artery. When an embolism blocks the blood supply to a tissue or organ, the tissue the artery feeds dies (infarction). Without immediate and appropriate treatment, an embolism can be fatal.

Magnetic resonance imaging (MRI) An imaging technique that uses a large circular magnet and radio waves to generate signals from atoms in the body. These signals are used to construct images of internal structures.

A complication known as pacemaker syndrome develops in approximately 7% of pacemaker patients with single-chamber pacing systems. The syndrome is characterized by the low blood pressure and dizziness that are symptomatic of bradycardia. It can usually be corrected by the implantation of a dual-chamber pacing system.

Normal results

Pacemakers that are properly implanted and programmed can correct a patient's arrhythmia and resolve related symptoms.

Resources

ORGANIZATIONS

American Heart Association. 7320 Greenville Ave. Dallas, TX 75231. (214) 373-6300. http://www.americanheart.org.

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pacemaker

pacemaker In the healthy heart the cells of the sino-atrial (SA) node constitute the natural pacemaker, generating regular electrical signals which spread through the heart and cause it to beat. An artificial pacemaker is required if for any reason, usually heart block, this natural system is compromised, resulting in an irregularly or consistently slow heart rate (bradycardia).

An artificial pacemaker contains a battery and circuitry which produces electrical pulses of short duration capable of stimulating the heart. The device was invented by Wilson Greatbatch of the USA and patented in 1960 after surgeons in New York had made the first clinically successful implant in a 77-year-old man. The pulses are delivered via an electrode which makes direct contact with the heart muscle. Pacemakers can be made to stimulate the heart at a fixed rate irrespective of the intrinsic heart rate, or a demand pacemaker may be used which is capable of sensing the native rhythm and pacing the heart only when the sensed rate falls below a certain value. Recent advances in design have produced pacemakers that are capable of re-synchronizing atrial and ventricular activity, thus functioning as a defibrillator.

A temporary pacemaker can be fitted by placing the pacing electrode within the right ventricle. The electrode is introduced through a needle inserted into a large vein in an arm or the neck. The electrode is advanced, under X-ray monitoring, within the vein following its course back to the heart. Once the electrode is in contact with the inner surface of the heart it is connected to the pacemaker, which remains outside the body. A temporary pacemaker may be required in the short term for certain individuals after a heart attack, during cardiac surgery or general anaesthesia. A permanent pacemaker is fitted in people requiring long-term pacing. The electrode in a permanent pacemaker is also introduced into a vein, but the vein in this case is surgically exposed. The permanent pacemaker is sufficiently small to be placed in a small pouch formed within the muscle under the skin; it is con-nected to the pacing electrode. Less commonly, pacemakers may be implanted that can detect the onset of abnormal tachycardias (fast heart rate). The pacemaker can stimulate the heart in competition with the abnormal beat in an attempt to return it to a normal rhythm. More recently, power supplies capable of delivering high energies for defibrillation have been introduced. The pacemaker may last five to fifteen years, depending on the lifetime of the battery and the frequency of stimulation. More modern versions can retain a microchip memory of their activity for periods of up to a year; this information can then be routinely ‘downloaded’ for analysis so that the physician has access to a detailed electrical history of the patient's heart. Pacemaker batteries can usually be recharged via an induction coil outside the skin so no further surgery is required. Modern pacemakers thus have increasingly sophisticated microprocessor-controlled ‘brains’. Like all such equipment, there is a risk of electrical interference by very powerful electrical devices; these risks are often signposted.

David J. Miller, and Niall G. MacFarlane


See also defibrillator; heart; heart block.

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Pacemaker

Pacemaker

The rhythmic, regular beating of the heart is controlled by a natural cardiac pacemaker called the sinoatrial node. This small patch of cells sends rhythmic bioelectric impulses along specific conducting fibers to the heart muscle. These impulses stimulate the muscles to contract and relax in a regular sequence.

If the heart muscle fails to receive the pacemaker's signals, it will not pump the blood. With no blood reaching the brain, lack of oxygen will quickly cause an individual to lose consciousness. Within a few more minutes, the individual dies, unless the heart muscle is stimulated to resume its beating.

Artificial Pacemakers

The idea of using electric impulses to restart the heart goes back at least as far as 1862. The first practical idea was that of American inventor Wilson Greatbatch, who envisioned an implantable pacemaker in 1951. His idea, however, could not be practically implemented until transistors became widely available in the late 1950s. In 1960, after two years of animal testing, Dr. William Chardack and his associates implanted Greatbatch's device in the chest wall of a human patient.

Pacemakers operate only when episodes of irregular heartbeat occur. They can be programmed to vary the heart rate according to the body's needs. So, for example, a slower heart rate is programmed during sleeping hours.

In situations where a pacemaker is only needed for a short time (such as when a person's heart rhythm is temporarily disturbed by a heart attack), the pacemaker's generating unit can be worn externally on a belt, rather than implanted.

Design of modern pacemakers has continued to improve. Modern lithium batteries last up to 15 years, while earlier mercury-zinc batteries had a life of only 20 months. Today's pacemakers can weigh as little as one ounce (30 grams), and are relatively easy to install.

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pacemaker

pacemaker
1. (or sinoatrial node) A small mass of specialized muscle cells in the mammalian heart, found in the wall of the right atrium near the opening for the vena cava. The cells initiate and maintain the heart beat: by their rhythmic and spontaneous contractions they stimulate contraction of the atria (see also atrioventricular node). The cells themselves are controlled by the autonomic nervous system, which determines the heart rate. Similar pacemakers occur in the hearts of other vertebrates.

2. An electronic or nuclear battery-charged device that can be implanted surgically into the chest to produce and maintain the heart beat. These devices are used when the heart's own pacemaker is defective or diseased.

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pacemaker

pacemaker (payss-mayk-er) n.
1. a device used to produce and maintain a normal heart rate in patients who have heart block. It consists of a battery that stimulates the heart through an insulated electrode wire (lead) attached to the surface of the ventricle (epicardial p.) or lying in contact with the lining of the heart (endocardial p.). A pacemaker may be temporary, with an external battery, or it may be permanent, when the whole apparatus is surgically implanted under the skin.

2. the part of the heart that regulates the rate at which it beats: the sinoatrial node.

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pacemaker

pace·mak·er / ˈpāsˌmākər/ • n. 1. an artificial device for stimulating the heart muscle and regulating its contractions. ∎  the part of the heart muscle (the sinoatrial node) that normally performs this role. ∎  the part of an organ or of the body that controls any other rhythmic physiological activity. 2. another term for pacesetter. DERIVATIVES: pace·mak·ing / -ˌmāking/ adj. & n.

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pacemaker

pacemaker (sino-atrial node) Specialized group of cells in the vertebrate heart that contract spontaneously, setting the pace for the heartbeat itself. If it fails, it can be replaced by an artificial pacemaker – an electronic unit that stimulates the heart by means of tiny electrical impulses.

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pacemaker

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