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Artificial pacemaker

From Academic Kids

A pacemaker (or "artificial pacemaker", so as not to be confused with the heart's natural pacemaker) is a medical device designed to regulate the beating of the heart. The purpose of an artificial pacemaker is to stimulate the heart when either the heart's native pacemaker is not fast enough or if there are blocks in the heart's electrical conduction system preventing the propagation of electrical impulses from the native pacemaker to the lower chambers of the heart, known as the ventricles. Generally, pacemakers do not treat fast rhythms of the heart.

Contents

History of the implantable pacemaker

The first external pacemaker was designed and built by the Canadian electrical engineer John Hopps in 1950. A substantial external device, it was somewhat crude and painful to the patient in use. A number of inventors, including Paul Zoll, made smaller but still bulky external devices in the following years. The pacemakers built in the late 1950s were bulky, relied on external electrodes, and had to be plugged into a wall outlet. External electric shocks were frequently too traumatic for young heart block patients, and the AC-operated pacemaker could fail during a power blackout.

Dr. C. Walton Lillehei, a pioneer in open heart surgery at the University of Minnesota Medical School and his colleagues set out to develop a better system, and Earl Bakken became closely involved in their work. When a power failure occurred in the Twin Cities and resulted in the death of one of Dr. Lillehei's young patients, the surgeon turned to Earl and for a battery back-up for the AC pacemakers.

Over the next few weeks, Earl developed a new kind of pacemaker that was not much larger than a paperback book. He borrowed parts from other electrical devices that he had in the shop. For the new device's circuitry, he relied on a design for a transistorized metronome he had seen in a trade publication. When finished, he had produced a pacemaker that was powered by mercury batteries, provided a 9-volt DC pulse, and could easily and comfortably be "worn" by young patients.

The original Bakken pacemaker was tested in the University of Minnesota's laboratory. The following day, it was applied to a pediatric heart block patient. The effect was instantaneous. The pacemaker immediately restored the child's heartbeat to near normal. Within days, the child's heart resumed a normal rhythm on its own, and the pacemaker was removed.

The development of the wearable, external, battery-powered pacemaker amounted to a leap forward in the treatment of heart block and other cardiac problems. It also signaled the beginning of a new era in the therapeutic application of electrical technology for patients around the world.

The first implantation into a human was made in 1958 by a Swedish team using a pacemaker designed by Rune Elmqvist and ke Senning. The device failed after three hours. A second device was then implanted which lasted for two days. The worlds first implantable pacemaker patient, Arne Larsson, survived the first tests and died in 2001 after having received 22 different pacemakers during his lifetime. In February 1960, an improved model relying on better materials was implanted in Montevideo, Uruguay. That device lasted until the patient died of other ailments, 9 months later. The early Swedish designed devices used rechargeable batteries, which were charged by an induction coil from the outside.

Devices constructed by the American Wilson Greatbatch entered use in humans from April 1960 following extensive animal testing. The first patient lived for a further 18 months. The early devices suffered from battery problems - every patient required an additional operation every 24 months to replace the batteries. See also Wickham heart pacemaker.

Pacemakers require wires (called leads) to both send the pacing pulses to the heart and sense the intrinsic rhythm of the heart. The first pacemakers required these leads to be placed surgically on the outer surface of the heart. In the mid 1960s, the first transvenous leads were placed. This allowed the placement of pacemakers without opening the thoracic cavity and therefore without the use of general anaesthesia.

The first American-made nuclear-powered pacemaker was developed and implanted at Newark Beth Israel Medical Center in Newark, New Jersey.

Indications for pacing

In most cases, the indication for permanent pacemaker placement is a slow heart rate (bradycardia) or a defect in the electrical conduction system of the heart (heart block) with associated symptoms. Typical symptoms of a slow heart rate include lightheadedness, poor exercise tolerance, and loss of consciousness. Individuals who have a slow heart rate but who are asymptomatic do not require a pacemaker. For instance, athletes typically have resting heart rates in the 40s without any deleterious effects.

If the slow heart rate is due to complete heart block, a pacemaker is indicated, since the heart rate can dramatically decrease without notice. Pacemakers can also be placed in patients at high risk for complete heart block.

Rarely, in people prone to ventricular fibrillation, a slow rhythm in the heart can lead to a ventricular fibrillation. In these people, preventing the slow rhythm can prevent ventricular fibrillation.

Methods of pacing

External pacing

External pacemakers can be used for initial stabilization of a patient, but implantation of a permanent internal pacemaker is usually required for most conditions. External cardiac pacing is typically performed by placing two pacing pads on the chest wall. Usually one pad is placed on the upper portion of the sternum, while the other is placed along the left axilla, near the bottom of the rib cage. When an electrical impulse goes from one pad to the other, it will travel through the tissues between them and stimulate the muscles between them, including the cardiac muscle and the muscles of the chest wall. Electrically stimulating any muscle, including the heart muscle, will make it contract. The stimulation of the muscles of the chest wall will frequently make those muscles twitch at the same rate as the pacemaker is set.

Pacing the heart via external pacing pads should not be relied upon for an extended period of time. If the person is conscious, he or she may feel discomfort due to the frequent stimulation of the muscles of the chest wall. Also, stimulation of the chest wall muscles does not necessarily mean that the heart is being stimulated as well.

Temporary internal pacing

An alternative to external pacing is the temporary internal pacing wire. This is a wire that is placed under sterile conditions via a central line. The distal tip of the wire is placed into either the right atrium or right ventricle. The proximal tip of the wire is attached to the pacemaker generator, outside of the body. Temporary internal pacing is often used as a bridge to permanent pacemaker placement. Under certain conditions, a person may require temporary pacing but would not require permanent pacing. In this case, a temporary pacing wire may be the optimal treatment option.

Permanent pacemaker placement

Placement of a permanent pacemaker involves placement of one or more pacing wires within the chambers of the heart. One end of each wire is attached to the muscle of the heart. The other end is screwed into the pacemaker generator. The pacemaker generator is a hermetically sealed device containing a power source and the computer logic for the pacemaker.

Most commonly, the generator is placed below the subcutaneous fat of the chest wall, above the muscles and bones of the chest. However, the placement may vary on a case by case basis.

Basic pacemaker function

Modern pacemakers all have two functions. They listen to the heart's native electrical rhythm, and if the device doesn't sense any electrical activity within a certain time period, the device will stimulate the heart with a set amount of energy, measured in joules.

Pacemaker naming code

The NASPE/BPEG generic (NPG) code is a pacemaker naming convention originally developed in 1974 that uses a 3-5 letter code to describe the main features of an artificial pacemaker. Each of the 5 positions signifies a particular aspects of pacemaker functionality. Using this scheme, a designation of VATOO would describe, for example, a pacemaker that sensed the atria and paced the ventricles in a triggered mode with no rate response or multisite pacing.

Position I II III IV V
Category Chamber(s) Paced Chamber(s) Sensed Response to Sensing Rate Response Multisite Pacing
Code O = none, A = atrium, V = ventricle and D = dual (A + V) O = none, A = atrium, V = ventricle and D = dual (A + V) O = none, T = triggered, I = inhibited and D = dual (T + I) O = none, R = rate modulation O = none, A = atrium, V = ventricle and D = dual (A + V)
Manufacturer's designation only S = single (A or V) S = single (A or V)

Table I. The Revised NASPE/BPEG Generic Code for Antibradycardia Pacing

Advancements in pacemaker function

When first invented, pacemakers controlled only the rate at which the heart's two largest chambers, the ventricles, beat.

Many advancements have been made to enhance the control of the pacemaker once implanted. Many of these enhancements have been made possible by the transition to microprocessor controlled pacemakers. Pacemakers that control not only the ventricles but the atria as well have become common. Pacemakers that control both the atria and ventricles are called dual-chamber pacemakers. Timing the contractions of the atria to precede that of the ventricles improves the pumping efficiency of the heart and can be useful in congestive heart failure.

Rate responsive pacing allows the device to sense the physical activity of the patient and respond appropriately by increasing or decreasing the base pacing rate via rate response algorithms.

Another advancement in pacemaker technology is left ventricular pacing. A pacemaker wire is placed on the outer surface of the left ventricle, with the goal of more physiological pacing than what is available in standard pacemakers. This extra wire is implanted to improve symptoms in patients with severe heart failure.

Devices with pacemaker function

Sometimes devices resembling pacemakers, called ICDs (implantable cardioverter-defibrillators) are implanted. These devices are often used in the treatment of patients at risk for sudden cardiac death. An ICD has the ability to treat many types of heart rhythm disturbances by means of pacing, cardioversion,or defibrillation.

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