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An artificial pacemaker (PM) is an electronic device implanted in the body to regulate the heart beat. It consists of a battery and electronic circuits enclosed in a hermetically sealed can. A PM treats abnormal heart rhythms called Cardiac dysrhythmia or arrhythmias, specifically slow arrhythmias called bradycardia. The PM delivers electrical stimuli over leads with electrodes in contact with the heart.

About 300,000 patients in the US have permanent pacemakers. Many more patients have been on a temporary PM during and after different operations, including coronary artery bypass surgery. The total number of people with various types of implanted pacemakers has crossed 3 million as of 2010. Many animals have had pacemakers installed.
Arrhythmia (incorrect rhythm of the heartbeat) results from a problem in the heart's electrical system. Electrical signals follow a certain pathway through the heart. It is the movement of these signals that causes a heart to contract, thus pump. The heart's generator of normal sinus rhythm, the sinoatrial node or SA node is the impulse-generating (pacemaker) tissue located in the right atrium of the heart. . During a slow arrhythmia, not enough electrical signals are being generated or are blocked from moving through the heart. The heart beats too slowly.
An artificial pacemaker can restore a heart to a normal rhythm. A pacemaker can also adjust its therapy to meet the body's needs. A PM does Cardiac Resynchronization Therapy (CRT). A PM is not designed to defibrillate the heart by the delivery of shocks, although some PMs can also be a hybrid and perform functions of a implantable cardioverter-defibrillator (ICD).

History

1700’s Luigi Galvani discovered when electrical energy was applied to a dissected frog’s leg, the muscle would contract.
1889 J A McWilliam reported in the British Medical Journal of his experiments in which application of an electrical impulse to the human heart in asystole (no heart activity) caused a ventricular contraction and that a heart rhythm of 60-70 beats per minute could be evoked by impulses applied at spacings equal to 60-70/minute.
1903 William Einthoven in 1924 won the Nobel prize for Medicine for the electrocardiogram or ECG or EKG. An EKG uses skin electrodes to capture and externally interpret and record the electrical activity of the heart over time .
Hymanotor
1932 Dr. Albert Hyman developed a machine, to be used as an artificial pacemaker in experimental animals. Some of it's characteristics:
A small source of electric current, i.e. a common flashlight battery
An interrupter mechanism
A timing device
A method of regulating the duration of the injected current
A suitable insulated needle to carry the current only to the right atrium of heart
Be portable
In 1933 Hyman developed the Hymanotor produced by Siemens that was improvement and more portable than his earlier model. It had several features, the needle electrode, a hand crank for winding the spring motor, a voltage control knob, a rate control mechanism with a thumb screw or button for switching between 30, 60, and 120 impulses per minute, a small neon lamp that flashed to indicate delivery of stimuli and a voltage meter.
The complexities of the pacemaker yielded a simple operational concept. The hand crank winds the spring motor which drives the magneto-generator at a governor controlled speed and causes the interrupter disc to rotate. The magneto-generator supplies current to a surface contact (referred to as a brush) which in turn makes intermittent contact with the rotating conductive surfaces on the interrupter disc, separated by an insulated surface. Each time such contact is made the illuminated neon lamp is extinguished and an electrical surge passes to the needle and then to the heart.
There was public debate on the morality of artificial biological machines to extend human life and Hyman's machine have may never tried on humans.

1949 John A. Hopps designed the first catheter electrode for cardiac stimulation, which was introduced via the right external jugular vein of the experimental animal and a vacuum tube operated external pacemaker. Atrial pacing was achieved and control of the cardiac rate was accomplished. The device that Hopps invented produced an electrical impulse to stop ventricular fibrillation (cardiac muscle rapidly quivers rather than contract properly). This experiment was done while Hopps at the University of Toronto’s Department of Surgery in 1949. He placed an electrode on an open chest of a dog through the heart. Along with this he placed another electrode on the dog’s body surface. PM-65 by Paul Zoll allowed the patient to ambulate
1952 Paul M. Zoll began work on an external pacemaker which was to stimulate the heart across the closed chest. The PM-65 pacemaker was line current operated and put out a maximum of approximately 150 volts. Output voltage and stimulation rates were controlled from the front panel of the pacemaker. The electrodes were two one-inch diameter metal discs placed on the right and left sides of the chest, held in place by a rubber strap and making contact via a conductive electrode jelly. By 1952 the first papers concerning the pacemaker were published and revolutionized the concept of resuscitation of the patient with heart block and asystole. Stimulation of an adult required approximately 100 volts, some sufficing with less and others requiring higher voltage. Stimulation was painful and required sedation. Prolonged stimulation produced local skin burns. The longest period of stimulation reported was 11 days. Beginning and end of stimulation was by manual switch operation. By 1956, the need for stimulation via myocardial wires placed during surgery caused the placement of an outlet labeled "internal" and from the output voltage was reduced to one tenth. A cart was needed which could go only as far as the extension cord could allow. Dr. Zoll said “We would put terrible, frightful electrodes on. They would stimulate all of the chest muscles and were kind of painful. But people who would otherwise have died were brought along. That was kind of miraculous.”

1957 Engineer Earl Bakken of Minneapolis, Minnesota, produced the first wearable external pacemaker for a patient of Dr. C. Walton Lillehei. This transistorized pacemaker, housed in a small plastic box, had controls to permit adjustment of pacing heart rate and output voltage and was connected to electrode leads which passed through the skin of the patient to terminate in electrodes attached to the surface of the myocardium of the heart.

1958 Arne Larsson was the first person ever to receive a fully implanted pacemaker system. Born May 26, 1915 and died December 28, 2001 in Stockholm, at age 86 years, thereby spending half of his life with a pacemaker. Arne Larsson´s first pacemakers were constructed on the initiative of his wife. After Arne's several heart attacks, having heard that animal experimentation with cardiac pacing was underway in the laboratories of the Karolinska Hospital she decided that a pacemaker might be beneficial for her husband. The first pacemaker was implanted by thoracotomy (cut open, major painful chest surgery) on October 8, 1958 and functioned for 3 hours following implantation. A second, identical unit, implanted the following morning, functioned somewhat longer. Nickel-cadmium cells, rechargeable from outside of the body, powered both pacemakers. By the end of his life Arne Larsson had used 22 pulse generators and 5 electrode systems. His heart worked well until his death of an unrelated malignancy, exemplifying what is commonplace today, but what was completely unknown at the time his first pacemaker was implanted. A pacemaker does not only preserve life, but also allows complete living.

1959 A transvenous lead of electrodes inserted via the cephalic vein was used first by a battery operated, external model 902M Atronic. The unit was capable of sensing spontaneous cardiac activity, of variation in output and stimulation rate and general evaluation of the impedance of the electrode system. A small meter indicated emission of stimuli or sensed events. A permanently attached cable delivered output to the electrodes. An audio output plug connector was available. Because the endocardial lead exited through the skin, it was fastened in place with stainless steel sutures which required frequent renewal. Though systemic infection did not occur the entry wound was frequently superficially infected, requiring cleansing and redressing. On November 8, 1962, after 41 months of such pacing he underwent thoracotomy for implantation of a pacemaker system, as a matter of potential convenience rather than to resolve pacing problems. He never fully recovered from surgery and died twenty days later.

1960 William Greatbatch came up with a viable implantable pacemaker using primary cells as a power source. It was known as the Chardack-Greatbatch implantable pacemaker. It used Mallory mercuric oxide-zinc cells (mercury battery) for its energy source, driving a two transistor, transformer coupled blocking oscillator circuit, all encapsulated in epoxy resin, then coupled to electrodes placed into the myocardium of the patient's heart. The main difference between this pacemaker and the Swedish one is the battery technology used. This patented innovation led to the Medtronic company of Minneapolis commencing manufacture and further development of cardiac pacemakers.
Mid 1960's Pacemakers were installed with pulse generator wires exiting sidewards and located in the shoulder instead of abdominally with longer wires.

1972 A lithium-iodine battery cell was developed that used two elements at near ends of the electrochemical scale, causing a high voltage of 2.8V and an energy density near the physical maximum. Unfortunately, it had an internal impedance which limited its current load to under 0.1 mA and was thus considered useless. William Greatbatch realized that since the reaction generates no gas, the cell can be hermetically sealed, huge energy verses weight, and the low amperage is sufficient (voltage can be increased with amplifiers), that this would be the ideal pacemaker battery.

1972 Radioisotopic (Nuclear) Generator (RP) Powered A major problem was short battery life, with lasting only 12 or 13 months. An exciting and reasonable alternate power source was a radioisotopic generator (RP). These American-made devices in were implanted from 1972 to 1988. The pacemaker had an expected life of 20 years. Availability of lithium-powered pacemakers with an expected life of about 10 years, and without the disadvantages of regulatory paperwork, eliminated the longevity advantage of RP's. In the hundreds of RP pacemakers patients no side effect could be attributed to the radioisotope.

1983 Medtronic steroid-eluting lead, are low stimulation threshold leads to improve longevity.

Other advances include leads, tips, sensing patient's level of activity, titanium cases, etc.

Pacemaker Leads

Design and Parts

Pacemaker Leads

The pacemaker leads are thin, insulated wires that carry electrical signals back and forth between the device and the heart. The leads can sense when the heart is beating too slowly and needs treatment. Different types of PMs may have one lead to either the atria or ventricle, two leads one each to a atria and ventricle or three leads to the right atria, right ventricle and third lead to the left ventricle. Sometimes the 3rd lead is high voltage that can work as a implantable cardioverter-defibrillator (ICD). All three lead types also have another electrode called the indifferent electrode. (like a ground wire in other applications) on the housing of the generator case. Leads have a built in resistance of 300 to 1200 Ohms. Unipolar Versus Bipolar. With Unipolar leads the embedded lead tip is the negative and the housing is the positive. With bipolar 2 wire leads, the tip is negative and the ring electrode which is exposed near the tip is positive so the current only flows through the heart muscle.
Lead end materials are platinum, titanium, silver, cobalt, alloys or a steroid eluting tined lead. Tips hold the lead in the cardiac muscle cells, passive fixation that uses barbs like an arrow head and active fixation screws into the heart muscle.
Pulse generator
Pulse generator is quite small, about the size of a silver dollar. It contains computerized parts that run on a battery. The device treats the heart by sending very small amounts of electrical energy to the heart through the leads.

Typical Specifications
Diameter: 44 mm (1.7”)
Thickness: 8 mm (.3”)
Weight: 26 grams
Volume: 11cc.
Pacing rate: 60 to 120 ppm (pulse per minute) to each lead
Pulse Width: 1.0 ms (milliseconds)
Pulse Voltage: 1.5 to 6 volts
Pulse Power peak: over 25 microamp
Refractory: 320 ms (milliseconds)
Pacing lead impedance: 300 to 1200 Ohms
Long term average drain: 25 microamp

Pacemaker, a sophisticated machine
All pacemakers have an internal clock that determines when the next pulse should be delivered. All pacemakers have an output circuit that, when triggered by the timer, emit a pulse of fixed duration and voltage (or current if so designed). Pacemakers that stimulate both atrial and ventricular tissue do so with two output circuits such that the ventricular output occurs after a programmable AV delay. The delay period is programmable in most permanent pacemakers. The device has sensors that can detect when you rest and need a slow heart rate, and when you exercise and need a faster heart rate.
Some pacemakers are capable of pacing and sensing in both the ventricle and the atrium. Such capability permits the pacemaker to ensure not only an adequate ventricular rate, but to preserve the atrial kick before each ventricular contraction. These pacemakers (DDD classification) guarantee a minimum atrial rate and also ensure that a ventricular contraction occurs within a specified amount of time after each atrial contraction. The electronics are complicated because it is not always appropriate for ventricular contraction to occur after every atrial contraction. For example, an atrial tachycardia might generate an excessive ventricular rate.
Now pacemakers have been devised that in some fashion sense the patient's level of activity and accordingly adjust the (demand) pacing rate (variable rate pacing).
Accelerometer
One such device uses a piezoelectric crystal in the unit to detect body motion transmitted from underlying muscles. The rate of pacing is determined by the sensed level of activity. Software "filters" screen out high (rigors, tremor, motor vehicle driving) and low frequency (respiration, heartbeat) vibrations. The accelerometer responds to activity in the frequency range of typical physiologic activity (1–10 Hz). An algorithm translates the measured acceleration in this range into a rate increase above the Lower Rate Limit.
Pacemaker Code Chart
Minute Ventilation (MV)
Another method of determining the presence of physical activity utilizes detection of the respiratory rate. The MV sensor for rate adaptation is derived by means of transthoracic impedance measurement. Approximately every 50 ms (20 times a second or 20 Hz), the device will drive a tiny current excitation waveform between the selected (atrial or ventricular) lead ring electrode and the pacemaker case. On the case is the third electrode called the indifferent electrode.
The average excitation current that is delivered to the tissue is approximately 0.5 micro-amp. When the lungs are full of air the amp flow will be slightly lower than when empty of air. The computer can create a wave form and count the respiratory rate. These pacemakers adjust the minimum pacing rate according to the respiratory rate, on the logic that rapid breathing indicates exertion. The filtered waveform is then processed to obtain the total volume measurement. The device keeps a 4-hour moving average (baseline) of these measurements as well as a short-term (approximately 30-second) moving average (acute), which is updated every 8.0 seconds. The difference between the baseline and the acute MV measure represents the relative increase or decrease in MV, which is translated by the algorithm into a rate increase over the Lower Rate Limit.

Battery
Typical pacemaker has usable Battery Capacity of 1 amphour. To compare that, an automobile has a 100 amphour battery at 12 volts.
2.8-V solid-state lithium-iodine battery. Jet Propulsion Laboratory developed this technology in 1967 and the first patients trials with lithium-iodine battery powered pacemakers were in 1972.
Operating Voltage: 1.5 to 6.0 volts

A Cardiac electrophysiologist is a cardiologist who specializes in pacemakers.

It is amazing to think about the idea of taking over the management of the human heartbeat and the ensuing quest to design an ever better machine that would emulate the natural functions ever more perfectly.