Pacemaker W hat is it?

A pacemaker is an implanted medical device that stimulates the heart muscle with precisely timed discharges of electricity, which cause the heart to beat in a manner very similar to the natural heart rhythm. Pacemakers are prescribed for people whose hearts beat too slowly or irregularly.

A pacemaker consists of the following components:

Battery:the pacemaker battery is the power supply. It is a small, sealed, lithium battery, that will generally last for many years (the average battery lifetime is 8 years). The energy from the battery is delivered as tiny electrical impulses that stimulate the heart.

Circuitry:the circuitry is a kind of miniature computer inside the pacemaker. It controls the timing and intensity of the electrical impulses delivered to the heart.

Case: the battery and circuitry are sealed inside a metal case.

Connector block: the plastic (epoxy) connector, which lies on top of the pacemaker's metal case, provides the connection between the pacemaker and the lead(s).

The leadThe pacing lead is an insulated wire that carries the electrical impulse to the heart, and carries information about the heart’s natural activity back to the pacemaker. One end of the lead is connected to the connector block. The other end is usually inserted through a vein and placed in the right ventricle or the right atrium. Either one or two leads are used depending on the type of pacemaker. At a heart rate of 70 beats per minute, the lead will bend about 100,000 times a day! Therefore, leads are extremely flexible and strong, so that they can withstand the twisting and bending caused by movement of the body and of the beating heart.

az

How does a pacemaker work? A pacemaker has two essential tasks: pacing and sensing.

Pacing means that the pacemaker paces the heart in case the heart’s own rhythm is interrupted, irregular, or too slow.

Sensing means that the pacemaker monitors the heart’s natural electrical activity. If a pacemaker senses a natural heartbeat it will not stimulate the heart.

What types of pacing are there? Depending on heart condition, doctor prescribes which chambers should be paced. Pacemakers are designed for either (rate responsive) single chamber or (rate responsive) dual chamber pacing.

Single chamber pacing Dual chamber pacing

In single chamber pacing, either the right atrium or the right ventricle is paced. Only one lead is used. The pacemaker senses (monitors) electrical activity in either the atrium or the ventricle and determines whether or not pacing is needed.

In dual chamber pacing, the pacemaker senses (monitors) electrical activity in both the atrium and the ventricle and determines whether or not pacing is needed. Dual chamber pacemakers help the upper and lower chambers of heart to beat in their natural sequence. This way, a paced heartbeat mimics a natural heartbeat.

Rate responsive pacing Rate responsive pacemakers can be single chamber or dual chamber pacemakers. Rate responsive pacing is needed when heart cannot adjust its rate to meet the body’s demands. Normal heart rhythm slows down or speeds up many times during the day depending on the level of activity and other factors. It slows down while patent is resting or sleeping and it speeds up in response to exercise and excitement. A rate responsive pacemaker uses (a) special sensor(s) that can recognize body changes.

Block diagram of a typical pacemaker

Components and Materials of ConstructionA connector block,

made of polyurethane, is located at the top of the pacemaker. It serves to attach the pacemaker to the pacemaker lead. Formerly, glass materials were used to comprise the connector block.The pulse generator is encased in ASTM grade1 titanium. Titanium replaced ceramics and epoxy resin, which were used for encapsulation of some pacemakers in the past, with silicone rubber. This upgrade to titanium

allowed patients to safely use appliances such as microwave ovens, because titanium helps to shield the internal components and reduce the external electromagnetic interference

.A pacing lead is vital to the pacemaker system, because it transmits the electrical signal from the pacemaker to the heart and information on the heart activity back to the pacemaker.

One or two leads may be used, depending on the type of pacemaker.

One end of the lead is attached to the connector block of the pacemaker. The other end is inserted through a vein and placed in the right ventricle or right atrium of the heart. The lead is an insulated wire consisting of a connector pin, lead body, fixation mechanism and at least one electrode. The connector pin is the portion of the lead that is inserted into the connector block. The lead body is the insulated metal wire that carries electrical energy from the pacemaker to the heart. The lead must be able to withstand the flexing induced by the cardiac contractions in the warm and corrosive environment in the body. Thus, the materials used must be inert, nontoxic, and durable. The lead body must be flexible, noncorrosive, and durable. It must also be a good electrical conductor. The early lead body was insulated with polyethylene. Currently, the lead body is insulated with a more resilient material such as silicone rubber tubing or polyurethanes. Polyurethanes are generally stronger than silicone rubbers, which are easily damaged. The strength of polyurethanes enables a thinner lead to be used in the pacemaker and offers greater lead flexibility. Another advantage of polyurethanes is their very low coefficient of friction when wet. However, metal-ion-induced oxidation may degrade polyurethanes, while silicones are not affected by this mechanism of degradation. The fixation mechanism serves to hold the tip of the lead in place in the heart. Currently, either a nickel-cobalt alloy with a silver core helix or an electrically active platinum-iridium helix may be used to anchor the electrode of the lead to the surface of the heart.

Connector block

The electrode is located at the tip of the lead. It serves to deliver the electrical energy from the pacemaker to the heart and information about the natural activity of the heart back to the pacemaker. Electrodes may be composed of platinum, titanium, stainless steel, silver, or cobalt alloys. Titanium has been used because it forms a nonconducting oxide layer at the surface. This surface prevents the exchange of charge carriers across the boundary. Titanium also exhibits a high modulus of elasticity, high resistance to corrosion, and high durability. Electrodes may be coated with iridium oxide to prevent nonconductive layers from forming. The coated electrodes may also provide lower acute and chronic thresholds due to the reduced local inflammation.

Drug-Eluting Leads- Leads have developed immensely since they were first introduced. The earliest leads were attached to the outer surface of the heart. In the mid-1960s, transverse leads were introduced. They could be inserted through a vein leading to the heart, thus eliminating the need to open the chest cavity during implantation. In the 1970s, tined and active fixation leads were developed to replace smooth tip leads. The prongs on the tined leads and the titanium alloy screws in the active fixation leads provide a more secure attachment to the heart and are still used today

Electromagnetic Interference (EMI)

Implanted heart rhythm device is designed to work properly around most appliances and equipment. Most things we handle or work near every day do not cause a problem. However, people with all types of implanted devices do need to be aware that their device can be affected by electromagnetic interference (EMI).

What is EMI?EMI is the term used to describe the effect of an electromagnetic field on the operation of an implanted heart rhythm device. Electromagnetic fields are invisible lines of force due to a combination of electrical fields (produced by voltage) and magnetic fields (produced by current flow) that an object emits. EMI occurs when the signals from an electromagnetic field temporarily interfere with the intended operation of the implanted device.

How could EMI affect implantable devices (pacemaker or ICDs)?In some cases, an implanted device may sense the electromagnetic signals produced by some objects and misinterpret them as a rapid signal coming from patient heart.A pacemaker (including the pacemaker contained within a defibrillator) may interpret the signals as heart rhythm. It may respond by withholding its pacing.A defibrillator may interpret the signals as a heart rhythm that needs therapy. This could cause the device to deliver a shock that patent don’t need. In rare cases, the device could withhold a necessary shock.The effects of EMI are temporary. The closer the implanted device is to the item, the stronger the effect. The farther away, the less effect will be experienced. EMI effects do not usually harm the device.

How can a magnet affect implanted device?A magnet can also cause the implanted device to respond differently if the device gets within six inches of the magnet. Defibrillator will respond to a magnet based on how the medico has programmed the device to respond. A pacemaker will respond by temporarily pacing at a different pre-set rate.

Which items are not safe to use?• Body fat measuring scales• Magnetic mattresses or chairs• Jackhammers•  MRI scans• Diathermy