Failure Scenarios and Fault Tree Analysis of a Pacemaker
Heart disease is among the leading causes of deaths each year. However, many people with heart problems are increasing their longevity with pacemakers. A pacemakers main purpose is to keep the heart from beating too slowly thereby preventing the problems associated with slow heart rhythms (passing out, congestive heart failure, and death). Pacemakers are necessary because, while there are many medications that prevent the heart from going too fast, there are only a handful that make the heart beat faster. Medications that speed the heart rate are poorly tolerated and often associated with serious side effects. Pacemakers have become a reliable means of helping people live longer and improve their lifestyles despite having a slow heart rhythm.
The bodys natural pacemaker is a small mass of specialized cells in the top of the right atrium, or chamber, of the heart. It produces the electrical impulses that cause a heart to beat. A chamber of the heart contracts when an electrical impulse or signal moves across it. For a heart to beat properly, the signal must travel down a specific path to reach the ventricles. Natural pacemakers may be defective, causing the heart to beat too fast, too slowly, or irregularly. There may also be a blockage of the hearts electrical pathways. A pacemaker is a solution to these problems.
Patients require pacemakers for many different reasons. Most pacemakers are implanted to prevent the heart from beating too slowly0. Often, this slowness occurs because there is no cell in the heart that will beat fast enough to maintain proper function, or because there is a block somewhere in the electrical pathway which does not allow the electrical activity to spread to all of the necessary portions of the heart muscle. The underlying cause of this mishap may be scar tissue, most frequently from previous heart attacks. Sometimes it is simply caused by aging of the conduction system.
Pacemakers come in different shapes and sizes. All of them are small and lightweight. Depending on the patients heart condition, the physician will prescribe the number of chambers to be paced and a specific kind of pacing. A single-chamber pacemaker paces either the right atrium or the right ventricle with one lead. Single lead pacemakers are used primarily in three situations:
1. When the only problem is with the formation of the initial impulse in the atrium, simply placing a lead in the right atrium will start things off when it is needed, and the electrical impulse will then continue normally throughout the rest of the atrium and the ventricles.
2. When the patient is in chronic atrial fibrillation and the ventricle is going too slow. In this case, only a single lead is placed in the ventricle.
3. When the problem with a slow rate occurs only occasionally and for relatively brief periods of time, a single lead in the ventricle may be all that is utilized to provide brief help at those times.
A dual-chamber pacemaker has one lead in the right atrium and one in the right ventricle. This pacemaker keeps the upper and lower heart chambers contracting in their proper sequence.
This style is the most common pacemaker used today. A rate-responsive pacemaker is needed when a heart cannot appropriately increase its rate according to a persons needs. This type of pacemaker varies its rate depending upon a persons level of activity, respiration, or other factors. Rate-responsive pacing can be a part of single-chamber and dual-chamber pacing.
Normal heart rhythms slow down or speed up many times during the day. The heart beats slower while resting or sleeping. Exercise or emotional excitement makes the heart beat faster because the body requires a greater amount of blood circulation. Rate-responsive pacemakers use special sensors that recognize body changes and help the heartbeat speed up or slow down to meet the bodys changing needs. It mimics a hearts natural function. Rate-responsive pacemakers also use sensors that monitor the changes in the body other than heart rate (such as motion, frequency of respiration, etc.) and increase or decrease the heart rate accordingly based on the programmed values set by a physician. For people whose heart rate does not increase when needed, rate-responsive pacing:
1. Provides people with a more appropriate heartbeat for their current activity.
2. Allows people to perform more vigorous exercise.
3. Allows people to perform ordinary, daily activities more effectively.
4. Gives people a greater sense of well being in their day-to-day lives.
Rate-responsive pacemakers vary the heart rate in response to the bodys needs and its programmed settings, similarly to how a healthy heart works. Such variations in pacing rate allow patients to better perform their everyday activities.
A pacing system consists of the pacemaker, which is a small metal case that houses the battery and circuitry that power it, and the pacing lead.This lead is an insulated wire, connected to the pacemaker, which carries the electrical impulses from the pacemaker to the heart and relays information about the hearts natural activity back to the pacemaker.
A pacemaker consists of the battery, the circuitry, and the connector block.
The battery supplies the electrical energy for the pacemaker. The batteries are made of lithium and usually last for many years. The circuitry is like a small computer inside the pacemaker that transforms the energy from the battery into electrical pulses. The connector block is a transparent plastic connector, located on top of the pacemakers metal container, where the lead is attached to the pacemaker.
A pacing lead is an insulated wire that is connected to a pacemaker. The lead carries the electrical impulse from the pacemaker to the heart and information about the hearts natural activity back to the pacemaker. Leads are flexible in order to withstand the twisting and bending caused by body movement. One end of the lead is connected to the pacemaker at the connector block. The other end of the lead is inserted through a vein and placed in the right ventricle or the right atrium of the heart. One or two leads are used depending on the type of pacemaker prescribed. Pacing leads consist of a connector pin, a lead body, a fixation mechanism, and an electrode. The connector pin is the part of the lead that is inserted in to the connector block on the pacemaker. The lead body is an insulated metal wire that carries electrical energy from the pacemaker to the heart. The fixation mechanism is near the tip of the lead and it holds the lead in place in the heart. The electrode is located near the tip of the lead. It delivers electrical energy from the pacemaker to the heart tissue and relays information about the hearts electrical activity back to the pacemaker.
One of the most desirable features of a pacemaker is its ability to be programmed to behave in different manners according to the needs of the patient. This feature allows for the function of the device to be individualized. Furthermore, patients can check on their pacemakers via the telephone. When the telephone is placed over the pacemaker, the pacemaker sends out sounds that can be carried over telephone wires. When unscrambled, an electrocardiographic tracing appears. To make sure the pacemaker is capable of pacing even when the patients own rate is high, a magnet is present which makes the pacer send electrical impulses regardless of the patients underlying rate. The magnet rate contains information about the status of the pulse generators battery. Each manufacturer programs this function in a different but characteristic way. A certain magnet rate indicates the need for elective or immediate replacement for each model.
Although pacemakers are useful life-extending devices, they are not foolproof. Many precautions must be taken for patients with pacemakers, including: battery maintenance, avoiding microwaves, arc welding (which can create a high-energy field that can reprogram pacemakers), power-generating equipment, powerful magnets, and cellular phones.
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