Н. В. Моина ю. Б. Генина т. В. Шульженко чтение английской научнотехнической литературы
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Pacemakers: What You Should Know About ThemOver the past few decades, cardiac pacemakers have become quite advanced, quite safe, and quite effective, and the people who receive them generally can lead entirely unrestricted lives. If you have a pacemaker, or have been told you need one, this article should help you understand what a pacemaker does, and what you might expect from it. A pacemaker is a small but very sophisticated electronic device that is implanted under the skin to help regulate the heart beat. Pacemakers are used to keep a person's heart rate from going too slowly, and to help coordinate the beating of the cardiac chambers: the atria and ventricles. Pacemakers are useful in many cardiac conditions that disrupt the heart's normal electrical system. They are most commonly used in people who have inappropriately slow heart rates (or bradycardia) caused by sick sinus syndrome or heart block. Pacemakers usually eliminate the symptoms often caused by bradycardia including weakness, fatigue, lightheadedness, dizziness, or loss of consciousness. In addition, a new kind of specialized pacemaker has proven useful in improving cardiac function in some patients with heart failure. These devices are called cardiac resynchronization therapy (CRT) devices. Pacemakers do not take over the work of the heart. After you have a pacemaker, your heart still does all its own work. Rather, the pacemaker merely helps to regulate the timing and sequence of your heart beat. Pacemakers consist of two major parts: the generator and the leads. The generator is essentially a tiny computer (along with a battery and other electronic components), housed in a hermetically sealed titanium container. Most modern pacemaker generators are roughly the size of a 50-cent piece, and approximately three times as thick. A lead is a flexible, insulated wire that carries electrical signals back and forth between the pacemaker generator and the heart. One end of the lead is attached to the generator, and the other end is inserted through a vein into the heart. Most pacemakers today use two leads; one is placed in the right atrium and the other in the right ventricle. Pacemakers are implanted under local anesthesia. The generator is placed under the skin, beneath the collar bone. The leads are threaded through a nearby vein, advanced to the appropriate position within the heart, and their ends are plugged into the generator. The implantation procedure usually takes 30 minutes to an hour. Once implanted, the pacemaker works by monitoring the heart's electrical activity, and deciding whether and when to "pace". If your heart rate becomes too slow, the device paces by transmitting a tiny electrical signal to the heart muscle, causing it to contract. Pacing can be done from the right atrium, the right ventricle, or both. The pacemaker decides on a beat-to-beat basis whether it needs to pace, and if so, in which chambers it should pace. Its intelligent pacing makes sure that that an appropriate heart rate is always present, and that the work of the cardiac chambers is always coordinated. Pacemakers are "programmable", which means that the specific functions they perform can be altered at any time. Programming a pacemaker is done by wirelessly transmitting new instructions to the generator, using a special device called a "programmer". For instance, your doctor can easily reprogram your pacemaker to change the rates at which it will pace your heart. In the early days of pacing, pacemakers were only able to pace at one specific heart rate. Whenever the patient's own intrinsic heart rate dropped below that pre-set rate (say, 70 beats per minute), the pacemaker would begin to pace at that fixed rate. But today, almost all pacemakers have the ability to vary the rate at which they pace, depending on your immediate needs. These pacemakers are called "rate-responsive pacemakers". Rate-responsive pacemakers can use one of several technologies to determine the optimal heart rate, but two in particular have proven quite useful. One of these is the activity sensor, which detects body movement. The more active you are, the faster the pacemaker will pace your heart (within a range of heart rates that is set by your doctor). The other method commonly used to vary the rate of pacing is a breathing sensor, which measures your rate of breathing. The faster your breathing, the more active you are (presumably), and faster the pacing (again, within a pre-set range). Either of these technologies allow rate-responsive pacemakers to mimic the normal, moment-to-moment changes in heart rate provided by a normal heart rhythm. In the majority of people with pacemakers, the heart's own electrical system is actually generating most of the heart beats. The pacemaker is there mainly as a "safety valve", to prevent occasional episodes of inappropriate bradycardia. In other people the pacemaker works mainly in the rate-responsive mode, to allow the heart rate to increase appropriately during exercise. While they are at rest, the pacemaker is usually not pacing. Rate-responsive pacing allows them to be much more active with much less fatigue. Still other people have more severe forms of bradycardia, and may require pacing virtually all the time. These people often will quickly develop severe symptoms if their pacemaker should ever stop functioning normally. They are said to be "pacemaker-dependant". |