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Different types of pacemakers can be used externally or internally and may have one or more chamber leads. Electronic pacemakers may vary according to the type of action required. The type of pacemaker chosen generally depends on the condition requiring treatment. Physicians generally program all of these devices during placement and may adjust a pacemaker periodically after insertion.
Transcutaneous external cardiac pacing (TEP) employs an external pacemaker, which cardiologists might use temporarily to adjust bradycardia, sinus bradycardia, or atrioventricular node blocks. They might also be used to regulate tachycardia. Though similar to defibrillators, external pacemakers are not generally used for asystole, or the absence of a heart rhythm. This kind of pacemaker usually includes large pads that contain the electrodes.
One pad is placed on the chest over the apex of the heart, and the other is placed on the chest, over the inner scapula region. The pacemaker can deliver up to 200 milliamperes of current for periods of up to 40 milliseconds. Physicians might use these types of pacemakers on conscious or unconscious patients, though a conscious patient would probably be given an analgesic and possibly a sedative.
Cardiac resynchronization therapy (CRT) requires an internal pacemaker, which generally consists of a pulse generator about the size of a matchbox that contains a battery and a microcomputer. The programmable circuitry within the device senses and times the heart rhythm. It can also emit electrical impulses to initiate a muscle contraction. Pulse generators can have one, two, or three leads, with one end attached to the unit and the other inserted into the cardiac muscle. Physicians usually install single chamber pacemakers, or pacemakers with only one lead, in patients suffering from atrial or ventricular rhythm anomalies.
Cardiac surgeons may implant dual chamber pacemakers, or devices having two leads, into the right atrium and the right ventricle. Biventricular pacemakers generally have three leads, which are installed into the right atrium, the right ventricle, and the left ventricle. These devices might be implanted in patients suffering from advanced heart blocks or congestive heart failure.
Standard types of pacemakers deliver an electrical impulse when the device senses that the heart rhythm has dropped below a certain rate. Some types of pacemakers generate impulses on demand, adjusting the heart rhythm during times of decreased or increased physical activity. Defibrillator pacemakers also emit electrical impulses when the internal sensor detects a potentially fatal heart rhythm.