The modern permanent pacemaker is powered by a lithium battery that has an approximate lifetime of 8 to 12 years. Most units are preset for rates around 70, with a pacing interval of 0.84 s. The demand pacemaker has a built-in refractory period (0.2 to 0.4 s) during which it will not sense. This prevents it from being inhibited by its own stimulus. Most demand pacemakers have a magnetic switch, which temporarily converts the pulse generator from the demand mode to the fixed-rate mode when a magnet is held over the unit. In this way, the pacing rate can be quickly determined, but the magnet should be applied for only short periods to avoid initiating tachyarrhythmias. The rate and stimulus strength can be reset by noninvasive means in programmable pacemakers. Some of the more sophisticated pacemakers can be interrogated to indicate events such as tachyarrhythmias and conversions. Since pacemaker complexity varies, the manufacturer supplies an identification card with each unit that patients should carry.
Temporary pacemakers are powered by 9-V radio-type batteries. On these pacemakers, there are settings for the mode (fixed or demand), rate (40 to 140), and stimulus strength (0.2 to 20 mA). During emergency pacing, initial settings should be in the demand mode with a rate around 70 and stimulus strength around 3.0 mA. The negative terminal should be connected to the distal electrode.
The transvenous intracardiac electrode may be either unipolar or bipolar. The unipolar setup has the negative electrode within the heart and the positive electrode in the chest wall. Permanent pacemakers using the unipolar setup have the positive electrode in their surface covering. Temporary pacemakers using the unipolar setup have their positive electrode connected to a needle implanted in the skin of the anterior thorax. With the bipolar setup, both electrodes are within a few millimeters of each other and both lie within the heart. Transvenous electrodes are placed most commonly into the apex of the right ventricle. This causes an LBBB pattern when pacing. Different catheters are used depending on the clinical situation. Semirigid catheters (6 or 7 Fr) are inserted through a venous puncture or cutdown. They usually require fluoroscopy for correct placement. Semifloating (3 or 4 Fr) or flexible balloon-tipped catheters (3 or 5 Fr) can be introduced and directed into the right ventricle without fluoroscopy, using blood flow. Flexible catheters can become dislodged by patient or cardiac movement and are usually replaced with semirigid catheters within 24 h.
Transthoracic cardiac pacing has been largely replaced by transcutaneous pacing. Transthoracic pacing is fraught with many complications, including coronary artery laceration and cardiac tamponade in addition to pneumothorax (Fig 2.4.-.4.3).
Transcutaneous electrodes are self-adhesive pads that are usually placed with the negative electrode over the left anterior precordium and the positive electrode over the left infrascapular area. Transcutaneous pacing is then initiated by using the lowest current setting, which is increased until electrical capture is achieved. Most patients can be paced with 100 mA, but some may require up to 200 mA. The newer pacemakers cause much less discomfort, can be applied faster, and combine defibrillator and pacing functions from the same pads.
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