Malfunction of a permanent pacemaker can be categorized as either (1) failure to sense, (2) failure to pace, (3) oversensing, or (4) combinations of the first three. With current lithium batteries and reliable circuitry, most pacemaker malfunctions are due to problems with the electrodes and not the result of battery exhaustion or pulse-generator failure.
Failure to sense may occur when the voltages of the patient's own intrinsic QRS complex is too low to be detected by the sensing circuit of the pacemaker. Changing from a bipolar to a unipolar setup (if possible) may help the pacemaker sense the intrinsic cardiac activity. Failure to sense may cause the pacemaker to become a fixed-rate pacemaker, discharge during the T wave, and trigger serious dysrhythmias.
Failure to pace may occur when tissue reaction around the electrode makes the myocardium insensitive to the electric discharge that is generated by the pacemaker. It is common for the pacing threshold to increase during the first few weeks after insertion, but further rises are infrequent.
Failure to both sense and pace may be due to battery exhaustion, fracture of the wires in the catheter, or displacement of the electrodes. Battery exhaustion is indicated when the pacing rate slowly decreases. With lithium batteries, such decreases usually occur years before actual battery exhaustion. Greater than a 10 percent change from the initial rate is an urgent indication for replacement. Catheter wire fracture may cause either sustained or intermittent interruption in electrical conductivity. Sudden onset of symptoms and/or bradyarrhythmias suggests catheter fracture. Catheter fractures are rarely seen on routine chest radiographs. The transvenous electrode is usually positioned in the right ventricular apex, with a characteristic appearance on the chest radiograph and ECG. Displacement can be suggested when changes on radiographs or ECG occur.
Oversensing is used to describe the situation where the pacemaker senses electrical activity not associated with atrial or ventricular depolarizations; it is thus inhibited and generation of the pacemaker impulse is suppressed. Causes of oversensing include physiologic electrical activity (T waves, muscle potentials), external electromagnetic interference, and signals generated by the interaction of different portions of the pacing system. Unipolar electrodes are more sensitive to physiologic electrical activity and electromagnetic interference than bipolar electrodes.
Under certain conditions, pacemakers may initiate tachyarrhythmias despite functioning as designed; this usually results from an intrinsic depolarization occurring during the pacemaker refractory period, therefore not being sensed, and the pacemaker firing soon thereafter and initiating a reentrant tachycardia. In this setting, maintenance of the dysrhythmia does not require further participation of the pacemaker. Dual-chamber pacemakers can also induce and sustain dysrhythmias. In this situation, emergent treatment requires reprogramming the pacemaker, if possible, or converting to synchronous mode by placing a magnet over the pulse generator.
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