Inappropriate Pauses During Bradycardia Pacing in a Third-Generation Implantable Cardioverter Defibrillator

Many ICD devices have the capability for back up bradycardia pacing. Because of the use of a single sensing algorithm for both bradycardia and tachycardia functions, they may be prone to certain "sensing errors." Following implantation of an ICD in a patient with long QT syndrome, "inappropriate" pauses were noted during bradycardia pacing, which were exactly twice the programmed pacing cycle length. This was due to an automatic increase in the device's sensitivity during pacing, a characteristic of the automatic gain control of this particular ICD. Proper recognition of this ICD's special features, known as "lower threshold crossing," allowed noninvasive rectification of the problem and prevented these pauses.