Different Automatic Mode Switching in DDDR Pacemakers

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Mode-switching algorithms are designed to alleviate symptoms related to tracking of atrial arrhythmias, that may result in inappropriately rapid or irregular ventricular pacing[1–19]. The ideal mode-switching algorithm should discriminate sinus tachycardia, a rhythm that should be tracked, from pathological atrial arrhythmias, rhythms that generally should not be tracked. In order to minimize symptoms related to the occurrence of atrial arrhythmias, the mode-switching algorithm should change quickly from a tracking to a non-tracking mode at the onset of the pathological atrial rhythm and remains in this mode until the arrhythmia terminates. Once sinus rhythm has been restored, the pacemaker should revert quickly to the normal atrial tracking mode. There are several potential causes of symptoms that relate to mode switching. First, an irregular paced ventricular intervals at the onset of an atrial arrhythmia before conversion to a non-tracking mode. Second, failure of the device to convert to a non-tracking mode because of intermittent undersensing of the atrial electrocardiogram may result in continued irregular or rapid ventricular pacing [20]. Third, inappropriate reversion to a tracking mode despite persistence of an atrial arrhythmia may also be caused by intermittent undersensing of the atrial electrocardiogram. Fourth, an overly sensitive mode-switching algorithm may result in loss of atrio-ventricular (AV) synchrony in sinus rhythm [2,11,17,19]. Finally, intrinsic AV conduction of an atrial arrhythmia may produce symptoms that are unrelated to the pacemaker [21]. Although all manufacturers of dual chamber pacemakers offer devices that provide mechanisms for managing the occurrence of atrial arrhythmias, the mode-switching algorithms that are available differ significantly in their sensitivity, specificity, and speed of mode conversion at the onset and termination of atrial arrhythmias. There are potential compromises between sensitivity and specificity with these algorithms, the balance of which may determine the frequency of arrhythmia-related symptoms. Atrial-based pacing is associated with a risk of developing atrial fibrillation lower than ventricular-based pacing for patients with sinus node dysfunction [22-25].