[Mode-switching algorithms: programming and usefulness]

Automatic mode switching is defined as the ability of a pacemaker to reprogram itself from tracking to non-tracking mode in response to atrial tachyarrhythmias, and to regain tracking mode as soon as the tachyarrhythmia terminates. In contrast to upper rate behavior, mode switching does not only limit atrial tracking at a certain rate but actively drives the ventricular pacing rate back to lower rate or sensor rate as long as the atrial tachyarrhythmia persists. In contrast to DDD with mode switch, AV synchrony may be lost in DDIR mode if the sinus rate exceeds the sensor rate. DDD pacing with mode switching represents a valuable option in patients with AV block and paroxysmal atrial tachyarrhythmias. It may prevent the transition from paroxysmal to permanent atrial fibrillation after AV node ablation to a higher extent than VVI(R) pacing. On the other hand, patients with sinus node disease and normal AV conduction may benefit from DDIR mode with long AV interval. Mode switching should provide a rapid, sensitive and specific detection of atrial tachyarrhythmias, fast switch to non-tracking mode without ventricular pacing at the upper rate limit, adequate ventricular rate during the atrial tachyarrhythmia, rapid, sensitive and specific detection of conversion to sinus rhythm and fast switch back to tracking mode. In addition, oscillations between DDD and DDI mode with sudden ventricular rate changes should be avoided. MODE-SWITCHING ALGORITHMS: To achieve these aims, different mode-switching algorithms have been developed which all show specific disadvantages: reliable but slow response to atrial tachyarrhythmias, fast but unspecific switch to non-tracking mode, mode oscillations, inclination to inadequate mode-switching due to ventricular far-field sensing, failure to perform modeswitching during atrial flutter or intermittent atrial undersensing. Some of these problems can be avoided by careful atrial lead implantation providing atrial signals above 2 mV and avoiding ventricular far-field signals. Programming of mode-switching related parameters (e.g. atrial rate and number of fast beats required for mode switch), atrial blanking times, and atrial sensitivity can solve some of the problems with mode switching. Clinical results show a strong influence of device programming and atrial undersensing on mode-switching performance. Some data suggest a superiority of fast mode-switching algorithms with regard to clinical symptoms. However, loss of AV synchrony during sinus rhythm due to premature or inadequate mode switching may limit the benefit of fast mode switching.

Improved performance may be achieved by a combination of different mode-switching algorithms (e.g. one algorithm for detection of atrial fibrillation, another one for detection of atrial flutter). In addition, programmability of several algorithms (e.g. mean atrial rate, beat-to-beat, x out of y) within the same device and atrial cycle-dependent sensitivity adjustment similar to automatic gain control in implantable defibrillators may further increase the clinical use of automatic mode switching.