Mode-switching algorithms: Programming and usefulness

Background: Automatic mode switching is defined as the ability of a pacemak er to reprogram itself from tracking to nontracking mode in response to atr ial tachyarrhythmias, and to regain tracking mode as soon as the tachyarrhy thmia terminates. In contrast to upper rate behavior, mode switching does n ot only limit atrial tracking at a certain rate but actively drives the ven tricular pacing rate back to lower rate or sensor rate as long as the atria l tachyarrhythmia persists. In contrast to DDD with mode switch, AV synchro ny 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 transitio n from paroxysmal to permanent atrial fibrillation after AV node ablation t o a higher extent than VVI(R) pacing. On the other hand, patients with sinu s node disease and normal AV conduction may benefit from DDIR mode with lon g AV interval. Mode switching should provide a rapid, sensitive a nd specif ic detection of atrial tachyarrhythmias, fa st switch to non-tracking mode without ventricular pacing at the upper rate limit, adequate ventricular ra te during the atrial tachyarrhythmia, rapid, sensitive and specific detecti on of conversion to sinus rhythm and fast switch back to tracking mode. In addition, oscillations between DDD and DDI mode with sudden ventricular rat e changes should be avoided. Mode-Switching Algorithms: To achieve these aims, different mode-switching algorithms have been developed which all show specific disadvantages: relia ble but slow response to atrial tachyarrhythmias, fast but unspecific switc h to nontracking mode, mode oscillations, inclination to inadequate mode-sw itching due to ventricular far-field sensing, failure to perform modeswitch ing during atrial flutter or intermittent atrial undersensing. Some of thes e problems can be avoided by careful atrial lead implantation providing atr ial signals above 2 mV and avoiding ventricular far-field signals. Programm ing of mode-switching related parameters (e. g. atrial rate and number of f ast beats required for mode switch), atrial blanking times, and atrial sens itivity can solve some of the problems with mode switching. Clinical result sshow a strong influence of device programming and atrial undersensing on m ode-switching performance. Some data suggest a superiority of fast mode-swi tching algorithms with regard to clinical symptoms. However, loss of AV syn chrony during sin us rhythm due to premature or inadequate mode switching m ay limit the benefit of fast mode switching. Further Developments: Improved performance may be achieved by a combination of different mode-switching a lgorithms (e. g. one algorithm for detection of atrial fibrillation, anothe r one for detection of atrial flutter). In addition, programmability of sev eral algorithms (e. g. mean atrial rate, beat-to-beat, x out of y) within t he 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.