- management of dual-chamber pacemaker recipients with atrial arrhythmias is complex given the risk of rapid ventricular pacing at the upper rate limit by tracking atrial rhythm
- mode switch refers to the ability to switch automatically from an atrial tracking mode (DDD or VDD) to a non-atrial tracking mode (DDI/VDI/VVI) to prevent high-rate ventricular pacing which can compromise hemodynamic stability
- at the onset of the arrhythmia, rapid ventricular pacing can be observed before mode switch occurs; ventricular pacing rate then gradually decreases from the maximum synchronous rate to the rate-response driven rate or the programmed lower rate limit to avoid a sudden drop in ventricular rate
- upon termination of the atrial tachyarrhythmia, mode switching reverts to the programmed synchronous pacing mode
- the ideal mode switch algorithm combines the following characteristics: – fast triggering to avoid prolonged rapid ventricular pacing during the initial detection phase of the atrial arrhythmias – ability to restore AV synchrony and to return rapidly to a synchronous mode after termination of the arrhythmic episode – high diagnostic accuracy despite the presence of atrial electrograms of variable rate and amplitude – ability to avoid inappropriate mode switch in response to far-field sensing, noise or sinus tachycardia
- the ability to accurately detect an atrial arrhythmia is dependent on the level of programmed atrial sensitivity and the duration of the absolute refractory periods
- detection of atrial arrhythmias remains the greatest challenge for mode switch algorithms and requires programming of high level of atrial sensitivity to avoid undersensing of low amplitude atrial signals during atrial fibrillation; as the atrial channel becomes very sensitive, oversensing of noise, myopotentials or far-field R wave may cause inappropriate mode-switching
- different mode-switching parameters are programmable: atrial rate threshold, duration before mode switching occurs, number of required fast atrial cycles, nontracking mode to which the mode switch occurs, pacing rate during switching, counter of long intervals for mode switching to revert
- higher pacing rate (70-80 bpm) is usually programmed during mode switch to compensate for loss of active atrial contribution and loss of atrioventricular synchrony; rate modulation is also activated
- the pacemaker automatically records the diagnostic data of the various arrhythmic episodes to assist in the management of these patients
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