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Mode Switching

The device provides mode switching to change pacing modes as a result of atrial tachycardias and to avoid tracking of non-physiologic atrial rates due to paroxysmal atrial arrhythmias. Different parameters are programmable.



Intervention rate

The Intervention rate is the minimum atrial rate at which mode switching will occur.

 Switch to Pacing

Switch to pacing is the mode the device reverts to during mode switch. The choices for mode switch modes are based on the programmed device mode.

Onset Criterion

The mode switch onset criterion uses an X of 8 rolling counter with a default X value of 5. This means that 5 out of the last 8 atrial events must be faster than the programmed intervention rate for mode switch to occur.


Resolution criterion

The resolution criterion uses an X of 8 rolling counter with a default X value of 5. This means that 5 out of the last 8 atrial events must be slower than the programmed intervention rate for a return to the programmed pacing mode.


Basic rate during mode switching

This refers to the basic pacing rate while mode switching is active. The value selected is added to the programmed basic rate value to become the basic rate during mode switch. By default this value is +10 bpm.


Rate stabilization during mode switching

This feature is designed to minimize sudden rate changes in the ventricle that can occur with atrial fibrillation and intact conduction. The device determines a four-beat ventricular rate average and provides ventricular support pacing any time the rate goes below the averaged rate minus 10 bpm.


2:1 Lock-In protection

2:1 Lock-In Management is an expansion to the Mode Switch feature. During an episode of atrial tachycardia or atrial flutter, if the AV delay and far-field protection intervals are programmed such that every second intrinsic atrial event falls within the far-field protection window, the pacemaker may not switch and inappropriate fast tracking up to the upper tracking rate may occur. The 2:1 Lock-in protection feature has been designed to detect the tachycardia and promote Mode Switching.

The 2:1 Lock-In Management feature consists of suspicion, confirmation and termination phases:


The pacemaker suspects 2:1 Lock-In when the following criteria are met:

  • the ventricular paced response rate is greater than 100 bpm
  • 8 consecutive V-pace—A-sense (VpAs) sequences have occurred with an average length shorter than the 2:1 Lock-In VA Length Criterion; this VA Length Criterion is based on the AV delay (AsVp) and far-field protection intervals (FFB)
  • the mean deviation of these 8 VpAs intervals is less than the 2:1 Lock-In Stability Criterion, defined as 50 ms


When the suspicion criteria have been met, the AV delay is increased by the programmed far-field protection interval (up to a maximum of 300 ms). If the event sensed in the FFP window moves with the ventricular paced event during the extension of the AV Delay, it is a cross-talk event due to ventricular pacing. However, if it does not move with the ventricular paced event when the AV Delay is extended, it is an intrinsic atrial event (atrial flutter).

  • if an atrial event is detected within the AV delay and the detected atrial rate is less than the programmed Mode Switch Detection rate, a 2:1 Lock-In is confirmed
  • otherwise, 2:1 Lock-In is not confirmed, and the AV delay is gradually decreased to the programmed value; the 2:1 Lock-In Management feature is suspended for 120 seconds


  • the pacemaker immediately switches to a non-atrial tracking mode when the confirmation criteria have been met
  • a mode switch events is recorded in memory