Automatic mode switching due to atrial arrhythmias in Medtronic pacemakers

Automatic mode switch for atrial arrhythmias

Mode switch is an ON / OFF programmable function designed to prevent the tracking of paroxysmal atrial tachycardias.

            Mode switch to non-atrial tracking mode (DDD to DDIR)

The criteria prompting a switch from an atrial synchronous to DDIR mode vary among the range of implanted pacemakers.

• in the Adapta series, the pacemakers switch to DDIR mode when out of 7 consecutive atrial intervals (AS-AR or AR-AS), 4 are shorter than the programmed detect rate interval

• in the Advisa / Ensura series the pacemaker switches to DDIR mode when 1 or both the following conditions is (are) met: – the median of 12 consecutive atrial intervals is shorter than the programmed AT/AF interval – the AT/AF counter is ≥ 3; the AT/AF counter of is incremented by 1 point, each time ≥ 2 atrial events are sensed between 2 ventricular sensed or paced events

            Mode switch to atrial tracking (DDIR to DDD or VDD)

The criteria which lead to the switch from DDIR to the atrial synchronous mode also vary depending on the series of pacemakers implanted.

• Adapta models: the pacemaker switches to the atrial tracking mode if the last 7 A-A intervals are longer than the maximum synchronous rate interval, or if 5 atrial pacing events occur
• Advisa / Ensura models: the pacemaker switches to the atrial tracking mode if 1 of the following criteria is met:- identification of normal sinus rhythm (or normal paced rhythm) for 5 consecutive ventricular intervals – the median atrial cycle is longer than the AT/AF sensing cycle – the AT/AF counter is < 27 for 3 minutes

Blanked flutter search

In the Adapta models, the mode switch algorithm has a programmable blanked flutter search setting based on the detection of short atrial cycles occurring on a 2:1 basis in the PVAB.  Depending on its rate and on the setting of the AV delay, 1 out of 2 atrial events may fall in the PVAB. Consequently, 1 out of 2 flutter waves is labeled AS and is followed by pacing, while the other atrial event falls in the PVAB and is neither sensed nor counted by the device. This may continue and cause a prolonged, rapid ventricular paced rhythm without diagnosis of flutter.

The algorithm of blanked flutter search is activated when 8 consecutive AS-AS intervals are:

• shorter than twice the sum of the AV delay + the PVAB
• shorter than twice the programmed interval of atrial arrhythmias detection

If these 2 conditions are met, the device suspects the presence of blanked flutter and extends the PVARP of the next cycle to unmask the next atrial event (AR) in the refractory period, suppress ventricular pacing and uncover the next flutter wave (AS).

This algorithm is not available in the most recent devices since the modification of the post-ventricular atrial blanking.

Non-competitive atrial pacing (NCAP)

Aim: to prevent the triggering of an atrial tachyarrhythmia by an atrial pacing stimulus that falls within the atrium’s vulnerable period.

• when NCAP is programmed ON, a sensed atrial refractory event falling in the PVARP starts a programmable NCAP during which no atrial pacing may occur
• if a sensor-driven or a lower rate pacing stimulus is scheduled to occur during the NCAP, the VA interval is extended until the NCAP expires
• if no pacing stimulus is scheduled to occur during the NCAP, the timing is unaffected and pacing occurs at the end of the VA interval, unless inhibited
• an atrial refractory sensed event occurring during the NCAP starts a new NCAP
• when an atrial pacing stimulus is delayed by the NCAP operation, the pacemaker attempts to maintain a stable ventricular rate by shortening the paced AV delay that follows, down to a minimum of 30 ms
• the paced AV delay, however, will not be shortened beyond 30 ms
• the NCAP interval is 400 ms for 1 paced cycle each time a PVC response or an anti-PMT intervention occurs

Atrial preference pacing

Aim: atrial preference pacing (APP) is designed to lower the incidence of atrial tachyarrhythmias by forcing atrial pacing at a rate slightly higher than the spontaneous rhythm.

• the device responds to variations in the atrial rate by accelerating the pacing rate until it reaches a stable paced rhythm that is slightly faster than the spontaneous rate
• after each sensed, non-refractory atrial event, the device shortens the atrial pacing interval by a programmed decremental value
• if the next atrial event is another non-refractory sensed event, the pacing interval is further decremented
• this overdriving function continues until the pacing rate surpasses the spontaneous rate, resulting in a paced atrial rhythm
• the programmed upper rate provides a limit to the APP rate
• after a programmable period of atrial pacing at 100%, the pacemaker gradually decreases
• the pacing rate, like a smoothing function, in search of the next spontaneous sinus cycle, which restarts the APP
• the atrial pacing pulses delivered by APP are labeled PP (proactive pacing) on the event markers

Atrial rhythm stabilization

Aim: atrial rhythm stabilization (ARS) is a programmable function designed to prevent long sinus pauses, which usually follow atrial extrasystoles. Short-long-short cycle sequences can be arrhythmogenic and facilitate the onset of atrial tachyarrhythmias.

• when activated by the sensing of an atrial extrasystole, the device delivers a pacing pulse at the premature interval, increased by a percentage of that interval (defined by a programmed Interval Percentage Increment parameter)
• the atrial pacing rate is then gradually decreased until the return of the spontaneous rate, the programmed pacing rate or the sensor rate
• the Maximum Rate parameter sets an upper rate limit for ARS
• the atrial pacing pulses delivered for ARS are labeled PP (proactive pacing) on the event markers

Post mode switch overdrive pacing

Aim: post mode switch overdrive pacing (PMOP) can be added to the mode switch function to pace the atria rapidly after the cessation of an episode of atrial tachyarrhythmia. The emergence of a slow heart rate after the end of an arrhythmia is associated with an increased risk of very early recurrence.

• after the return of sinus rhythm and switch to DDD/VDD mode, the device increases the pacing rate beat-by-beat until it reaches the programmed Overdrive Rate
• it then smooths the return to the programmed atrial tracking mode by gradually slowing the rate until reaching the programmed pacing rate, the lower programmed rate or the sensor rate

Response to conducted AF

Aim: the response to conducted AF is an algorithm designed to promote a regular ventricular rate during episodes of AF to prevent long diastoles.

• during an episode of AF, the pacemaker modifies the beat-by-beat pacing rate in order to pace near the mean intrinsic ventricular rate
• his eliminates long pauses and diminishes the irregularity of the ventricular rate
• the response to conducted AF function adjusts the pacing rate, such that it is faster in presence of sensed ventricular events and slower in presence of ventricular pacing pulses
• the increase in pacing rate caused by the response to conducted AF is limited by the programmed maximum rate
• depending on the programmed response level value (a higher response level value results in a higher percentage of ventricular pacing and faster alignment with the patient’s own ventricular response rate), the device accelerates the pacing rate in response to a sensed event and slows the pacing rate in response to a pacing pulse