Algorithms favoring AV conduction

Basic concepts



There is exhaustive evidence which show the detrimental effects of chronic right ventricular pacing. Any reduction in unnecessary right ventricular pacing also prolongs battery life and extends life of the pacemaker. Different modes have been proposed by the manufacturors to reduce the pacing percentage without compromising patient security during loss of atrioventricular intrinsic conduction. These modes are specifically designed for patients with sick sinus syndrome with preserved atrioventricular conduction or only paroxysmal loss of conduction. We will discuss the specificities of these modes as proposed by each manufacturor.



BIOTRONIK: Vp suppression algorithm or DDD-ADI mode

This algorithm is very similar to the AAI-SafeR (MicroPort CRM-Sorin) algorithm. The pacemaker functions in ADI(R) mode and switches to DDD(R) mode when atrioventricular (AV) conduction fails. 



Criterias for mode switch DDD(R) to ADI(R)

When the pacemaker functions in DDD mode, it will search for intrinsic conduction every 30 seconds. In case of absent intrinsic ventricular activation the search interval will be extended to every 1, 2, 4, 8, 18 ... 128 minutes, with a maximum of 20 hours. During this search, the pacemaker extends the AV delay to 450 ms during 8 cycles. The pacemaker switches to ADI when at least 6 out of 8 cycles are sensed ventricular events. During this phase, there will be no blocked P-waves. However, the extended AV interval may favour the occurence of pacemaker mediated tachycardias. When 15 mode switches occur in one hour, the Vp suppression function will be disabled for the duration of 20 hours.


Mode switch criteria for ADI(R) to DDD(R)

When the pacemaker functions in ADI mode, various criteria will result in a mode switch to DDD. 




In response to first degree AV blok (PR interval > 450 ms during two cycles), the pacemaker switches to the programmed AV delay. The 450 ms is not programmable and is equal for sensed or paced atrial activity.


As these mode switches are not saved into the memory of the device, it is not possible to evaluate the EGMs.



The RYTHMIQ algorithm corresponds to an AAI(R) mode (actually ADI) with back-up VVI mode during periods with normal AV conduction. When a loss of AV synchronization occurs, the algorithm switches the pacemaker to DDD(R) mode. When AV conduction recovers, the algorithm will switch the pacemaker back to AAI(R) mode with VVI backup.

The device delivers VVI back-up pacing at a rate 15 beats lower than the programmed lower rate. However, the lowest paced rate will never be lower than 30 beats per minute or faster than 60 beats per minute. Obviously, in case of normal AV conduction, there is no ventricular stimulation.



Mode switch from AAI(R) to VVI back-up to DDD(R) mode

The device waits for slow ventricular intervals. All ventricular events sensed or paced slower than the basic AAI(R) pacing rate with an added 150 ms is considered as a slow interval. When 3 intervals out of a rolling window of 11 are considered slow, the device automatically switches to DDD(R) mode.  


Mode switch from DD(R) to AAI(R) with VVI back-up

When the pacemaker functions in DDD mode, the AV Search+ algorithm is used to evaluate intrinsic AV conduction. When active, the AV delay is prolonged for 25 intervals. When the device observes only up to 2 paced ventricular complexes in the last 10 intervals, the pacemaker switches to AAI(R) mode with VVI back-up. In this phase, there are no blocked P-waves. The temporary extension of the AV delay favours the induction of pacemaker mediated tachycardias.





It is possible to evaluate the function of RYTHMIQ as the mode switches are saved into the memory of the device. We have analyzed over a thousand RYTHMIQ episodes and found that there are a lot of unnecessary mode switches, according to underlying conduction disease. Read all about or study here.



The Managed Ventricular Pacing mode (AAI<=>DDD), allows an AAI (in fact ADI) mode while continiously evaluating AV conduction. The function of this algorithm used to relatively simple in older devices with a sole criterium for mode switch to DDD mode. In newer generations, mode switched may also occur during long PR-intervals or during pauses. During ADI mode, the parameters as programmed for single chamber atrial pacing are applicable. However, ventricular sensing is active to ensure evaluation of AV conduction.

Mode switch of ADI(R) to DDD(R)

In case of a single blocked atrial event, the device remains in ADI mode and delivers a ventricular stimulus 80 ms after the sensed atrial event. So a single non-conduction P-wave does not result in a mode switch to DDD. 

In case of two blocked atrial events in a rolling window of 4 atrial events are not conducted (blocked), the device will switch to DDD mode. Again, a ventricular back-up stimulation occurs (AV delay of 80 ms) after every blocked P-wave. The algorithm does not allow two consecutive P-waves to be non-conducted. The maximal ventricular pause depends on the atrial rate and the programmed lower rate.

The second generation MVP algorithm may also switch to DDD when the PR interval exceeds a programmable value. The device measures the PR interval (AS/AP-VS) over 4 consecutive cycles. If the average of the last four PR/AR exceeds this limit, the device switches to DDD(R) mode. By default, the long PR function of the MVP algorithm is programmed to OFF. It is probably necessary to reserve the programming of this feature for patients with symptoms on exertion..The major modification incorporated into the new version of the MVP algorithm. is that after a ventricular extrasystole or blocked atrial activity, an advanced atrial pacing followed by ventricular pacing with short AV delay is delivered to avoid a prolonged ventricular pause. The atrium is paced at least 420 ms, 600 ms at most, after the "blocked" atrial activity (spontaneous or paced). The duration of the AA interval is contingent on the previous measurements of the PR intervals (420 ms if PR normal or <320 ms). Ventricular pacing is subsequently delivered 80 ms after atrial pacing. 

Mode switch of DDD(R) to ADI(R) 

After having switched to DDD mode, the devices regularly check for AV conduction, evaluating the possibility to swtich to ADI mode. The first check occurs after 1 minute after the mode switch to DDD. During this verification, the device switches to ADI mode for a single interval. When the following A-A interval includes a sensed ventricular event, the check succeeded and the device remains in ADI mode. When the check fails (no sensed ventricular event betwee two atrial events), the device continues in DDD mode and performs a new check after double the time (2, 4, 8 ... minutes until a maximum of 16 hours). 

When the patient is in complete AV block, the device will function in DDD mode in a persitent manner. Every 16 hours, a check will be performed which will result in a blocked P-wave. All programmed parameters for DDD mode are applicable. 


  1. the pacemaker functions in ADI(R) mode 
  2. after a blocked P-wave, there is a ventricular back-up stimulus with an AV delay of 80 ms. 
  3. the pacemaker switches to DDDR mode since 2 out of 4 P-waves are non-conducted. 


  1. DDDR mode
  2. conduction check succeeded with a sensed ventricular event
  3. pacemaker remains in ADI(R) mode


  1. DDDR mode, pacemaker will perform an AV conduction check
  2. there is no AV conduction
  3. DDDR mode continues

In Medtronic devices, these mode switches are not saved into the memory so it is not possible to evaluate the function of MVP in this way. 

On the newer platforms , it is possible to obtain the number of switches that have occurred since the last follow-up. The device also provides on the report detailed information on the last 10 commutations with the type of switching, switching dates and times, average ventricular rate, the duration of AV interval. No EGM of the switching episode is recorded.


ABBOTT: VIP algorithm

For Abbott devices, there is not a specific algorithm like previously described for other pacemaker companies. The VIP algorithm is based on AV hysteresis up to 450 ms when the atrial rate is below 110 beats per minute. Extended AV delays mean that when there is no sensed ventricular activation at the end of this delay, the ventricle is paced. Therefore, there will be no blocked P-waves. The extended AV delays are associated with the induction of pacemaker mediated tachycardias. 

Activation of VIP algorithm

Extension of the AV delay allows for intrinsic AV conduction. 


We see in this example that in intervals with prolonged AV delay, there is AV conduction and the device will keeps its AV delay prolonged. 

Deactivation of VIP


In this exmple the AV conduction degrades and three intervals are associated with ventricular pacing with a prolonged AV delay. This induces a shortening of the AV delay to a programmed value of 220 ms. 

Programmation of the VIP algorithm

3 parameters may be programmed.  

  • VIP Extension™ which corresponds to an extension of the AV delay in relation to the programmed AV delay (sensed or paced) to allow for intrinsic AV conduction. 
  • The interval of AV delay extension of search for AV conduction (30 seconds, 1, 3, 5, 10 or 30 minutes)
  • the number of cycles during which the AV delay is prolonged during the AV conduction search (1, 2 or 3 cycles)


It is possible to visualise the histogramme of AV conduction in the memory of the device. 


It is also possible to programme a specific remote monitoring alert when the percentage of ventricular pacing surpasses a programmed threshold.



The SafeR mode is a specific mode allowing ADI mode (during preserved AV conduction) with mode switch to DDD during various degrees of AV conduction disease (first, second , complete AV block, and pauses). 


“ADI” to DDD mode switch

The pacemaker functions in ADI mode with permanent evaluating of the AV conduction. It will switch to DDD mode in function of four criteria. 

Third degree (complete) AV block

Mode switch to DDD when two consecutive atrial events are blocked (not programmable). 


Second degree AV block

Mode switch to DDD when 3 atrial events are blocked in a rolling window of 12 events. Not programmable. 



First degree AV block

Mode switch to DDD when the PR interval exceeds the programmed long PR interval for 6 times in a row.


For this criteria, it is possible to program a PR limit during exercise (mode switch only during exercise or during exercise and during rest. It is possible to program a maximal PR interval during exercise and during rest.



Ventricular pauses

Mode switch to DDD when a ventricular pause exceeds a programmed value (2, 3 or 4 seconds). 

Switch from DDD back to “ADI”

The pacemaker switches back to ADI in two circumstances. 

After 12 sensed ventricular events (not programmable). 



Systematically after every 100 paced cycles (not-programmable). 

It is therefore possible to observe blocked P-waves during these new searches. 



Persistent mode switch to DDD mode

In case of the following criteria the device remains in DDD mode until the following morning (08 AM). :

More than 45 episodes of mode switch of ADI to DDD per day

More than 15 episodes of ADI to DDD in three consecutive days 

More than 50% of the time spent in DDD mode during one hour. 


So the pacemaker will switch to AAI mode every morning at 8 o clock and when 12 consecutive R waves are detected. 

One of the strong points of this algorithm is that it is possible to visualise the EGMs and to compare the different AV problems.