Pacing modes

Device: PM Field: Pacing Modes

1. Basic concepts

Depending on the number of leads and device model implanted, various pacing modes are available. The functions, advantages and disadvantages associated with each pacing mode must be familiar to the caregivers in charge of the follow-up, in order to optimize the programming, taking into consideration the specific characteristics of the various device recipients.


The combined efforts by the North American (NASPE) and British (BPEG) societies of pacing and electrophysiology have defined the international code (NBG) for the classification of the various pacing modes. The pacing modes are assigned a 4- or 5-letter code that describes their primary function.

  • The first letter defines the ventricular (V), atrial (A), both (D), single chamber (S) or no (0) pacing site(s);
  • The second letter defines the sensing site(s) using the same letters;
  • The third letter defines the pacing mode, inhibited (I), triggered (T), both (D), or none of these (0);
  • The fourth letter indicates the presence (R) or absence (0) or rate responsiveness;
  • The fifth letter defines atrial (A), ventricular (V) or dual (D: A+V) multisite stimulation, if present, and (0) if absent;


 Single chamber pacemakers

The asynchronous modes SOO (VOO and AOO)

  • Single chamber pacing
  • No sensing
  • Asynchronous pacing

    The A00 and V00 modes pace asynchronously at a fixed programmed rate, without inhibition by spontaneous events in the paced cavity. The only interval that needs to be programmed is the pacing interval (or the rate = 60,000/pacing interval). It is the most primitive pacing mode, the only mode available in the very first programmable pacemakers.
    Electrocardiographic characteristics of V00 mode: regular ventricular stimuli without interference by the intrinsic activity.
    Electrocardiographic characteristics of A00 mode: regular atrial stimuli without interference by the intrinsic activity.
    A00 and V00 are the respective magnet modes of the corresponding single chamber atrial and ventricular pacing modes. The V00 mode is also the magnet mode of the VDD mode.
    The programming of S00 is obsolete as permanent modes, and it is now limited to test modes. They were initially programmed to prevent the sensing of, and inhibition by, electromagnetic interferences in pacemaker-dependent patients. However, this pacing mode is associated with a risk of arrhythmogenesis by delivery of pacing pulses on the T wave.

The SSI/ SSIR modes

SSI is a single chamber mode, that is VVI if the lead is implanted in the ventricle and AAI if the lead is in the atrium. In both VVI and AAI, a pacing pulse is delivered at the end of a programmable escape interval. If a spontaneous event is sensed, pacing is inhibited and an escape interval is reset. In absence of spontaneous event, the device paces at the end of a programmed interval


The VVI/VVIR mode

  • Ventricular pacing
  • Ventricular sensing
  • Inhibition by the sensing of a ventricular signal outside the refractory periods

    The VVI mode paces a single chamber at the programmed pacing rate, unless inhibited by a sensed event. Sensing is limited to the ventricle. An escape interval is initiated after all ventricular sensing. It is noteworthy that the spontaneous sensing of a ventricular signal by the pacemaker can be delayed with respect to the onset of the QRS complex on the surface ECG. This is particularly apparent in the presence of right bundle branch block, as the QRS is sensed when the ventricular depolarization wavefront passes by the cathode in the right ventricle.
    The first component of the RR cycle, whether after sensing or after pacing, is the refractory period, during which the pacemaker cannot react to a sensed signal. A signal occurring during this refractory period does not reset the escape interval. This refractory period is needed to prevent double sensing of a same spontaneous or paced QRS complex. A signal sensed after the refractory period inhibits pacing and resets the escape interval.
    Typical electrocardiographic findings: ventricular pacing (wide QRS) with inhibition of pacing by spontaneous QRS complex.



The VVIR mode paces a single ventricular chamber in a rate responsive manner, i.e. varies the pacing rate according to activity, tracked by a sensor.

The VVI/VVIR mode is particularly indicated for patients with chronotropic incompetence in permanent atrial fibrillation, for whom atrial sensing and pacing are useless. It can also be programmed in patients who present with episodes of paroxysmal bradycardia. The back up rate is programmed below the spontaneous rate of the patient and the pacemaker intervenes only when pauses occur.

   The AAI/AAIR mode

  • Atrial pacing
  • Atrial sensing
  • Inhibited by sensing of atrial signals outside the refractory period.

AAI is a single chamber atrial pacing mode at the programmed rate unless inhibited by a sensed event. Sensing is limited to the atrium. The programming and functions of the device are nearly the same as for a VVI pacemaker. However, the programming of sensitivity must be higher (lower programmed values), as the amplitude of atrial complexes is often lower than that of ventricular complexes. Furthermore, the refractory period must be longer, to eliminate all ventricular oversensing. The sensing of an R wave by an AAI pacemaker slows the pacing rate, because each sensed R wave resets the escape interval. In the presence of crosstalk (sensing of the R wave), the pacemaker can be programmed to a less sensitive value or the refractory period can be lengthened.
Typical electrocardiographic observations: atrial pacing inhibited by sensing of spontaneous atrial activity.


AAIR is a rate responsive, single chamber, atrial pacing mode.
The preferred indication for an AAI/AAIR pacemaker is the presence of pure sinus node dysfunction with preservation of normal AV conduction. It is, therefore, important, at the time of implantation, to verify the preservation of normal AV conduction. The presence of 1:1 AV conduction beyond a pacing rate of 120 bpm associated with a normal PR interval and normal QRS morphology suggests that AV conduction is preserved, in which case the risk of AV block is <1% per year.

The SST (VVT ou AAT) modes

  • Ventricular or atrial pacing
  • Ventricular or atrial sensing
  • Pacing triggered by R or P wave sensed outside the refractory period

In AAT and VVT modes, the programmed paced rate is the slowest rate seen; however an event sensed outside the refractory periods triggers an immediate paced event in the corresponding chamber. This mode is safe, as pacing occurs systematically in the natural, absolute refractory period of the ventricular or atrial myocardium. The 3 important programmable parameters are the back-up rate, the maximum pacing rate and the refractory period. The last 2 prevent the running away of the pacemaker in case of sensing of rapid signals.

Typical electrocardiographic observations in VVT: in the absence of intrinsic cardiac activity, the ventricles are paced at a fixed cycle length. The occurrence of a spontaneous ventricular complex outside the refractory periods triggers a pacing stimulus inside the QRS. The QRS is either identical to the spontaneous QRS (pseudo-fusion) or intermediate between the spontaneous and the paced QRS (fusion).
Typical electrocardiographic observations in AAT: same as the ventricle but in the atrium.

These pacing modes are rarely used today. Pacing triggered by sensing is a useless waste of energy and acceleration of the battery drain. These modes can be used temporarily to prevent pacing inhibition in a pacemaker-dependent patient presenting with oversensing. They also allow the verification of the quality and timing of sensing when assessing device function.



The DOO mode

  • Asynchronous dual chamber pacing
  • No sensing

The D00 mode paces AV sequentially at the programmed back-up rate without inhibition by intrinsic events.
The AV and VA delays are fixed and are not reset, as sensing does not exist.
Typical electrocardiographic observations: regular atrial pacing followed by ventricular pacing without interference by the intrinsic atrial or ventricular activity.


D00 is a programmable mode but D00 is also the mode seen when a magnet is placed over the pacemaker, except for the VDD mode (V00 magnet mode, as atrial pacing is nonexistent).

The DDD/DDDR mode

  • Dual (atrial and ventricular) pacing
  • Dual (atrial and ventricular) sensing
  • Dual response (inhibited and triggered) to sensing

DDD is the standard programming of dual chamber pacemakers or resynchronization devices. It ensures AV synchronization at rest as well as during exercise during sensed or paced atrial rhythms. Spontaneous atrial activity occurring outside the refractory periods inhibits atrial pacing. Spontaneous ventricular activity outside the refractory periods inhibits ventricular pacing.
Each paced or sensed atrial event outside the refractory periods starts an AV delay and the programmed pacing rate.
The AV delays which follow sensed and paced atrial events are programmable independently; a shortening of the AV delay can be programmed when the rate increases (rate-adaptive AV delay) or can be modified according to the spontaneous conduction intervals (AV delay + hysteresis).
A paced ventricular event might follow a sensed atrial event up to the programmed synchronous upper rate or maximum tracking rate: the limit at which point the pacemaker is no longer allowed to pace the ventricle in synchrony with the atrial rate.

Typical electrocardiographic observations: the tracings may show spontaneous and paced atrial events as well as spontaneous and paced ventricular events. A paced ventricular event could be synchronous with either an intrinsic or a paced atrial event. Schematically, both chambers might be paced, or any combination of paced and sensed events from athe atrium and ventricle.


In DDDR mode, the pacemaker follows the fastest rate, whether spontaneous atrial or sensor-driven. The maximum tracking rate and the maximum sensor-driven rate are independently programmable.
The DDDR mode is expected to fit the needs of all pacemaker recipients.

The DDI/DDIR mode

  • Dual pacing (atrial and ventricular)
  • Dual sensing (atrial and ventricular)
  • Inhibition by sensed events (atrial and ventricular)

The atria are paced at the slowest programmed rate.
After atrial pacing, the ventricle is paced at the end of the AV delay if no ventricular event is sensed during this delay. 
An atrial event sensed outside the refractory periods inhibits atrial pacing and never triggers an AV delay.
Thus, after atrial sensing and in absence of spontaneous AV conduction, the ventricle is paced at the back-up rate, dissociated from atrial sensing, as in the case of a VVI pacemaker. The typical appearance hinges on the absence of P-synchronous ventricular pacing. AV synchronization is ensured only when the atrium is paced. In case of an atrial arrhythmia, rapid ventricular pacing never occurs since a spontaneous atrial event never triggers an AV delay.


The DDI mode may be indicated in the case of frequent atrial arrhythmias in a patient presenting with bradycardia-tachycardia syndrome.
This pacing mode is not suitable for patients who present with complete AV block and a rapid spontaneous atrial rate, as the spontaneous P waves do not initiate a synchronous AV delay. This pacing mode can be programmed in patients presenting with complete AV block and sick sinus syndrome: if sinus node dysfunction is present, AV pacing will occur; during an arrhythmic episode, the rapid spontaneous atrial activity is not followed by rapid ventricular pacing. This mode is particularly useful when the fallback or mode-switching algorithm fails.

The VDD mode

  • Ventricular pacing
  • Dual sensing (atrial and ventricular)
  • Ventricular pacing triggered by atrial sensing, inhibited by atrial and ventricular events.

Sensing occurs in the atrium and in the ventricle, while pacing is limited to the ventricle. In VDD, pacing is synchronized to atrial sensing. In the absence of atrial activity, effective VVI pacing is seen. 
The ventricle is paced synchronously to the atrium up to the programed maximum tracking rate.

Typical electrocardiographic manifestations: the atrium is never paced; therefore, if sinus pauses or arrest occurs, ventricular pacing becomes uncoupled from the atrium (pseudo-VVI).


Therefore, since the atrium cannot be paced, the VDD mode is not suitable for patients who present with sinus node dysfunction; it is, however, appropriate for patients who suffer from complete AV block, in presence of normal sinus node and chronotropic function. It is also possible to implant a specific VDD stimulation system using a single pacing/sensing ventricular lead with two floating electrodes that sense the atrium.

The DDT mode

  • Identical to DDD, used as a temporary test mode.
  • Offers in addition ventricular and atrial pacing in response to the sensing of all ventricular or atrial events that occur outside the refractory periods.

The AAI <=> DDD or MVP mode

  • Atrial pacing with rescue ventricular pacing
  • Atrial and ventricular sensing
  • In case of loss of AV conduction, switch to DDD mode
  • If AV conduction returns, the device returns to AAI mode (it is in fact an ADI mode)

The MVP mode, AAI<=>DDD, enables AAI pacing while monitoring AV conduction. In case of persistent loss of AV conduction, pacing switches to DDD mode. When AV conduction returns, pacing returns to AAI mode.

Preserved AV conduction: the device stays in AAI mode. During AAI pacing, the settings associated with single chamber atrial pacing are applicable. Ventricular sensing is active.

Transient loss of AV conduction: the device delivers a rescue ventricular pulse 80 ms after the AA escape interval in response to an A-A interval that is missing a sensed ventricular event.

Sustained loss of AV conduction: in absence of a ventricular event in response to 2 of the 4 of the most recent A-A intervals, the device confirms as loss of AV conduction and switches to DDD mode. It delivers ventricular back-up pacing pulse at 80 ms.

Sustained loss of AV conduction: in absence of ventricular response to 2 of the 4 latest A-A intervals,
the device confirms as loss of AV conduction and switches to DDD mode.

Return of AV conduction: after switch to DDD mode, the device regularly monitors AV conduction pending the return to AAI mode. The first verification of AV conduction occurs 1 minute after switch to DDD mode. While verifying, the device switches to AAI pacing for one cycle. If the A-A interval includes a sensed ventricular event, the monitoring of sensing was successful and the device stays in AAI mode. If the A-A interval does not include a sensed ventricular event, the monitoring of conduction has failed and the device returns to DDD mode. The interval that separates each verification of conduction doubles (2, 4, 8...minutes, up to a maximum of 16 hours).

Complete AV block: the device functions persistently in DDD mode, verifying the presence of AV conduction every 16 h, resulting in a single missing cycle. All the settings associated with the DDD mode are applicable.






The adverse effects of long-term right ventricular pacing have been confirmed by multiple observations. The MVP mode lowers considerably the percentage of ventricular paced events without jeopardizing the patient’s safety at the onset of AV conduction abnormalities. The main indication is, therefore, the presence of sinus node dysfunction with AV conduction preserved or only intermittently abnormal.



2. Specificities by company