Traditional pacing modes

Depending on the number of leads and the device model, different pacing modes can be programmed. Choosing the mode is the first essential step in programming a single- or dual-chamber pacemaker, since it directly affects the other parameters; it takes into account the patient’s age, indication for implantation, intrinsic rhythm, exercise capacity, chronotropic response and comorbidities. The clinician in charge of follow-up must know how each mode works, with its advantages and limitations.

Schematic and clinical examples. Each mode is illustrated by a schematic example (the timing principle) and, in the tinted box, a clinical example. All clinical examples (from Essentials of cardiac pacemakers, chapter 3) come from the same patient, recorded 3 days after implantation of an Adapta (Medtronic) dual-chamber pacemaker: a 63-year-old man with paroxysmal atrial fibrillation, right bundle branch block, 3 syncopes and a prolonged HV interval (82 ms). Lead I carries the event markers (AS/VS = atrial/ventricular sensing, AP/VP = atrial/ventricular pacing, AR = atrial event in a refractory period), followed by the intervals (ms) and leads III and II.

The international NBG code

Combined efforts by the North American (NASPE) and British (BPEG) societies of pacing and electrophysiology have defined the international NBG code (last revised in 2002). Each mode is designated by a 4- or 5-letter code describing its basic operation; in practice the code is shortened to 3 or 4 letters, and most pacemakers are programmed in VVI, VVIR, DDD or DDDR.

Letter	Describes	Codes
1st	Chamber(s) paced	V = ventricle, A = atrium, D = dual (A + V), O = none
2nd	Chamber(s) sensed	V, A, D; O = none (asynchronous operation)
3rd	Response to sensing	I = inhibited, T = triggered, D = dual (I + T), O = none
4th	Rate modulation	R = rate response (sensor); simply omitted when off: DDD, not DDDO
5th (optional, rarely used)	Multisite pacing	O = none, A = atrium, V = ventricle, D = dual (A + V)

Inhibited (I): a sensed intrinsic event withholds pacing.
Triggered (T): a sensed event triggers pacing (AAT, VVT). In the dual (D) response of VDD and DDD, an atrial sensed event triggers ventricular pacing if no ventricular event is sensed within the AV delay (atrial tracking).
DDI is a non-tracking mode: an atrial sensed event never triggers ventricular pacing.
O is the letter O, not the digit zero: AOO, VOO, DOO, ODO.
S (single chamber, A or V) is a manufacturers’ designation only: SSI = AAI or VVI, depending on the lead position.

Single-chamber pacing

Asynchronous modes: AOO and VOO

Single-chamber pacing
No sensing
Asynchronous pacing

AOO and VOO deliver asynchronous pacing at the programmed lower rate: with no sensing, intrinsic events never inhibit pacing (regular spikes on the ECG), and the only timing parameter is the pacing rate (rate = 60,000 / pacing interval in ms). Asynchronous modes (AOO, VOO, DOO) are the most primitive pacing modes, the only ones available in the first programmable devices.

In non-dependent patients, spontaneous and paced activity compete (parasystole) and stimuli may fall in the vulnerable period, with a risk of atrial fibrillation (atrial pacing) or, very rarely outside myocardial ischemia or metabolic disorders, ventricular fibrillation (R-on-T). A rate above the spontaneous rate promotes capture and reduces, without abolishing, this risk. Asynchronous modes are therefore used only temporarily: in pacing-dependent patients undergoing surgery with electrocautery (to avoid inhibition and asystole) or an MRI scan, and as the classical magnet response (see below).

VVI/VVIR mode

Ventricular pacing
Ventricular sensing
A ventricular event sensed outside the refractory period inhibits pacing

VVI mode provides single-chamber ventricular pacing at the programmed lower rate unless inhibited by a sensed ventricular event, each sensed or paced event starting a new escape interval; in VVIR mode, the rate follows the patient’s activity measured by a sensor. Ventricular sensing often occurs after the onset of the surface QRS, especially with right bundle branch block (the signal is sensed only when activation reaches the right ventricular lead). Signals falling in the refractory period that follows each sensed or paced event do not reset the escape interval, which prevents double sensing of the QRS and T-wave sensing.

Schematic example of VVI

Figure – VVI mode delivers ventricular pacing at the programmed lower rate, with inhibition when intrinsic ventricular events are sensed; after every sensed event, the lower rate interval (escape interval) restarts. AP/AS: atrial paced/sensed event; VP/VS: ventricular paced/sensed event.

Clinical example of VVI

Figure – VVI mode at 60 bpm in the example patient: permanent ventricular pacing (VP) with 1:1 retrograde atrial conduction, a retrograde P wave following each paced QRS complex.

VVI prevents ventricular bradycardia and is primarily indicated in atrial fibrillation with a slow ventricular response, where atrial sensing and pacing are useless. With a lower rate below the spontaneous rate (e.g., 40 bpm), it works in sentinel mode, pacing only when required, which suits patients needing infrequent pacing. In sinus node dysfunction, a lower rate above the sinus rate makes ventricular pacing permanent, with retrograde conduction when VA conduction is intact: the atria contract against closed AV valves, with retrograde flow into the pulmonary veins and venae cavae, which can cause pacemaker syndrome (dyspnea, orthopnea, neck pulsations, palpitations, chest pain).

AAI/AAIR mode

Atrial pacing
Atrial sensing
An atrial event sensed outside the refractory period inhibits pacing

AAI, the atrial counterpart of VVI, provides atrial pacing at the lower rate unless inhibited by an atrial sensed event (sensor rate in AAIR). Programming is nearly the same as in VVI, but atrial signals are smaller, requiring a higher sensitivity (lower programmed value), and the refractory period must be longer to prevent far-field R-wave oversensing, which would slow the pacing rate by resetting the escape interval. Such oversensing is corrected by reducing atrial sensitivity and/or lengthening the refractory period.

Schematic example of AAI

Figure – Typical ECG tracing during atrial pacing and capture, with inhibition of pacing during sensed intrinsic atrial activity (AR: atrial event in the refractory period). AP/AS: atrial paced/sensed event; VP/VS: ventricular paced/sensed event.

Clinical example of AAI

Figure – AAI mode at 60 bpm: atrial pacing (AP) at the end of the escape interval (1000 ms); a premature atrial contraction is sensed (AS) and inhibits atrial pacing. A second, non-conducted one is also sensed but no ventricular pacing follows: AAI cannot pace the ventricle, hence its contraindication in AV conduction disorders.

The preferred indication of AAI(R) is pure sinus node dysfunction. It is formally contraindicated in permanent or paroxysmal AV conduction disorders and should be avoided in patients with vagal syncope or carotid sinus syndrome. If it is considered, AV conduction must be assessed: 1:1 AV conduction during atrial pacing at 120–130 bpm, with a normal PR interval and normal QRS complexes, is a good sign of preserved conduction.

What the guidelines say. Single-lead atrial pacing (AAI/AAIR) is now rarely chosen: the 2021 ESC guidelines make DDD(R) the first-choice mode in sinus node dysfunction, since AAIR led to more paroxysmal AF and twice as many reoperations in the DANPACE trial (AV block developing in 0.6–1.9% of patients per year). With a DDD pacemaker, minimizing unnecessary ventricular pacing through programming is recommended (class I, level A): see below.

Triggered modes: AAT and VVT

Atrial or ventricular pacing
Atrial or ventricular sensing
A sensed event outside the refractory period triggers immediate pacing in the same chamber

In AAT and VVT, pacing occurs at the lower rate, but a sensed event outside the refractory period triggers an immediate pulse in the same chamber instead of inhibiting it; this stimulus is harmless, falling in the natural refractory period of the myocardium. The maximum pacing rate and the refractory period prevent the device from running away with rapid signals. On the ECG in VVT, a spike appears within each sensed QRS, whose morphology is unchanged (pseudo-fusion) or slightly altered (fusion); AAT is the atrial equivalent.

Useful in older pacemakers with unipolar sensing, highly susceptible to interference, triggered modes are now rarely used as a primary mode (they also waste battery energy). They remain useful temporarily to prevent inhibition in a pacemaker-dependent patient during surgery with electrocautery, and for diagnosis: each spike marks the moment of sensing on the ECG, to check sensing and the absence of far-field oversensing. The dual-chamber triggered mode, DDT, is likewise a temporary sensing test mode.

Dual-chamber pacing

ODO: sensing without pacing

In ODO mode, the device senses the atrium and the ventricle but never paces, whatever the intrinsic rate; markers and electrograms show sensing in both chambers (in the example patient: sinus rhythm sensed AS-VS, wide QRS complexes and premature atrial contractions). ODO is programmed only temporarily, for diagnostic purposes such as checking sensing, or in non-pacing-dependent patients scheduled for an MRI scan; never permanently, for safety reasons, unless pacing is considered detrimental.

DOO: asynchronous dual-chamber pacing

Asynchronous dual-chamber pacing
No sensing

DOO mode delivers asynchronous AV sequential pacing at the programmed lower rate, without inhibition by intrinsic atrial or ventricular events. The AV and VA intervals are fixed and are not reset, since sensing is not possible. Its risks and uses are those of the asynchronous modes described above.

Schematic example of DOO

Figure – DOO mode delivers sequential atrioventricular pacing at the lower rate without inhibition by intrinsic activity: the AV and VA delays are fixed and are not reset by spontaneous events, since sensing is disabled. AP/AS: atrial paced/sensed event; VP/VS: ventricular paced/sensed event.

Clinical example of DOO

Figure – DOO mode at 40 bpm (1500 ms between cycles): asynchronous atrial and ventricular pacing with no sensing. A spontaneous QRS complex is ignored; the stimuli that follow it do not capture (refractory period), and the ventricular stimulus falls on the T wave, in the vulnerable period.

Magnet response. A magnet closes the reed switch of the pulse generator, classically causing asynchronous pacing at the magnet rate, which reflects battery status: AOO or VOO in single-chamber devices, VOO for the VDD mode, DOO for the DDD and DDI modes. But magnet mode and rate depend on the manufacturer, model, settings and battery status: the response is programmable in Abbott, Boston Scientific and Biotronik pacemakers, can be programmed off in some devices, or may trigger electrogram storage or a threshold search instead. For example, the default AUTO magnet mode of Biotronik pacemakers gives only 10 asynchronous beats at 90 bpm before returning to the programmed mode: for surgery, it must be reprogrammed to ASYNC.

VDD mode

Ventricular pacing
Dual-chamber sensing (atrium and ventricle)
Atrial sensing triggers ventricular pacing after the AV delay; ventricular sensing inhibits it

VDD senses the atrium and the ventricle but paces only the ventricle. It is a tracking mode: an atrial sensed event triggers ventricular pacing after the AV delay, up to the maximum tracking rate. When the atrial rate is slower than the lower rate, VVI pacing is delivered at the lower rate, since atrial pacing is impossible. VDD is mostly used with single-lead systems, a ventricular lead sensing the atrium through two floating electrodes on its intra-atrial portion, but it is also a programmable option in many dual-chamber pacemakers.

Schematic example of VDD

Figure – The ventricle is paced tracking the sensed atrium up to the maximum tracking rate; in the absence of atrial sensed events, the pacemaker operates as in VVI mode. AP/AS: atrial paced/sensed event; VP/VS: ventricular paced/sensed event.

Clinical example of VDD

Figure – VDD mode at 60 bpm: each sensed atrial event (AS) triggers ventricular pacing (VP); when the atrial rate falls below the lower rate, VP occurs after 1000 ms (VVI behavior), and the retrograde atrial activity sensed in the post-ventricular atrial refractory period (AR) triggers neither an AV delay nor ventricular pacing.

VDD may be indicated in complete AV block with preserved sinus node function, not in sinus node dysfunction. A low lower rate, possibly with rate hysteresis, promotes spontaneous atrial activity and avoids VVI behavior and retrograde conduction. With a floating dipole, atrial sensing is sometimes less reliable than with an atrial lead, especially during deep breathing and exercise, which also degrades atrial arrhythmia detection and memory data; in AV block, loss of atrial sensing on exertion causes a sudden drop to the lower rate, with possible pacemaker syndrome and a risk of pacemaker-mediated tachycardia. VDD is now also available in some leadless pacemakers, whose accelerometer detects atrial contraction.

DDI/DDIR mode

Dual-chamber pacing (atrium and ventricle)
Dual-chamber sensing (atrium and ventricle)
Sensed events inhibit pacing (atrial and ventricular); no atrial tracking

DDI can be thought of as DDD without P-wave tracking. Atrial pacing occurs at the lower rate, followed by an AV delay and ventricular pacing if no ventricular event is sensed. A sensed atrial event inhibits atrial pacing but, unlike in DDD, starts no AV delay: AV synchrony is kept with atrial pacing but lost in AV block with sensed P waves, the ventricle then being paced at the lower rate, dissociated from atrial activity (pseudo-VVI behavior). The paced rate never exceeds the lower rate (or the sensor rate in DDIR), so fast atrial arrhythmias never cause rapid ventricular pacing.

Schematic example of DDI

Figure – A sensed atrial event inhibits atrial pacing and does not trigger an AV delay: without intrinsic AV conduction, the ventricle is paced at the programmed lower rate, dissociated from the sensed atrial event, as in VVI mode. AP/AS: atrial paced/sensed event; VP/VS: ventricular paced/sensed event.

Clinical example of DDI

Figure – DDI mode at 60 bpm: atrial and ventricular pacing at the lower rate (AP-VP), with an AV delay after atrial pacing; after a spontaneous atrial event (AS), atrial pacing is inhibited but no AV delay starts, and the ventricle is paced at the lower rate regardless of the P wave (pseudo-VVI behavior).

The lower rate should be high enough, possibly with rate response, to favor atrial pacing and avoid spontaneous P waves, pseudo-VVI behavior and retrograde conduction. DDI is commonly used as the mode-switch mode, to avoid rapid ventricular pacing during atrial arrhythmias. It can be programmed long term in patients combining complete AV block, fast AF episodes and sinus node dysfunction (AP-VP pacing), is useful when mode switching works suboptimally, and may be considered in patients with repeated pacemaker-mediated tachycardia. It does not suit complete AV block with a normal, faster sinus rate, since P waves start no AV delay.

DDD/DDDR mode

Dual-chamber pacing (atrium and ventricle)
Dual-chamber sensing (atrium and ventricle)
Dual response to sensing (inhibition and triggering)

DDD is the most commonly used pacing mode, appropriate for sinus node dysfunction and/or AV block, and the standard mode of dual-chamber and resynchronization pacemakers; it maintains AV synchrony at rest and during exercise. Intrinsic atrial or ventricular activity outside the refractory periods inhibits pacing in that chamber, and each paced atrial event, or atrial event sensed outside the refractory periods, starts an AV delay ending with ventricular pacing if no ventricular event is sensed. DDD can thus deliver AP-VP, AP-VS, AS-VP (atrial tracking) or AS-VS (complete inhibition) sequences.

AV delays after sensed and paced atrial events are programmed separately; they can shorten at higher rates (rate-adaptive AV delay) or lengthen to favor intrinsic conduction. Ventricular pacing tracks sensed atrial events up to the maximum tracking rate, above which the ventricle is no longer paced at the atrial rate. In DDDR mode, the pacemaker follows the faster of the atrial and sensor rates; the maximum tracking rate and the maximum sensor rate are programmed separately.

Schematic example of DDD

Figure – Each paced or sensed atrial event starts an AV delay; a paced ventricular complex may follow an intrinsic atrial event up to the maximum tracking rate, above which the pacemaker is not allowed to pace the ventricle at the atrial rate. AP/AS: atrial paced/sensed event; VP/VS: ventricular paced/sensed event.

Clinical example of DDD

Figure – DDD mode at 60 bpm: alternating AS-VP cycles (AV delay 120 ms after a sensed P wave) and AP-VP cycles (AV delay 150 ms after atrial pacing). The interval between two VPs is then 30 ms longer than the lower rate interval: the difference between the paced and sensed AV delays.

In this patient, follow-up would probably show a percentage of ventricular pacing close to 100%. One of the priorities of programming is to avoid unnecessary right ventricular pacing without compromising patient safety.

Avoiding unnecessary right ventricular pacing

Prolonged right ventricular pacing promotes deleterious remodeling (increased ventricular volumes) and atrial arrhythmias, and reducing it prolongs battery life. A high percentage of ventricular pacing in a patient implanted for sinus node dysfunction should therefore prompt a discussion of specific algorithms. Permanently long AV delays reduce ventricular pacing but impair ventricular filling and favor pacemaker-mediated tachycardia and 2:1 block on exercise. Other options are AV hysteresis (e.g., VIP, Abbott) and modes switching between AAI/ADI and DDD: MVP (Medtronic), RYTHMIQ (Boston Scientific), SafeR (MicroPort) and Vp Suppression (Biotronik). With these modes, occasional blocked P waves can be normal operation; they should be avoided in high-grade AV block.

ECG tip. The surface ECG alone does not identify the programmed mode: a repeated sequence of atrial pacing with spontaneous ventricular activation (AP-VS) may correspond to DDD, AAI, AAT or DDI modes, to a manufacturer-specific mode preserving spontaneous ventricular activation, or to AV delay hysteresis.

Overview of pacing modes

Mode	Key use	Main risks / limits
ODO	Diagnostics (sensing check); MRI in non-pacing-dependent patients	No pacing: never for long-term use
AOO / VOO / DOO	Surgery with electrocautery or MRI in pacing-dependent patients; magnet mode	Pacing in the vulnerable period (R-on-T, AF)
AAT / VVT	Older devices; surgery; sensing checks	Rarely used today
AAI(R)	Pure sinus node dysfunction	Contraindicated in AV conduction disorders; DDD(R) preferred (ESC 2021)
VVI(R)	AF with slow ventricular response	Pacemaker syndrome (retrograde conduction)
VDD	AV block with preserved sinus node function	Less reliable atrial sensing (floating dipole)
DDI(R)	Atrial arrhythmias (mode switch); repeated pacemaker-mediated tachycardia	No AV synchrony on sensed P waves
DDD(R)	Sinus node dysfunction and/or AV block	Unnecessary right ventricular pacing