NBG (NASPE/BPEG) pacing code

Cardiac pacing modes are designated according to the NASPE/BPEG Generic (NBG) code, developed by the North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group. The NBG code uses three to five letters to describe how a pacemaker paces, senses, and responds to intrinsic cardiac activity.

 

1st letter – Chamber paced

• A = atrium

• V = ventricle

• D = dual (atrium + ventricle)

• O = none
   

2nd letter – Chamber sensed

• A = atrium

• V = ventricle

• D = dual (atrium + ventricle)

• O = none
   

3rd letter – Response to sensing

• I = inhibited (pacing is withheld when an intrinsic event is sensed)

• T = triggered (a sensed event triggers pacing)

• D = dual (inhibited + triggered)

• O = none (no response, i.e. no sensing in that chamber)

 

O here means that the device will still sense internally (for diagnostics), but those sensed events are not used to change when it paces.

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4th letter – Rate modulation

• R = rate response/adaptive-rate pacing enabled

• 0 or omission = no rate response
   

5th letter – Multisite pacing

• 0 or omission = not used

• A = multisite atrial pacing

• V = multisite ventricular pacing

• D = multisite pacing in both atrium and ventricle
   

In clinical practice, a three-letter code is most commonly used. The 4th letter (rate response) is added if rate response/adaptive-rate pacing is enabled (e.g. DDDR). The 5th letter is rarely explicitly written. Multisite ventricular pacing in modern practice usually refers to cardiac resynchronization therapy (CRT), where a left ventricular lead is paced in addition to (or occasionally instead of) a right ventricular lead, rather than pacing the RV alone. Multisite atrial pacing (e.g. biatrial pacing) has been studied but is rarely used in routine clinical practice.

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Most common cardiac pacing modes

AAI(R)

Atrial demand pacing.
The pacemaker paces the atrium when the atrial rate falls below the programmed lower rate. Intrinsic atrial activity sensed outside the refractory period inhibits atrial pacing. The ventricles are never paced, and intact AV conduction is required.

Typical use: patients with sinus node dysfunction (SND) and reliably normal AV conduction.

 

VVI(R)

Ventricular demand pacing.
The pacemaker paces the ventricle when the ventricular rate is below the programmed lower rate and inhibits pacing when intrinsic ventricular activity is sensed. AV synchrony is not preserved during pacing.

Typical use: patients with permanent atrial fibrillation and bradycardia, or patients in whom atrial lead implantation or dual-chamber pacing is not appropriate (e.g. frailty, high comorbidity burden, limited life expectancy, difficult venous access, or high infection/lead-related risk), where the incremental benefit of an atrial lead is small compared with the added procedural and long-term risk.

 

DDD(R)

Dual-chamber tracking mode.
The device can pace and sense both atrium and ventricle. Sensed and paced atrial events trigger ventricular pacing after a programmed AV delay if no intrinsic ventricular activation occurs. Sensed events in either chamber inhibit unnecessary pacing in that chamber. This mode preserves AV synchrony and limits the paced ventricular rate to a programmed upper tracking rate.

Typical use: AV block or sinus node disease where AV synchrony and chronotropic competence (the heart’s ability to increase and decrease its rate appropriately in response to physiological demand) are to be maintained.

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DDI(R)

Dual-chamber non-tracking mode.
The device paces and senses both chambers, but sensed atrial events do not trigger ventricular pacing; they only inhibit further atrial pacing. Paced atrial events trigger AV delay and ventricular pacing if no intrinsic ventricular activation occurs.

AV synchrony can be maintained at lower heart rates—either when both atrium and ventricle are paced at the programmed lower rate or when intrinsic AV conduction follows paced/sensed atrial activity—while still preventing rapid ventricular tracking of atrial tachyarrhythmias. At higher atrial rates above the set upper limit, however, DDI no longer ensures AV synchrony because sensed P waves do not trigger ventricular pacing.

Typical use: patients with brady-tachy syndrome or frequent atrial arrhythmias where ventricular rate tracking of atrial activity is undesirable. Often used as a “problem solving” mode if mode switch does not work optimally.

 

DOO

Dual-chamber asynchronous pacing.
The device paces atrium and ventricle sequentially at a fixed rate with a fixed AV delay and no sensing or response to intrinsic activity. Pacing is AV-synchronous but asynchronous to the native rhythm. Used primarily as a temporary mode during procedures or in environments with strong electromagnetic interference.

 

VOO

Ventricular asynchronous pacing.
The device paces the ventricle at a fixed rate without sensing or response to intrinsic activity. This mode is mainly used as a temporary safety or magnet mode, but it carries a risk of competition with intrinsic ventricular activity and R-on-T pacing if the patient has their own rhythm.

 

Pacemaker Modes Explained

AOO and VOO, Asynchronous single-chamber modes

• Single-chamber pacing

• No sensing in the paced chamber

• Fixed-rate asynchronous stimulation, independent of the intrinsic rhythm
   

In AOO mode, the lead is in the atrium and the device delivers atrial pacing at a fixed rate without any sensing.

In VOO mode, the lead is in the ventricle and the device delivers ventricular pacing at a fixed rate without sensing.

 

Because there is no sensing, intrinsic atrial or ventricular events never inhibit pacing. 

 

Asynchronous VOO/AOO pacing is conceptually the simplest mode: the key programmable parameter is the pacing rate.

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In many devices, magnet application converts a single-chamber device to an asynchronous mode such as AOO or VOO at a manufacturer-specific “magnet rate”.

 

When intrinsic activity is present, a pacing spike can occasionally fall during the vulnerable phase of ventricular repolarisation (the T wave). This R-on-T pacing may trigger serious ventricular arrhythmias (e.g. VT/VF), which is why VOO/AOO are usually reserved for short, controlled situations (e.g. surgery with heavy electromagnetic interference, magnet mode) and not for routine long-term programming.

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AOO mode: behaviour in sinus node dysfunction (intact AV conduction)

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AOO mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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VOO mode: behaviour in sinus node dysfunction (intact AV conduction)

 

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VOO mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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VVI / VVIR, Ventricular demand pacing

• Ventricular pacing

• Ventricular sensing

• Inhibition of pacing by sensed ventricular events
   

VVI mode delivers ventricular pacing at or above the programmed lower rate. When an intrinsic ventricular event is sensed outside the refractory period, the scheduled pace is inhibited and the lower-rate interval is reset. Only ventricular activity is sensed.

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Sensing typically occurs after the onset of ventricular activation, i.e. after the beginning of the QRS complex. This is especially evident in right bundle branch block, where the wavefront must propagate to the right ventricular lead before it is sensed.

 

Immediately after a sensed or paced ventricular event, a ventricular refractory period is active. Events within this refractory period are not sensed and do not reset the pacing interval, which helps avoid double-counting of the same QRS complex and T-wave oversensing. Ventricular events sensed after the refractory period inhibit pacing.

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Oversensing is when a pacemaker “sees” extra signals and interprets them as myocardial activity — for example extra cardiac muscle activity, T waves, or electrical interference.
Because the pacemaker "sees" these are real beats, it inhibits pacing unnecessarily, leading to pauses or bradycardia even though the patient actually needs a pacing pulse.

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VVIR mode is VVI pacing with rate response enabled. An activity sensor adjusts the pacing rate according to patient movement or other physiologic signals.

 

Clinical use: Very common in patients with permanent atrial fibrillation and symptomatic bradycardia, where atrial sensing has no value. May be used in patients with intermittent or paroxysmal bradycardia, with the lower rate programmed below the intrinsic sinus rate so pacing occurs only during pauses or significant bradycardia.

 

In AF, the atria are electrically chaotic (typical atrial rate 350-600bpm) and mechanically ineffective, so atrial sensing doesn’t provide helpful timing information for ventricular contraction. Atrial sensing would just require additional atrial lead and sensing the atrium would just use battery
 

VVI mode: behaviour in sinus node dysfunction (intact AV conduction)

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VVI mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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AAI / AAIR, Atrial demand pacing

• Atrial pacing

• Atrial sensing

• Sensed atrial events outside the refractory period inhibit atrial pacing
   

In AAI mode, the pacemaker delivers atrial pacing at the programmed lower rate except when intrinsic atrial events are sensed, in which case pacing is inhibited and the timing cycle is reset.

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Because only the atrium is sensed, atrial refractory periods must be configured to avoid far-field sensing of ventricular events. If R-waves are inappropriately sensed by an AAI device, they may reset the lower rate interval and reduce the effective pacing rate.

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Far-field sensing: pacemaker lead picks up electrical signals coming from a distant chamber (e.g. atrial lead sensing the ventricular QRS, or ventricular lead sensing atrial activity) instead of just the local myocardium it’s meant to monitor.

 

AAIR mode adds rate response, allowing the atrial pacing rate to increase with physical activity.

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AAI(R) should be used only in carefully selected patients, because many with sinus node dysfunction will eventually develop AV conduction disease. For this reason, most centres prefer a dual-chamber device (DDD(R)) with algorithms to minimise unnecessary RV pacing, and reserve AAI(R) mainly for younger patients with clearly reliable AV conduction and a normal QRS.

 

Clinical use: AAI/AAIR may be considered in sinus node dysfunction with reliably intact AV conduction. Before choosing this mode, it is important to assess AV conduction, for example demonstrating 1:1 AV conduction during atrial pacing at ~120–130 bpm with normal PR intervals and QRS complexes.
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AAI mode: behaviour in sinus node dysfunction (intact AV conduction)

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AAI mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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VVT and AAT, Triggered single-chamber modes

• VVT: ventricular pacing and ventricular sensing

• AAT: atrial pacing and atrial sensing

• Triggered response: a sensed event triggers pacing in the same chamber (in addition to basic pacing at the lower rate)
   

In AAT and VVT modes the pacemaker delivers basic pacing at the lower rate and a sensed intrinsic event in the same chamber triggers an immediate pacing stimulus in that chamber, typically within its refractory period, so the triggered pulse is usually hemodynamically benign.

 

Key programmable parameters:

• lower pacing rate

• maximum pacing rate

• refractory period (to prevent inappropriate rapid pacing)
   

Clinical use: These modes are rarely used because triggering after each sensed event leads to increased and unnecessary battery drain. VVT/AAT may have a niche role in a small group of patients with very problematic oversensing, in whom they can prevent inappropriate inhibition of pacing by oversensed events

VVT essentially behaves like VVI, but instead of withholding a pulse after sensing an intrinsic beat, it delivers one—usually without significant hemodynamic effect.

 

In CRT systems, the LV channel often behaves in a VVT-like way: an RV event (sensed or paced) triggers an LV pace after a short VV delay, thereby resynchronizing LV activation. Globally, the device is programmed in a mode such as DDD or VVI; the VVT-like behaviour applies only to the LV channel and does not mean the entire device is in a “VVT CRT mode.”

 

​​​AAT mode: behaviour in sinus node dysfunction (intact AV conduction)

 

 

 

 

 

 

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AAT mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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VVT mode: behaviour in sinus node dysfunction (intact AV conduction)

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VVT mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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DOO, Asynchronous dual-chamber pacing

• Dual-chamber pacing

• No sensing in either chamber

• No response to intrinsic activity
   

DOO mode delivers sequential AV pacing at a fixed rate, with fixed AV and VA intervals that are not reset by intrinsic events, since sensing is disabled. The device therefore maintains AV-synchronous but globally asynchronous pacing relative to the native rhythm.

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Clinical use: Mainly used temporarily, for example during surgery when significant electromagnetic interference (e.g. electrocautery) is anticipated in a pacemaker-dependent patient, during certain device checks, or troubleshooting situations.
 

DOO mode: behaviour in sinus node dysfunction (intact AV conduction)

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DOO mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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DDD / DDDR, Dual-chamber tracking mode

• Dual-chamber pacing (atrium and ventricle)

• Dual-chamber sensing

• Dual response to sensing (inhibition and triggering)
   

ESC and ACC guidelines consider DDD/DDDR the default mode in many patients with AV block and/or SND where atrial pacing is required, provided that unnecessary RV pacing is minimized. DDD is the standard programming mode in most dual-chamber and CRT pacemakers. It preserves AV synchrony both at rest and during exercise by coordinating atrial and ventricular pacing.

 

Basic behaviour in sinus rhythm:

• A sensed atrial event (P wave) outside the atrial refractory period inhibits atrial pacing and starts an AV delay. If no intrinsic QRS occurs before the end of the AV delay, a ventricular pacing pulse is delivered.

• A sensed ventricular event inhibits ventricular pacing and resets the timing cycle.
   

Separate AV delays are typically programmed for sensed atrial events and paced atrial events.
 

AV delays may shorten automatically at higher heart rates (rate-adaptive AV delay) or be lengthened to promote intrinsic AV conduction in appropriate patients.

 

The ventricle tracks the atrium up to the programmed upper tracking rate (UTR). Above this rate, the device cannot track each P wave 1:1 and uses specific rate-limiting behaviours. 

 

In DDDR mode, the pacemaker follows the fastest rate, being either the intrinsic atrial rate or the rate indicated by the movement sensor. The maximal tracking rate and the maximal sensor rate are to be programmed separately.

 

When atrial tachycardia, atrial flutter/fibrillation, or other SVT occurs and the atrial rate remains below or near the UTR, many atrial events fall outside the post-ventricular atrial refractory period (PVARP) and are sensed, leading the device to maintain 1:1 tracking (each sensed P wave initiating an AV delay followed by ventricular pacing if no intrinsic QRS occurs). 

 

PVARP = Post-Ventricular Atrial Refractory Period. It’s a time window after each ventricular event (paced or sensed) during which the atrial channel ignores signals/does not sense. This prevents tracking, for example, of retrograde P-waves and ventricular events.

 

This results in rapid ventricular pacing up to the UTR, which may cause symptoms. If the atrial rate exceeds the UTR, the device can no longer track every atrial event and employs rate-limiting behaviour such as “pacemaker Wenckebach,” in which progressive prolongation of the effective AV interval and dropped ventricular responses produce a pattern analogous to Mobitz I block, and higher-degree block (e.g. 2:1 or 3:1 atrial–ventricular response), thereby limiting the ventricular rate to around the UTR by allowing only a fraction of atrial events to be tracked.

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In atrial fibrillation, atrial activity is very rapid (350-600bpm), irregular, and often low amplitude. Some atrial signals are sensed outside refractory periods and may still trigger ventricular tracking up to the UTR, while others fall within refractory periods or are too small to be detected. This may result in an irregular and sometimes rapid ventricular pacing pattern that can be highly symptomatic. To prevent this, modern DDD devices incorporate automatic mode switch algorithms: once an atrial tachyarrhythmia such as AF or flutter is detected according to programmed criteria, the pacemaker switches from a tracking mode (DDD) to a non-tracking mode (e.g. DDI or VVI), so that ventricular rate is governed by device programming rather than chaotic atrial activity.

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In re-entrant SVTs involving the AV node, such as AVNRT or AVRT, atrial and ventricular activation are nearly simultaneous and the tachycardia circuit itself determines the ventricular rate; in this setting the pacemaker typically senses frequent ventricular events and inhibits pacing (behaving functionally like VVI).

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DDD mode: behaviour in sinus node dysfunction (intact AV conduction)

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DDD mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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DDI / DDIR, Dual-chamber non-tracking mode

• Dual-chamber pacing

• Dual-chamber sensing

• Inhibition only (no triggering / no atrial-to-ventricular tracking)
   

In DDI atrial pacing occurs at the programmed lower rate. After an atrial pacing pulse, a ventricular pacing pulse is delivered after the programmed AV delay unless a ventricular event is sensed in that interval.

 

A sensed atrial event inhibits atrial pacing but does not start an AV delay. Thus, intrinsic atrial events do not trigger ventricular pacing. If AV conduction fails after a sensed atrial event, the device will pace the ventricle at the lower rate, functioning effectively like VVI for that beat.

Because sensed atrial events do not trigger AV delays, atrial tachyarrhythmias do not increase the ventricular pacing rate. Ventricular pacing occurs at or near the programmed lower rate if intrinsic AV conduction is not working and V-pacing is needed.

 

Clinical use: DDI is indicated in patients with frequent atrial arrhythmias (e.g. brady-tachy syndrome) where tracking atrial activity would cause inappropriate rapid ventricular pacing especially if mode switch does not work optimally. It is not suitable for patients with complete AV block and high atrial rates, because atrial events are not tracked, and AV synchrony is not maintained at faster sinus rates.

 

In DDI with complete AV block and a fast atrial rate, the pacemaker senses many rapid P waves. Sensed atrial events only inhibit atrial pacing and do not start an AV delay to trigger ventricular pacing. Because the AV node is blocked, no ventricular beats are conducted and thus no QRS complexes are sensed. With nothing in the ventricle to reset its timing, the device’s ventricular channel just runs on its own internal clock and delivers ventricular pacing at the programmed lower rate, effectively behaving like a VVI pacemaker. DDI in complete AV block would lead to ventricular pacing at the programmed base rate without atrial tracking.

 

In some patients with complete AV block and sinus node dysfunction, DDIR may be programmed so that:

• in sinus rhythm with slow intrinsic rate, dual-chamber pacing maintains AV synchrony;

• during atrial tachyarrhythmias, the fast atrial activity is not tracked and the ventricular rate remains controlled.

• This mode is particularly useful when the mode switching algorithm functions suboptimally. 

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In DDIR with complete AV block, the ventricle does not follow the intrinsic atrial rate. Even if the atrium is at, for example, 100 bpm, sensed P waves only inhibit atrial pacing and do not trigger or inhibit ventricular pacing, so the ventricle simply paces at the device’s own rate (lower rate or sensor-driven rate if rate response is active). Thus, DDIR (not DDIO) may be used even in 3rd degree AV block.

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The main reasons a mode-switch algorithm can work suboptimally:​

Atrial undersensing (missed AF/atrial flutter)

• Poor P-wave amplitude, scarred atrium, or suboptimal lead position → the device doesn’t see enough atrial events.

• AF/atrial flutter may be present, but the atrial rate never crosses the detection threshold → no mode switch, and the pacemaker continues to track some of the fast atrial beats in DDD.

Atrial oversensing / noise

• Myopotentials, far-field R-waves, lead noise, or EMI can be interpreted as atrial events.

• The device may falsely think there is AF and mode-switch inappropriately, or oscillate in and out of mode switch. Leads to unstable behaviour and symptoms.

Atrial flutter rates near the detection boundary

• Typical flutter (e.g. ~240–300 bpm) with variable conduction and refractory periods can produce atrial sensing patterns that confuse the algorithm.

• Sometimes the atrial rate appears just below the programmed “atrial tachy” detection criteria, so no mode switch is triggered, but enough P waves are tracked to make the patient symptomatic.

Very brief, frequent atrial tachy runs

• If atrial tachyarrhythmias are short and frequent, they may be too brief to satisfy the detection duration needed for mode switch.

• The pacemaker may track some fast atrial beats before it decides to switch mode, leading to bursts of rapid ventricular pacing.
   

Because DDI does not track atrial events at all, it bypasses these sensing/algorithm limitations:

You keep dual-chamber pacing in sinus rhythm (when the atrium is slow and can be paced) and AV-synchronous ventricular pacing for atrial rates between the programmed lower and upper limits, but during atrial arrhythmias the ventricle is not driven by the chaotic atrial rate, even if mode switch fails or misbehaves.

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In a patient with SSS + sinus rhythm + good AV conduction, most of the time DDD and DDI look quite similar. The key differences between DDD and DDI are about what happens when things are not perfect.​

in Sinus rhythm with the atrial rate between lower and upper limits, in DDD each intrinsic P wave inhibits A pacing and starts an AV delay. If conduction is normal, QRS appears before the end of AV delay → no V-pace. If there’s intermittent AV block or very prolonged PR, the device steps in with a V-pace at the end of the AV delay.
Conversely in DDI mode, Intrinsic P waves inhibit A pacing, but do not start an AV delay for V. If AV conduction is normal → QRS happens early, so pacing is inhibited → looks similar to DDD on ECG. If there’s intermittent block or long PR, the backup V-pace comes at the set low rate (eg 60bpm). This results in that pacing comes late relative to the P wave and awkward AV timing. Thus, here the difference shows up mainly when AV conduction is borderline or starts to deteriorate over time. DDD guarantees neat beat-by-beat AV support; DDI only gives “backup” V pacing on its own schedule.

 

Atrial tachyarrhythmias is where the difference between DDD and DDI stops being modest. DDD tracks atrial rate to the ventricle up to the upper tracking rate. This can cause rapid ventricular pacing near the upper limit in atrial tachycardia/flutter/fibrillation if mode switch isn’t perfect.
In DDI the pacemaker does not track atrial events at all. Ventricular rate is driven by intrinsic conduction and/or the device’s own lower rate, not by atrial sensing. Thus in DDI the ventricle is protected from atrial tachyarrhythmias causing fast paced V-rates.

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DDD is chosen as the default because it provides reliable AV synchrony if AV conduction worsens (which often happens over years). DDI is more of a problem-solving mode: used when atrial tachyarrhythmias, oversensing, or mode-switch problems make DDD problematic.
In real, long-term patients (with evolving conduction disease and arrhythmias), the advantages and disadvantages of DDD vs DDI become much more obvious.

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DDI/DDIR modes are also commonly used as the destination mode after automatic mode switch in DDD devices during atrial tachyarrhythmias.
 

DDI mode: behaviour in sinus node dysfunction (intact AV conduction)

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DDI mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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VDD, Ventricular pacing with dual-chamber sensing

• Ventricular pacing

• Dual-chamber sensing (atrial and ventricular)

• Atrial sensing triggers ventricular pacing after AV delay if no intrinsic ventricular activity; both atrial and ventricular sensing can inhibit pacing in that chamber
   

VDD can be delivered using a single-pass lead with atrial sensing electrodes located proximally in the atrium and a ventricular electrode in the ventricle.

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In VDD sensing occurs in both the atrium and ventricle. Pacing occurs only in the ventricle. Sensed atrial events start an AV delay and trigger ventricular pacing if no intrinsic ventricular activation occurs. If no P-waves are detected, the device behaves as VVI, pacing the ventricle at the lower rate.
 

Thus, VDD provides AV-synchronised ventricular pacing in patients with intact sinus node function: the ventricle is paced in synchrony with sensed P waves until the maximal tracking rate is reached.

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Clinical use: VDD is not appropriate in patients with sick sinus syndrome because atrial backup pacing is impossible. It is most suitable for patients with complete AV block and normal sinus node function and chronotropic competence.
 

VDD mode: behaviour in sinus node dysfunction (intact AV conduction)

 

 

 

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VDD mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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DDT, Dual-chamber triggered pacing

• Dual-chamber pacing (atrium and ventricle)

• Dual-chamber sensing

• Triggered response in both chambers
   

In DDT mode, any sensed atrial or ventricular event triggers pacing in the corresponding chamber. This mode is used as a temporary testing mode to evaluate sensing and pacing behaviour, not as a chronic clinical pacing mode.

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DDT mode: behaviour in sinus node dysfunction (intact AV conduction)

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DDT mode: behaviour in AV block (second- or third-degree) with normal sinus node function

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