Introduction

A cardiac pacemaker is a rhythm management device whose purpose is to maintain an appropriate heart rate and rhythm synchrony, detect the heart’s intrinsic electrical activity, and deliver pacing impulses when needed. Understanding pacemaker function requires knowledge of both basic physiology and the device’s internal timing cycles.

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Pacemaker function is not based only on whether the device delivers a pacing spike or not. It depends on how the device times pacing, detects intrinsic rhythm, protects itself against erroneous sensing, and adapts its behavior to the patient’s own rhythm, physical activity, and possible atrial or ventricular arrhythmias.

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Basic Concepts

Pacing threshold means the lowest pacing impulse output that produces capture. It is not constant and may change even during the same day. It is affected by factors such as medication, circadian rhythm, sympathetic tone, and disease progression. For this reason, a safety margin is used in pacing.

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Sensing amplitude means the size of the patient’s intrinsic cardiac signal measured by the pacemaker lead. In the atrium, this is the P-wave amplitude; in the ventricle, it is the R-wave amplitude. The unit is mV. The higher the sensing amplitude, the better the pacemaker “sees” the heart’s intrinsic electrical signal. Sensing amplitude is not constant. It is affected by factors such as lead position, tissue contact, respiration, rhythm, medication, electrolytes, and disease progression. For this reason, a margin is used in sensing settings: device sensitivity is programmed so that the device detects true P or R waves, but not noise, T waves, or far-field signals.

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Upper tracking rate means the highest rate at which a DDD/DDDR pacemaker is allowed to track atrial events to the ventricle. For example, if the patient has AV block and the atrial rhythm accelerates, the pacemaker can pace the ventricle after each P wave only up to this limit. When the atrial rate exceeds the upper tracking rate, not all P waves are tracked to the ventricle. Instead, the device begins to limit ventricular pacing, for example with Wenckebach-type behavior or 2:1 tracking. This protects the ventricle from being driven too rapidly by the pacemaker during atrial tachycardia or atrial fibrillation.

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Upper sensor rate means the highest rate to which the pacemaker’s rate response function is allowed to increase pacing based on sensor input. The sensor may estimate exertion, for example, based on movement or minute ventilation. If the patient needs an increase in pacing rate during exercise, the device may increase pacing, but not above the upper sensor rate. This differs from the upper tracking rate: the upper tracking rate relates to atrial tracking, whereas the upper sensor rate relates to sensor-driven increases in pacing rate.

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The basic function of a pacemaker is described using a few key terms:

Pacing interval means the time between paced complexes.

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Escape interval means the time from a sensed intrinsic event to the next paced event.

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Alert period is the part of the pacing interval or escape interval during which the pacemaker responds to sensed events.

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Refractory period is the period during which the pacemaker does not respond to sensed events.

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Functional non-capture means a situation in which the pacing spike falls within the heart’s intrinsic refractory period and does not cause depolarization.

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Pacemaker Timing Cycles

Atrial Channel

In principle, the atrial channel has three main timing cycles, although they may consist of several components:

• atrial alert period

• AV delay or PV delay

• PVARP

PVARP (post-ventricular atrial refractory period) makes the atrial channel refractory after a ventricular event.

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In some devices, the atrial refractory period consists of two components:

• PVAB (post-ventricular atrial blanking period)

• PVARP

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During PVAB, the atrial channel is completely “blind” and does not see any events. During PVARP, the atrial channel can see events but does not respond to them. Thus, PVAB + PVARP together form the refractory period of the atrial channel.

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The purpose of PVAB is to prevent the atrial channel from seeing false signals caused by a ventricular event. When VP or VS occurs, the atrial lead may briefly see the ventricular event as a “far-field” signal, that is, a far-field R-wave signal. If the device interpreted this as a true atrial event, it could incorrectly assume that an AS had occurred in the atrium, leading the device to count too many atrial events. This may result in erroneous atrial high-rate episode markings or even mode switch. On the other hand, if a false AS falls outside PVARP, a DDD device may start the AV delay as if after a true P wave. The result may be pacemaker-mediated tachycardia.

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Therefore, immediately after a ventricular event, the atrial channel is briefly closed:

VP/VS → PVAB, during which the atrial channel sees nothing → PVARP

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Important distinction:

PVAB = the atrial channel is completely blind. The event is not detected or marked.

PVARP = the atrial channel may detect the event, but it is not tracked to the ventricle. It may be marked, for example, as a refractory atrial event.

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Thus, PVAB is intended especially to eliminate very early artifacts and far-field R waves. PVARP, in contrast, is intended more to prevent the device from tracking, for example, a retrograde P wave and initiating pacemaker-mediated tachycardia.

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The atrial channel sequence is therefore repeatedly:

atrial alert period → sensed or paced AV delay → PVAB + PVARP → atrial alert period

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Ventricular Channel

In principle, the ventricular channel has three main timing cycles:

• ventricular alert period

• ventricular blanking period / crosstalk detection window

• ventricular refractory period

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The ventricular refractory period begins after a sensed or paced ventricular event, during which the ventricular channel does not respond to sensed events.

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VBP, or ventricular blanking period, and the crosstalk detection window follow one another immediately after atrial pacing. During VBP, the ventricular channel is completely blind. During the crosstalk detection window, the ventricular channel sees events but does not respond to them. In practice, these intervals are triggered only after a paced atrial event, not after an intrinsic atrial impulse.

 

The ventricular alert period begins when the ventricular refractory period ends.

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Ventricular Blanking Period

In a DDD device, especially after atrial pacing, the logic is as follows:

AP → ventricular blanking period → crosstalk detection window → ventricular alert period → VS or, if needed, VP → VRP

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The ventricular blanking period means the time during which the ventricular channel is completely blind.

 

After atrial pacing, the atrial spike may also be visible in the ventricular channel. This is called crosstalk. If the device interpreted the atrial spike as a ventricular event, it could inhibit necessary ventricular pacing. Therefore, immediately after AP, the ventricular channel is briefly closed.

 

During this time, the device does not see ventricular events, and the signal is not marked as VS.

 

The disadvantage is that if a true QRS happens to fall exactly within the blanking period, the device does not see it. If this were normal atrioventricular conduction after AP, a QRS falling immediately within the ventricular blanking period would require practically impossibly rapid AV conduction. If post-AP ventricular blanking is, for example, 30–50 ms, a true QRS conducted from the AP cannot occur that quickly. Physiological AV conduction time is usually clearly longer, for example approximately 120–220 ms depending on the situation.

 

Therefore, the real problem is usually not “excessively rapid normal AV conduction,” but rather that an intrinsic QRS was already about to occur just when AP was delivered. For example, the pacemaker delivers an atrial pacing spike, AP, but the patient’s own ventricular beat is already beginning because of a junctional rhythm, a premature ventricular beat, or intrinsic conduction.

 

If the QRS falls within the ventricular blanking period, the device does not mark it as a VS event. It may then continue counting the AV delay and later deliver VP even though the ventricle has already been activated. Often, this VP falls within the ventricular refractory period and does not cause a new beat.

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Crosstalk Detection Window

The crosstalk detection window, also called the crosstalk sensing window or safety window, usually occurs immediately after the ventricular blanking period.

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In this window, the ventricular channel listens again. However, if it detects a signal very soon after atrial pacing, the device considers it suspicious: it may be a true QRS, but it may also be crosstalk caused by the atrial spike.

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Therefore, the device usually does not treat it as an ordinary VS event that would simply inhibit ventricular pacing. Instead, it often delivers ventricular safety pacing with a shortened AV delay. The shortened AV delay prevents a late VP from falling on the T wave if the detected event was actually a true ventricular beat. In that case, the early safety VP usually does not produce capture. An event detected in the crosstalk window leads to ventricular pacing with a shortened AV interval, whereas an event detected later in the alert period inhibits the ventricular pulse.

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The idea is that if the signal was crosstalk, the safety VP protects the patient from asystole; and if the signal was a true QRS, the safety VP usually falls within the ventricular refractory period and does not cause a new beat.

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Crosstalk logic is a post-AP phenomenon, not a post-AS phenomenon.

 

During AP, the device delivers an atrial pacing spike. This spike may also be seen in the ventricular channel and may be incorrectly interpreted as VS. This is called crosstalk. Therefore, after AP, the following protection is needed:

AP → ventricular blanking → crosstalk detection / safety window → ventricular alert period → VS or VP

 

After AS, the situation is different. AS means that the patient’s own P wave has been sensed in the atrium. Because the device does not deliver an atrial pulse, no large atrial pacing artifact is generated. Therefore, an actual crosstalk detection window is usually not needed in the same way.

 

After AS, the basic logic is:

AS → sensed AV delay → ventricular channel waits for VS → if VS occurs, VP is inhibited → if VS does not occur, VP is delivered

In other words:

AS → AV delay → VS or VP

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Some devices may also have a very short ventricular blanking period after AS, but its significance is not the same as after AP. It is not specifically intended to prevent atrial spike crosstalk, because there is no atrial spike. It may relate to internal channel protection within the device or to avoiding far-field effects from the atrial signal.

 

In practice:

After AP: crosstalk protection is needed because the atrial spike may be visible in the ventricular channel.

After AS: there is usually no actual crosstalk problem; the device simply starts the AV delay and waits for the patient’s own QRS.

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Ventricular Alert Period

The ventricular alert period is the final part of the AV delay, during which the ventricular channel is normally active and “alert.”

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If a ventricular event occurs during this interval, the device interprets it as a true intrinsic ventricular beat:

VS → planned VP is inhibited

 

This is the normal DDD logic:

AS/AP → AV delay → if VS occurs, VP is not delivered

AS/AP → AV delay → if VS does not occur, VP is delivered

 

If sensing occurs during the alert period, ventricular output is inhibited.

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Ventricular Refractory Period

The ventricular refractory period, or VRP, begins after a ventricular event, that is, after VS or VP.

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During VRP, signals detected by the ventricular channel are not treated as normal new VS events. Its purpose is to prevent the ventricular channel from counting the same ventricular beat multiple times or interpreting the T wave as a new QRS.

 

VRP therefore protects especially against:

• double counting of the R wave

• T-wave oversensing

• detection of afterpotentials after a pacing spike

• incorrect interpretation of noise signals

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AV Delay

The sensed AV delay should generally be approximately 20–30 ms shorter than the paced AV delay.

 

This is because AS and AP are not the same physiological “starting point.”

 

In the AS situation, the pacemaker detects the patient’s own P wave only after atrial activation has already started and the electrical wavefront has reached the lead. Thus, when the device marks AS, atrial activation does not begin at that moment; it is already underway.

 

In the AP situation, the pacemaker delivers an atrial impulse. Atrial activation then begins practically from the pacing electrode area and must spread through the atria toward the AV node.

 

Therefore, the sensed AV delay can be shorter because the intrinsic P wave is already “on its way” toward the AV node when the pacemaker detects it. The paced AV delay needs slightly more time because atrial depolarization must first spread from the pacing site through the atria.

 

The goal is for the true atrioventricular timing to be approximately similar after both an intrinsic P wave and atrial pacing.

 

Rate-responsive AV delay (RRAVD) means a function in which the pacemaker shortens the sensed or paced AV delay as the heart rate increases. This is usually controlled by the device’s own algorithm if the function is activated.

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Atrial-Based and Ventricular-Based Timing

Pacemakers may operate using either atrial-based timing or ventricular-based timing logic. This usually cannot be programmed; it is a structural characteristic of the device. Its practical significance is seen especially when analyzing ECGs.

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Few modern dual-chamber pacemakers are “purely” atrial-based or “purely” ventricular-based. Most modern devices are hybrid in some way in order to avoid the problems of both pure timing logics.

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Ventricular-Based Timing

The atrial escape interval, or AEI, also called the V-to-A interval, is an essential concept in ventricular-based timing devices. It means the time from a ventricular event to the next atrial pacing impulse.

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AEI can be calculated by subtracting the programmed paced AV delay from the programmed pacing interval. For example:

base rate 60/min = base interval 1000 ms
paced AV delay 200 ms
AEI = 1000 − 200 = 800 ms

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In ventricular-based timing devices, AEI, or the V-to-A interval, remains constant.

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Atrial-Based Timing

In atrial-based timing devices, the constant interval is the A-to-A interval, meaning the time from one atrial event to the next atrial paced event, provided there is no intervening sensed atrial event. In this logic, the A-to-A interval remains constant.

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Maximum Tracking Rate and Upper Rate Behavior

Maximum tracking rate, or MTR, is the highest rate at which the pacemaker can pace the ventricles in response to atrial events, that is, track the intrinsic atrial rate. MTR operates through the upper rate timing cycle so that ventricular pacing is delayed until the upper rate limit interval has fully elapsed, even if the programmed AV delay would allow ventricular pacing earlier.

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Upper rate behavior occurs in a dual-chamber tracking mode when the atrial rate becomes high and mode switch is not active.

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If an atrial event falls outside PVARP and starts the sensed AV delay, but ventricular pacing at the end of the AV delay would violate the MTR limit, the device prolongs the sensed AV interval until the upper rate interval has been completed. This is called the pacemaker Wenckebach response.

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Pacemaker Wenckebach resembles physiological Wenckebach behavior:

• the sensed AV interval gradually prolongs

• eventually one P wave falls within PVARP and is not tracked

• the next AV delay shortens again

• the cycle starts over

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TARP

TARP, or total atrial refractory period, equals PVARP + sensed AV delay.

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TARP means the total period during which a new atrial event cannot lead to new ventricular tracking in a DDD pacemaker.

 

AS → sensed AV delay → VP/VS → PVARP

This entire period forms an interval within which a new P wave cannot initiate new tracked ventricular pacing.

 

For example, if the sensed AV delay is 150 ms and PVARP is 250 ms, then:

TARP = 150 + 250 = 400 ms

 

This means that after AS, the next atrial event can become eligible for tracking only after approximately 400 ms.

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TARP is important because it partly determines how fast a DDD device can track the atrial rhythm to the ventricle. If TARP is 400 ms, then when the atrial rate rises above approximately 150/min, all P waves can no longer be tracked. Some P waves fall within TARP, and the device therefore does not track them to the ventricle.

 

Practical marker logic:

AS = intrinsic P wave is sensed
→ sensed AV delay begins
→ if no intrinsic QRS occurs, VP is delivered
→ after VP, PVARP begins
→ a P wave occurring during PVARP is not tracked
→ only a new AS occurring after PVARP can start a new AV delay

 

TARP is the time from AS to the point at which the next atrial event can again be eligible for tracking. The longer the sensed AV delay or PVARP, the longer the TARP, and the lower the atrial rate at which the pacemaker begins to lose 1:1 tracking.

 

Thus, if the atrial rate exceeds the rate determined by TARP, multiblock occurs. In this situation, every second, every third, or another atrial event is sensed appropriately, but some events fall within TARP and are not tracked.

 

Pacemaker multiblock occurs when every second, third, or another atrial event is properly sensed while intervening atrial events fall within the refractory timing and are not tracked.

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Pacemaker Wenckebach

Pacemaker Wenckebach occurs when the intrinsic atrial rate exceeds the programmed maximum tracking rate, or MTR, but remains below the 2:1 block rate determined by the total atrial refractory period, or TARP. In this situation, the pacemaker progressively prolongs the AV interval until one atrial event is not tracked, limiting the ventricular paced rate to the programmed upper tracking limit.

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Pacemaker Wenckebach is the preferred upper rate behavior because it limits the ventricular pacing rate to the upper rate more smoothly than an abrupt transition to 2:1 tracking.

 

When the atrial rate exceeds MTR, or the upper tracking rate, the pacemaker is no longer allowed to track every P wave to the ventricle in a full 1:1 ratio. The device must therefore somehow prevent excessively rapid ventricular pacing.

 

In practice, the alternatives are:

1. Pacemaker Wenckebach
   The device gradually prolongs the AS–VP interval until one atrial event is not tracked.

2. 2:1 tracking / 2:1 block
   The device tracks only every second atrial event to the ventricle.

 

Pacemaker Wenckebach is usually better because it preserves the ventricular rate closer to MTR and does not suddenly drop the heart rate by half.

 

Example:

MTR 130/min
Patient’s sinus rhythm during exercise 140/min

If the device immediately switched to 2:1 tracking: 140 / 2 = 70/min

During exercise, this would be an excessively slow ventricular rate, and the patient could experience dizziness, fatigue, or exertional dyspnea.

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With pacemaker Wenckebach, the device can instead keep the ventricular rate closer to the upper limit, for example approximately 120–130/min, while still preventing ventricular pacing from exceeding the programmed MTR.

 

Pacemaker Wenckebach is possible only if the atrial rate is above MTR but below the 2:1 limit determined by TARP. If the atrial rhythm is even faster, P waves repeatedly fall within PVARP, forcing the device into 2:1 tracking.

 

Pacemaker Wenckebach therefore occurs when the atrial rhythm is faster than MTR but slower than the 2:1 tracking limit determined by TARP.

 

MTR = the highest rate at which the pacemaker is allowed to track the atrium to the ventricle.

 

For example:

MTR 130/min → 60 / 130 = 0.460

Thus, the pacemaker is not allowed to deliver tracked VP impulses more frequently than approximately every 460 ms.

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TARP = sensed AV delay + PVARP.

For example:

sensed AV delay 150 ms
PVARP 250 ms

Therefore:

TARP = 150 ms + 250 ms = 400 ms

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Based on TARP, the 2:1 tracking limit is:

60 / 0.4 = 150/min

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The rhythm ranges are then:

If the atrial rhythm is below 130/min
→ the pacemaker can track 1:1 normally.

If the atrial rhythm is 130–150/min
→ the atrial rhythm is above MTR, but the P waves do not yet continuously fall within TARP.
→ pacemaker Wenckebach occurs.

If the atrial rhythm is above 150/min
→ P waves occur so frequently that some fall within the TARP interval in a 2:1 or higher ratio.
→ 2:1 tracking often occurs, meaning that every second P wave is not tracked. The ventricular response then drops rapidly, for example from 150 bpm to 75 bpm.

 

Pacemaker Wenckebach occurs only as long as the intrinsic atrial rate is greater than MTR but less than the rate determined by TARP.

 

To avoid multiblock and favor Wenckebach, the following may be considered when clinically appropriate:

• increase MTR

• shorten TARP by shortening the sensed AV delay or PVARP

• program rate-responsive AV delay

 

Mode switch may also reduce the clinical significance of multiblock because the device stops tracking the rapid atrial rhythm.

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Rate-Responsive PVARP/VREF

As a reminder, PVARP and VRef/VRP are both protective intervals after a ventricular event.

PVARP = post-ventricular atrial refractory period, meaning an atrial refractory period after a ventricular event. It applies to the atrial channel.

 

VRef / VRP = ventricular refractory period. It applies to the ventricular channel.

 

VP/VS → PVARP begins in the atrial channel
VP/VS → VRP/VRef begins in the ventricular channel

 

Thus, they begin from the same event, but in different channels.

 

Both PVARP and the ventricular refractory period may change with heart rate. In some devices, rate-responsive PVARP/VREF can be programmed, in which case the refractory periods shorten as the heart rate increases.

 

In a more detailed implementation, this function may:

• activate only when the heart rate is at least 90/min

• shorten PVARP, for example by 2 ms for each 1/min increase in heart rate

• aim to maintain a 25 ms difference between PVARP and VREF

• continue shortening until the programmed “shortest PVARP/VREF” value is reached

 

Shortest PVARP/VREF can usually be programmed within a range of 120–230 ms, with a nominal value of, for example, 200 ms.

 

Patients who may benefit particularly from this function include:

• patients in whom mode switch is important

• active and athletic patients for whom a higher-than-usual MTR is desired

• patients who have or may develop competitive pacing

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Capture Control and Automatic Capture Verification

Capture control means the pacemaker’s automatic method of ensuring and adjusting that the pacing spike actually depolarizes the myocardium, that is, produces capture. The pacemaker does not detect mechanical contraction; it evaluates capture based on the electrical response.

 

The basic principle is:

pacing spike → myocardium depolarizes → evoked response = capture

pacing spike → no depolarization → no evoked response = loss of capture

 

The evoked response is the local electrical response caused by the pacing pulse, detected by the device through its own lead after the pacing artifact.

 

How does the pacemaker detect capture?

The pacemaker delivers a normal pacing pulse. Immediately after the pulse, there is a very short blanking period because the pacing artifact is large. After this, the device opens an early sensing window in which it looks for an evoked response.

• If the response is found, capture has occurred.

• If the response is not found, the device interprets the situation as loss of capture.

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In many systems, a backup pulse with higher output is then delivered rapidly.

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Automatic Threshold Measurement

Threshold search may function, for example, as follows:

• output is started at a high level

• output is reduced stepwise

• capture is maintained

• capture is eventually lost

• a level slightly above this is defined as the threshold

• the device programs output above the threshold with a working margin

 

In AutoCapture-type systems, the working margin may be, for example, 0.25 V above the threshold. If capture is not detected, the system may deliver, for example, a 4.5 V backup pulse.

 

The working margin is not the same as the conventional safety margin. The working margin is an internal operating method of the automatic algorithm, whereas the clinical safety margin more broadly means a programmed margin above the pacing threshold. This programmed clinical margin is often twice the voltage of the capture threshold, whereas the working margin is usually approximately 0.25 V above the measured pacing threshold.

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Capture Control Sensing Window

In capture control, the device opens a very early sensing window after the ventricular pacing pulse. However, this does not mean that the ordinary ventricular blanking parameter has simply been “shortened.” This is a separate logic based on evoked response detection.

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In AutoCapture-type operation, there may be, for example:

• approximately 15 ms of blanking

• then approximately a 45 ms evoked response window during which the device searches for the evoked response signal

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Why Is Capture Detection Technically Difficult?

A pacing pulse produces a large polarization artifact at the electrode–tissue interface. The device must distinguish between:

• pacing artifact / residual polarization

• true evoked response

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This is the central technical challenge in capture detection.

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Atrial Capture Control

Atrial capture control is more difficult than ventricular capture control because the atrial evoked response is smaller and far-field signals interfere more easily. Therefore, not all atrial algorithms are based directly on the evoked response. Some use more indirect inferences, such as AV conduction, P/R synchrony, or resetting of sinus rhythm.

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Motion Sensors and Rate Response

There are mainly three technologies for motion sensors:

• accelerometer

• vibration sensor, or piezoelectric sensor

• minute ventilation sensor

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Rate response can often be programmed to:

• on

• off

• passive

 

Passive means that the device records sensor signals but does not respond to them. This can be used when evaluating pacemaker behavior in relation to the patient’s movement and daily activity before actually activating the function.

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Maximum sensor rate means the highest rate at which the pacemaker can pace under sensor control.

 

Many pacemakers have automatically adjusting rate-responsive AV/PV delays and rate-responsive PVARP/VREF functions that modify timing cycles at higher rates.

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Mode Switch

Mode switch means a function in which a dual-chamber pacemaker changes from a tracking mode, such as DDD(R) or VDD(R), to a non-tracking mode, such as DDI(R) or VVI(R), if the atrial rate becomes too high, for example during atrial fibrillation or atrial tachycardia. The goal is to prevent tracking of a rapid atrial rhythm to the ventricles.

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During mode switch, the base rate is often set slightly higher than the usual lower rate so that the transition from an atrial-driven rhythm to a non-tracking mode does not cause too large a hemodynamic drop.

 

Reasons for the commonly programmed higher mode switch base rate include:

• AV synchrony is lost

• a sudden collapse in ventricular rate is avoided

• symptoms such as dizziness, fatigue, and dyspnea may be reduced

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Hysteresis

The lower rate limit, or base rate, defines the pacing rate when there is no intrinsic activity. The hysteresis rate defines the intrinsic heart rate below which the pacemaker begins pacing again.

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For example, the device may be programmed as follows:

• base rate 70/min

• hysteresis rate 60/min

 

In this case, the device does not begin pacing immediately when the intrinsic rate falls below 70/min. It begins pacing only when the heart rate falls below 60/min.

 

Hysteresis is based on the idea that intrinsic activity is usually better than paced activation if the rate is adequate. Therefore, hysteresis attempts to allow the patient’s own rhythm to emerge.

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Hysteresis With Search

In some devices, hysteresis includes a search function. In this case, after a certain number of cycles, the device extends one pacing cycle according to the hysteresis rate. If intrinsic activity is detected, the device remains inhibited. If intrinsic activity is not detected, the device continues pacing at the base rate.

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Hysteresis with search:

• maximizes the use of intrinsic rhythm

• may reduce unnecessary pacing

• may prolong device longevity

• may be useful in patients whose intrinsic rhythm is adequate but appears intermittently

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Neurocardiogenic Syncope

Hysteresis can also be programmed in an atypical way in the treatment of neurocardiogenic syncope. In this case, the following may be used:

• a low hysteresis rate

• a higher-than-usual base rate

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In this way, the device activates only in connection with a sudden rate dropout and provides relatively rapid pacing support.

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VIP / Positive AV Hysteresis Technology and Preference for Intrinsic Conduction

Ventricular Intrinsic Preference, or VIP, and the similar AICS, or autointrinsic conduction search, are designed to reduce unnecessary right ventricular pacing while maintaining adequate ventricular support.'

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The VIP algorithm works by:

• prolonging the programmed sensed or paced AV delay at programmed intervals

• searching for an intrinsic ventricular event

• if VS appears during the prolonged AV delay, the prolonged AV delay may remain in effect

• if intrinsic conduction is no longer detected over several cycles, the device returns to the original settings

 

The goal is to favor intrinsic conduction and minimize unnecessary RV pacing.

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Negative AV Hysteresis With Search

Negative AV Hysteresis with Search is a feature that, in contrast, aims to favor ventricular pacing. This may be useful in special situations in which hemodynamics benefit from more continuous pacing, such as biventricular pacing in CRT therapy.

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The operating principle is that if an intrinsic ventricular event is detected during the normal AV delay, the device shortens the next sensed or paced AV delay, for example with a −40 ms offset. This increases the likelihood of ventricular pacing.

 

Like regular hysteresis, Negative AV Hysteresis may also include a search function in which the AV delay is periodically prolonged to search for intrinsic conduction.

 

This function is not clinically relevant for most pacemaker patients, but it may be useful in selected situations.

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Pacemaker-Mediated Tachycardia

Pacemaker-mediated tachycardia, or PMT, means tachycardia maintained by the pacemaker. PMT requires:

• a dual-chamber pacemaker

• retrograde conduction

• a triggering event, most commonly PVC

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Other common triggers include:

• atrial capture loss

• atrial undersensing

• other losses of AV synchrony

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PMT Mechanisms

All PMT mechanisms share the same basic idea: PMT begins when AV synchrony is disrupted and a retrograde P wave occurs after a ventricular event. The pacemaker senses this retrograde P wave as an atrial event and begins to track it. This may lead to an “endless loop” circuit in a DDD/VDD-type tracking mode.

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In PMT, the retrograde P wave falls within the atrial alert period, the atrial channel senses it, and a new AV delay and ventricular pacing are initiated:

VP → retrograde P → AS → AV delay → VP → retrograde P

 

Retrograde conduction is most common in patients with preserved AV conduction and sinus node disease, but it may also occur in complete AV block.

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PVC as a Trigger of PMT

A PVC occurs before the normal atrial-driven ventricular event and disrupts the normal A–V sequence. The PVC may conduct retrogradely to the atria. If the retrograde P wave falls outside PVARP, the pacemaker interprets it as a true atrial event and paces the ventricle after the AV delay. A PMT circuit may then begin. PVC is the most common trigger of PMT.

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Atrial Capture Loss as a Trigger of PMT

The pacemaker delivers an atrial impulse, but the atrium does not depolarize. The expected intrinsic atrial depolarization therefore does not occur, but the device still paces the ventricle after the programmed AV delay. This ventricular pacing may conduct retrogradely to the atria, in which case the retrograde P wave is sensed and tracked. PMT may result.

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Atrial Undersensing as a Trigger of PMT

An intrinsic P wave occurs, but the pacemaker does not sense it. The device “thinks” that no atrial event has occurred and may deliver atrial or ventricular pacing with inappropriate timing. This can disrupt normal AV synchrony and lead to a situation in which a ventricular event conducts retrogradely to the atria. If the retrograde P wave is sensed as a tracking event, PMT may begin.

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Other Losses of AV Synchrony

This is an umbrella term for situations in which the normal sequence of atrial and ventricular events is disrupted. Examples include an excessively long AV delay, atrial or ventricular premature beats, situations after magnet use, or another sensing disturbance. The essential point is that a ventricular event occurs without an appropriate preceding atrial event, and it may conduct retrogradely to the atria. When the pacemaker tracks this retrograde atrial signal, a PMT circuit may begin.

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Prevention and Treatment of PMT

Most PMT episodes can be prevented by programming a sufficiently long PVARP.

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There are also several algorithms for preventing PMT, such as:

• automatic PVARP prolongation after PVC

• VP–AS tracking, in which the device evaluates the possibility of a stable retrograde circuit

• PMT termination algorithms, in which the device temporarily changes timing or briefly interrupts ventricular pacing

 

“A pace on PMT” means an atrial pacing pulse intended to break the retrograde circuit by making atrial tissue or the AV node/VA pathway refractory. In practice, however, the more common method of terminating PMT is PVARP prolongation.

 

Retrograde conduction may appear and disappear and may not be evident at every follow-up. Therefore, the possibility of retrograde conduction should be assessed especially if AS–VP complexes are disproportionately frequent and occur at relatively high rates.

 

Thus, if many AS–VP sequences are seen at a high rate in a DDD pacemaker, it is worth considering whether the AS events are definitely true sinus P waves — or whether they could be retrogradely conducted P waves.

 

In a patient with AV block, this is completely normal:

sinus P → AS → AV delay → VP

 

But in retrograde conduction, the pattern may be different:

VP → impulse conducts backward to the atria → retrograde P wave → AS → AV delay → VP

In this situation, the strip shows many AS–VP complexes, often at a relatively stable and high rate, for example close to the upper tracking rate. It is especially important to examine the VP–AS interval. If each VP is followed by AS after almost the same delay, for example:

VP → 280 ms → AS
VP → 275 ms → AS
VP → 282 ms → AS

this suggests retrograde conduction.

 

If, on the other hand, the AS events vary naturally and fit the patient’s own sinus rhythm, normal atrial tracking is more likely.

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AF Suppression and Atrial Overdrive Pacing

Pacemaker AF suppression usually means an atrial overdrive pacing algorithm in which the device paces the right atrium slightly above the patient’s own sinus rate in order to reduce atrial premature beats, pauses, and short–long sequences. The goal is to maintain a high atrial pacing percentage and a more stable atrial rhythm.

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The algorithm does not “defibrillate” the atrium and is not an acute method for terminating AF. It is a preventive function. Some devices may also have atrial ATP / reactive ATP, which may help in organized atrial tachycardias but is less effective in chaotic atrial fibrillation.

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Early studies of atrial overdrive pacing showed a small absolute benefit in reducing symptomatic AF burden, but later results have been variable.

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According to the 2023 AHA/ACC/ACCP/HRS AF guideline, AF-suppressing atrial pacing algorithms generally do not reduce the incidence or progression of AF. They are not recommended for routine use; the recommendation is Class 3: No Benefit.

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Atrial Undersensing

In a dual-chamber pacemaker, undersensing may also appear in diagnostics as an abnormally high PVC count, because the pacemaker defines PVC as two consecutive ventricular events without an intervening atrial event. If the patient has intrinsic atrial activity and the PVC counter is high, atrial undersensing is one possible explanation.

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Causes of undersensing include:

• inappropriate programming

• lead dislodgement, especially in the acute phase

• connector problem

• lead damage, such as insulation failure or conductor fracture

• component failure

• Undersensing can often be corrected by making the device more sensitive, that is, by programming sensitivity to a lower mV value. Excessive correction should be avoided, however, because too much sensitivity may cause oversensing.

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Oversensing

Oversensing may appear on ECG or EGM as long pauses or “missing beats.” The pacemaker appears to pace less than it should. In diagnostics, oversensing may also appear as an excessively low proportion of paced events or, on ECG/EGM, as cardiac activity below the programmed base rate.

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Oversensing often does not appear in the device’s stored episodes because the pacemaker may interpret false signals as true sensed events and consider its own inhibition appropriate. Suspicion often arises based on ECG findings, symptoms, a low pacing percentage, abundant sensing, abnormal VS–VS intervals, or real-time EGM.

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Oversensing therefore means that the pacemaker is inhibited even though it should be pacing.

 

Causes of oversensing include:

• sensitivity programmed too high

• crosstalk

• myopotentials

• electromagnetic interference

• polarization potentials

• lead dislodgement in the acute phase

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Correcting oversensing often requires reducing sensitivity, that is, increasing the mV value.

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