Atrial pacing pulses might be sensed in the V channel, a phenomenon also known as “crosstalk”. This inappropriate ventricular sensing (VS) will result in ventricular inhibition: should the patient be pacemaker dependent, this ventricular inhibition will cause asystole. Thus, VS in a narrow window following atrial pacing (AP) will trigger a safety pacing to warrant VP and prevent pauses or asystole. However, a ventricular premature beat might also happen just simultaneous with an AP. In this case, if ventricular safety pacing was delivered after a normal AVI, i.e. 200ms, it might pace during a vulnerable period and trigger a ventricular arrhythmia. Therefore, ventricular safety pacing occurs after a short, preset interval (110ms in Biotronik2 and Medtronic,3 120ms or programmed AVI if shorter in Abbott,4 95ms in Sorin5) so that the ventricles are paced and captured, in case of crosstalk, or it falls in a refractory period, in case of a ventricular premature beat (Fig. 1).

Fig. 1.

Ventricular safety pacing. Dual-chamber pacemaker (in a patient with atrial fibrillation and undersensing of waves). The first two beats show sequential atrioventricular pacing at the programmed atrioventricular interval (AVI) (blue double-headed arrow), with the second being a fusion (narrower than purely paced QRS). The third beat shows atrial pacing simultaneous with a conducted beat sensed in the ventricular channel, thus triggering a ventricular safety pacing with sequential atrioventricular pacing at a much shorter AVI.

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This feature is intended to increase patient safety by guaranteeing ventricular capture6 as well as to reduce battery drain by adjusting pulse amplitude to the lowest value required to maintain capture.7 Some manufacturers (Abbott ACap and RVCap Confirm, Biotronik, Boston Scientific) provide beat-to-beat capture confirmation by assessing the evoked response of the action potential after myocardial capture.8 If capture does not occur in a particular beat, a backup safety pulse of greater amplitude and/or duration is provided. Other devices only perform regular searches of pacing thresholds (Abbott AutoCapture,4 Medtronic, Sorin). When evaluating the ventricular threshold, intrinsic conduction and fusion must be ruled out, so V will be paced after a short AVI. The test VP is followed by a backup VP with higher output to warrant capture in case the testing stimulus fell below the capture threshold (Fig. 2). In this case, regularity is key: threshold measurements are performed by repeating runs of baseline cycles alternating with test cycles (Table 1).

Fig. 2.

Threshold search. This VVI Medtronic Sensia pacemaker is programmed at a lower rate limit of 55bpm. Every four cycles (see lower rhythm strip), a test pace (dashed arrow) is followed at 110ms by a backup pace (solid arrow).

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This feature is used mainly by Abbott and Biotronik in cardiac resynchronization therapy since it targets to maximize VP. In Abbott devices, the AVI after a VS is shortened for 31 cycles, and if another VS occurs meanwhile, that shortening will be kept for 255 cycles up to 120ms shorter than programmed AVI.4 Biotronik shortens the programmed AVI up to 50ms.2

Pacemakers can adapt AVI in a physiological way, shortening it at faster heart rates. Usually, both rest (longer) and exercise (shorter) AVI values can be preset and changes between them are made in a linear fashion by the pacemaker; other devices, such as Biotronik, allow for customization of the profile of atrioventricular (AV) adaptation. Thus, an AVI as short as 65ms at high atrial rates can be expected.12

Medtronic pacemakers try to avoid AP shortly after an atrial premature contraction where it might trigger atrial arrhythmia.13 Hence, an AS (atrial sensing) shortly after a ventricle (during the post-ventricular atrial refractory period) will trigger a 300ms interval that might delay the next AP. To balance the cardiac cycle for this delay in AP, the AV interval might be shortened up to a minimum 30ms. Sorin uses the WARAD (Window of Atrial Rate Acceleration Detection) algorithm, whose functioning is more complicated since it depends on the basal rate, timing of atrial premature contraction and existence of previous atrial premature contractions.16 To summarize, WARAD will pace the atria 500ms after an atrial premature contraction, and the ventricle 80ms thereafter (Fig. 3).

Fig. 3.

Sorin WARAD. This algorithm prevents and recognizes atrial arrhythmia. After an atrial premature contraction (arrow), the atrium is paced after 500ms regardless of the programmed lower rate limit, and the ventricle is paced after 80ms (shortest exercise atrioventricular interval).

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This is the simplest form of promoting intrinsic conduction. AV hysteresis algorithms simply lengthen AVI on a periodic basis to allow for existent but slow AV conduction to occur. If there is VS within the extended interval, that new AVI will be kept: otherwise, pacing returns to the preset AVI until the next checking round. In some cases, if no intrinsic conduction is observed after several verifications, the algorithm is disabled until the device is interrogated. These algorithms have shown to reduce VP, while other clinical benefits remain unproven.17 Their main feature, that differentiates them from other V pacing reduction algorithms, is that in AV hysteresis every atrium is followed by a ventricle, either paced or sensed. A comprehensive list of their different registered names and rules of operation is provided in Table 2.

This could probably be the most common pseudo-malfunction encountered in Holter-ECG, since intrinsic AV conduction assessment occurs several times during a 24-h recording. In some cases, they may cause short pauses or atrial cycles without their corresponding ventricle (Fig. 4, Fig. 5, Fig. 6). These algorithms have proved useful to reduce the percentage of VP in patients with relatively preserved AV conduction (i.e. sinus node dysfunction, Wenckebach AV block during exercise or paroxysmal AV block).17 Their different implementations have been proven more effective than AV hysteresis in reducing VP.19,20 They follow different rules depending on the manufacturer (Table 3). As a whole, they operate as atrial-managed modes with VP backup: hence, they are sometimes referred to as AAI↔DDD. When some conditions are met, such as a number of A without V, maximal pause duration or AVI, they switch to a DDD pacing mode. Boston Scientific's Rhythmiq is a particular case thereof.12 It is conceived as AAI(R) operating at a lower rate or sensor rate, plus a backup VVI running behind at atrial rate minus 15 beats per minute (bpm), but neither slower than 30 nor faster than 60bpm (Fig. 5).

Fig. 4.

Medtronic Managed Ventricular Pacing. Upper: Holter recording showing sinus rhythm or atrial pacing (asterisk) at a lower rate limit. When an A–A elapses with no conducted QRS, a ventricular pace is delivered at a lower rate limit plus 80ms. Modified with permission from Higueras et al.18 Lower: Pacemaker operating in AAI mode with long (∼600ms) intrinsic AV interval. New Medtronic pacemakers have corrected such behavior.

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Fig. 5.

Boston Rythmiq. Patient with a dual-chamber pacemaker programmed at 60bpm. See atrial pacing at lower rate limit with intrinsic atrioventricular conduction at 60bpm (blue line). There is an atrial premature complex (arrow), but it is not followed either by sequential ventricular pacing (VP) after an atrioventricular delay or by VP at lower rate limit. Contrarily, VP occurs at 45bpm (1400ms, dashed red line), that is 15bpm less than lower rate limit. Rythmiq functions as an AAI-managed mode with VVI running in the background at 15bpm slower than lower rate limit.

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Fig. 6.

Sorin SafeR. Patient with complete atrioventricular block. The device is operating in DDD mode tracking sinus rhythm when it performs a conduction test, allowing two consecutive P waves (arrows) not to be conducted to the ventricles, then it resumes DDD operation.

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Pacemakers can become more physiological by increasing their lower rate according to metabolic demands, which is particularly useful in patients with chronotropic incompetence or pacemaker dependency. For that purpose, all current devices use activity sensors based on motion (accelerometers) to measure patient activity and adapt their pacing rate. Besides, some current device families from Boston Scientific (RightRate: Accolade, Essentio, Proponent and Altrua 2) and Sorin (TwinTrace: Reply, Kora and Symphony), as well as legacy Medtronic Kappa, use blended sensors that integrate information from a minute ventilation sensor.21 This minute ventilation uses sub-threshold pacing pulses to measure cyclic changes in transthoracic impedance to estimate the respiratory rate and adapt their rate accordingly. In any case, activity sensors can increase the pacing rate up to a preset value, usually in the 120–130bpm range. The operation of the widely extended accelerometer sensor can be verified by “fooling” the sensor by gently tapping on the device for a few seconds.

In some circumstances, a sudden drop in heart rate might occur, as in paroxysmal AV block or a pause following atrial tachycardia in bradycardia–tachycardia syndrome. Other rhythm disorders such as atrial fibrillation might be symptomatic from the very irregularity of heart rhythm. Rate smoothing adapts the pacing rate not to a lower fixed rate but to a percentage of the previous R–R interval, thereby softening any abrupt changes in heart rate (Fig. 7).

Fig. 7.

Rate smoothing. The upper tracing shows atrial fibrillation conducted with rapid ventricular rate. A rate smoothing algorithm does not allow the heart rate to drop abruptly, so lower rate limit is temporarily increased to allow minimum deviations from previous heart rate. If such algorithm is turned off, as in the lower tracing, sudden changes in heart rate may occur when pacing only at lower limit. Modified with permission from Ruiz Pizarro et al.22

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To measure ventricular thresholds, there must be VP. Therefore, some manufacturers (Boston Scientific, Biotronik) overdrive intrinsic heart rate, up to a maximal predefined value (usually maximum tracking or sensor rate or 110bpm) (Fig. 8 and Table 1). A backup pace is usually provided to prevent asystole in case of loss of capture during these measurements (Fig. 2).

Fig. 8.

Threshold search in a Boston pacemaker under atrial fibrillation. See rapid ventricular response (narrow, irregular QRS) overdriven by the pacemaker (wider, regular QRS) for some seconds until intrinsic heart rhythm is resumed. These tests occurred every 21h: see the time stamp on the left, over 3 consecutive days.

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In vasovagal and carotid sinus syncope, a component of hypotension is added to bradycardia. Thus, a pacemaker merely working at a lower rate might not overcome the hemodynamic depression and would not avoid syncope. Different algorithms (Table 4) have been devised to detect the sudden bradycardia that precedes syncope and pace at a faster rate (intervention rate) aiming to compensate hypotension with tachycardia and to alleviate symptoms (Fig. 9). With the same spirit but with a different rationale, Biotronik's CLS (Closed Loop Stimulation) does not depend on a previous drop in heart rate. It uses sub-threshold current to measure myocardial inotropy during the cardiac cycle and estimate sympathetic tone, according to which pacing rate is adapted, modifying cardiac output and closing the loop of autonomic feedback.23

Fig. 9.

Medtronic Rate Drop Response. The lower rhythm strip shows progressive bradycardia (blue arrow) followed by sudden onset of fast pacing (red arrow). The highlighted epoch (enlarged above) shows how heart rate changes from sinus rhythm at ∼50bpm, to atrial pacing at 100bpm (red circle).

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After raising a great hype in the 90s, these algorithms fell from widespread use. Their goal is to prevent atrial tachyarrhythmia by keeping a constant pace in the atria: however, their usefulness is yet to be proven.24 For instance, atrial preference pacing overdrives intrinsic sinus rhythm. After any spontaneous AS, Boston's pacemakers will accelerate heart rhythm by shortening the cardiac cycle by 10ms, and intrinsic heart rate will be searched by lengthening the cardiac cycle 10ms every 4 cycles.12 Abbott, Biotronik (accelerates 8bpm, searches every 20 cycles),2 Medtronic (30ms shortening, searches decreasing 20ms every 20 cycles),13 and Sorin have similar functions. Other algorithms overdrive atrial rhythm after an episode of atrial tachyarrhythmia or prevent post-extrasystolic pause by shortening the cycles following an atrial premature contraction.

If atrial tachycardia or fibrillation is detected, the pacemaker switches to a non-tracking mode, i.e. VDI/DDI (R). In some cases, to compensate for the reduction in cardiac output secondary to the loss of active atrial emptying, the lower ventricular rate will be increased to 70bpm (Boston12), 80bpm (Abbott4) or 10bpm over the lower rate limit (Biotronik2). This mode switching relies on appropriate detection of atrial tachycardia (also known as atrial high rate episodes, AHRE), since it is triggered by a predefined number of fast sensed atria (over 1602–18012 AS per minute). Thus, if AS is inaccurate, AHRE will not be recognized and irregular tracking and pacing of the atria may occur. This is to be suspected whenever the ECG shows atrial and VP at irregular intervals.

This behavior belongs to implantable, transvenous, cardiac defibrillators. Ventricular tachycardia can be terminated either by high-energy shocks or by transiently pacing faster than the tachycardia, taking control over it (“entraining”) and terminating it. Ventricular spikes can be seen regularly spaced, i.e. pacing at a constant speed (burst), or in decreasing distance, i.e. in accelerating progression (ramp) (Fig. 10). The programming of anti-tachycardia pacing (type, rate, duration, etc.) is outside of the scope of this review. Anyway, it can be suspected when pacing is of short duration and unusually fast (over 150bpm and even up to 270bpm).

Fig. 10.

Antitachycardia pacing in an implantable cardioverter defibrillator. The tracing starts with ventricular tachycardia that prompts an anti-tachycardia pacing (ramp, see how the spikes are progressively faster up to a minimum 200ms), followed by transient acceleration and extinction of the arrhythmia, resuming normal ventricular pacing.

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To avoid the deleterious effect of VP, the device allows an intrinsic heart rate lower than the minimum pacing rate. In other words, it will wait for the heart rate to fall to a lower value before pacing, but that pacing will start at a faster rate than the intrinsic one.

They are described in section above.

The lower rate is decreased during some prespecified hours (Biotronik, Medtronic) or under some levels of patient activity (Abbott, Sorin), further mimicking the physiological chronotropic response. This function is programmed off by default. If activated, note that rate is adapted to the internal device clock: should the patient travel to a different time zone, the pacemaker will go to sleep at an inconvenient local time.

In endless-loop tachycardia treatment (ELT), a tachycardia is established in the presence of VP, retrograde ventriculoatrial conduction and AS that triggers another VP, perpetuating a reentrant tachycardia where the AV node is the retrograde limb and the pacemaker acts as the antegrade limb. ELT must be suspected in dual-chamber pacemakers when there is VP at upper tracking rate (120–130bpm) with no AP (Figs. 11 and 12).

Fig. 11.

Endless loop tachycardia (ELT), onset. The electrocardiogram strip (above) starts with sinus rhythm and ventricular pacing, followed by a premature atrial contraction (arrow) that is tracked to the ventricles with a longer AVI (199ms) not to violate the upper rate interval (130bpm, 460ms). This ventricular pace propagates retrogradely to the atrium and starts an ELT at the upper rate interval. Device interrogation (below) shows in detail such phenomenon (from upper to lower: marker channel, internal clock timing, atrial electrogram, and ventricular electrogram). This patient was shown to have 1:1 ventriculoatrial conduction at rates over 160bpm.

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Fig. 12.

Endless loop tachycardia (ELT), termination. Device interrogation (above) shows atrial sensing (AS) tracked to the ventricles (VP) during tachycardia until one of the atria is discarded (AR): the next pacing shows sequential atrioventricular pacing (see text for details). The electrocardiogram (below) shows the characteristic sudden drop in heart rate.

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To declare an ELT, as opposed to appropriate tracking of sinus tachycardia, the device might cross-check the appropriateness of that tachycardia with the information provided by the activity sensor.13 Other devices3,4 use an elegant approach: the timing of a VP is changed to confirm the presence of ventriculoatrial linking and ELT. In either case, the interruption of the antegrade limb will terminate the episode. An AS is not tracked, so a VP is withdrawn,2,5,12,13 the loop is broken and the heart rhythm drops back to normal. Abbott devices pace sequentially atrium and ventricle, with the AP occurring at 330ms after the last AS4 (Fig. 12).

A thorough review of pacemaker behavior under magnet application has been published elsewhere.25 As a summary, a pacemaker responds to magnet application by pacing in asynchronous mode (V00) at a preset rate that depends on the manufacturer and battery status. In other cases, when previously programmed so, magnet application will elicit no apparent response, either because “Magnet mode” has been turned off or because the device has been configured to store an internal electrogram. There are a couple of notable exceptions to this rule: as preset, Biotronik devices will function in asynchronous mode only for 10 beats and then return to their normal parameters; and some Abbott and Pacesetter legacy devices (i.e. Microny) have a function named VARIO that performs a threshold test with 16 VP at 100bpm and constant output followed by 15 VP at 120bpm with progressive reduction in pacing voltage (Fig. 13).

Fig. 13.

St. Jude VARIO magnet mode (and failure to capture). This electrocardiogram from a Pacesetter pacemaker shows asynchronous pacing spikes at 100bpm followed by others at 120bpm with a decreasing amplitude. Plus, this patient had a malfunctioning lead with failure to capture.

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Many current pacemakers have a dedicated magnetic resonance imaging, or electrocautery mode. It is essentially an asynchronous mode to prevent interference and oversensing that may lead to inappropriate inhibition or tracking. Upon activation, it can usually be programmed to any manually selected rate. Some devices will revert to previous pacing mode after a preset time, and others will even detect the magnetic field and enter “magnetic resonance imaging mode” only when approaching the gantry.

A premature ventricular beat coincidental with an AP could elicit a ventricular safety pacing with a pacing spike right on the QRS, giving an impression of undersensing. Nothing farther from the truth: that spike is a safety mechanism already described above.

Different pacemakers recognize patterns of repetitive signals at high frequencies (over 20Hz for Sorin, 25Hz for Boston, 100Hz for Abbott). When such signals are sustained long enough, the device turns to an asynchronous mode at lower rate or sensor rate. The cause and management of such interference, from electromagnetic interference to myopotential oversensing, is beyond the scope of this review.

Cardiac resynchronization therapy devices always try to maximize VP. So, when there is a ventricular premature beat or supraventricular rhythm conducted to the ventricles, there will be VS. Some cardiac resynchronization therapies will pace from the left ventricular lead in response to this VS in the right ventricular channel, willing to resynchronize as much as possible (Fig. 14). It can be differentiated from true failure to sense because the pacing spike appears earlier than expected at the programmed pacing rate.

Fig. 14.

Triggered ventricular pacing. The patient had a cardiac resynchronization therapy functioning in VVIR mode at ∼70bpm with high density ventricular premature beats (asterisks). The cardiac resynchronization therapy responded to sensing of every ventricular premature beat by pacing from the left ventricular lead: this triggered pacing occurs, therefore, earlier than expected (arrow). Reproduced with permission from Fernández-Vega et al.26

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Digital ECG recorders have band-pass filters to remove baseline respiration and movement wander, myopotentials and electrical powerline noise. However, these filters also suppress pacemaker signals, so current ECGs do not represent “true” pacing spikes but they rather depict an artificial pacing spike when they find a signal with a particular bandwidth, voltage, and duration criteria.27 Similarly, any artifact meeting such criteria will be signaled as a pacing stimulus, which can be a nuisance to the clinician. Some pacemakers have features that use sub-threshold pulses to obtain information about the device or the patient; such pulses might be recorded in the ECG, showing a regular though un-understandable behavior with the appearance of failure to sense/capture (Fig. 15). In other cases, just random, ambient noise can be picked by the ECG recorder. There is no common feature or pattern that can be recognized in these cases, but we must be aware of such possibility.

Fig. 15.

False pacing spikes. The electrocardiogram (ECG) of a patient with a single-chamber Biotronik Lumax ICD showed pacing spikes after every QRS (upper ECG); a recording was later repeated, activating manually ventricular pacing, with a clearly different QRS morphology (middle ECG). In this case, the false pacing spikes were due to the home monitoring intrathoracic measurement that sends 1024 sub-threshold pulses every hour, 100ms after every ventricular event (sensed or paced). The lower ECG shows a trace of a patient with a dual-chamber Sorin Kora: AAI-based pacing can be observed, with spikes randomly distributed (asterisks) throughout the tracing. Simultaneous device interrogation showed no pacing other than atrial.

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As explained before, some pacemakers perform a regular assessment of their capture threshold (Table 1). By definition, this means that some pacing artifacts will not capture, but usually, a backup pace will then be provided. However, Medtronic cardiac resynchronization therapy devices are somewhat different. They assess the capture threshold daily using iterations of three support pulses followed by one test pulse. Right ventricular test pulses are followed 90ms later by a backup pulse to ensure capture, as opposed to left ventricular test pulses, which lack such backup.14 Therefore, loss of capture during the assessment of left ventricular threshold will cause a pause in the absence of intrinsic atrioventricular conduction or escape rhythm.