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© The European Society of Cardiology 2007. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Evolution of pacing for bradycardia: Autocapture

Mauro Biffi1,*, Johannes Sperzel2, Cristian Martignani1, Angelo Branzi1 and Giuseppe Boriani1

1 Institute of Cardiology, University of Bologna and Azienda Ospedaliera S.Orsola-Malpighi, Via Massarenti 9 40138 Bologna, Italy
2 Kerckhoff Klinik, Bad Nauheim, Germany

* Corresponding author. Tel: +39 051 345898; fax: +39 051 344859. E-mail address: mbiffi{at}aosp.bo.it


   Abstract
Top
 Abstract
Introduction
 Technological background
 Operative behaviour of AVC...
 Clinical results of stimulation...
 Funding
 References
 
Many useful technological improvements have occurred since the first pacemaker implantation in 1958. A new paradigm of cardiac stimulation has been offered by the development of AutocaptureTM, which allows the implanting physician to adjust to the patient's clinical needs, thanks to device automaticity. Automatic verification of capture (AVC) is the ability of a cardiac pacing device to determine whether a delivered pacing stimulus results in stimulation of the myocardium, and, consequently, to adapt pacing output according to the measured threshold. The first single-chamber pacemaker featuring this capability for ventricular stimulation, termed Autocapture, was released by Pacesetter in 1994 (a subsidiary of St Jude since 1994). In 1999 the first Autocapture-featured dual-chamber pacemaker was also released by Pacesetter-St Jude. Autocapture is the most extensively studied algorithm for AVC, being the first to be developed. Over 1 000 000 Autocapture-featured units have now been implanted worldwide. More recently, Autocapture or automatic verification of atrial stimulation has also been proposed at the atrial level in DDDR devices and automatic verification of left ventricular stimulation has been implemented in devices for cardiac resynchronization therapy.

Key Words: Autocapture • Evoked response • Automatic threshold measurement • Pacemaker


   Introduction
Top
 Abstract
 Introduction
Technological background
 Operative behaviour of AVC...
 Clinical results of stimulation...
 Funding
 References
 
Automatic verification of capture (AVC) is the ability of a cardiac pacing device to determine whether a delivered pacing stimulus results in the stimulation of the myocardium, and consequently to adapt pacing output according to the measured threshold. The first single-chamber pacemaker featuring this capability for ventricular stimulation, termed AutocaptureTM, was released by Pacesetter in 1994 (a subsidiary of St Jude since 1994).1,2 In 1999, the first Autocapture-featured dual-chamber pacemaker (DDDR) was also released by Pacesetter-St Jude.3 Only recently, a DDDR pacemaker capable of automatically verifying atrial stimulation has been released by Medtronic.4 The first CRT-D device with automatic verification of left ventricular (LV) stimulation by Medtronic entered clinical use in June 2006. The newly released Zephyr pacemaker by St Jude Medical featuring the ACapTM algorithm is the first to provide beat-to-beat verification of atrial stimulation.


   Technological background
Top
 Abstract
 Introduction
 Technological background
Operative behaviour of AVC...
 Clinical results of stimulation...
 Funding
 References
 
AVC is a concept that covers several algorithms for cardiac stimulation. It is an important device featured with implications for safety, automated remote follow-up of stimulation, and battery longevity.5–7 The ability of the pacing output to vary following threshold stimulation is pivotal, since the threshold may change according to physiological conditions, disease conditions, and maturation of the lead/tissue interface.8–11 This has led physicians to programme pacing output amplitudes two to three times greater than the measured pacing threshold (the so-called safety margin) to ensure consistent cardiac capture. Implementation of a capture verification feature requires a reliable method to assess that myocardial systole has occurred following each delivered stimulus. Evaluation of the cardiac depolarization [or evoked response (ER)] following the pacing pulse is a reliable surrogate for myocardial systole, and is the most common method for implementing AVC features.12,13 This approach is challenged by the ability to discriminate between the myocardial ER signal and the pace-induced residual voltage (or pacing artefact or polarization) remaining on the lead/tissue interface. The relative magnitude of ER, as opposed to pacing artefact magnitude, may vary greatly depending on the physical characteristics of the pacing leads and on tissue–electrode interface. Several approaches have been investigated to decrease the effect of pace-induced residual voltage on ER detection, such as modification of the lead electrode design,12 increased signal processing of the ER,13,14 reduction of the pacing output coupling capacitor circuitry,15,16 or modification of the ER-sensing vector.17,18

Depending on the approach to overcome pacing artefact and reliably detect the ER, some types of limitations are observed in current AVC algorithms (Table 1). The approach to decrease artefact magnitude by the use of low-polarization leads makes Autocapture strictly dependent on lead type and configuration, it being mandatory to elicit ventricular stimulation tip to can, and to achieve ER detection tip to ring1–3,19,20 (bipolar leads are needed); only Zephyr pulse generators allow Autocapture with both uni- and bipolar low-polarization leads. The reduced coupling capacitor (RCC) technology has been developed to overcome this limitation, and achieve AVC regardless of lead type or pacing/sensing configuration.15,16 The RCC technology increases the rate of decay of polarization artefact, governed by the pacing output circuitry and myocardial load impedance, to minimize the effective polarization on the lead tissue interface.15–16 However, the ability of the RCC method to reliably sense the ER with all lead types is related to the current delivered at the lead–tissue interface and, accordingly, the pacing output is limited to voltages ≤3.5 V with a fixed pulse duration of 0.4 ms.16 Splett et al.17 found that creating independent vectors for pacing and ER sensing in implantable cardioverter defibrillator leads can abolish (true bipolar leads) or minimize (integrated bipolar leads) post-pacing artefact, allowing a greater adjusted output compared with the RCC method.


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  		Table 1 Clinical requirements for an automatic verification of capture algorithm: technological approach and unmet limitations

 
The importance of minimizing post-pacing artefact is greater when atrial AVC is concerned; a small atrial signal would make any algorithm prone to ER underdetection in the event of a small ER/polarization ratio. Butter et al.18 studied a modification of the RCC approach for atrial AVC, in which the pacing and sensing vectors were isolated, creating an independent pace/sense (IPS) configuration. The IPS method further reduces pace-induced residual voltage and allows for an increased pulse amplitude range,18 but requires unipolar pacing and a bipolar lead for ER sensing by an independent vector. A unique approach for atrial AVC has been reported by Sperzel et al.4; atrial threshold measurement is not based on the detection of the atrial ER, but on indirect evidence of atrial capture. This is based on reset of the atrial cycle in the event that the patient has intrinsic atrial activity, or on conduction of the paced atrial beat to the ventricle in the event of dependency on atrial stimulation with 1:1 atrioventricular (AV) conduction. This algorithm (ACM) has no dedicated lead requirement, but can perform only in a dual-chamber mode and is limited in the event of dependency on stimulation in both chambers.4 The novel ACap algorithm can overcome this limitation: atrial ER is detected beat-to-beat by the analysis of the post-depolarization integral (tip to can vector), thanks to low-polarization atrial leads, and allows automatic atrial output adjustment with backup stimulation in the event of loss of atrial capture during threshold testing. Because of minimization of polarization, the atrial ER magnitude can be reliably detected.13

A pioneering study to apply Autocapture to the ventricle paced second during biventricular stimulation demonstrated the feasibility of detecting LV ER during epicardial pacing by the transvenous route, and led to the investigation of the concept of AVC in cardiac resynchronization therapy (CRT).21 Goetze et al.22 have recently reported a reliable performance of both the RCC and the IPS method in an acute study, albeit the latter proved more effective (capable of supporting AVC in 96% of patients vs. 86%), being less sensitive to LV pacing vector and stimulation current, thus capable of managing LV output at greater pulse amplitude and width. Importantly, the methods were able to perform throughout all the pacing settings (AV and V–V intervals) used in clinical practice, hence are suited to be implemented in a CRT device. Diotallevi et al.23 have investigated the feasibility of AVC for biventricular pacing based on ER morphology during an acute study: the method proved sensitive to interventricular offset, possibly yielding a >10% false-positive beats classification in particular settings. The only algorithm for automatic detection and adjustment of LV stimulation (LVCM) available in clinical practice is based on conduction of LV-paced beats to the right ventricle24 (not on detection of LV ER), hence the algorithm is not limited by lead type nor by the current delivered at the electrode–tissue interface. Capture is determined when the right ventricular (RV) sense occurs because of interventricular conduction (LV–RV conduction). LV–RV conduction times are separated from AV conduction times in tracking modes, and from intrinsic depolarization in non-tracking modes (during atrial arrhythmias). Because of its particular setting, maximum adapted output can be 6 V at 1.5 ms, save for the risk of phrenic stimulation.

Despite these major technological developments, limitations to meet clinical requirements still exist (Table 1) mostly because of the clinical setting and lead characteristics, so that an ideal AVC algorithm capable of tracking the pacing threshold over all pacing scenarios is not available. On the other hand, the application of cardiac stimulation to a continuously growing aged population demands improved automaticity of pacing devices. It appears from Table 1 that available technologies are suited to different applications depending on the pacing situation. It is thence conceivable that, in the future, multiple AVC methods will be applied automatically to manage pacing output in each chamber to overcome all the limitations imposed by the particular ‘stimulation settings’.


   Operative behaviour of AVC algorithms
Top
 Abstract
 Introduction
 Technological background
 Operative behaviour of AVC...
Clinical results of stimulation...
 Funding
 References
 
Most of the algorithms measure stimulation threshold either periodically or in the event of repetitive loss of capture, then adapt the pacing output to minimize current drain while providing improved safety by capture monitoring. From a clinical perspective, two operational aspects are important: reliable detection of stimulation and maximum adjusted threshold in the AVC modality. As for the former, two main features distinguish available algorithms for AVC stimulation: beat-to-beat capture verification and backup pulse delivery in the event of loss of capture. The pivotal point to warrant safety is beat-to-beat detection of ER, which represents heart depolarization. The ventricular, atrial, and LVCM algorithms by Medtronic and the ventricular autothreshold by ELA differ, as neither ER monitoring (aside from scheduled threshold measurements) nor backup pulse in case of loss of capture is provided. This approach stands on the assumption that pacing threshold variability is minimal in the short term,11,25 but poses a relevant safety issue in the event of an unexpected threshold increase. Algorithms based on continuous ER verification perform an initialization test to measure ER amplitude and assess the feasibility of the AVC modality. This test runs automatically in ACap and other algorithms, once AVC is chosen as the output modality, whereas it needs to be commanded for Autocapture (ER sensitivity default is 49.7 mV, much greater than that observed in clinical practice). ER sensitivity, once programmed, is a fixed value in Autocapture, hence the algorithm may turn off if ER amplitude decreases below the programmed value, whereas it is beat-to-beat updated in the Boston algorithm. ACap, Biotronik, Medtronic, and ELA algorithms update ER sensitivity at scheduled intervals (programmable for ACap, Biotronik, and Medtronic) together with pacing threshold measurement, or in the event of repetitive loss of capture or of mode, polarity, and pulse duration reprogramming. A practical implication is to measure ventricular ER amplitude at routine St Jude pacemaker follow-up, or in the rare instances of sudden threshold change to ensure adequate ER sensitivity.

It must be noted that neither measurement of ER amplitude nor ER sensitivity programming is made available to clinicians by algorithms other than Autocapture, as all of them work in a closed-loop manner. Closed-loop management of ER is an obvious advantage, as the physician is never required to perform any testing, except in the event of an inadequate ER. In this situation, the algorithm will automatically revert to a high-pacing amplitude until ER magnitude returns to values suitable for AVC modality, or a formal device follow-up is performed, possibly wasting considerable battery longevity. Thus, useless reversion to a high-pacing output is the main unwanted effect of failure to detect the ER. Most commonly, underestimation of ER amplitude may occur because of fusion/pseudofusion beats1–3,26; St Jude, Boston, and Biotronik algorithms are provided with fusion avoidance features to prevent useless backup stimulations and cyclic recalculations of pacing threshold. In response to fusion detection (Boston) or to backup pulse delivery (St Jude, Biotronik), the AV interval is prolonged, so that intrinsic conduction is promoted and fusion avoided in the following beats. Biotronik algorithm modulates AV interval (either lengthening or shortening) to promote intrinsic activity or to achieve ventricular pacing without fusion. Fusion phenomenon is not relevant to Medtronic and ELA algorithms, since no beat-to-beat monitoring of paced events occurs. Differently from all other manufacturers, Boston pacemakers use a dedicated channel to display ventricular ER during threshold testing and throughout device follow-up; thus, misinterpretations because of intracardiac electrogram limitations are avoided, and ER can be used as an alternative to surface ECG.

In the event of loss of capture, backup stimulation is delivered beat-to-beat at 4.5 V (5 V in Victory and Zephyr pacemakers) by St Jude algorithms, at threshold +1.5 V (minimum 3.5 V, up to 5 V) by Boston algorithm, and at 1 ms duration with the actual pacing amplitude by Biotronik algorithms. This feature requires ER to be continuously detected, which is the operational mode of St Jude Autocapture and ACap, Boston Automatic Capture Verification, and Biotronik Automatic Capture Control. Once stimulation threshold is measured, based on ER detection, pacing output is automatically adjusted to threshold and a definite amount (0.25 V for Autocapture, self-adjusting to maintain a 1.7:1 safety margin in ACap, 0.5 V for Boston, 0.5–1.2 V for Biotronik). In Medtronic algorithms, pacing output is adapted following a safety margin (threshold x 1.5–4, programmable), and a lowest adapted output (programmable), whichever is the highest. Moreover, an acute phase duration may be programmed during lead ‘maturation’ before the algorithm begins output management. Tuning of this algorithm is needed to achieve optimal balance between patient safety and device longevity, because of the absence of backup stimulation. The Medtronic LVCM algorithm adds a programmable amount (0.5–2 V) to measured LV threshold.

The second pivotal aspect of AVC algorithms is the maximum adapted pacing threshold; AVC benefits battery longevity the more the stimulation threshold increases, compared with a 100% safety margin programming (Figures 1–3). Maximum adapted pacing threshold varies significantly among AVC algorithms, depending on the effect of delivered current on the ability to detect the ER. The RCC technology by Boston is not suitable to detect ER at high-pacing outputs,15,16 hence the AVC algorithm switches to retry mode at 5 V until threshold decreases, or eventually shuts off if it does not decrease, when ventricular threshold is >3 V at 0.4 ms. Medtronic atrial and ventricular automatic capture management provides output adjustment based on measured threshold and programmed safety margin (2.5 V at 0.4 ms with a 2:1 safety margin, 3.25 V at 0.4 ms with a 1.5:1 safety margin). As in the Boston algorithm, pulse duration is limited to 0.4 ms, ER detection being sensitive to pacing output. If the threshold increases above the maximum adapted value, a safe pacing output is set: 5 V at 1 ms. The ELA autothreshold sets ventricular output at 2 x threshold, minimum 1.5 V not programmable, up to 4 V at 0.5 ms. The maximum adapted threshold is 2 V at 0.5 ms. The safety margin is non-programmable, and the maximum adapted threshold is limited because of the effect of higher-pacing outputs on ER detection ability. In this algorithm, ER detection is based on the assumption of a linear relationship between pacing output and polarization, which can be lost above 5 V and at a pulse duration >0.5 ms (limit of this technology). Autocapture and ACap can adjust the pacing threshold up to 3.875 V regardless of pulse duration and revert to 5 V in the event of a higher or an undetectable threshold. The use of dedicated leads allows polarization to be <1 mV in a wider range of pacing outputs compared with other algorithms. Thus, ER detection is rather insensitive to pulse duration (Figure 1), and allows a higher adapted pacing threshold compared with other algorithms (Table 2). The Biotronik algorithm maximum adapted threshold is limited by the effective ability to detect ER at any given output, which may decrease at high-pacing outputs. Once effective detection of ER is confirmed during the initialization test, a maximum adapted threshold up to 5.9 V at 0.5 ms is allowed. Duration is limited to 0.5 ms to warrant an effective backup pulse at 1 ms (doubled pacing current). This algorithm reverts to the maximum adapted output (programmable up to 6.4 V) +1.2 V in the event of a high or undetectable threshold. Because of its particular method (conduction to the right ventricle, no dependence on ER detection), the LVCM by Medtronic can adjust the LV threshold up to a programmable maximum output (6 V at 1.5 ms), which is set depending on patient characteristics (phrenic threshold, lead stability). A programmable safety margin (0.5–2 V) is added to measured threshold: pacing output, then varies with threshold changes and can increase up to the maximum programmed output.


Figure 1
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  		Figure 1 Evoked response and automatic threshold measurements by Autocapture at different pulse durations in a pacemaker-dependent patient with a chronic high stimulation threshold (3.38 V at 0.4 ms, left lower panel), close to maximum adjusted amplitude (3.875 V). Evoked response magnitude is insensitive to pulse duration and amplitude (upper panels), thus allowing operation at 1 ms. Output is conveniently set 2.5 V at 1 ms (right lower panel), and allows further adaptation in the event of threshold increase.

 

Figure 3
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  		Figure 3 Projected pacemaker longevity in the same patient as Figures 1 and  2 (Vp = 100%, Ap = 0%, ventricular impedance 650 {Omega}, average heart rate 70 b.p.m.) based on threshold measured at both pulse durations in three different settings: Autocapture, fixed output with 50% safety margin (1.5 x threshold), fixed output with 100% safety margin (2 x threshold). Battery voltage is 2.8 V; at 1 ms the use of voltage multipliers is avoided by Autocapture, which explains the increased advantage over fixed outputs.

 

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  		Table 2 Main features of available algorithms for automatic verification of capture

 
When considering the characteristics of the AVC algorithms, it should be kept in mind that the maximum adapted threshold is limited because ER detection is heavily affected by the post-pacing artefact. In fact, the highest adapted thresholds are allowed by algorithms independent of ER, or by those that can achieve low-polarization artefacts at high-pacing output. It must be noted that if the threshold cannot be measured after several attempts, or a high threshold is found or transient conditions do not allow a reliable threshold measurement, all these algorithms ensure patient safety by reverting to a high-pacing output, and try threshold measurement later to switch back to the AVC modality. A summary of AVC algorithms is shown in Table 2.


   Clinical results of stimulation by AVC
Top
 Abstract
 Introduction
 Technological background
 Operative behaviour of AVC...
 Clinical results of stimulation...
Funding
 References
 
Autocapture is the most extensively studied algorithm for AVC, being the first to be developed. Over 1 000 000 Autocapture-featured units have been implanted worldwide, following the initial experiences.1–3

ER detection
Consistent with these observations, Autocapture applicability ranges from 90 to 96% of patients1–3,20,26 because of unsuitable ER amplitude in a minority, detected after pacemaker implantation. Hence, ER measurement should be part of the implantation procedure to warrant 100% Autocapture applicability; lead placement should be accomplished in a perspective different from usual that is to attain an adequate ER amplitude. Accordingly, manufacturers should make ER measurements available in their own pacing system analysers. In published reports, ER stability at 1 year was good; ER decrease and underdetection over time were a rare occurrence (<0.01% of patients).2,20,26 True ER undersensing accounted for 0.5% of backup pulse delivery,1 whereas ER underestimation occurred mainly because of fusion/pseudofusion beats in 0.4–1.4% of paced beats,1–3 or in 3.5% of patients.26 Importantly, backup stimulation elicited because of fusion/pseudofusion accounted for >60% of total backup pulses1–3,20,26 (87% in Clarke's study1), being most likely in those patients with the greatest amount of intrinsic cardiac activity. Of totally recorded fusion/pseudofusion beats, 22.5% elicited backup pulses in Clarke's pioneering experience with the first Autocapture release.1 Because of the implementation of fusion avoidance features, this problem is negligible in the current release of St Jude, Boston, and Biotronik algorithms. The applicability of Boston AVC algorithm is comparable with Autocapture, as reported by Sperzel et al.,27 whereas limited experience has been reported with other algorithms. Ventricular capture management failed to detect ER in 3.4–6% of patients in small studies,25,30 mainly because of inadequate ER or fusion beats. Atrial capture management has proved capable of providing daily threshold verification in 64% of patients, applicability improving to 90% when threshold measurement once weekly was deemed an acceptable result.4 This is far from optimal, as expected by its features (indirect determination of atrial threshold). LVCM by Medtronic showed 86% applicability over 83 patients in the pioneering download study24 when evaluated on a daily threshold measurement, whereas it was 94% when weekly threshold determination was deemed acceptable.

Backup stimulation
Autocapture safety was reproducibly demonstrated in all studies1–3,20,26: loss of capture was always correctly followed by an effective backup pulse. Overall backup pulses accounted for a maximum of 1.1% of paced beats,1–3 which appears as an overestimation of true loss of capture, because of fusion/pseudofusion occurrence in the first Autocapture release.1,2 The low-output stimulus as threshold +0.25 V provided >99% effective stimulation in these studies; indeed, Sperzel et al.27 have recently reported <0.2% backup pulse delivery when the output is adjusted to threshold +0.5 V. In no case was backup pulse delivered during the ventricular vulnerable period because of spontaneous R-wave undersensing.

Threshold tracking
A high correlation between automatically measured and manually determined thresholds has been uniformly reported.1–3,20,26 Although technical improvements in lead manufacture have ensured that pacing threshold variability in the long term is truly modest,11,26 unexpected changes are encountered in 25% of patients7 because of the abovementioned factors.1,8,9,11,12,19 It is noteworthy that the behaviour of pacing threshold over long-term follow-up was unknown until the introduction of AVC-featured pacemakers, which record and display threshold trending over time, providing a detailed history. Beyond the basic advantage of safety compared with a fixed ventricular output, there are multiple benefits of AVC for patient care. First, improved safety is ensured for those patients prevalently paced with moderate to high stimulation thresholds, because of continuous capture monitoring and backup pulse delivery. Secondly, these patients gain the most in terms of battery longevity, compared with a 100% safety margin programming of pacing output. In patients with pacing thresholds between 1.5 and 3.5 V a 2:1 safety margin programming is expected significantly to reduce battery longevity (because of the necessary use of voltage multipliers) compared with threshold +0.25 V programming, which allows >99% effective stimulation. This aspect has never been considered in studies of projected longevity,5–7 but indeed represents the true clinical situation in a subgroup of patients paced in the long term. In fact, during the year following pacemaker implantation, ventricular threshold was ≥1.5 V in at least 25% of patients, and ≥2.5 V in 5%, aside from the acute phase.7,25 In projected longevity studies,5–7 the expected benefit of Autocapture with respect to a fixed output with a 100% safety margin was 8–11 months on average. Pacing output programming by a 100% safety margin was 1.8–2.5 V (thresholds ranged from 0.9 to 1.2 V) based on threshold measurements 2–6 months after implantation, a far different situation from a pacing threshold between 1.5 and 2.5 V, which recommends a 3–5 V output unless AVC is applied. In this different setting, the longevity would increase more than 12 months by AVC.7 No important gap between projection studies and clinical practice exists, suggesting an underestimation of AVC effect on pacemaker longevity. Longevity should indeed be evaluated on actual threshold, which can change over time, more than on projections based on the assumption of a chronically stable threshold. A threshold increase may be observed more frequently during longer patient follow-up (Figure 2) without a detectable cause, and highlights the limitations of a fixed pacing output. Because of the possibility of detection of ER at any pulse duration (Figure 1), Autocapture makes it possible to decrease battery drain by pacing at the lowest amplitude, thus avoiding the use of voltage multipliers: as shown in Figure 3, the net longevity gain with respect to a 50% safety margin-fixed output (5 V at 0.4 ms) would reach 26 months (25% longevity increase). The net longevity gain compared with a 100% safety margin would indeed be 86 or 92% (60 or 62 months), respectively, at 0.4 and 1 ms duration. An improvement of Autocapture could be the automatic determination of ER and pacing threshold at different pulse durations in the event of a high threshold, with automatic reprogramming to the appropriate setting so as to avoid the use of voltage multipliers.


Figure 2
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  		Figure 2 Threshold trends of the same patient prior to (A) and following (B) device follow-up and pulse duration reprogramming from 0.4 to 1 ms.

 
In paced patients operated on because of congenital heart disease, Cohen et al.29 have also reported a significantly increased longevity (average 13 months, range 6–19) for ventricular-capture-managed output compared with a fixed output in 14/27 patients. These observations change the perspective from which AVC is examined, and make it a more helpful feature the more pacing threshold increases.

The difference between real clinical practice and projection studies is emphasized when CRT devices are concerned, whose life span is seriously compromised by three output channels. AVC is of paramount importance for LV pacing when coronary veins suitable for lead placement are limited and lead to a high LV pacing threshold site, or when a high LV pacing threshold is found at the optimal anatomic site for CRT, or when a high LV threshold is found by changing the pacing configuration to avoid phrenic stimulation. A 100% safety margin programming of LV output would dramatically impact CRT cost-effectiveness in such instances, and could also not be feasible in patients with a phrenic threshold ≤ safety margin. Beyond increased device longevity and assurance of constant therapy delivery (CRT is an on/off phenomenon and depends on LV stimulation), avoidance of phrenic stimulation may indeed be achieved by automatically adjusted LV output, programmed at threshold +0.5 V (as available in the LVCM algorithm24) according to the findings by Sperzel et al.27 and Biffi et al.28 Third, increased device longevity by automated follow-up of pacing output translates into a reduction of complications carried by device replacement (infections, lead extraction procedures), and into a decreased cost of pacing.5,7 Although these latter reports suffer the limitations of projection studies, it is very likely that the economic advantage of AVC will be underestimated, rather than overestimated, as >50% of pacemakers are never reprogrammed after implantation,30 and pacing threshold may increase over time.7 Eventually, technological evolution targeted to improve patient care also benefits physicians. In fact, the availability of reliable algorithms for automatic adjustment of stimulation and sensitivity, and for lead integrity monitoring, allows physicians to focus in depth on pacemaker-generated diagnostics (stored arrhythmia recordings, heart rate, sensor-driven rate and patient activity, heart failure-related measurements, etc.), which are more clinically relevant to patient care than inspecting the pacing system itself.

Many useful technological improvements have occurred since the first pacemaker implantation in 1958. A new paradigm of cardiac stimulation has indeed been settled by the development of Autocapture, which allows the implanting physician to be closer to the patient's clinical needs by means of device automaticity. The recent release of Zephyr pacemakers has extended beat-to-beat AVC to both chambers in a DDDR pacemaker, with the aims of increased patient safety and pacing system longevity. It must be expected that, on the basis of available technology, a CRT device capable of AVC in all paced chambers will enter clinical use in the very near future.


   Funding
Top
 Abstract
 Introduction
 Technological background
 Operative behaviour of AVC...
 Clinical results of stimulation...
 Funding
References
 
The present article was partly supported by a grant from ‘Fondazione Luisa Fanti Melloni’, Bologna, Italy.


   Acknowledgments
 
Conflict of interest: none declared.


   References
Top
 Abstract
 Introduction
 Technological background
 Operative behaviour of AVC...
 Clinical results of stimulation...
 Funding
 References
 

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