Europace Advance Access originally published online on July 13, 2007
Europace 2007 9(8):645-650; doi:10.1093/europace/eum130
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PACING AND LEAD EXTRACTION
Long-term experience with AutoCapture®-controlled epicardial pacing in children
1 Division of Pediatric Cardiology, University Children's Hospital, Steinwiesstrasse 75, 8032 Zurich, Switzerland; 2 Biostatistics Unit, University Zurich, Switzerland; 3 Division of Congenital Cardiovascular Surgery, University Children's Hospital, Zurich, Switzerland
Manuscript submitted 16 April 2007. Revision received 11 June 2007. * Corresponding author. Tel: +41 44 2667747; fax: +41 44 2667981. E-mail address: maren.tomaske{at}kispi.uzh.ch
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Abstract |
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Aims To examine the feasibility and safety of AutoCapture (AC)-controlled pacing with epicardial leads in children, and study the effects on device longevity.
Methods A total of 62 children were prospectively enrolled. Pre-discharge testing precluded AC function in six children. In 56 (90%) children, devices with AC-controlled pacing were followed up to 9years. Calculated battery life in AC-controlled pacing was compared with theoretical calculations, using a two-fold stimulation output of measured thresholds.
Results In 53 of 56 children, no differences were observed for evoked response signals (13.3 vs. 11.5mV, P = 0.20) or lead polarization safety margins (5.5 vs. 4.1, P = 0.25) at 6-month and 4-year follow-up. A crossover to conventional pacing was required in 3 of 56 children. AC-controlled pacing prolonged the calculated battery life up to 15% for the identity and integrity devices with 0.95A h capacity, compared with theoretical conventional settings (P = 0.008). In patients with ventricular pacing thresholds >1.5V at 0.5ms, battery life was increased by 30% compared with theoretical conventional settings (P < 0.001).
Conclusion AC-controlled pacing with epicardial leads is feasible and safe in children during long-term follow-up. An adequate lead polarization safety margin persists in most patients. Calculated battery life was prolonged up to 15% with AC-controlled pacing. Patients with high or fluctuating pacing thresholds benefit the most.
Key Words: Pacemaker, AutoCapture, Safety, Battery service life, Epicardial pacing leads, Pediatric
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Introduction |
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Prolongation of device longevity has been an important goal since the introduction of permanent pacing.1
,2 Major problems in the paediatric population have been caused by the large size of the devices,3 as well as physiologically high pacing rates and high pacing thresholds resulting in early battery depletion.4,5
One of the central issues in research was the development of energy-saving pacing features to delay device replacement. In 1994, the first single-chamber device with AutoCapture® (AC) algorithm was introduced (Microny, Pacesetter, Solna, Sweden). The principle of AC-controlled pacing is automated, beat-by-beat ventricular capture verification, by monitoring the evoked response signal (ERS) resulting from myocardial depolarization.6,7 The algorithm comprises continuous monitoring of ventricular capture and adjustment of the stimulation output at 0.25–0.3V above the actual ventricular threshold. Therefore, the drainage of current is diminished, although the pacing safety margins may be markedly decreased.
Studies in adults with AC devices connected to transvenous leads proved the algorithm to be safe, with a prolonged battery service life.6,8 Theoretically, the use of an AC device connected to epicardial leads in children would also reduce energy consumption and improve battery life span. As children face a lifelong dependency on permanent pacing, they would benefit the most. A first report with epicardial pacing leads connected to single-chamber devices in AC-controlled pacing has been encouraging.9 Nevertheless, there is little data about appropriate AC function and energy saving during long-term follow-up.
The purpose of this study was to examine the long-term reliability and practicability of the AC algorithm in children, and to assess the effects of adjusted output on device longevity.
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Methods |
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Patients
Between December 1996 and August 2006, with hospital Ethics Committee approval and informed consent, 62 children receiving single- or dual-chamber devices with AC function were prospectively enrolled. Devices were connected to bipolar steroid-eluting epicardial pacing leads (Medtronic CapSure Epi 10366 or 4968, Medtronic, Inc., Minneapolis, MN, USA). At pre-discharge testing, AC function could not be applied in six patients. Reasons for conventional programming of the devices were inadequate ERS (n = 4), and intolerable pocket stimulation caused by backup pulses and high stimulation output (n = 2). They were excluded from further analysis.
The AC function was established after device implant in 56 of the 62 (90%) children, who make up the study group. Pre-discharge Holter recordings were performed in all patients.
Devices, AC algorithm, and leads
Either single- or dual-chamber AC device (St. Jude Medical, Sylmar, CA, USA) was implanted, and programmed in single- (VVI/VVI-R) or dual-chamber (DDD/DDD-R) pacing modes. Details are depicted in Table 1. For the ventricular leads, pulse delivery, impedance measurement, pacing threshold, and ERS detection were unipolar, whereas R-wave sensing was applied by bipolar configuration. Two parameters are measured during device interrogation and are crucial for a safe AC function: a sufficient ERS of at least 2.5mV and a low lead polarization signal (PS) smaller than 4mV. In addition, an accurate discrimination between both parameters has to be present: the evoked response safety (ERS) margin (ERS to ER sensitivity) has to be 
1.8:1. Finally, the lead polarization safety margin (ER sensitivity to PS) has to be 1.7:1.9,10
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A subanalysis of the impact of epicardial right ventricular (RV) or left ventricular (LV) pacing sites on measured ERS and lead polarization safety margin were performed.
Surgical technique
Access to the implantation of the epicardial leads was either via a subxyphoidal incision (n = 10) to reach the RV apex, or via a left lateral thoracotomy (n = 34) to reach the LV-free wall11 and corresponding atria. In the case of concomitant cardiac surgery, the epicardial leads were implanted via midline sternotomy (n = 12). Standard surgical suture fixation of the leads was used. The devices were positioned either in the abdominal rectus sheath (n = 47), in a left thoracic muscular pocket (n = 7), or subpectorally (n = 2). Lead impedances, sensing and pacing thresholds, ERS, and PS were measured intra-operatively.
Telemetry data at implant and follow-up
For the atrial and ventricular leads, impedances, P- and R-wave amplitudes, pacing thresholds, ERS, and PS were obtained. Furthermore, stimulation output, pulse energy, and battery current were noted. By using the previously published energy formula,12 voltages of lead threshold and stimulation output were calculated for a standard value of 0.5ms pulse duration (V at 0.5ms) to allow for comparison. Measurements were taken at implant, at hospital discharge, at 1, 3, and 6months after implant, and every 6 months thereafter. Holter recordings were performed in all patients before discharge. During follow-up, Holter recordings were routinely performed in those patients with congenital heart disease to rule out arrhythmias.
Battery service life calculation
Battery service life calculation at last follow-up was performed assuming stable threshold and impedance. In children with >99% ventricular pacing, mean heart rate was determined by total beats per observation period stored in the device. Holter recordings in the last 6months were used to determine mean heart rate, if storage in the device memory was not available, or in the presence of <99% of ventricular pacing. Calculations of battery service life were based on diagnostic data retrieved from device memory and actual programming. The feature of storing electrograms potentially expending current drain was used in none. The percentage of atrial and ventricular pacing was included into the calculations.
The atrial and ventricularly drained current was derived by the following formulas13,14:
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Estimated longevity of the device was then calculated:
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Battery voltage was set at 2.576 V for all devices (H. Zerlik, personal communication). The amount of housekeeping current for each device in the single- or dual-chamber mode was found in the user manuals (St Jude Medical, Zurich, Switzerland), taking into account the higher current drain of the dual-chamber devices and the rate adaptive pacing modes (VVI-R and DDD-R).
Calculated battery service life with AC-controlled pacing was compared with theoretical conventional settings for each patient, using bipolar measurements for impedances, and stimulation outputs twice the measured thresholds at last follow-up.
Statistical analysis
Estimated freedom from re-intervention was plotted with Kaplan–Meier analysis during the whole observation period.
A follow-up period of 4years was statistically analysed. Data are presented as medians ([interquartile range], range). A P-value of <0.05 was considered statistically significant.
For statistic analysis, mean values for all measured variables were calculated at 6-month and 4-year follow-up. Using the Wilcoxon signed-rank test, intra-individual changes in continuous variables between the two follow-up intervals were evaluated. Furthermore, individual mean values of measured variables were calculated for each patient's course over a follow-up period of 4years. Mann–Whitney U-tests were used for analysing differences in continuous or ordinal variables between independent groups. Differences in the calculated and theoretical battery service life were analysed using a t-test. All statistical analyses were performed using the Statistical Package for Social Sciences (SPSS for Windows, Inc., Version 14.0.1, Chicago, IL, USA).
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Results |
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Patients' demographic data, clinical characteristics, surgical data at implantation, and re-interventions
The study group consisted of 32 male and 24 female patients [age, 8.0years (range 0.0–18.4years); weight, 20.0kg (range 3.0–68.0 kg). Median follow-up was 3.0years (range 0.1–9.0years)]. Clinical characteristics, indications for permanent pacing, and surgical data at implantation are depicted in Table 1. No pacemaker system-related mortality or adverse events were observed. In 10 of 56 children, a device exchange was required during the follow-up period. Kaplan–Meier estimates of freedom from re-intervention at 2 and 5years were 95.3 and 73.9%, respectively (Figure 1). Reasons for device exchange were battery depletion (n = 2), premature device exchange at the time of lead replacement due to a persistent threshold rise (n = 2), lead replacement due to fracture or insulation break (n = 4), or cardiac surgery for right ventricle–pulmonary artery conduit exchange (n = 2).
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Measured device telemetry data at various follow-up visits
Measured device telemetry data and percentage of pacing at various follow-ups are given in detail in Table 2. To discover the potential changes during follow-up, differences in the measured variables between the 6-month and the 4-year follow-ups were analysed. No relevant or significant differences were observed for impedances, P- and R-wave amplitudes, pacing thresholds, or stimulation outputs (pulse amplitude, pulse energy) of the atrial and ventricular leads, as well as ERS, PS, or battery current (Table 2). A subanalysis of the impact of single- or dual-chamber pacing modes revealed no difference in individual mean ventricular threshold value (0.80 vs. 0.67V at 0.5ms, P = 0.43), but for individual mean battery current (8.5 vs. 11.8 µA, P < 0.001). Furthermore, no significant differences in the measured device telemetry data were observed among devices placed in the rectus sheath or in the left thoracic muscular pocket.
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Sufficient lead polarization safety margins at implantation were obtained [5.27 (1.7–24.9)]. A decrease in the ERS resulting in a lead polarization safety margin <1.7:1 required a crossover to conventional pacing after 1year (n = 2) and 3years (n = 1). No difference between the 6month and 4year follow-up was observed for ERS (P = 0.20) and PS (P = 0.99) (Table 2). Furthermore, lead polarization safety margins remained stable between the 6 month and 4 year follow-up (P = 0.25) and above the cut off value of 1.7:1 (Figure 2). A subanalysis of the impact of the ventricular pacing site demonstrated no difference between the epicardial right and LV pacing site for individual mean ERS (P = 0.53) and individual mean lead polarization safety margin (P = 0.31).
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Pre-discharge Holter recordings demonstrated an adequate AC function in all 56 patients. During follow-up, a total of 43 Holter recordings were performed in the 35 children with congenital heart disease. Analysis demonstrated an appropriate AC function in all Holter recordings, independently of ventricular threshold or ventricular threshold fluctuations. In one patient, Holter recording revealed repeated ventricular capture loss due to insulation break of the ventricular electrode. No patient suffered from syncope during the observation period.
Battery service life calculations
Median percentage of atrial pacing was 27–35% (range, 1–100%) during follow-up. The percentage of ventricular pacing was 99% in 50 of 56 children (89%).
Three devices requiring crossover to conventional settings were excluded from battery service life calculations. One of these three devices was exchanged due to battery depletion after 6.4years. Detailed battery service life calculations for devices with AC-controlled pacing are given in Table 3.
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For the identity devices with battery capacities of 0.55 and 0.95A h, calculated battery service life with AC-controlled pacing was increased by 0.7years (8%) and 1.2years (10%), respectively, compared with theoretical, conventional settings. Calculated battery service life extension with AC-controlled pacing for the Integrity devices was 1.7years (15%) (Table 3).
Of importance, a comparison of the calculated battery service life in AC-controlled pacing vs. theoretical conventional settings for those patients with ventricular pacing thresholds 
1.5V at 0.5ms and those >1.5V at 0.5ms revealed a significant difference. Battery service life of the devices with a capacity of 0.95A h was increased by 30% in patients with ventricular pacing thresholds above 1.5V at 0.5ms [median battery life extension 1.1years (for thresholds 1.5V at 0.5ms) and 2.4years (for thresholds >1.5V at 0.5ms); P < 0.001].
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Discussion |
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Lead technology with steroid-eluting low polarization electrode surface designs and threshold tracking devices have improved safety, efficiency, and generator longevity.11
,15,16 The first clinically successful implementation of threshold tracking was the AC algorithm.17 As physiological factors and pharmacological agents may influence pacing thresholds,18,19 the stimulation output for conventionally programmed devices is set with a two- to three-fold safety margin. To compensate for threshold fluctuations, ventricular capture in the AC algorithm is verified on a beat-to-beat basis. Thus, a stimulation output just above the actual threshold can be maintained, and minimizes the current drain. At the same time, a potentially higher stimulation safety margin is provided.
Significant predictors of battery longevity are programmed pacing rates, stimulation output, pacing mode, battery capacity, and internal current of the device.20 Several clinical studies in adults with transvenous leads have proved the safety of the AC algorithm. An initial activation of the AC function is possible in 82–95% of the patients6,21–24 and is maintained in 81–90%7,22–24 of the patients. A recent clinical study in 20 children with transvenous leads25 observed an initially activated AC function in 95% of the children who could be maintained in 70% during a mean follow-up of 5years. Furthermore, an 1.2–1.7-fold extended device longevity was found, when AC-controlled pacing was compared with conventional settings with stimulation outputs twice the pacing threshold.7,22,26
Different circumstances in the paediatric population, mainly small vessels or cardiac abnormalities, often preclude the use of transvenous electrodes and favour epicardial pacing leads. Two case reports in children with epicardial leads demonstrated stable ERS and PS during short-term follow-up.27,28 However, a clinical study with bipolar steroid-eluting epicardial pacing observed unfavourable results with considerably low ERS and an activated AC algorithm in only three of eight patients.24 In contrast, an observation during up to 18 months follow-up demonstrated stable ERS and a maintained AC function in 86% of the children.8 However, there is little data concerning mid- and long-term performance of AC-programmed devices connected to epicardial pacing leads in children.
In our study, we prospectively analysed a total of 62 children. In 90% of the children, the AC algorithm could be established since implant. Furthermore, it could be maintained up to 9years in 85% of our study cohort with stable ERS and PS over time. Lead polarization safety margins remained above the cut off value of 1.7:1. These encouraging results are comparable with the previously published data in patients with transvenous leads.21–24
Of special interest is the similar ERS or lead polarization safety margin of the right and LV pacing site. Clinical trials have demonstrated superior haemodynamics in LV pacing29 compared with chronic RV apex pacing. Thus, epicardial leads are increasingly implanted on the left ventricle in the paediatric population.
To rule out arrhythmias and verify the safety of AC-controlled pacing, Holter recordings were performed routinely in those patients with congenital heart disease. All recordings confirmed an appropriate AC function, even in those children with high ventricular thresholds or high daily ventricular threshold fluctuations, monitored by a trend graph included into the AC feature.
Our study confirms the results from previous studies,8 indicating that AC-programmed devices connected to bipolar epicardial pacing leads in children result in a prolonged calculated battery service life. AC-controlled pacing extended calculated battery service life up to 15%, compared with theoretical calculations of conventional pacing with stimulation outputs twice the pacing threshold. These results were obtained even though dual-chamber pacing modes were predominantly used in our study cohort, with a median percentage of atrial pacing up to 35%. Elevated battery consumption in dual-chamber devices can be expected due to the additional battery current caused by atrial stimulation at conventional stimulation output settings. Furthermore, the presence of fusion or pseudofusion beats in patients with intrinsic atrioventricular conduction30 may result in an insufficient or absent ERS, and consequently unwarranted current drain caused by automatic backup pulses.31 As only 4 of 44 children with dual-chamber pacing in our cohort exhibited intrinsic atrioventricular conduction with either intermittent atrioventricular block or long PR intervals, the later aspect can be neglected in our patient cohort. The benefit of battery service life extension with AC-controlled pacing was even more pronounced, up to 30%, compared with theoretical, conventional settings in those children with pacing thresholds >1.5V at 0.5ms. These findings concur with the results demonstrated in an adult cohort with transvenous pacing leads.22
Study limitations
A main limitation of this study is that battery service life calculations are theoretical. Variables influencing battery service life such as pacing parameters, thresholds, and mean heart rates may change in a growing child. This and additional ancillary device functions or pre-implant current drain32 may result in a difference between real and estimated device longevity. As device replacement in the pacemaker-dependent child is scheduled well before complete battery exhaustion, and may be influenced by family and hospital logistics, there is an inevitable shortfall of longevity. In three patients with AC-controlled pacing, a device exchange due to battery depletion was required. However, two of these exchanges were scheduled prematurely at the time of lead replacement due to persistent threshold rise. Measured battery impedance at exchange was 2.5 and 2.7
, respectively, indicating a substantial remaining capacity of the device.2 Thus, a comparison between real and estimated device longevity was only reasonable in one device, demonstrating an underestimated battery service life of 2.2months.
A further limitation is the fact that the six children excluded due to conventional pacing since hospital discharge could not serve as a control group. Mean ventricular thresholds in the children with conventional stimulation outputs at a two- to three-fold safety margin were significantly higher than in those with AC-controlled pacing. Thus, stimulation output would have led to a biased and definitely shorter calculated battery service life.
Conclusions
The use of AC-programmed devices is practicable and safe in children with bipolar epicardial leads during an observation period up to 9years. During follow-up, a sufficient and stable lead polarization safety margin well above the cut off value of 1.7:1 was seen in 85% of our cohort, independently of the pacing site. Substantial prolongation of the calculated battery service life was seen, even though dual-chamber pacing modes prevailed. The benefit was even more pronounced in those children with higher ventricular thresholds.
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Acknowledgements |
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The authors would like to thank Heiko Zerlik, Manager study co-ordination (St Jude Medical, Zurich, Switzerland), for his valuable support in data supply and ongoing discussions.
Conflict of interest: none declared.
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References |
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