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Europace Advance Access originally published online on June 15, 2007
Europace 2007 9(8):662-668; doi:10.1093/europace/eum100
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© The European Society of Cardiology 2007. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

PACING AND LEAD EXTRACTION

Do daily threshold trend fluctuations of epicardial leads correlate with pacing and sensing characteristics in paediatric patients?

Maren Tomaske1,*, Paul Harpes2, Ali Dodge-Khatami3, Nadja Amacker1 and Urs Bauersfeld1

1 Division of Pediatric Cardiology, University Children's Hospital, Steinwiesstrasse 75, 8032 Zurich, Switzerland; 2 Biostatistic Unit, University of Zurich, Zurich, Switzerland; 3 Division of Congenital Cardiovascular Surgery, University Children's Hospital, Zurich, Switzerland

Manuscript submitted 10 February 2007. Accepted after revision 23 April 2007.

* Corresponding author. Tel: +41 44 2667519; fax: +41 44 2667981. E-mail address: maren.tomaske{at}kispi.unizh.ch


   Abstract
Top
 Abstract
Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Aims To evaluate whether the magnitude of daily ventricular pacing threshold fluctuations ({Delta}fluctuation) in trend graphs of stored diagrams correlate with ventricular threshold and sensing changes over time.

Methods and results A total of 56 children received AutoCapture® devices (St. Jude Medical, Sylmar, CA, USA) connected to steroid-eluting epicardial leads. Maximum lead age at study closure was 12.2 years (median 4.0). Telemetry data and daily {Delta}fluctuation were obtained every 6 months. Regression slope coefficients and mean values of repeated measurements were calculated for each patient's course. High daily {Delta}fluctuation correlated with higher pacing thresholds ({rho} = 0.68, P < 0.001), lower impedances ({rho} = –0.38, P = 0.004), and a {Delta}fluctuation-incline ({rho} = 0.34, P = 0.01) over time. Furthermore, a {Delta}fluctuation-incline correlated with a pacing threshold-incline ({rho} = 0.34, P = 0.01). No correlation was observed for ventricular sensing. Higher daily {Delta}fluctuation were observed if lead age was > 5 years compared with ≤ 5 years (0.75 vs. 0.55 V@0.5 ms, P = 0.028).

Conclusion High amplitudes of daily {Delta}fluctuation correlate with higher and increasing pacing thresholds and lower impedances. Theoretically, this results from electrode microinstability on the epicardial surface. A decrease of the steroid-eluting potency of the electrode can be hypothesized to cause higher daily {Delta}fluctuation beyond a lead age of 5 years. Potential implications of marked daily {Delta}fluctuation are short-term follow-up and lead replacement in the presence of high pacing thresholds.

Key Words: Pacemaker, Threshold tracking, AutoCapture, Epicardial pacing leads, Paediatric


   Introduction
Top
 Abstract
 Introduction
Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Pacing thresholds are affected by physiologic and pharmacologic factors.1Go–3Go To guarantee capture, stimulation output of conventionally programmed devices is set with a two–three-fold safety margin. In order to reduce and optimize output energy, the intention of automatic threshold detection with automatic output adjustment emerged.4Go,5Go The first clinically successful implementation of threshold tracking was the AutoCapture® algorithm based on the detection of the cardiac-evoked response signal.6Go To adjust for threshold fluctuations, ventricular capture is verified on a beat-to-beat basis with permanent output regulation and backup safety pulse delivery following an ineffective stimulation. Combined with this algorithm, pacing threshold trends are stored in the device memory.7Go,8Go Thus, pacing threshold fluctuations for a given period can easily be retrieved from the trend graphs.

A major challenge for epicardial leads is an adequate fixation technique of the electrode in order to maintain good chronic electrical performance. A loose fixation might result in relative motion on the epicardial surface and elevated pacing thresholds. A radiographically invisible microinstability of the electrode can be suspected in the presence of excessive fluctuations of pacing thresholds in the trend graphs.9Go–10Go Furthermore, the question of how long steroid-eluting leads are capable to maintain their effect, to prevent inflammation and fibrosis at the electrode–issue interface,11Go–14Go is not satisfactorily answered.

Even though threshold trend graphs comprise a valuable research tool, there are no data about the magnitude of ventricular pacing threshold fluctuation during long-term follow-up. The purpose of this study is to evaluate whether daily amplitudes of ventricular threshold fluctuation correlate with ventricular threshold and sensing changes over time.


   Patients and methods
Top
 Abstract
 Introduction
 Patients and methods
Results
 Discussion
 Conclusion
 References
 
Study patients
With hospital ethical committee approval and informed consent, 56 children who received AutoCapture® programmed devices (St. Jude Medical, Sylmar, CA, USA) between December 1996 and August 2006 were enrolled into the study. Devices were connected to bipolar steroid-eluting epicardial pacing leads (Medtronic CapSure Epi 10366 or 4968, Medtronic, Inc., Minneapolis, MN, USA). The lead tips of the bipolar steroid-eluting epicardial leads consist of a cathode (electrode surface area 6 mm2) responsible for the AC detection and an anode (electrode surface area 14 mm2) responsible for unipolar pulse delivery in the AC setting.

Children with less than 3 stored threshold trend graphs during follow-up, or an accumulation of fusion beats making a distinct determination of daily {Delta}fluctuation impossible, were excluded. In the event of a ventricular lead replacement, follow-up was completed at that point. Four patients exhibited an exchange of the AutoCapture® device, with a device placement in the same location of the pre-existing abdominal pocket. One patient lost to follow up after 2.6 years. All children were allowed to perform physical activity within normal limits. None of the patients suffered from heart failure.

For analysis of the impact of an underlying heart disease, patients were subdivided into three groups: normal cardiac anatomy (NCA, n = 23), congenital heart disease (CHD, n = 24), and complex single ventricle (SV, n = 9). All patients with congenital heart disease underwent prior corrective or palliative cardiac surgery. Diagnosis of the underlying heart disease in those 15 patients with lead implantation before an AutoCapture® device was connected was distributed as follows: NCA n = 5, CHD n = 3, and SV n = 7, including 5 right (RV) and 10 left (LV) ventricular leads.

To evaluate the lead age-related course of amplitudes of ventricular thresholds, the date of lead implantation was set as day 0. In 15 of the 56 (27%) children, AutoCapture ® devices were connected to previously implanted ventricular bipolar steroid-eluting epicardial leads. They were implanted 4.81 years (3.7–6.5) before elective device exchange with an AutoCapture® device.

Devices, stored diagrams, and telemetry data during follow-up
Implanted AutoCapture ® devices are depicted in Table 1. The AutoCapture® algorithms of all devices were described previously in detail.7Go,8Go Threshold trend graphs with 24-h sample intervals were used to evaluate daily amplitudes of ventricular threshold fluctuation ({Delta}fluctuation). The average of the three highest and three lowest deflections in 6-monthly printouts of threshold trend graphs were taken as upper and lower margins of the amplitude. Excluded were typical single deflections in the presence of backup pulses, due to fusion or pseudofusion beats. Daily {Delta}fluctuation were measured from the printouts of threshold trend graphs by two authors (U.B., N.A.).


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  		Table 1 Devices and baseline characteristics of 56 children

 
Telemetry data included R-wave amplitudes, ventricular pacing thresholds, impedances, and evoked response signals (ERS) and were performed at routine device interrogation every 6 months. R-wave sensing in the AutoCapture® algorithm was bipolar, whereas ventricular pacing threshold and ERS detection, impedance measurement, and pulse delivery were unipolar. By using the previously published energy formula,15Go voltages of ventricular pacing thresholds and daily {Delta}fluctuation were calculated for a standard value of 0.5 ms pulse duration (V@0.5 ms) to allow for comparison. Clinical studies have demonstrated an excellent correlation between pacing thresholds measured by AutoCapture, and those measured by traditional VARIO feature during pacemaker interrogation.7Go

Surgical technique
Access for implantation of the epicardial leads was either via a subxyphoidal incision to reach the right ventricular apex or via left lateral thoracotomy to reach the left ventricular free wall and corresponding atria, as described in detail previously.16Go In those children with concomitant cardiac surgery, the epicardial leads were implanted via midline sternotomy at the time of cardiopulmonary bypass surgery (n = 4). The implant technique of the leads was standardized: each triangular electrode consisted of a suture pad with two suture holes and grooves. Fixation was done with non-absorbable sutures. For the distal and proximal suture, a single knot technique was used. Sutures were placed perpendicularly to the epicardium to avoid tissue trauma near the electrode. The device was implanted either in the abdominal rectus sheath (n = 46), in a left thoracic muscular pocket (n = 8), or subpectorally (n = 2).

Statistical analysis
A follow-up period of 8 years was statistically analysed. Due to too small number of patients with SV, no separate statistical analysis was done for this group. Data are presented as medians [(interquartile range), range]. A P-value < 0.05 was considered statistically significant.

To determine an individual persistent change or an individual average level, slope coefficients from a regression over individual repeated measurements (inclinei) and individual mean values of measured variables (meani) were calculated for each patient's course. Correlations between variables were measured by Spearman's correlation. Mann–Whitney U tests were used for analysing the differences in continuous variables between independent groups. Wilcoxon signed-rank test was used for within group changes of continuous variables between a specific time period and within group changes between the mean values at different time periods (meant). All statistical analyses were performed using the Statistical Package for Social Sciences (SPSS for Windows, Version 14.0.1, Inc., Chicago, IL, USA).


   Results
Top
 Abstract
 Introduction
 Patients and methods
 Results
Discussion
 Conclusion
 References
 
Patients' demographic data, clinical characteristics, and surgical data at implantation
Demographic data, clinical characteristics, indications for permanent pacing as well as surgical data are depicted in Table 1.

Stored diagrams and telemetry data during follow-up
Daily {Delta}fluctuation, R-wave amplitudes, ventricular impedances, pacing thresholds, and ERS during follow-up are given in detail in Table 2. High meani daily {Delta}fluctuation correlated with higher meani pacing thresholds ({rho} = 0.684, P < 0.001), lower meani impedances ({rho} = –0.379, P = 0.004), and an inclinei of daily {Delta}fluctuation over time ({rho} = 0.335, P = 0.01). Furthermore, an inclinei of daily {Delta}fluctuation correlated with an inclinei of pacing thresholds ({rho} = 0.341, P = 0.01). Neither the meani nor inclinei of daily {Delta}fluctuation correlated with meani ventricular sensing ({rho} = –0.130, P = 0.36 and {rho} = 0.134, P = 0.34) and meani ERS ({rho} = –0.005, P = 0.97 and {rho} = 0.071, P = 0.61). Similarly, the inclinei of daily {Delta}fluctuation did neither correlate with the meani nor the inclinei of lead polarization signals ({rho} = 0.123, P = 0.19 and {rho} = 0.221, P = 0.09). One patient with simultaneous lead and device implantation was an exception. He exhibited trend graphs with high daily {Delta}fluctuation [2.9 V@0.5 ms (2.2–3.5)] since electrode implant, with comparatively high ventricular impedances up to 979 Ohm, and pacing thresholds up to 2.0 V@0.5 ms.


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  		Table 2 Telemetry data during follow-up with date of lead implantation set as day 0

 
The individual incline and mean of measured variables, subdivided for the ventricular pacing site and underlying heart disease, are given in Table 3. No difference for any measured variable was observed for the ventricular pacing site. In contrast, a higher meani daily {Delta}fluctuation as well as lower meani impedance were found in children with CHD compared with those with NCA (Figure 1A and B). For patients with SV, there was a trend for a comparatively higher inclinei of daily {Delta}fluctuation and an impedance declinei. However, the number of patients with SV was too small to allow for statistical analysis.


Figure 1
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  		Figure 1 Individual mean (meani) (A) daily {Delta}fluctuations and (B) impedances for the underlying heart disease in patients with single ventricle (n = 9), congenital heart disease (n = 24), and normal cardiac anatomy (n = 23). Data are given as medians [(interquartile range), range]. For children with congenital heart disease, higher individual mean daily {Delta}fluctuations (*P = 0.02) and lower individual mean impedances (**P = 0.008) were found.

 


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  		Table 3 Individual regression slope coefficients (inclinei) and individual mean values (meani) for the ventricular pacing site and underlying heart disease

 
Lead age-related course of {Delta}fluctuation
To evaluate a time-related trend of daily {Delta}fluctuation, patients with trend graphs before as well as beyond a lead age of 5 years (intersection: n = 13) were tested (Figure 2). Analysis between the mean values at the different time periods revealed higher meant for daily {Delta}fluctuation if lead age was > 5 years than for those ≤ 5 years (0.75 vs. 0.55 V@0.5 ms, P = 0.028). A separate analysis of those 41 children with simultaneous lead and AutoCapture® device implant revealed an increased difference of the meant for daily {Delta}fluctuation after 5 years compared with the 1-year follow-up [difference: 0.28 V@0.5 ms (–0.14–2.46)] (Figure 3).


Figure 2
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  		Figure 2 Example of amplitudes of daily ventricular threshold fluctuation in a patient 4 and 10 years after implant of the ventricular lead: the stored trend graphs display initially stable, small daily {Delta}fluctuation with an apparent increase at last follow-up. These original data are shown in volt at 0.4 ms pulse duration.

 


Figure 3
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  		Figure 3 Difference of meant for daily {Delta}fluctuation during the first 6 years after lead implantation compared with the 1-year follow-up in 41 children. Data are given as median [(interquartile range), range].

 

   Discussion
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
Conclusion
 References
 
A major benefit of the AutoCapture® algorithm is the accurate determination of pacing thresholds and thereby assurance of patient safety.17Go In addition to physiological and pathological factors that are well known to alter pacing thresholds,1Go–3Go,18Go there are specific risk factors for patients with congenital heart disease. Pacing thresholds have been demonstrated to be influenced by scars and adhesions due to prior cardiac surgery.19Go Moreover, they might be influenced by cyanosis, elevated pressure or volume overload of the right ventricle, or ventricular inversion.

The valuable ability of the AutoCapture® feature to retrieve continuous monitoring of ventricular pacing thresholds has facilitated to evaluate the clinical status of patients, such as worsened congestive heart failure,7Go or an acute and persistent ventricular threshold rise due to direct current cardioversion.20Go Furthermore, the storage and retrieval of information about threshold fluctuations has served as a research tool of pacing physiology at acute high altitude exposure.21Go Atrial and ventricular pacing threshold trend graphs (Thera pacemaker, Medtronic Inc.) were studied for different transvenous pacing leads during a follow-up of 25 months.22Go Represented by threshold variations, the study demonstrated a three-staged lead maturation with a slight but consistent threshold rise beyond 1 month of implant. Of importance, the overall amplitude of daily threshold fluctuations during this short-term follow-up was < 0.2 V@0.5 ms.

However, there is no data about continuous monitoring of ventricular pacing threshold fluctuations as a measure to access epicardial lead performance during long-term follow-up. This diagnostic valuable tool might assist as decision criterion, managing patients with chronic epicardial leads in the clinical setting.

In this study, we analysed the trend graphs and telemetry data of a total of 56 children with AutoCapture® programmed devices connected to bipolar steroid-eluting epicardial pacing leads. They were followed to a maximum lead age of 12.2 years. The main finding of an analysis for each patient's course was that high daily {Delta}fluctuation correlated with higher and increasing pacing thresholds and lower lead impedances. Furthermore, an incline of daily {Delta}fluctuation correlated with an incline of pacing threshold.

As demonstrated in previous case reports, a microinstability of the electrode leading to high fluctuations of pacing thresholds in the trend graphs is suggestive.9Go–10Go Microinstability provokes continued or prolonged tissue irritation with continued migration and activation of macrophages and fibroblasts.22Go Although lead fixation was done with a single knot technique, a relative motion of the lead on the epicardial surface cannot be excluded. A fast conducting interface between the electrode and epicardial surface, such as local oedema as a possible aetiology for the observed correlation between high daily {Delta}fluctuation and lower impedances, can be anticipated. A further circumstance known to cause a steady impedance decline is a deterioration of the insulation of epicardial leads and comparatively might influence the daily {Delta}fluctuation over time. In contrast, in the mentioned patient with high daily {Delta}fluctuation and higher impedances since electrode implant, a fibrous capsule due to inflammation between the electrode and epicardial surface is more likely.

The observed higher daily {Delta}fluctuation and decreased impedance in children with CHD and prior surgery, compared with those without, remains unclear. No differences were found between the right or left ventricular pacing site.

Interestingly, ventricular sensing did not correlate with daily {Delta}fluctuation. Neither unipolar ERS detection nor lead polarization signals of the epicardial leads correlated with an incline of daily {Delta}fluctuation. This might reflect a difference of lead tip behaviour between the anode and cathode. As the anode is responsible for unipolar pulse delivery, the chronic release of current drain crossing the epicardial lead–tissue interface itself may cause electrical injury associated with morphological changes,23Go,24Go whereas the cathode remains unaffected.

Remarkably, a difference of daily {Delta}fluctuation did not occur until a lead age of 5 years. A significantly higher daily {Delta}fluctuation beyond this lead age could represent diminished steroid elution, and therefore reduced protection of the electrode–tissue interface. In clinical studies, stable low acute and chronic pacing thresholds were observed up to 6 years in steroid-eluting epicardial leads25Go–28Go and up to 10 years in steroid-eluting transvenous leads.29Go An additional reason for high daily {Delta}fluctuation over time, leading to microinstability of the lead, could result from lead stretch due to somatic growth or physical activity in our paediatric population.

As demonstrated, high daily {Delta}fluctuation correlated with higher ventricular pacing thresholds. However, only a slight but consistent increase of pacing thresholds or daily {Delta}fluctuation was observed over time. The median daily {Delta}fluctuation was ranging between 0.56 and 0.84 V@0.5 ms, and median ventricular pacing threshold was 0.67–0.84 V@0.5 ms during the study period. This denotes that, even though microinstability of the electrode may be present, it cannot easily be read from one single device interrogation. In the clinical routine, an increase of daily {Delta}fluctuation can be retrieved by comparison of subsequent printouts of threshold trend graphs during follow-up, as demonstrated in Figure 2.

Study limitations
A main limitation of this study is the use of seven different devices with AutoCapture® features. In the clinical setting, we observed a slightly lower pacing threshold measurement for the Identity or Integrity, compared with the Affinity devices. When relative amplitudes of threshold fluctuation are correlated, expressed in individual mean or incline, no impact of this phenomenon should be observed.

Clinical implications
Potential clinical implications are improvements in lead designs with a long-lasting potency of steroid elution. Furthermore, high magnitudes of daily ventricular threshold fluctuation might warrant an adequate follow-up with shortened intervals. An elective lead replacement in leads with high pacing thresholds and additional high magnitudes of daily ventricular threshold fluctuation at the time of a device exchange due to battery depletion is suggestive.


   Conclusion
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
References
 
Magnitudes of ventricular threshold fluctuation are predictors of ventricular pacing characteristics. High amplitudes of threshold fluctuation correlated with higher and increasing pacing thresholds and lower impedances. Theoretically, this may result from microinstability of the epicardial electrode with continuous tissue irritation. A fast conducting electrode–tissue interface, such as local oedema, can be anticipated as a possible aetiology for the correlation between high daily {Delta}fluctuation and lower impedances. Higher daily {Delta}fluctuation beyond a lead age of 5 years could represent diminished steroid elution, and therefore reduced protection of the electrode–tissue interface.

Conflict of Interest: none declared.


   References
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
[1] Preston TA, Fletcher RD, Lucchesi BR, Judge RD. Changes in myocardial threshold. Physiologic and pharmacologic factors in patients with implanted pacemakers. Am Heart J (1967) 74:235–42.[CrossRef][Web of Science][Medline]

[2] Preston TA, Judge RD. Alteration of pacemaker threshold by drug and physiological factors. Ann NY Acad Sci (1969) 167:686–92.[CrossRef][Web of Science][Medline]

[3] Dohrmann ML, Goldschlager NF. Myocardial stimulation threshold on patients with cardiac pacemaker: Effect of physiologic variables, pharmacologic agents, and lead electrodes. Cardiol Clin (1985) 3:527–37.[Medline]

[4] Preston TA, Bowers DL. The automatic threshold tracking pacemaker. Med Instr (1974) 8:322–5.[Medline]

[5] Preston TA, Bowers DL. Clinical applications of the threshold tracking pacemaker. Am J Cardiol (1975) 36:322–6.[CrossRef][Web of Science][Medline]

[6] Sermasi S, Marconi M, Libero L, Moracchini PV, Rusconi L, Mininno A, et al. Italian experience with AutoCapture in conjunction with a membrane lead. Pacesetter Automatic Control of Energy and Membrane Automatic Threshold Evaluation (Pacemate) Study Group. Pacing Clin Electrophysiol (1996) 19:1799–1804.[CrossRef][Medline]

[7] Clarke M, Liu B, Schüller H, Binner L, Kennergren C, Guerola M, et al. Automatic adjustment of pacemaker stimulation output correlated with continuously monitored capture thresholds. Pacing Clin Electrophysiol (1998) 21:1567–75.[CrossRef][Medline]

[8] Duru F, Bauersfeld U, Schüller H, Candinas R. Threshold tracking pacing based on beat by beat evoked response detection: clinical benefits and potential problems. J Intervent Card Electrophysiol (2000) 4:511–22.[CrossRef][Web of Science][Medline]

[9] Candinas R, Huemmer A, Varga L, Schoenbeck M. Diagnose und Verlaufskontrolle einer Mikrodislokation bzw. Mikroinstabilität mit Hilfe der Funktion Autocapture. Herzschrittmacher (1996) 16:160–3.

[10] Liebold A, Birnbaum DE. Autocapture as a diagnostic tool: Detection of a lead dislodgement. Z Kardiol (1997) 86:521–3.[CrossRef][Web of Science][Medline]

[11] Mond HG, Stokes KB. The electrode-tissue interface: The revolutionary role of steroid elution. Pacing Clin Electrophysiol (1992) 15:95–107.[CrossRef][Medline]

[12] Radovsky AS, Van Vleet JF. Effects of dexamethasone elution on tissue reaction around stimulating electrodes of endocardial pacing leads in dogs. Am Heart J (1989) 117:1288–98.[CrossRef][Web of Science][Medline]

[13] Hamilton R, Gow R, Bahoric B, Griffiths J, Freedom R, Williams W. Steroid-eluting epicardial leads in pediatrics: improved epicardial thresholds in the first year. Pacing Clin Electrophysiol (1991) 14:2066–72.[CrossRef][Medline]

[14] Cohen MI, Bush DM, Vetter VL, Tanel RE, Wieand TS, Gaynor JW, et al. Permanent epicardial pacing in pediatric patients: seventeen years of experience and 1200 outpatients. Circulation (2001) 103:2585–90.[Abstract/Free Full Text]

[15] Hamilton RM, Chiu C, Gow RM, Williams WG. A comparison of two stab-on unipolar epicardial leads in children. Pacing Clin Electrophysiol (1997) 20:631–6.[CrossRef][Medline]

[16] Dodge-Khatami A, Kadner A, Dave H, Rahn M, Prêtre R, Bauersfeld U. Left heart atrial and ventricular pacing through a left lateral thoracotomy in children: a safe approach with excellent functional and cosmetic results. Eur J Cardio Thoracic Surg (2005) 28:541–45.[CrossRef]

[17] Sperzel J, Kennergren C, Biffi M, Smith M, Knops M, Gill J, et al. Clinical performance of ventricular automatic capture verification algorithm. Pacing Clin Electrophysiol (2005) 28:933–7.[CrossRef][Medline]

[18] Hughes HC, Tyers GFO, Torman HA. Effect of acid-base imbalance on myocardial pacing threshold. J Thorac Cardiovasc Surg (1975) 69:743–6.[Abstract]

[19] Karpawich PP, Walter H, Hakimi M. Chronic performance of a transvenous steroid pacing lead used as epi-myocardial electrode. Pacing Clin Electrophysiol (1998) 21:1486–8.[CrossRef][Medline]

[20] Kurisu S, Inoue I, Kawagoe T, Ishihara M, Shimatani Y, Mitsuba N, et al. Documentation of acute increase in ventricular capture threshold after direct current cardioversion with AutoCaptureTM threshold record. Pacing Clin Electrophysiol (2005) 28:1009–10.[CrossRef][Medline]

[21] Weilenmann D, Duru F, Schonbeck M, Schenk B, Zwicky P, Russi EW, et al. Influence of acute exposure to high altitude and hypoxemia on ventricular stimulation thresholds in pacemaker patients. Pacing Clin Electrophysiol (2000) 23:512–5.[CrossRef][Medline]

[22] Danilovic D, Ohm O. Pacing threshold trends and variability in modern tined leads assessed using high resolution automatic measurements: Conversion of pulse width into voltage thresholds. Pacing Clin Electrophysiol (1999) 22:567–87.[CrossRef][Medline]

[23] Antunez SF. Maturation of the electrode-endomyocardium interface. Rev Esp Cardiol (1990) 43(Suppl. 2):3–12.

[24] Gregory JW. The pathology of cardiac pacing. In: Cardiovascular Pathology.—Malcom DS, ed. (1991) 2nd. Edinburgh: Churchill Livingstone. 1429–60.

[25] Cutler NG, Karpawich PP, Cavitt D, Hakimi M, Walters HL. Steroid-eluting epicardial pacing electrodes: six-year experience of pacing thresholds in a growing paediatric population. Pacing Clin Electrophysiol (1997) 20:2943–8.[CrossRef][Medline]

[26] Horenstein MS, Hakimi M, Walters HL, Karpawich PP. Chronic performance of steroid-eluting epicardial leads in a growing pediatric population: a 10-year comparison. Pacing Clin Electrophysiol (2003) 26:1467–71.[CrossRef][Medline]

[27] Beaufort-Krol G, Mulder H, Nagelkerke D, Waterbolk TW, Bink-Boelkens M. Comparison of longevity, pacing, and sensing characteristics of steroid-eluting epicardial versus conventional endocardial pacing leads in children. J Thorac Cardiovasc Surg (1999) 117:523–8.[Abstract/Free Full Text]

[28] Fortescue EB, Berul CI, Cecchin F, Walsh EP, Triedman JK, Alexander ME. Comparison of modern steroid eluting epicardial and thin transvenous pacemaker leads in paediatrics and congenital heart disease patients. J Intervent Cardiac Electrophysiol (2005) 14:27–36.[CrossRef][Web of Science][Medline]

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M. Tomaske, P. Harpes, N. Woy, and U. Bauersfeld
The efficacy of ventricular pacing with device automaticity in paediatric patients
Europace, July 1, 2008; 10(7): 838 - 843.
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