Ventricular rate determines early bradycardic electrical remodeling

Ventricular rate determines early bradycardic electrical remodeling Fumiaki Suto, MD, PhD,a Wei Zhu, MD,a Sean A. Cahill, MBiotech,a Ilana Greenwald, BSc,a André L.C. Navarro, MD,a Gil J. Gross, MDa,b a

From the Cardiovascular Research Programme, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada. b Cardiology Division, Hospital for Sick Children; Department of Pediatrics and Heart & Stroke/Richard Lewar Centre of Excellence, University of Toronto. OBJECTIVES The purpose of this study was to isolate chronic ventricular rate as the primary determinant of early bradycardic ventricular electrical remodeling. BACKGROUND Ventricular repolarization delay predisposing to potentially lethal tachydysrhythmias occurs during chronic bradycardia. Prolonged QT intervals and torsades de pointes are associated with down-regulated ventricular myocyte delayed rectifier potassium (K⫹) currents. METHODS Transcatheter AV node ablation in rabbits was followed by chronic right ventricular pacing at either 140 bpm (n ⫽ 16) or the near-physiologic rate of 280 bpm (n ⫽ 9). ECG QT intervals were assessed in vivo at days 0 and 8 of paced AV block. Repolarizing currents in isolated left and right ventricular myocytes were assessed using whole-cell patch clamp technique. RESULTS Bradycardic rabbits had increased steady-state QT intervals (230 ⫾ 6 ms vs 206 ⫾ 7 ms [mean ⫾ SE], day 8 vs day 0; P ⬍ .001). Biventricular myocyte expression of the delayed rectifier K⫹ currents IKr and IKs was down-regulated in bradycardic rabbits, with no change in the transient outward current Ito or inwardly rectifying current IK1. None of these changes were observed in rabbits paced at 280 bpm. Pause-dependent torsades de pointes was documented in one bradycardic animal on day 8. No heart failure or ventricular hypertrophy was apparent. CONCLUSIONS Bradycardic ventricular electrical remodeling proceeds independently of structural remodeling, heart failure, or AV synchrony and is prevented by maintenance of near-physiologic ventricular rate. KEYWORDS Bradycardia; Ion channels; Remodeling; Repolarization; Ventricular arrhythmias (Heart Rhythm 2005;2:293–300) © 2005 Heart Rhythm Society. All rights reserved.

Introduction Chronic bradycardia in a variety of settings is associated with ventricular repolarization delay predisposing to lifethreatening tachydysrhythmias, most notably torsades de pointes (TdP).1 Marked ECG QT interval prolongation complicated by TdP has been described in both young2 and older3 patients with acquired complete AV block. QT interval prolongation is frequently observed and is predictive of Dr. Suto was the recipient of a Research Fellowship and of the Evelyn McGloin Award from the Heart and Stroke Foundation of Ontario (HSFO). This work was supported by HSFO Grant-in-Aid No. NA5044. Address reprint requests and correspondence: Dr. Gil J. Gross, Cardiology Division, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. E-mail address: [email protected]. (Received August 10, 2004; accepted December 14, 2004.)

poor outcomes in bradycardic infants with congenital complete AV block.4 QT interval normalization and TdP suppression are observed in response to pacing therapy in these and other bradycardic patients5 with repolarization delay. Although the efficacy of cardiac pacing in preventing bradycardia-dependent TdP has long been recognized, the importance of pacing rate per se has been assessed only recently in the clinical setting.6 Complete AV block with uncontrolled bradycardic intrinsic escape rhythms in both canine7 and rabbit8 models is associated with progressively impaired ventricular repolarization and predisposition to TdP in vivo and with downregulated ventricular myocyte delayed rectifier K⫹ currents in vitro. Ventricular hypertrophy9 and heart failure8 are concomitants of bradycardia in these models. We developed a novel complete AV block model with experimentally variable ventricular rate to elucidate time

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doi:10.1016/j.hrthm.2004.12.020

294 course and mechanisms of bradycardia-mediated ventricular electrical remodeling.10 We report here the early onset of rate-dependent remodeling in complete AV block rabbits paced chronically at approximately half the physiologic sinus rate but not at near-physiologic heart rate. Early bradycardic remodeling is not associated with heart failure or hypertrophy in this model. Most significantly, electrical remodeling mediated by down-regulation of delayed rectifier K⫹ currents is prevented by maintenance of paced ventricular rates in the physiologic range. These results establish the primacy of chronic ventricular rate in the development and prevention of bradycardic electrical remodeling.

Methods This investigation conformed with the Guide to the Care and Use of Experimental Animals published by the Canadian Council on Animal Care (2nd edition, 1993) and was approved by the Hospital for Sick Children Animal Care Committee.

Complete AV block model Transcatheter radiofrequency complete AV block induction and permanent endocardial ventricular lead placement in healthy young adult male New Zealand White rabbits (weight 3.0 –3.5 kg) have been described.10 Following subcutaneous interscapular pacemaker implantation, demand right ventricular (RV) pacing was instituted at either 140 bpm (16 animals) or 280 bpm (nine animals). Control rabbits (n ⫽ 11) underwent intracardiac catheter manipulation without conduction system disruption, and an endocardial pacing system was implanted but not activated. M-mode echocardiography was performed as previously described.10

In vivo studies QRS duration and QT interval measurement All ECG recordings were obtained during ventricular pacing at 140/min to obviate the need for rate correction and to maximize T-wave visibility. QRS complex durations and QT intervals were measured offline by two authors who were blinded to each animal’s chronic pacing rate using averaged values from three consecutive beats. Continuous ambulatory heart rhythm monitoring An implantable ECG telemetry system (Data Sciences International UA-10, St. Paul, MN, USA) was used to continuously monitor heart rhythm. The transmitter was implanted in the interscapular pacemaker pocket. One lead was extended forward subcutaneously around the neck, yielding a bipolar recording configuration simulating surface ECG lead II. Acquired data were captured and analyzed using DSI software (DQ ART2.1). Arrhythmic events were

Heart Rhythm, Vol 2, No 3, March 2005 detected using automated software features and then analyzed by visual inspection.

In vitro studies Myocyte isolation Left ventricular (LV) and RV ventricular myocytes were isolated as described elsewhere.11 The excised heart was retrogradely perfused for 10 min with nominally Ca2⫹-free HEPES buffered saline containing the following (in mM): 132 NaCl, 4 KCl, 1.2 MgCl2, 10 HEPES, 10 D-glucose, and 0.5% BSA; pH adjusted to 7.4 with NaOH. Perfusate then was switched to Ca2⫹-free HEPES-buffered saline containing collagenase 1 mg/mL (Worthington Class 2, Lakewood, NJ, USA) and protease 0.1 mg/mL (Sigma type XIV, Sigma Chemical Co., St. Louis, MO, USA) for 15 to 20 minutes, followed by 4-minute washout with Ca2⫹-free HEPES-buffered saline. All perfusates were gassed with 100% O2 and maintained at 37°C. Ventricles were removed and teased apart with forceps. After gauze filtration, dispersed myocytes were resuspended in sequentially higher Ca2⫹ concentrations (0.05, 0.1, 0.2, 1.0, 1.8 mM Ca2⫹ HEPESbuffered saline) and stored at room temperature for study within 10 hours of isolation. Electrophysiologic recording Conventional whole-cell patch clamp was performed as previously described.12,13 Quiescent rod-shaped myocytes selected for study were perfused at 1 to 2 mL/min (1-mL total bath volume) with extracellular solution containing the following (in mM): 135 NaCl, 5.4 KCl, 1.0 CaCl2, 1.0 MgCl2, 10.0 HEPES, and 10.0 D-glucose; pH adjusted to 7.4 with NaOH. Micropipettes had resistances of 1.5 to 2.5 M⍀ when filled with intracellular solution containing the following (in mM): 145 KCl, 5 NaCl, 5 K2EGTA, 10 HEPES, 4 MgATP; pH adjusted to 7.2 with KOH. L-type Ca2⫹ current was blocked with extracellular CoCl2 (2 mM). Na⫹ current was inactivated with 100-ms conditioning voltage steps to ⫺35 or ⫺50 mV. With each change in extracellular solution, at least 5 mL was perfused through the bath to allow for equilibration prior to recording. All experiments were carried out at physiologic temperature (35– 37°C). Delayed rectifier currents were measured as peak density of tail current elicited by repolarization to ⫺30 mV following 3-second depolarizing voltage steps from a holding potential of ⫺80 mV.11,14 The rapidly (IKr) and slowly (IKs) activating components of delayed rectifier K⫹ current were measured as the E-4031– (5 ␮M; Wako, Osaka Japan)15 and chromanol 293B– (C293B, 10 ␮M; Hoechst AG, Frankfurt Germany)16,17 sensitive tail current components, respectively. Transient outward current (Ito) and inwardly rectifying current (IK1) were recorded and analyzed as previously described.18,19 Normalized data from multiple cells were pooled to determine fitted parameters, including time constants and midpoint potentials.

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Paired echocardiographic measurements in five complete AV block rabbits paced at 140 bpm Day 0

Right ventricular diastolic chamber dimension (cm) Interventricular septal diastolic wall thickness (cm) Left ventricular diastolic chamber dimension (cm) Left ventricular posterior wall thickness in diastole (cm) Left ventricular systolic ejection fraction (%) Left ventricular systolic fractional shortening (%) Cardiac output (L/min)

0.60 0.30 1.78 0.26 59.4 28.2 0.92

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

Day 8 0.14 0.07 0.19 0.06 13.4 6.7 0.17

0.72 0.30 1.84 0.28 54.2 27.0 0.74

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

P value 0.22 0.07 0.09 0.11 12.1 6.7 0.11

Data analysis

Results

Results are reported as mean ⫾ SE. Curve fitting and statistical procedures were performed using Origin 7.0 (Microcal Software, Northampton, MA, USA), yielding time constants and midpoint potentials for pooled data where appropriate. Statistical analysis was based on the paired or unpaired Student’s t-test or one-way ANOVA followed by Bonferroni test for multiple pairwise comparisons. P ⬍ .05 was considered significant.

Absence of heart failure and ventricular hypertrophy

NS NS NS NS NS NS NS

None of the animals in the control group or in the group with complete AV block paced at either 140 or 280 bpm showed evidence of heart failure, such as reduced oral intake or intrathoracic effusions at cardiectomy. Echocardiographic assessment on days 0 and 8 of complete AV block revealed no significant change in LV dimensions or function in animals paced chronically at either 140 bpm (Table 1) or 280 bpm (not shown). Moreover, cell membrane capacitance recordings showed no change in individual RV or LV myocyte size (Figure 1).

QT interval prolongation Rabbits paced at 140 bpm demonstrated significant QT interval prolongation from 206 ⫾ 7 ms immediately after complete AV block induction to 230 ⫾ 6 ms 8 days later (P ⬍ .001; Figure 2A). No significant QT interval prolongation was observed during this time interval in rabbits paced at 280 bpm (200 ⫾ 6 ms on day 0 vs 196 ⫾ 5 ms on day 8, P ⫽ .57; Figure 2B). Considering the possibility that repolarization could be altered by the paced ventricular activation sequence in a

Figure 1 Cell capacitance measurements obtained in the wholecell patch clamp configuration in right ventricular (RV) and left (LV) ventricular myocytes of control rabbits (cont) and complete AV block (CAVB) rabbits paced for 8 days at either 280 or 140 bpm. No significant differences were noted among control and paced CAVB categories within each ventricle (i.e., RV or LV), indicating cell sizes did not change appreciably in response to CAVB, bradycardia, and/or pacing. However, group-specific comparisons between LV and RV myocytes (e.g., control LV vs control RV) yielded significant differences (P ⬍ .01) in the control group and in CAVB rabbits paced at 140/min, suggesting effective separation of RV from LV myocytes. Number of myocytes is indicated on each bar.

Figure 2 QT interval measurements obtained on days 0 and 8 after complete AV block induction from six-lead surface ECG recorded during steady-state acute pacing at 140 bpm in anesthetized individual animals paced chronically at either 140 bpm (A) or 280 bpm (B). Complete QT interval data as shown were available for 15 of 16 rabbits chronically paced at 140 bpm and for 7 of 9 rabbits chronically paced at 280 bpm. Two points joined by a line represent paired measurements recorded in one animal.

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Heart Rhythm, Vol 2, No 3, March 2005 myocytes from rabbits paced at 140 bpm was significantly smaller than that in control rabbits. Tail current densities in myocytes of rabbits paced at 280 bpm were almost indistinguishable from those of control rabbits. Therefore, specific analysis of IKr and IKs was restricted to comparison between control myocytes and those from complete AV block rabbits paced at 140 bpm (Figure 4).

No significant change in Ito or IK1

Figure 3 Pause-dependent torsades de pointes (TdP) initiation. Tracing shows complete AV block with dissociated atrial beats (A) occurring regularly at a sinus cycle length of 210 ms. Ventricular pacing spikes (PS) occur at the programmed interval of 430 ms. The first two pacing spikes in the series are followed by QRS complexes, but the third pacing spike (PS*) fails to capture the ventricle, resulting in a prolonged pause (“long” interval in “longshort” sequence). After the fourth pacing spike once again captures the ventricle, TdP onset terminates the “short” interval in the “long-short” sequence. TdP electrograms transiently inhibit the pacemaker. The record demonstrates numerous similar episodes, ultimately deteriorating to ventricular fibrillation and then asystole.

rate-dependent and/or time-dependent fashion, either directly or via the remodeling process, we compared QRS complex durations on days 0 and 8 during pacing at 140 and 280 bpm in seven animals paced chronically at 280 bpm. Although very slight and statistically insignificant rate-related QRS complex shortening was observed on day 0 (81 ⫾ 3 ms at 140/min vs 78 ⫾ 3 ms at 280/min) and day 8 (81 ⫾ 3 ms at 140/min vs 79 ⫾ 2 ms at 280/min), no discernible timerelated change in QRS duration at either pacing rate was noted. We also measured QRS complex duration on days 0 and 8 during chronic pacing at 140 bpm in 13 rabbits. Once again, no significant change (82 ⫾ 5 ms on day 0 vs 81 ⫾ 7 ms on day 8) was observed, confirming that QT interval prolongation in these bradycardic rabbits was attributable solely to delayed repolarization.

TdP on continuous telemetric ECG monitoring We encountered sporadic cases of late postprocedural sudden death in complete AV block rabbits with no apparent antecedent illness or significant postmortem findings. Selected complete AV block animals subsequently underwent implantation of telemetric ECG monitoring devices. In one complete AV block rabbit paced at 140 bpm and found dead in its cage early on the morning of day 8, multiple episodes of ultimately lethal TdP with pause-dependent onset were recorded (Figure 3).

Reduced IKr and IKs expression after 8 days of bradycardic pacing The total delayed rectifier tail current density at membrane test potentials positive to ⫹20 mV in RV and LV

The transient outward K⫹ current Ito is down-regulated in a number of pathophysiologic states.20 We found no significant difference in RV or LV Ito peak current density between control and complete AV block cardiac myocytes paced at either 140 or 280 bpm (Figure 5A–C). Voltage dependence of steady-state Ito inactivation and time dependence of Ito recovery from inactivation did not differ among the three groups (not shown). The inwardly rectifying current IK1 contributes importantly to the final phases of myocyte action potential repolarization and to maintenance of the resting diastolic transmembrane potential. IK1 current densities were similar in all control and complete AV block myocytes obtained from either ventricle (Figure 5D and F).

Discussion Clinical association between bradycardia and impaired repolarization QT interval prolongation and associated predisposition to TdP have been well documented in bradycardic patients with congenital4 and acquired1–3 complete AV block and in other patients with bradycardia attributed to antiarrhythmic medication use and central nervous system injury.5 Although the efficacy of ventricular pacing for TdP suppression has long been recognized empirically,5 the role of bradycardic electrical remodeling in pathogenesis of repolarization delay and TdP predisposition is the subject of much more recent investigations in animal models (discussed later). Interestingly, in a comparison of eight complete AV block patients who experienced TdP prior to initiation of chronic pacing with six patients who had no TdP, Kurita et al1 found that significant QT interval prolongation could still be unmasked at acute pacing rates less than 60 bpm after at least 2 months of chronic pacing at physiologic rates only in the TdP group but not in the TdP-free group. They concluded that the TdP patients had inherently abnormal repolarization responses to bradycardia.1 An alternative hypothesis related to our findings is that bradycardic electrical remodeling can be prevented by physiologic rate pacing but, once established, is slow and/or difficult to reverse. This hypothesis is supported by the findings of Peschar et al,21 who found persistent repolarization abnormalities in dogs undergoing institution of ventricular pacing after 8 weeks of

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Relationship between ventricular hypertrophy and bradycardic electrical remodeling Myocardial hypertrophy occurs in response to chronic bradycardia and is associated with altered ventricular repolarization.20 Electrical remodeling in chronic complete AV block has been linked with echocardiographic and morphometric evidence of ventricular hypertrophy, prompting some investigators to suggest repolarizing current downregulation is an expression of myocyte hypertrophy.7,9,22 However, at least two reports have demonstrated a temporal if not causative dissociation between hypertrophy and repolarization abnormalities in the intact dog with complete AV block. Peschar et al21 showed persistence of repolarization abnormalities in the face of resolution of bradycardia-induced hypertrophy following institution of RV pacing in their complete AV block dogs. Meanwhile, Schoenmakers et al23 found QT interval prolongation and TdP inducibility that were disproportionate to the degree of cardiac hypertrophy after 2 weeks of complete AV block without pacing.

Figure 4 Delayed rectifier currents in isolated myocytes. A: Representative family of tracings elicited with a series of 3-second depolarizing voltage steps from a holding potential of ⫺80 mV, followed by repolarization to ⫺30 mV to elicit outward tail current. Alternating tracings elicited at 10-mV increments are omitted for clarity. Inset on this and subsequent raw tracings schematically illustrates voltage protocol used to elicit currents. B, C: Current density-voltage relations for total right ventricular (B) and left ventricular (C) tail currents in myocytes obtained from control rabbits or complete AV block rabbits paced for 8 days at either 280 or 140 bpm. *P ⬍ .05, complete AV block rabbits paced at 140 beats/min vs. control. **P ⬍ .05, complete AV block rabbits paced at 140 beats/min vs control and vs complete AV block rabbits paced at 280 bpm. D: Representative tracing of delayed rectifier activating and tail currents with step potential of ⫹30 mV, illustrating current reduction by extracellular exposure to 5 ␮M E-4031 alone, followed by the combination of 5 ␮M E-4031 plus 10 ␮M chromanol 293B (C293B). The E-4031–sensitive tail current is understood to represent IKr, whereas the component blocked by addition of C293B is considered to represent IKs. E, F: Current density-voltage relations for IKr and IKs in right ventricular (E) and left ventricular (F) myocytes obtained from control rabbits or complete AV block rabbits paced for 8 days at 140 bpm. *P ⬍ .05, **P ⬍ .01 complete AV block rabbits paced at 140 bpm vs control. Numbers in parentheses indicate number of data points (myocytes) for each group.

complete AV block without pacing. Unfortunately, Kurita et al did not report prepacing complete AV block or bradycardia duration in their patients.

Figure 5 Transient outward current Ito and inwardly rectifying current IK1 in isolated myocytes. A: Representative family of Ito tracings. B, C: Peak current density-voltage relations for Ito in right ventricular (B) and left ventricular (C) myocytes from control rabbits or complete AV block rabbits paced for 8 days at either 280 or 140 bpm. D: Representative family of IK1 tracings. E, F: Peak current density-voltage relations for IK1 in right ventricular (RV) (E) and left ventricular (LV) (F) myocytes.

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Temporal dissociation between structural remodeling and cardiac myocyte repolarizing K⫹ channel expression has been observed in nonbradycardic models, including postmyocardial infarction rat ventricle,24 pressure-overload mouse heart,25 and canine ventricular tachypacing-induced congestive heart failure.26 In our rabbit model with variably rate-compensated ventricular bradycardia, ECG and cellular manifestations of repolarizing current down-regulation are already well established 8 days after complete AV block induction, at which time no evidence of significant ventricular hypertrophy is seen. These observations support the notion that electrical remodeling can proceed independently of structural remodeling.

finding that again could be related to the degree and/or duration of bradycardia. Of note, seven of their nine animals from which myocytes were obtained exhibited evidence of heart failure, including ascites and pleural effusions. Heart failure was not apparent in any of the rabbits in our study. We documented significant biventricular reductions in IKr and especially IKs expression after 8 days of bradycardic pacing. Our finding of LV IKr down-regulation is consistent with that reported by Tsuji et al.8 The apparent discrepancy relative to canine data recorded in midmyocardial myocytes7 might be related to interspecies differences.

QT interval prolongation and predisposition to TdP

Chronic ventricular pacing affects the expression of canine cardiac myocyte repolarizing currents, especially Ito,28,29 and may play a role in the repolarization changes reported here. However, some evidence argues against this theory. The observed alterations in ionic current expression did not differ significantly between the LV and RV in our paced rabbits or with respect to changes reported by other investigators in nonpaced complete AV block rabbits.8 Moreover, changes induced by electrical stimulation would predictably be accentuated by the frequency of stimulation, whereas our complete AV block rabbits paced at 280 bpm showed almost no evidence of repolarizing current remodeling in vivo or in vitro. One possible explanation for the apparent lack of pacinginduced Ito down-regulation in our model is related to the previous observation that Ito is thought to be carried chiefly by Kv1.4 in the rabbit,30 –32 whereas pacing-induced Ito downregulation in the dog ventricle is clearly attributable to reduced Kv4.3 expression.28,29 Evidence points to the importance of ␤-adrenergic stimulation in IKs expression, especially in the setting of reduced IKr.33 We did not assess or attempt to manipulate autonomic state in vivo, nor did we determine the ventricular myocyte IKs response to application of a ␤-adrenergic agonist such as isoproterenol. Given that IKr is down-regulated in bradycardic animals and that the stress of bradycardia likely enhances their intrinsic autonomic state, residual IKs may play a more prominent repolarization “rescue” role in these animals than suggested by its marked reduction in unstimulated myocytes. The possibility of a temporal or mechanistic link between ventricular electrical remodeling and functional hemodynamic adaptation to chronic bradycardia was not explored in this study. The significance of the apparent decline in echocardiographically estimated cardiac output in bradycardic animals between days 0 and 8 (Table 1), as it might relate to the remodeling process, is unclear.

Volders et al7 demonstrated progressive repolarization delay over a 9-week time course in their canine chronic complete AV block model and marked susceptibility to TdP induction.22 Spontaneous lethal TdP has been recorded.27 Tsuji et al8 observed significant QT interval prolongation and a high incidence of spontaneous TdP among 34 rabbits after 3 weeks of complete AV block. In the present study, we demonstrate significant QT interval prolongation in complete AV block rabbits paced for 8 days at half the usual physiologic rate but not in rabbits paced at a nearly normal rate. Because all QT interval measurements were made at the same paced ventricular rate, they are more directly and rigorously comparable between groups than data obtained from complete AV block animal models with variable intrinsic ventricular escape rates. Although provocative evaluation of TdP inducibility was not undertaken in our rabbits, spontaneous pause-dependent TdP was documented. The apparently low incidence of TdP in our rabbits compared with the 71% incidence reported by Tsuji et al8 may be ascribed to the more profound degree of bradycardia (mean ventricular rate ⬃ 75 bpm vs 140 bpm in our bradycardic group) and longer duration of follow-up (21 vs 8 days) in their animals. The great majority of TdP episodes in their animals occurred beyond the first week of bradycardia.8

Alterations in ventricular myocyte repolarizing currents Reduced expression of ventricular myocyte IKr and IKs is an established feature of chronic bradycardic remodeling in the dog7 and the rabbit.8 Volders et al7 noted substantial biventricular IKs reduction, with IKr down-regulation limited to the RV in their canine model. Tsuji et al8 documented comparable reductions of approximately 50% in both IKr and IKs expression in LV apical myocytes of rabbits maintained at profoundly bradycardic intrinsic ventricular escape rates for 3 weeks. No significant changes in Ito were documented. They did report increased IK1 expression in bradycardic rabbits, a

Study limitations

Conclusion Ventricular electrical remodeling in acquired heart block is dependent on bradycardia and independent of AV synchrony or heart failure. This is of particular interest in light

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of large-scale randomized clinical trials showing no significant difference in cardiovascular mortality between patients treated with ventricular pacing and those receiving dual-chamber pacing.34 We observed changes in repolarization early after onset of bradycardia and prior to the appearance of hypertrophic structural remodeling. All of these changes were fully prevented by artificial maintenance of a near-physiologic ventricular activation rate, indicating that ventricular rate is the primary determinant of electrical remodeling in chronic bradycardia.

Acknowledgments We thank Marvin Estrada for expert technical assistance. Permanent pacing equipment was generously provided by Medtronic Canada Ltd. C293B was a gift from Dr. Uwe Gerlach and Dr. Hans-Jochen Lang of Aventis AG.

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