Propagation velocity kinetics and repolarization alternans in a free-behaving sheep model of pacing-induced atrial fibrillation
1 Service of Cardiology, BH10-982, CHUV, Rue du Bugnon 46M, 1011 Lausanne, Switzerland; 2 Signal Processing Institute, EPFL, 1015 Lausanne, Switzerland; 3 Service of Cardiovascular Surgery, CHUV, 1011 Lausanne, Switzerland
* Corresponding author. Tel: +41 21 3140104; fax: +41 21 3140013.E-mail address: etienne.pruvot{at}chuv.ch
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Abstract |
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Aims: Experimental models have reported conflicting results regarding the role of dispersion of repolarization in promoting atrial fibrillation (AF). Repolarization alternans, a beat-to-beat alternation in action potential duration, enhances dispersion of repolarization when propagation velocity is involved.
Methods and results: In this work, original electrophysiological parameters were analysed to study AF susceptibility in a chronic sheep model of pacing-induced AF. Two pacemakers were implanted, each with a single right atrial lead. Right atrial depolarization and repolarization waves were documented at 2-week intervals. A significant and gradual decrease in the propagation velocity at all pacing rates and in the right atrial effective refractory period (ERP) was observed during the weeks of burst pacing before sustained AF developed when compared with baseline conditions. Right atrial repolarization alternans was observed, but because of the development of 2/1 atrioventricular block with far-field ventricular interference, its threshold could not be precisely measured. Non-sustained AF was not observed at baseline, but appeared during the electrical remodelling in association with a decrease in both ERP and propagation velocity.
Conclusion: We report here on the feasibility of measuring ERP, atrial repolarization alternans, and propagation velocity kinetics and their potential in predicting susceptibility to AF in a free-behaving model of pacing-induced AF using the standard pacemaker technology.
Key Words: Atrial fibrillation, Repolarization alternans, Remodelling, Pacemaker
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Introduction |
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High-frequency focal discharges have been involved in the initiation of paroxysmal1
and maintenance of permanent2 atrial fibrillation (AF) in humans. Some patients, however, evolve rapidly towards sustained AF, whereas others may exhibit only bouts of atrial tachycardia for years before AF settles (if any). The mechanical and electrical remodelling process that promote AF in humans is not yet fully understood.3,4 It has been shown experimentally that most re-entrant arrhythmias require a critical amount of dispersion of refractory periods5 and a critical slowing of propagation velocity.6 Some experimental models are based on the induction of AF by means of stimulation of vagal trunks, which has been shown to increase dispersion of refractory periods.7 In contrast, in pacing-induced models of AF that mimic atrial high-frequency foci,8,9 the increase in AF susceptibility over time is not paralleled by any increase in dispersion of refractory periods.9 Measurements of the refractory period, however, have been performed by using a single extra stimulus. This technique bears its own limitation because it cannot evaluate any increase in dispersion of repolarization that may arise at rapid heart rates. Repolarization alternans, a beat-to-beat alternation in action potential duration, enhances dispersion of refractory periods dynamically10 when propagation velocity is involved,11,12 is associated with an increase risk of ventricular arrhythmias,13 and involves intracellular Ca2+ alternans.14 It is unknown whether repolarization alternans plays a role in maintaining AF and whether the involvement of propagation velocity at slower heart rates decreases atrial repolarization alternans threshold. We hypothesize that the increased susceptibility to AF that takes place during the time course of pacing-induced atrial electro-anatomical remodelling is associated with a gradual reduction in the atrial repolarization alternans threshold and a gradual decrease in the atrial propagation velocity. |
Methods |
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Our experimental protocol complies with and has been approved by the Swiss Federal Veterinary Office. Two pacemakers (VitatronTM), each with a single lead screwed into the right atrium (RA, Figure 1 A), were implanted in five sheep. The first pacemaker was used to record a broadband (sampling frequency 800 Hz and 0.4 Hz high-pass filter) unipolar atrial electrogram (EGM), as shown in Figure 1 B. The pacemaker impulse (S) is followed by right atrial depolarization (Ra) and repolarization (Ta) waves. A far-field ventricular depolarization (Rv) is also seen. Electrogram and subcutaneous ECG were recorded with a Holter device and transmitted to a computer by Bluetooth. The second pacemaker was used to deliver long-term intermittent burst pacing and electrophysiology (EP) protocols.
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Signal analysis
In order to extract different parameters (repolarization alternans threshold and propagation velocity), identification of timing of each event must first be extracted from EGM signals. These different types of events are pacemaker impulse, atrial depolarization and repolarization waves, and far-field ventricular depolarization wave. The pacemaker impulse constitutes the highest frequency component of the EGM. The impulse detection is carried out as follows: (i) application of a high-pass filter to the EGM signal. A Chebyshev filter was used (cutoff frequency 180 Hz); (ii) application of a threshold (=85% of the maximum amplitude of the filtered signal); (iii) identification of pacemaker impulses as local maxima. Atrial depolarization waves were then identified as the local minima (window length of 50 ms), following the pacemaker impulse. A template matching approach was used to identify the timing of T-waves. In order to reduce noise and artefacts, a fourth-order polynomial was fitted on each T wave segment. A threshold (85% of maximal amplitude) was applied to the subcutaneous ECG to detect the far-field ventricular depolarization. They were then identified by the local maxima.
Experimental procedure
Three different pacing protocols, named S1S1, S1S2, and burst pacing, were used in the experiments. The first two were measurement protocols and the last one was used for electrical remodelling of the atria until sustained AF. The S1S1 protocol, used to determine atrial repolarization alternans threshold, included trains of 400 beats starting at 400 ms, with 10 ms decrement until loss of 1/1 atrial capture. The S1S2 protocol, used to determine restitution of atrial activation time (AT), consisted of 20 beats at 400 ms (S1), followed by delivery of a pre-mature beat (S2) decremented by 10 ms until loss of atrial capture. Activation time was defined as the time interval measured between the S2 impulse (S) and the following atrial depolarization (Ra) for each S1S2 interval. The RA effective refractory period (ERP) was defined as the longest S1S2 interval that failed to capture the RA. The pacing protocol leading to sustained AF consisted of intermittent sequences of burst pacing of 5 s duration followed by 2 rest period, mimicking AF triggered by high-frequency focal discharges.1 In two sheep, the burst pacing interval was programmed at 10 ms above the RA ERP as measured with the S1S2 protocol during the time course of pacing-induced atrial remodelling; 1/1 RA capture was ascertained before animal discharge. Measurements of RA ERP and propagation velocity made before burst pacing activation were taken as baseline conditions. The sheep were studied at 2-week intervals for 3 months, or until sustained AF was observed in those submitted to burst pacing.
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Results |
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None of the five implanted sheep developed non-sustained AF at baseline (i.e. before burst pacing protocol activation) during measurement protocols (S1S1 and S1S2). Burst pacing was activated in two sheep in which sustained AF was successfully induced after 4 and 6 weeks, respectively. Interestingly, both sheep developed non-sustained AF after 2 weeks of burst pacing during both protocols for coupling intervals between 140 ms and atrial ERP (
110 ms), as shown in Figure 2.
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Activation time kinetics and effective refractory period
Pacing threshold and voltage amplitude (2.5 V at 0.5 ms) remained stable over time in the two sheep undergoing burst pacing. Activation time and its restitution kinetics were determined on a 2-week basis in three sheep at baseline and in the two of them in which sustained AF developed during burst pacing. Figure 3 A shows a representative example of AT measurements. AT remained stable for a large range of S1S2 intervals; an increase was noted (corresponding to a slowing of propagation velocity) as the S1S2 interval reached the last 30 ms before RA ERP. Figure 3 B shows that RA ERP gradually decreased during the time course of burst pacing from 160 ms at baseline down to 90 ms after 4 weeks of pacing, corresponding to an overall 40% reduction. Figure 3 B also shows a representative example of AT restitution kinetics at baseline and after 2 and 4 weeks of burst pacing before sustained AF was induced. Each AT curve was fitted by an exponential function using a minimum least-squares approach: y=a exp (–x/
)+c, where x is the S1S2 interval and y is the corresponding AT. The amplitude a, the offset c, and the time constant are the parameters to be estimated. The offset c corresponding to the AT at rest (i.e. 400 ms pacing CL) remained stable for a large range of S1S2 intervals before and during electrical remodelling. Importantly, AT increased progressively during the time course of burst pacing from 13 ms at baseline to 17 ms and 21 ms after 15 and 30 days of electrical remodelling, respectively, indicative of a gradual slowing of propagation velocity. In addition, the amplitude a increased with burst pacing, which illustrates a further slowing of propagation velocity for coupling intervals close to RA ERP; AT measured 25 ms at baseline increased to 30 and 44 ms after 15 and 30 days of burst pacing, respectively, corresponding to 70% increase. No significant evolution in the time constant was observed with burst pacing, suggesting that the range of S1S2 intervals at which AT increased remained stable over time. Knowing the distance between pacing and recording electrodes, we were able to estimate propagation velocity over time. Of note, no significant change in interelectrode distance was observed during the time course of electrical remodelling, but our ability to measure subtle changes was limited. A summary of the ERP and propagation velocity values before and during burst pacing is presented in Table 1. At baseline, the mean propagation velocity and ERP were 103 cm/s and 150 ms, respectively; both variables progressively decreased during the time course of burst pacing. Propagation velocity reached 103 cm/s at baseline and decreased to 88 and 66 cm/s after 2 and 4 weeks of burst pacing, respectively. Importantly, propagation velocity further decreased for S1S2 interval 10 ms above RA ERP, from 61 cm/s at baseline to 47 and 33 cm/s after 2 and 4 weeks of pacing, respectively.
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Atrial repolarization alternans
In two of the four implanted sheep, alternans of atrial repolarization was reliably observed during S1S1 at coupling intervals near atrial ERP during 4/1 AV block. Figure 4 A shows a representative example of atrial repolarization alternans. The top row shows a bipolar subcutaneous ECG derivation with 4/1 AV block. The bottom row shows the intracardiac EGM. Note the alternation of atrial repolarization (Ta) as emphasized by the arrows, and the far-field ventricular depolarization took place every four atrial beats. In general, atrial repolarization alternans and its threshold could not be established because of 2/1 atrioventricular block with far-field ventricular interference impinging on the atrial repolarization wave every-other-beat at most rapid pacing rates. In some instances, repolarization alternans appeared unstable during 2/1 AV block with periods of overt alternation subsiding progressively until next episode. In some episodes, the relationship between the pacing stimulus and the far-field ventricular activation was constant, suggesting that repolarization wave truly alternated as shown in Figure 4 B.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionFundingAcknowledgementsReferences |
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We report here for the first time in a free-behaving sheep model of pacing-induced AF the feasibility of recording atrial ERP, alternation of RA repolarization, and propagation velocity using the standard pacemaker technology. Atrial repolarization alternans was observed near RA ERP; its threshold, however, could not be reliably determined because of far-field ventricular interference. In addition, other important changes took place during the time course of pacing-induced remodelling, including a gradual decrease in the atrial ERP over time associated with a progressive reduction in propagation velocity at any pacing rates, with an additional marked slowing near atrial ERP.
Dispersion of repolarization, repolarization alternans, and susceptibility to re-entrant arrhythmias
It has been shown experimentally that re-entrant arrhythmias require a critical amount of dispersion of refractory periods1,2 to settle. Former models of pacing-induced AF used continuous atrial burst pacing to remodel the atria. A decrease in ERP and propagation velocity and an increase in atrial size were reported over time that paralleled the susceptibility to AF,8 but no change in dispersion of refractory periods was noticed.9 Measurements of dispersion of refractory periods, however, were made during steady-state conditions, i.e. with delivery of a single premature beat to determine and compare atrial ERP at different atrial locations. In contrast, models based on vagal nerve trunks stimulation to promote AF showed a positive association between the amount of dispersion of repolarization and AF susceptibility and duration.7 Repolarization alternans, a beat-to-beat alternation in action potential duration, enhances dispersion of refractory periods and susceptibility to re-entrant ventricular arrhythmias.10 Importantly, repolarization alternans is a rate-dependant phenomenon that appears above critical heart rates.10,13,14 In patients with atrial flutter, those presenting alternans of action potential duration (APD) were more susceptible to AF at high pacing rates than those without alternans.15 Furthermore, atrial cells are used to study the cellular mechanisms involved in APD alternans; their susceptibility to repolarization alternans has been attributed to the paucity of T-tubules.16
Our findings demonstrate the feasibility of measuring in vivo repolarization alternans based on unipolar signals as provided by the standard pacemaker technology. Atrial repolarization alternans was reliably seen at high pacing rates during 4/1 AV block. In addition, repolarization alternans appeared intermittent as shown in Figure 4 B, which suggests variation of propagation velocity on a longer time scale.12
Intermittent repolarization alternans has been observed clinically17 and is associated with discordant alternans,12 a condition of high arrhythmogenicity.18 However, we were not able to establish whether repolarization alternans threshold decreased during the time course of atrial remodelling because of far-field ventricular interference. Nevertheless, our findings suggest that atrial repolarization alternans might be a mechanism by which dispersion of repolarization transiently increases, promoting wavebreaks and AF at rapid rates.
Propagation velocity and susceptibility to atrial fibrillation
Kuo et al.6 reported that besides a critical amount of dispersion of refractory periods, re-entrant arrhythmias require a critical slowing of propagation velocity to settle. Others reported subtle slowing of propagation velocity, especially near ERP following atrial burst pacing that tended to persist after restoration of sinus rhythm.9 Acute dilation of the RA produced a slowing of propagation velocity only at rapid pacing rates, whereas conduction did not change at slower rates.19 In chronic models of AV block, propagation velocity remained largely unchanged, whereas duration of burst-induced AF increased when compared with control animals.20 Importantly, area of slow conduction and conduction block were only observed during rapid pacing or during AF but were not seen at slower rates.20,21 Our preliminary results suggest that alteration of propagation velocity may be an important contributor to the susceptibility to AF in a model based on intermittent burst pacing simulating high-frequency focal discharges. Propagation velocity decreased over time at slow pacing rates, but it displayed a much greater reduction as atrial ERP declined. Therefore, the gradual decrease in ERP may promote slow conduction for short-coupled intervals, a mechanism by which atrial premature beats may start propagating across the atria. Although we did not observe any difference in interelectrode distance, our ability to measure subtle changes was limited. Thus, an increase in atrial size due to atrial burst pacing8 may have contributed in part to the slowing of propagation velocity observed in the present study. The range of propagation velocity slowing during delivery of extrastimuli, however, increased over time, as shown in Figure 3, a measurement independent of any increase in atrial size. These findings, nevertheless, need to be confirmed using accurate measurements of atrial size and interelectrode distance.
Propagation velocity and repolarization alternans
Interestingly, Qu et al.11 showed in simulated tissues that propagation velocity had to be engaged for discordant alternans to take place. Later on, these findings were confirmed experimentally in strands of Purkinje fibres.12 Only discordant alternans, where islands of APD were out of phase, increased dispersion of repolarization to the point where wavebreaks and re-entry could emerge.11,18 In the present study, propagation velocity declined gradually during the time course of electroanatomical remodelling, with a steep slowing near atrial ERP that paralleled susceptibility to AF. Because atrial repolarization alternans was measured in a single point and its threshold could not be determined reliably, the role of repolarization alternans and propagation velocity in enhancing dispersion of refractory periods and susceptibility to AF in vivo could not be established. Future studies are warranted in which ventricular rate will be controlled by AV block. Figure 5 shows the feasibility of ablating the AV junction in one representative animal. In conclusion, these preliminary results show the feasibility of measuring ERP, atrial repolarization alternans, and propagation velocity kinetics and their potential in predicting susceptibility to AF in a free-behaving model of pacing-induced AF using the standard pacemaker technology.
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This work was supported in part by The Swiss National Fund for Scientific Research, The CardioMet Pôle, and Vitatron.
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Acknowledgements |
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The authors wish to thank A. van Oosterom for constructive criticism and suggestions on the manuscript.
Conflict of interest: none declared.
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References |
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