Mechanism of spontaneous termination of stable atrial flutter in the canine sterile pericarditis model

Machine learning-derived cycle length variability metrics predict spontaneously terminating ventricular tachycardia in implantable cardioverter defibrillator recipients

Aims

Implantable cardioverter defibrillator (ICD) therapies have been associated with increased mortality and should be minimized when safe to do so. We hypothesized that machine learning-derived ventricular tachycardia (VT) cycle length (CL) variability metrics could be used to discriminate between sustained and spontaneously terminating VT.

Methods and results

In this single-centre retrospective study, we analysed data from 69 VT episodes stored on ICDs from 27 patients (36 spontaneously terminating VT, 33 sustained VT). Several VT CL parameters including heart rate variability metrics were calculated. Additionally, a first order auto-regression model was fitted using the first 10 CLs. Using features derived from the first 10 CLs, a random forest classifier was used to predict VT termination. Sustained VT episodes had more stable CLs. Using data from the first 10 CLs only, there was greater CL variability in the spontaneously terminating episodes (mean of standard deviation of first 10 CLs: 20.1 ± 8.9 vs. 11.5 ± 7.8 ms, P < 0.0001). The auto-regression coefficient was significantly greater in spontaneously terminating episodes (mean auto-regression coefficient 0.39 ± 0.32 vs. 0.14 ± 0.39, P < 0.005). A random forest classifier with six features yielded an accuracy of 0.77 (95% confidence interval 0.67 to 0.87) for prediction of VT termination.

Conclusion

Ventricular tachycardia CL variability and instability are associated with spontaneously terminating VT and can be used to predict spontaneous VT termination. Given the harmful effects of unnecessary ICD shocks, this machine learning model could be incorporated into ICD algorithms to defer therapies for episodes of VT that are likely to self-terminate.

Inflammatory signalling in atrial cardiomyocytes: a novel unifying principle in atrial fibrillation pathophysiology

Inflammation has been implicated in atrial fibrillation (AF), a very common and clinically significant cardiac rhythm disturbance, but its precise role remains poorly understood. Work performed over the past 5 years suggests that atrial cardiomyocytes have inflammatory signalling machinery - in particular, components of the NLRP3 (NACHT-, LRR- and pyrin domain-containing 3) inflammasome - that is activated in animal models and patients with AF. Furthermore, work in animal models suggests that NLRP3 inflammasome activation in atrial cardiomyocytes might be a sufficient and necessary condition for AF occurrence. In this Review, we evaluate the evidence for the role and pathophysiological significance of cardiomyocyte NLRP3 signalling in AF. We first summarize the evidence for a role of inflammation in AF and review the biochemical properties of the NLRP3 inflammasome, as defined primarily in studies of classic inflammation. We then briefly consider the broader evidence for a role of inflammatory signalling in heart disease, particularly conditions that predispose individuals to develop AF. We provide a detailed discussion of the available information about atrial cardiomyocyte NLRP3 inflammasome signalling in AF and related conditions and evaluate the possibility that similar signalling might be important in non-myocyte cardiac cells. We then review the evidence on the role of active resolution of inflammation and its potential importance in suppressing AF-related inflammatory signalling. Finally, we consider the therapeutic potential and broader implications of this new knowledge and highlight crucial questions to be addressed in future research.

A Novel Tool for the Identification and Characterization of Repetitive Patterns in High-Density Contact Mapping of Atrial Fibrillation

Introduction

Electrical contact mapping provides a detailed view of conduction patterns in the atria during atrial fibrillation (AF). Identification of repetitive wave front propagation mechanisms potentially initiating or sustaining AF might provide more insights into temporal and spatial distribution of candidate AF mechanism and identify targets for catheter ablation. We developed a novel tool based on recurrence plots to automatically identify and characterize repetitive conduction patterns in high-density contact mapping of AF.

Materials and Methods

Recurrence plots were constructed by first transforming atrial electrograms recorded by a multi-electrode array to activation-phase signals and then quantifying the degree of similarity between snapshots of the activation-phase in the electrode array. An AF cycle length dependent distance threshold was applied to discriminate between repetitive and non-repetitive snapshots. Intervals containing repetitive conduction patterns were detected in a recurrence plot as regions with a high recurrence rate. Intervals that contained similar repetitive patterns were then grouped into clusters. To demonstrate the ability to detect and quantify the incidence, duration and size of repetitive patterns, the tool was applied to left and right atrial recordings in a goat model of different duration of persistent AF [3 weeks AF (3 wkAF, n = 8) and 22 weeks AF (22 wkAF, n = 8)], using a 249-electrode mapping array (2.4 mm inter-electrode distance).

Results

Recurrence plots identified frequent recurrences of activation patterns in all recordings and indicated a strong correlation between recurrence plot threshold and AF cycle length. Prolonged AF duration was associated with shorter repetitive pattern duration [mean maximum duration 3 wkAF: 74 cycles, 95% confidence interval (54-94) vs. 22 wkAF: 41 cycles (21-62), p = 0.03], and smaller recurrent regions within repetitive patterns [3 wkAF 1.7 cm² (1.0-2.3) vs. 22 wkAF 0.5 cm² (0.0-1.2), p = 0.02]. Both breakthrough patterns and re-entry were identified as repetitive conduction patterns.

Conclusion

Recurrence plots provide a novel way to delineate high-density contact mapping of AF. Dominant repetitive conduction patterns were identified in a goat model of sustained AF. Application of the developed methodology using the new generation of multi-electrode catheters could identify additional targets for catheter ablation of AF.

Self-restoration of cardiac excitation rhythm by anti-arrhythmic ion channel gating

Homeostatic regulation protects organisms against hazardous physiological changes. However, such regulation is limited in certain organs and associated biological processes. For example, the heart fails to self-restore its normal electrical activity once disturbed, as with sustained arrhythmias. Here we present proof-of-concept of a biological self-restoring system that allows automatic detection and correction of such abnormal excitation rhythms. For the heart, its realization involves the integration of ion channels with newly designed gating properties into cardiomyocytes. This allows cardiac tissue to i) discriminate between normal rhythm and arrhythmia based on frequency-dependent gating and ii) generate an ionic current for termination of the detected arrhythmia. We show in silico, that for both human atrial and ventricular arrhythmias, activation of these channels leads to rapid and repeated restoration of normal excitation rhythm. Experimental validation is provided by injecting the designed channel current for arrhythmia termination in human atrial myocytes using dynamic clamp.

Fetal Atrial Flutter: Electrophysiology and Associations With Rhythms Involving an Accessory Pathway

BACKGROUND

Atrial flutter (AFl) accounts for up to one third of all fetal tachyarrhythmias and can result in premature delivery, hydrops, and fetal death in 10% of cases; however, the electrophysiology of AFl in utero is virtually unstudied.

METHODS AND RESULTS

In this observational study, we reviewed 19 fetal magnetocardiography studies from 16 fetuses: 15 fetuses (21-38 weeks' gestation) referred with an echocardiographic diagnosis of AFl and 1 fetus (20 weeks' gestation) referred with a diagnosis of tachycardia that was shown by fetal magnetocardiography to have transient AFl in addition to atrioventricular reciprocating tachycardia. Thirteen fetuses showed AFl during the fetal magnetocardiography session, including 4 that presented prior to the third trimester. Five fetuses had incessant AFl; all but 1 of the others with AFl showed additional significant rhythms. Specifically, AFl showed a strong association with rhythms involving an accessory pathway: atrioventricular reciprocating tachycardia, blocked reentrant premature atrial contractions, and ventricular preexcitation. The observed initiations and terminations of AFl most often involved reentrant premature atrial contractions. Spontaneous termination of AFl showed AFl cycle length oscillations. Nine fetuses with 2:1 AFl also showed periods of 4:1 conduction or variable conduction that oscillated between 2:1 and 4:1; however, 3:1 AFl was relatively rare.

CONCLUSIONS

Fetal AFl can occur as early as midgestation and is often accompanied by atrioventricular reciprocating tachycardia and other rhythms associated with an accessory pathway. The findings depict critical differences in the electrophysiology of AFl in the fetus versus the neonate.

Termination of a tachyarrhythmia by flunarizine is not a specific marker for a triggered mechanism

BACKGROUND

Prior studies have indicated that tachyarrhythmia termination by flunarizine demonstrates a triggered mechanism. This concept was not confirmed in atrial tachyarrhythmias.

OBJECTIVE

The purpose of this study was to test the hypothesis that flunarizine will not terminate reentrant atrial flutter (AFL).

METHODS

We administered flunarizine (2 mg/kg intravenously over 2 minutes) in 11 episodes of reproducibly inducible, sustained AFL in eight canines with sterile pericarditis. If flunarizine terminated AFL, we studied AFL reinducibility. We also studied pacing thresholds, refractoriness, and intra-atrial conduction time during closed-chest studies and pacing at selected cycle lengths (CLs) from selected sites before and after flunarizine administration. Atrial mapping (510 electrodes) assessed the epicardial activation sequence during AFL and its termination in six episodes. Four AFL episodes were studied in the closed-chest state.

RESULTS

Flunarizine increased AFL CL in all episodes (mean 21 ms; range 7-49 ms), which is explained by slowing conduction in the AFL reentrant circuit, principally in the area of slow conduction. AFL was terminated in 10/11 episodes after drug initiation (mean 3.7 minutes; range 0.5-6.5 minutes) by block in the area of slow conduction. AFL was then not immediately reinducible until >20 minutes after drug administration. Flunarizine had no meaningful effect on atrial pacing thresholds for capture or refractoriness and only affected conduction time in the area of slow conduction in the reentrant circuit.

CONCLUSIONS

Flunarizine (1) causes progressive slowing and block in the area of slow conduction of the AFL reentrant circuit in the canine sterile pericarditis model and (2) is effective in terminating reentrant AFL and so is not a specific marker for a triggered mechanism.

The interrelationship between atrial fibrillation and atrial flutter

For a long time, it has been known that atrial fibrillation and atrial flutter have a close clinical interrelationship. Recent electrophysiological studies, especially mapping studies, have significantly advanced our understanding of this interrelationship. Regarding the relationship of atrial fibrillation with atrial flutter: Atrial fibrillation of variable duration precedes the onset of atrial flutter in almost all instances. During the atrial fibrillation, the functional components needed to complete the atrial flutter reentrant circuit, principally a line of block between the venae cavae, are formed. If this line of block does not form, classical atrial flutter does not develop. If this line of block shortens or disappears, classical atrial flutter disappears. In fact, it is fair to say that the major determinant of whether atrial fibrillation persists or classical atrial flutter develops is whether a line of block forms between the venae cavae. Regarding the relationship of atrial flutter with atrial fibrillation: Studies in experimental models and now in patients have demonstrated that a driver (a rapidly firing focus or a reentrant circuit of very short cycle length) can cause atrial fibrillation by producing fibrillatory conduction to the rest of the atria. When the driver is a stable reentrant circuit of very short cycle length, it is, in effect, a very fast form of atrial flutter. There probably is a spectrum of reentrant circuits of short cycle length, i.e., "atrial flutter," that depend, in part, on where the reentrant circuit is located. When the cycle length of the reentrant circuit is so short that it will only activate small portions of the atria in a 1:1 manner, the rest of the atria will be activated rapidly but irregularly, i.e., via fibrillatory conduction, resulting in atrial fibrillation. In short, there are probably several mechanisms of atrial fibrillation, one of which is due to a very rapid atrial flutter circuit causing fibrillatory conduction. In sum, atrial fibrillation and atrial flutter have an important interrelationship.

Interrelationships Between the Autonomic Nervous System and Atrial Fibrillation

Mechanisms responsible for atrial fibrillation are not completely understood but the autonomic nervous system is a potentially potent modulator of the initiation, maintenance, termination and ventricular rate determination of atrial fibrillation. Complex interactions exist between the parasympathetic and sympathetic nervous systems on the central, ganglionic, peripheral, tissue, cellular and subcellular levels that could be responsible for alterations in conduction and refractoriness properties of the heart as well as the presence and type of triggered activity, all of which could contribute to atrial fibrillation. These dynamic inter-relationships may also be altered dependent upon other neurohumoral modulators and cardiac mechanical effects from ventricular dysfunction and congestive heart failure. The clinical implications regarding the effects of the autonomic nervous system in atrial fibrillation are widespread. The effects of modulating ganglionic input into the atria may alter the presence or absence of atrial fibrillation as has been highlighted from ablation investigations. This article reviews what is known regarding the inter-relationships between the autonomic nervous system and atrial fibrillation and provides state of the art information at all levels of autonomic interactions.

Inter-relationships between atrial flutter and atrial fibrillation

It has been appreciated for a long time that atrial flutter and atrial fibrillation have a clinical relationship. Now, with the technological advances that permit more sophisticated electrophysiological studies, especially mapping studies, we have significantly advanced our understanding of this interrelationship. Regarding the relationship at atrial fibrillation to atrial flutter: Atrial fibrillation of variable duration (very brief to prolonged episodes) precedes the onset of atrial flutter in most instances. It seems that during the period of atrial fibrillation, the functional components of the atrial flutter reentrant circuit are formed. This is principally a line of block between the venae cavae. If this line of block does not form, classical atrial flutter does not form. And if this line of block shortens or disappears, classical atrial flutter disappear as well. In fact, it might be said that the major difference in whether classical atrial flutter or atrial fibrillation develops is whether a line of block forms between the venae cavae. Regarding the relationship of atrial flutter to atrial fibrillation: Studies have demonstrated that a driver (a single focus or reentrant circuit of very short cycle length) can be responsible for causing atrial fibrillation by producing fibrillatory conduction to the rest of the atria. In experimental models and now beginning to be demonstrated in patients, this driver may be a stable reentrant circuit of very short cycle length, i.e., a fast form of atrial flutter, if you will. In fact, there is probably a spectrum of these short cycle lengths that depend, in part, on where the reentrant circuit (i.e., "atrial flutter") exists. When the stable reentrant circuit is of sufficiently short cycle length, it will only activate small portions of the atria in a 1 : 1 manner. The rest of the atria will be activated irregularly, resulting in atrial fibrillation. Unstable reentrant circuits can also do the same thing. In short, it appears that there are several mechanisms of atrial fibrillation, one of which is due to a form of very rapid atrial flutter.

Characterization of the anatomy and conduction velocities of the human right atrial flutter circuit determined by noncontact mapping

OBJECTIVES

This study was done to characterize human right atrial (RA) flutter (AFL) using noncontact mapping.

BACKGROUND

Atrial flutter has been mapped using sequential techniques, but complex anatomy makes simultaneous global RA mapping difficult.

METHODS

Noncontact mapping was used to map the RA of 13 patients with AFL (5 with previous attempts), 11 with counterclockwise and 2 with clockwise AFL. "Reconstructed" electrograms were validated against contact electrograms using cross-correlation. The Cartesian coordinates of points on a virtual endocardium were used to calculate the length and thus the conduction velocity (CV) of the AFL wave front within the tricuspid annulus-inferior vena cave isthmus (IS) and either side of the crista terminalis (CT).

RESULTS

When clearly seen, the AFL wave front split (n = 3) or turned in the region of the coronary sinus os (n = 6). Activation progressed toward the tricuspid annulus (TA) from the surrounding RA in 10 patients, suggesting that the leading edge of the reentry wave front is not always at the TA. The IS length and CV was 47.73 +/- 24.40 mm (mean +/- SD) and 0.74 +/- 0.36 m/s. The CV was similar for the smooth and trabeculated RA (1.16 +/- 0.48 m/s and 1.22 +/- 0.65 m/s, respectively [p = 0.67]) and faster than the IS (p = 0.03 and p = 0.05 for smooth and trabeculated, respectively).

CONCLUSIONS

Noncontact mapping of AFL has been validated and has demonstrated that IS CV is significantly slower than either side of the CT.

Dynamics of action potential head-tail interaction during reentry in cardiac tissue: ionic mechanisms

In a sufficiently short reentry pathway, the excitation wave front (head) propagates into tissue that is partially refractory (tail) from the previous action potential (AP). We incorporate a detailed mathematical model of the ventricular myocyte into a one-dimensional closed pathway to investigate the effects of head-tail interaction and ion accumulation on the dynamics of reentry. The results were the following: 1) a high degree of head-tail interaction produces oscillations in several AP properties; 2) Ca(2+)-transient oscillations are in phase with AP duration oscillations and are often of greater magnitude; 3) as the wave front propagates around the pathway, AP properties undergo periodic spatial oscillations that produce complicated temporal oscillations at a single site; 4) depending on the degree of head-tail interaction, intracellular [Na(+)] accumulation during reentry either stabilizes or destabilizes reentry; and 5) elevated extracellular [K(+)] destabilizes reentry by prolonging the tail of postrepolarization refractoriness.

Influence of propafenone on resetting and termination of canine atrial flutter

Previous studies on atrial flutter (AF) presumed that resetting was due to the prematurity effect (PE) in which the stimulated antegrade wavefront travels in the tail of the AF preexisting wavefront. We studied the collision effect (CE) between the AF and the stimulated retrograde wavefronts, its contribution to resetting, and its relationship to AF termination and how they are affected by the Class IC agent propafenone (PPF). A canine model of AF was created using a Y-shaped lesion in the right atrium in 14 dogs (33 +/- 3 kg). Five atrial bipolar electrodes were positioned around the tricuspid valve. In a subsequent set of 11 dogs, we used 16 bipolar electrodes for recording. AF was induced by burst pacing. Single and multiple stimuli were applied to measure conduction time and reset-response curves (RRCs). This was repeated after the administration of PPF (1 mg/kg loading dose for 10 minutes, followed by 1.8 mg/kg/per hour infusion). Three distinct mechanisms were found to contribute to the RRC: the PE, the CE, and heterogeneity. PPF stabilized the RRC, increased significantly the cycle length (CL), the duration of the effective refractory period, as well as the duration of the excitable gap. However, PPF did not alter the duration of the fully excitable portion. We studied 36 annihilations without and 48 with PPF. Transient fibrillation was found in 75% of the episodes without, compared to 22% with PPF. Other types of termination such as conduction block, CL oscillations, and reversal of activation were found for 25% of the episodes without and 78% with PPF. In many cases, conduction block and CL oscillations were associated with a failure of propagation of the stimulated antegrade wavefront in the region of collision. Termination by reversal of activation suggests that propagation was two dimensional and could not be represented by a one dimensional movement. The average coupling interval (in percent of CL), that induced fibrillation was not significantly different from that at which conduction block occurred. This suggests that transient fibrillation is associated with a weak CE rather than with rapid pacing. The CE is amplified by multiple stimuli and PPF. The incidence of transient fibrillation in AF annihilation diminishes with PPF as the CE becomes more important. This suggests that the evaluation of PE and CE in AF may be an indication of the risk of atrial fibrillation.

Factors determining clockwise and counterclockwise conduction patterns in atrial reentrant tachycardias: a rabbit model of atrial flutter

INTRODUCTION

In the development of atrial flutter due to reentry, the crista terminalis is supposed to pose a conduction barrier, but the role of its longitudinal conduction in determining the propagation pattern of the reentrant impulse is not known. In rabbit right atrial preparations, we induced reentrant atrial tachycardias and examined the effects of transverse section of the crista terminalis on the development and conduction patterns of arrhythmias.

METHODS AND RESULTS

Right atrial preparations from 12 albino rabbits were placed endocardial surface down in a chamber with an array of 48 bipolar electrodes to draw activation maps. A single premature stimulus was delivered to induce tachycardias at the free wall. In the control, five instances of tachycardia per preparation were induced and another five were induced after cutting the crista terminalis. In the control, the mean duration of tachycardia was 127.1+/-25.2 seconds. The tachycardia was counterclockwise in 39 of 60 instances, clockwise in 12, and undetermined in 4 defined as "atypical." After transverse section of the crista terminalis, the duration was prolonged to 372.6+/-30.4 seconds, but the conduction patterns were not changed. In the free wall, counterclockwise reentry had a broader wavefront and faster conduction than clockwise reentry.

CONCLUSION

Longitudinal conduction block at the crista terminalis contributed to maintenance of reentrant atrial tachycardias, but had no influence on their propagation patterns. Clockwise and counterclockwise rotation of impulses in reentrant tachycardias had different paths and velocities of the wavefront in the free wall of the right atrium.

Transverse conduction capabilities of the crista terminalis in patients with atrial flutter and atrial fibrillation

OBJECTIVES

In this study, the transverse conduction capabilities of the crista terminalis (CT) were determined during pacing in sinus rhythm in patients with atrial flutter and atrial fibrillation.

BACKGROUND

It has been demonstrated that the CT is a barrier to transverse conduction during typical atrial flutter. Mapping studies in animal models provide evidence that this is functional. The influence of transverse conduction capabilities of the CT on the development of atrial flutter remains unclear.

METHODS

The CT was identified by intracardiac echocardiography. The atrial activation at the CT was determined during programmed stimulation with one extrastimulus at five pacing sites anteriorly to the CT in 10 patients with atrial flutter and 10 patients with atrial fibrillation before and after intravenous administration of 2 mg/kg disopyramide. Subsequently, atrial arrhythmias were reinduced.

RESULTS

At baseline, pacing with longer coupling intervals resulted in a transverse pulse propagation across the CT. During shorter coupling intervals, split electrograms and a marked alteration of the activation sequence of its second component were found, indicating a functional conduction block. In patients with atrial flutter, the longest coupling interval that resulted in a complete transverse conduction block at the CT was significantly longer than that in patients with atrial fibrillation (285 +/- 49 ms vs. 221 +/- 28 ms; p < 0.05). After disopyramide administration, a transverse conduction block occurred at longer coupling intervals as compared with baseline (287 +/- 68 ms vs. 250 +/- 52 ms; p < 0.05). Subsequently, a sustained atrial arrhythmia was inducible in 15 of 20 patients. This was atrial flutter in three patients with previously documented atrial fibrillation and in eight patients with history of atrial flutter. Mapping revealed a conduction block at the CT in all of these patients.

CONCLUSIONS

It was found that the CT provides transverse conduction capabilities and that the conduction block during atrial flutter is functional. Limited transverse conduction capabilities of the CT seem to contribute to the development of atrial flutter.

What is the relationship of atrial flutter and fibrillation?

Animal models and human studies of atrial activation mapping and entrainment have considerably enhanced our understanding of the anatomical substrate for atrial flutter and created the basis for a definite cure with radiofrequency catheter ablation. As atrial flutter has now become a curable arrhythmia, emphasis is shifting to understand the most common arrhythmia: atrial fibrillation. Furthermore, from clinical observation, it is apparent that there is a relationship between atrial fibrillation and atrial flutter in patients with atrial arrhythmias. Techniques that have informed our understanding of the anatomical basis of atrial flutter may also be useful in understanding the relationship between atrial fibrillation and flutter, including animal models, clinical endocardial mapping, and intracardiac anatomical imaging. Thus, atrial anatomy and its relationship to electrophysiological findings, and the role of partial or complete conduction barriers around which reentry can and cannot occur, may be of importance for atrial fibrillation as well. Ultimately, the relationship between atrial fibrillation and atrial flutter may inform our understanding of the mechanisms of atrial fibrillation itself, and help to develop new approaches to device, catheter-based, and pharmacological therapy for atrial fibrillation.

Mechanisms for spontaneous termination of monomorphic, sustained ventricular tachycardia: results of activation mapping of reentrant circuits in the epicardial border zone of subacute canine infarcts

OBJECTIVES

The objective of this study was to determine why sustained ventricular tachycardias (VT) sometimes stop without outside intervention.

BACKGROUND

Sustained, monomorphic VT in patients with ischemic heart disease is often caused by reentrant excitation. These tachycardias can degenerate into rapid polymorphic rhythms or occasionally terminate spontaneously.

METHODS

Sustained VT was induced by programmed stimulation in dog hearts 4 to 5 days after ligation of the left anterior descending coronary artery. Activation in reentrant circuits in the epicardial border zone of the infarct was mapped using 192 to 312 bipolar electrodes.

RESULTS

Spontaneous termination of sustained VT always occurred when the reentrant wave front blocked in the central common pathway in reentrant circuits with a figure-of-eight configuration. Two major patterns of termination were identified from activation maps of the circuits that were not distinguishable from each other on the surface electrocardiogram: 1) Abrupt termination was not preceded by any change in the pattern of activation or cycle length. It could occur at different locations within the central common pathway, was not related to the directions of the muscle fiber orientation and was not caused by a short excitable gap. 2) Termination caused by premature activation (after a short cycle) either resulted from shortening of the functional lines of block around which the reentrant impulse circulated or was caused by wave fronts originating outside the reentrant circuit. In only one episode were oscillations of cycle length associated with termination.

CONCLUSIONS

The mechanisms for termination of reentry in functional circuits causing VT are different from those in anatomic circuits where oscillatory behavior precedes termination.

Mechanisms and medical management of patients with atrial flutter

Type I atrial flutter is due to reentrant excitation, principally in the right atrium. The standard ECG remains the cornerstone for its clinical diagnosis. Acute treatment should be directed at control of the ventricular response rate and, if possible, restoration of sinus rhythm. Radiofrequency catheter ablation therapy provides the best hope of cure, although atrial fibrillation may subsequently occur after an ostensibly successful ablative procedure. Alternatively, antiarrhythmic drug therapy to suppress recurrent atrial flutter episodes may be useful, recognizing that occasional recurrences are common despite therapy. Radiofrequency ablation of the His bundle ablation with placement of an appropriate pacemaker system may be useful in selected patients.

Atrial flutter: historical background

For five decades, the mechanism of atrial flutter remained controversial, with protagonists and antagonists of circus movement versus ectopic focus theories. The development of clinical electrophysiology in the 1970s and the observations made by many authors in various canine heart models supported the concept of atrial flutter as a reentrant wave confined to the right atrium. It was established that, in the common type of atrial flutter, the activation wavefront proceeds in a cranial direction over the right atrial septum and descends on the right atrial free wall in the caudal direction. A zone of slow conduction was identified inferiorly and posteriorly in the right atrium, target of the modern ablative techniques. The history of atrial flutter clearly illustrates the bidirectional flow of information and the mutual stimulation between the basic and the clinical levels, leading both to a better understanding of the nature of the arrhythmia and to new therapeutic approaches.

Alternation of atrioventricular nodal conduction time during atrioventricular reentrant tachycardia: are dual pathways necessary?

INTRODUCTION

Alternation of atrial cycle length and AV nodal conduction time (NCT) is often observed during AV reentrant tachycardia. Both AV nodal dual pathway and rate-dependent function have been postulated to be involved in this phenomenon. This study was designed to determine the respective role of these two mechanisms in the alternation observed in an in vitro model of orthodromic AV reentrant tachycardia.

METHODS AND RESULTS

The tachycardia was produced by detecting each His-bundle activation and stimulating the atrium after a retrograde delay, thereby simulating retrograde pathway conduction, in six isolated rabbit heart preparations. After a 5-minute stabilization period at a fast rate, the retrograde delay was decremented by 2 msec every minute until nodal blocks occurred. We observed a sequential alternation of the cycle length and NCT in four preparations in the short retrograde delay range. The magnitude of the alternation gradually increased as the retrograde delay was decreased and reached 4.6 +/- 0.5 msec during 1:1 conduction. The alternation increased further just prior to termination of the tachycardia by an AV nodal block. None of the preparations showed discontinuous AV nodal recovery curves. Moreover, an electrode positioned over the endocardial surface of the node showed that the alternation developed distally to the nodal inputs, which are believed to constitute a major component of dual pathways. A mathematical model predicted the alternation from known characteristics of rate-dependent nodal functional properties.

CONCLUSIONS

NCT and cycle length alternation can arise during orthodromic AV reentrant tachycardia when the retrograde delay is sufficiently short. The characteristics of the alternation, presence of continuous recovery curves, intranodal location of the alternation, and mathematical modeling suggest that the alternation is predictable from the known functional properties of the AV node without postulating dual pathway physiology.

Effects of azimilide dihydrochloride on circus movement atrial flutter in the canine sterile pericarditis model

INTRODUCTION

The effects of a Class III agent, azimilide dihydrochloride, on atrial flutter circuits were studies in a functional model of single loop reentrant atrial flutter using dogs, 3 to 5 days after production of sterile pericarditis.

METHODS AND RESULTS

A computerized mapping system was used to construct activation maps from 138 to 222 epicardial sites in the right atrium. Doses of 3, 10, and 30 mg/kg i.v. azimilide dihydrochloride were analyzed in 8 dogs in which sustained atrial flutter lasting more than 30 minutes was induced by burst pacing. Atrial flutter was always due to single loop circus movement reentry in the lower right atrium. At 3 mg/kg, azimilide dihydrochloride terminated atrial flutter in 2 dogs; however, atrial flutter was reinduced. At 10 mg/kg, atrial flutter was terminated in all 8 dogs but was reinduced in 4 dogs with slower rate. At 30 mg/kg, atrial flutter was terminated in the remaining 4 dogs and could not be reinduced. Atrial flutter cycle length always increased prior to termination. Isochronal activation maps showed that the increase in cycle length was due to additional conduction delays in the slow zone of the reentrant circuit. The site of termination was always located within the slow conduction zone situated in the lower right atrium between the line of functional conduction block and the AV ring. Effective refractory periods (ERPs) were measured at selected sites in the slow zone and normal zone at twice diastolic threshold for the 10 mg/kg dose. Azimilide preferentially prolonged ERP in the slow zone (42.4 +/- 20.1 msec, mean +/- SD) compared with the normal zone (23.3 +/- 15.4 msec, P < 0.0001). The increase in cycle length corresponded with the increase in ERP in the slow zone.

CONCLUSIONS

In a functional model of circus movement atrial flutter, azimilide dihydrochloride terminates and prevents reinduction of atrial flutter by a preferential increase in refractoriness leading to further conduction delay and conduction block in the slow zone of the functional reentrant circuit.

Conversion of atrial flutter by ibutilide is associated with increased atrial cycle length variability

OBJECTIVES

This study was designed to test the hypothesis that conversion of atrial flutter in humans by ibutilide, a new class III antiarrhythmic agent, is characterized by an increase in atrial cycle length variability.

BACKGROUND

Conversion of tachyarrhythmias has been associated with increased oscillations of cycle length.

METHODS

Electrograms and monophasic action potentials from the right atrium in 35 patients with spontaneous, sustained atrial flutter were recorded before, during and after intravenous ibutilide (0.005 to 0.025 mg/kg body weight, n = 25) or placebo (n = 10). Atrial cycle length, cycle length variability (coefficient of variation), diastolic interval and diastolic interval variability were measured from 10 consecutive cycles at baseline and 3 min before, 1 min before, 30 s before and immediately before conversion. Similar measurements were made in patients who received ibutilide or placebo but did not convert.

RESULTS

Ibutilide converted atrial flutter in 14 of 25 patients 25 +/- 16 min (mean +/- SD) after initiation of the infusion, whereas placebo converted no patients. Atrial cycle length was prolonged to the same extent in ibutilide converters and nonconverters (36 +/- 19 vs. 38 +/- 21 ms, p = NS) and was not affected by placebo. Beat-to-beat variability in atrial cycle length (baseline 1.2 +/- 0.7 vs. preconversion 7.3 +/- 4.9, p < 0.01) and diastolic interval (baseline 11 +/- 8 vs. preconversion 33 +/- 23, p < 0.05) increased significantly just before atrial flutter conversion and remained unchanged in ibutilide nonconverters and placebo group patients.

CONCLUSIONS

Ibutilide prolongs atrial cycle length, but conversion of atrial flutter by ibutilide is characterized by increased variability in atrial cycle length and diastolic interval.

Mechanisms of atrial flutter: implications for ablative therapy

Much has been learned about atrial flutter mechanisms from studies in animal models and in patients. In fact, it seems virtually always due to some form of reentry. Furthermore, it seems likely that there is more than one location of the atrial flutter reentrant circuit in patients, although the reentrant circuit in most instances of atrial flutter seems to be activation up the interatrial septum and then down the posterior right atrial free-wall. Other locations of the reentrant circuit may include the right atrial free-wall alone or the tricuspid valve annulus, among others. Resolution of this awaits better mapping data from human studies. Clearly, an understanding of mechanism is central to achieving effective ablation. Thus, if it is possible to identify a critical aspect or aspects of the atrial flutter mechanism vulnerable to therapy with ablative energy, effective treatment using ablative techniques should be successful.

Reentry: insights from theoretical simulations in a fixed pathway

This review article summarizes theoretical insights into the principles and mechanisms associated with reentrant activity in cardiac tissue. A mathematical ring model is used in computer simulations to investigate, at the cellular level, mechanistic aspects of initiation, perpetuation, and termination of reentry. Taking advantage of the ability to compute membrane processes in this model, we relate dynamic properties of the reentrant action potential (e.g., beat-to-beat alternans) to the underlying kinetics of membrane ionic channels. Effects on reentry of inhomogeneities in refractoriness, excitability, cellular coupling at gap junctions, and fiber cross-section are also studied.

Mechanism of interruption of atrial flutter by moricizine. Electrophysiological and multiplexing studies in the canine sterile pericarditis model of atrial flutter

BACKGROUND

Moricizine is said to have potent effects on cardiac conduction but little or no effect on cardiac refractoriness.

METHODS AND RESULTS

The effects of moricizine (2 mg/kg IV) on induced atrial flutter were studied 2 to 4 days after the creation of sterile pericarditis in 11 dogs. Ten episodes of stable atrial flutter before and after the administration of moricizine were studied in 9 dogs in the conscious, nonsedated state, and 7 episodes were studied in 6 dogs in the anesthetized, open chest state. In the conscious state, the effects of moricizine on atrial excitability, atrial effective refractory period, and intra-atrial conduction times were studied by recording during overdrive pacing of sinus rhythm from epicardial electrodes placed at selected atrial sites. Moricizine prolonged the atrial flutter cycle length in all the episodes, from a mean of 133 +/- 9 to 172 +/- 27 milliseconds (P < .001), and then terminated 7 of the 10 episodes. Moricizine increased the atrial threshold of excitability from a mean of 2.3 +/- 1.4 to 3.3 +/- 2.2 mA (P < .01) and prolonged intra-atrial conduction times (measured from the sulcus terminalis to the posteroinferior left atrium) from a mean of 58 +/- 6 to 64 +/- 5 milliseconds (P < .005). Prolongation of the atrial effective refractory period from 166 +/- 20 to 174 +/- 24 milliseconds (P < .05) was observed only at the sulcus terminalis site. In the open chest studies, administration of moricizine prolonged the atrial flutter cycle length from a mean of 150 +/- 15 to 216 +/- 30 milliseconds (P < .001) and then terminated the atrial flutter in all 7 episodes. As demonstrated by simultaneous multisite mapping from 95 bipolar sites on the right atrial free wall, the atrial flutter cycle length prolongation was either due to further slowing of conduction in an area of slow conduction in the reentrant circuit of the atrial flutter (5 episodes) or further slowing of conduction in an area of slow conduction plus the development of a second area of slow conduction (2 episodes). The change in conduction times in the rest of the reentrant circuit was negligible (10.9 +/- 8.7% of the total change). In all 7 episodes, the last circulating reentrant wave front blocked in an area of slow conduction.

CONCLUSIONS

Moricizine (1) prolongs the atrial flutter cycle length, primarily by slowing conduction in an area of slow conduction in the reentrant circuit, (2) terminates atrial flutter by causing block of the circulating reentrant wave front in an area of slow conduction of the reentrant circuit, and (3) effectively interrupts otherwise stable atrial flutter in this canine model. The reason for these effects of moricizine are not readily explained by its effects on global atrial conduction times and refractoriness studied during sinus rhythm. Local changes in conduction in an area(s) of slow conduction are responsible for both cycle length prolongation and atrial flutter termination rather than the traditional wavelength concept of head-tail interaction.