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(Circulation Research. 2000;87:1157.)
© 2000 American Heart Association, Inc.

Integrative Physiology

Role of Structural Barriers in the Mechanism of Alternans-Induced Reentry

Joseph M. Pastore, David S. Rosenbaum

From the Heart and Vascular Research Center and Department of Biomedical Engineering, MetroHealth Campus, Case Western Reserve University, Cleveland, Ohio.

Correspondence to David S. Rosenbaum, MD, Director, Heart and Vascular Research Center, MetroHealth Campus, Case Western Reserve University, 2500 MetroHealth Dr, Hamman 322, Cleveland, OH 44109-1998. E-mail drosenbaum{at}metrohealth.org

	Abstract

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Abstract—Previously, using an animal model of T-wave alternans in structurally normal myocardium, we demonstrated that repolarization can alternate with opposite phase between neighboring myocytes (ie, discordant alternans), causing spatial dispersions of repolarization that form the substrate for functional block and reentrant ventricular fibrillation (VF). However, the mechanisms responsible for cellular discordant alternans and its electrocardiographic manifestation (ie, T-wave alternans) in patients with structural heart disease are unknown. We hypothesize that electrotonic uncoupling between neighboring regions of cells by a structural barrier (SB) is a mechanism for discordant alternans. Using voltage-sensitive dyes, ventricular action potentials were recorded from 26 Langendorff-perfused guinea pig hearts in the absence (ie, control) and presence of an insulating SB produced by an epicardial laser lesion. Quantitative analysis of magnitude and phase of cellular alternans revealed that in controls, action potential duration alternated in phase at all ventricular sites above a critical heart rate (269±17 bpm), ie, concordant alternans. Also, above a faster critical heart rate threshold (335±24 bpm), action potential duration alternated with opposite phase between sites, ie, discordant alternans. In contrast, only discordant but not concordant alternans was observed in 80% of hearts with the SB, and discordant alternans always occurred at a significantly slower heart rate (by 68±28 bpm) compared with controls. Therefore, the SB had a major effect on the alternans–heart rate relation, which served to facilitate the development of discordant alternans. Whether a SB was present or not, discordant alternans produced considerable increases (by 170%) in the maximum spatial gradient of repolarization, which in turn formed the substrate for unidirectional block and reentry. However, by providing a structural anchor for stable reentry, discordant alternans in the presence of a SB led most often to sustained monomorphic ventricular tachycardia rather than to VF, whereas in the absence of a SB discordant alternans caused VF. SBs facilitate development of discordant alternans between cells with different ionic properties by electrotonically uncoupling neighboring regions of myocardium. This may explain why arrhythmia-prone patients with structural heart disease exhibit T-wave alternans at lower heart rates. These data also suggest a singular mechanism by which T-wave alternans forms a substrate for initiation of both VF and sustained monomorphic ventricular tachycardia.

Key Words: repolarization • reentry • alternans

	Introduction

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T-wave alternans is a beat-to-beat fluctuation in the amplitude of the electrocardiographic T wave that repeats once every other beat and has been closely associated with ventricular arrhythmias and sudden cardiac death. Visually apparent T-wave alternans is known to hasten cardiac electric instability in a surprisingly wide variety of experimental^{1 2 3 4} and clinical^{5 6 7} conditions. More recently, using sensitive signal-processing techniques, we found that the detection of microvolt-level, visually inapparent T-wave alternans is also a strong predictor of vulnerability to ventricular arrhythmias in patients.⁸ Interestingly, T-wave alternans has been closely associated with polymorphic ventricular tachycardia (VT),⁹ ventricular fibrillation (VF),¹⁰ torsade de pointes,^{7 11} and sustained monomorphic VT (SMVT)⁸ both in the absence of structural heart disease and in patients with advanced cardiomyopathy. Therefore, a greater understanding of the mechanisms underlying T-wave alternans may provide important new insights into the pathophysiology of sudden cardiac death in a variety of clinical and experimental circumstances.

Recently, we used an experimental model to demonstrate that T-wave alternans is caused by primary alternations in the repolarization phase of the action potential.¹⁰ Importantly, above a critical heart rate threshold, repolarization of action potentials from adjacent regions of the ventricle alternated with opposite phase, ie, discordant alternans, causing steep spatial gradients of repolarization that formed an electrophysiological substrate for functional block, reentry, and VF.¹⁰ It is noteworthy that discordant alternans occurred under conditions of normal intercellular coupling, indicating that regional differences in cellular ionic properties can overcome electrotonic effects that normally act to synchronize repolarization.^{12 13} However, the mechanisms responsible for triggering discordant alternans are unknown.

A major unresolved question is why patients with structural heart disease and T-wave alternans are at particularly high risk for ventricular arrhythmias. Clearly, patients with contractile dysfunction constitute the vast majority of cases of sudden cardiac death. Among patients with structural heart disease, those with T-wave alternans are more susceptible to inducible SMVT⁸ and spontaneous arrhythmic events.^{8 14} However, the role of structural heart disease in the mechanisms of alternans-induced reentry is unknown. Most forms of myocardial disease are associated with fibrotic barriers,¹⁵ alterations of fiber bundle architecture,¹⁶ and maldistribution of cardiac gap junctions,¹⁷ all of which are expected to produce electric uncoupling between fibers. Because cell-to-cell coupling plays an important role in synchronizing repolarization between cells, we hypothesize that electrotonic uncoupling by structural barriers (SBs) facilitates the development of discordant alternans. This may explain a mechanism by which SBs promote the development of critical dispersions of repolarization that cause functional block and reentry.

To test this hypothesis, we used high-resolution optical mapping in an experimental model of T-wave alternans. This model offered several advantages, as follows. (1) Action potentials could be recorded simultaneously while T-wave alternans was elicited on the ECG. (2) SB could be created with precise control using an argon ion laser in a preparation with previously characterized heterogeneities of repolarization. (3) Propagation, functional block, and dispersions of repolarization could be mapped directly with high spatial resolution. (4) In this model, both monomorphic and polymorphic arrhythmias could be induced. We found that electrotonic uncoupling produced by a SB greatly facilitates the development of discordant alternans, creating critical heterogeneities of repolarization that can form the substrate for a variety of reentrant arrhythmias. These data provide additional evidence linking T-wave alternans to a novel mechanism of arrhythmogenesis.

	Materials and Methods

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Experiments were performed in the guinea pig model of pacing-induced T-wave alternans¹⁰ in which a thin (800 µm) viable rim of left and right ventricular epicardium having normal electrophysiological properties was created by endocardial cryoablation in Langendorff-perfused hearts.^{10 18} After staining with voltage-sensitive dye, hearts were placed in an imaging chamber that maintained uniform temperature, eliminated motion artifacts without altering electrophysiological properties, and allowed ECG recordings from the volume conductor bath.^{10 18 19} To uncouple neighboring regions of cells having known differences in cellular restitution kinetics,²⁰ an insulating SB was produced by a computer-driven argon laser to produce a 2x10-mm SB (Figure 1) spanning from the epicardial surface transmurally to the endocardial cryoablation.

View larger version (84K): [in this window] [in a new window]   Figure 1. Figure 1. Langendorff-perfused guinea pig heart was stained with voltage-sensitive dye and action potentials were mapped from a 1.5x1.5-cm mapping area (square grid). Left ventricle was stimulated (square pulse symbol) over a broad range of CLs in absence (ie, control) and presence of a SB. ECGs were recorded from electrodes immersed in the coronary effluent. RA indicates right atrium; LA, left atrium; RV, right ventricle; LV, left ventricle; and LAD, left anterior descending coronary artery.

We have developed an optical action potential mapping system that is capable of resolving membrane potential changes as small as 0.5 mV simultaneously from 128 sites across the anterior epicardial surface of the guinea pig ventricle.^{18 20 21} An optical magnification of x1.2 was used that corresponded to a mapping area of 15x15 mm, 1.25-mm interpixel spatial resolution, and 1-ms temporal resolution.

Repolarization time (RT), activation times, and action potential duration (APD) were determined from previously described algorithms.^{18 20} Alternans of cellular RT was determined by measuring differences in local RT on consecutive beats. Because the error associated with measurement of RT was 3±4 ms, only RT alternans >10 ms was considered significant. A previously validated spectral analysis technique¹⁰ was used to measure ECG T-wave alternans. The method of Bayly et al²² was modified for optically recorded action potential maps to accurately quantify the magnitude and direction of conduction velocity and repolarization gradients at each recording site.

In all hearts (n=26), action potentials were recorded (Figure 1) over a broad range of steady-state pacing cycle lengths (CLs) until one-to-one capture failed or an arrhythmia was initiated. In each experiment, the left ventricular free wall was stimulated near the atrioventricular groove (Figure 1) to ensure that the sequence of epicardial activation was nearly identical before and after the introduction of the SB. This stimulation protocol was repeated in each experiment to confirm reproducibility of the findings. To ensure that laser energy did not alter properties of cells outside of the precisely demarcated SB, validation studies were performed in 15 hearts. Each heart served as its own control, as the identical stimulation protocol (described above) was performed before (ie, control) and after a SB was introduced. APD, repolarization gradients, conduction velocity, and ECG morphology were compared at the baseline stimulation CL of 400 ms in the absence and presence of a SB. The effects of SB on cellular alternans was determined in a subset of 5 of these experiments in which quantitative analysis of the magnitude and phase of cellular alternans was compared at each steady-state CL both before and after the introduction of a SB.

An expanded Materials and Methods section can be found in an online data supplement available at http://www.circresaha.org.

	Results

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Effect of SB on Baseline Electrophysiology
Although the electrophysiological properties of guinea pig epicardium were described in detail previously,^{19 20} it was important to confirm whether introduction of a SB changed the baseline characteristics of the model. Conduction velocity and APD were compared at a baseline heart rate of 150 bpm during control and in the presence of the SB. Conduction-velocity transverse to fibers measured during control (24±5 cm/second) and in the presence of the SB (25±7 cm/second) did not differ significantly (P=0.79, Wilcoxon matched pairs test). Furthermore, the SB did not significantly change baseline APD (Wilcoxon matched pairs test) on either the basal (208±14 before compared with 222±11 ms after the SB, P=0.07) or the apical (197±13 compared with 213±8 ms, P=0.08) side of the SB, nor was apex-to-base APD gradient changed by the SB (4.57±1.37 compared with 4.79±0.93 ms/mm).

There was no evidence of ischemia either during control or after the introduction of a SB as confirmed by the absence of triangulated action potentials and by histological examination using viability staining.

Effect of SB on Cellular Alternans
The effect of regional uncoupling produced by a SB on the magnitude and heart rate threshold of action potential alternans is illustrated in Figure 2. Panel A shows a representative example of the change in cellular alternans–heart rate relation at one ventricular site caused by a SB. Such changes were indicative of cellular alternans at all recording sites. At relatively slow heart rates (<200 bpm), no cellular alternans is present either in control or in the presence of SB. However, above a critical threshold heart rate, RT alternans occurs and increases markedly as heart rate further increases. Notice that the heart rate threshold for RT alternans is considerably lower in the presence of a SB (240 bpm) compared with control (300 bpm). Consequently, over a broad range of heart rates, the magnitude of RT alternans is amplified in the presence of a SB. The difference in cellular alternans produced by the SB is further emphasized by the superposition of even and odd action potentials shown in panel C. Note that the heart rate dependence of repolarization alternans in individual cells was profoundly influenced by the insulating SB located some distant from the cell. The SB also decreased the heart rate threshold for T-wave alternans measured from the ECG (not shown), suggesting that SBs can similarly explain changes in heart rate alternans relation manifest on the surface ECG.

View larger version (22K): [in this window] [in a new window]   Figure 2. Figure 2. Effect of SB on repolarization alternans. A, Relationship between local repolarization alternans and heart rate in absence (ie, control, open circles) and presence (filled circles) of SB. Optically recorded action potentials shown were recorded from the same site (filled pixel, Figure 1) as heart rate varied. Notice in both cases that repolarization alternans occurs above a threshold heart rate that is lower with SB (240 bpm) compared with control (300 bpm). Repolarization alternans is also demonstrated by superposition of even and odd (ie, sequential) action potentials in panel C. B, Effect of the SB on the discordant alternans heart rate threshold from 5 experiments. Notice that presence of SB significantly decreases the heart rate at which discordant alternans is elicited.

Effect of SB on Discordant Alternans Between Cells
In this investigation, the alternans heart rate threshold was defined as the slowest heart rate at which significant alternans was recorded at one or more ventricular sites, and the heart rate threshold for discordant alternans occurred at the heart rate at which 2 sites had significant alternans with opposite phase between them. As shown in Figure 2B, the SB significantly (P<0.05, Wilcoxon matched pairs test) decreased the heart rate threshold for discordant alternans (by 68±28 bpm) compared with controls. Also, in controls, a progression from no alternans to concordant alternans to discordant alternans always occurred as heart rate increased. In contrast, in the presence of a SB, a progression directly from no alternans to discordant alternans was observed in 80% of experiments. Consequently, the heart rate that produced discordant alternans in hearts with SBs (276±6 bpm) was essentially the same as the heart rate that produced concordant alternans in control hearts (269±17 bpm, P=0.35). Therefore, a SB greatly increased the propensity for discordant alternans. Preliminary experiments performed over long time periods (>3 hours) without introduction of a SB (ie, time controls) demonstrated stable electrophysiological properties of this preparation and, importantly, no change in the heart rate dependence or distribution of cellular alternans. Therefore, changes in magnitude or phase of cellular alternans that followed the application of the SB could not be attributed to degradation of the preparation over time.

We hypothesize that electrically insulating 2 regions of cells with heterogeneous intrinsic repolarization properties such as those known to exist across the epicardium²⁰ will influence the development of discordant alternans. Figure 3 demonstrates that electrically uncoupling 2 regions of functionally distinct myocardium by introducing a SB creates an anatomic substrate across which discordant alternans can form. Shown are RT alternans phase plots delineating regions of cells where local repolarization alternates in a short-long phase (Figure 3, black region) or a long-short phase (Figure 3, gray region). Areas without significant RT alternans are represented in white. All plots correspond to the slowest heart rate at which discordant alternans was recorded. Note that in all experiments, the SB forms at least a portion (and in experiments 1, 3, and 5, a large portion) of the line separating regions of cells alternating with opposite phase. Below each phase plot are representative action potentials from both the long-short (Figure 3, site A) and short-long (Figure 3, site B) regions demonstrating discordant alternans. Importantly, in the absence of a SB, stimulation at the identical heart rate produced concordant RT alternans in the same hearts. Therefore, the SB lowered the heart rate threshold for discordant alternans.

View larger version (28K): [in this window] [in a new window]   Figure 3. Figure 3. Effect of a SB on spatial orientation of discordant alternans shown as alternans phase plots. Alternans phase plots from 5 experiments show regions alternating in short/long fashion (black region) or long/short fashion (gray region). Areas without significant repolarization alternans are represented in white. Below each plot are action potentials from representative sites (labeled either A or B) within each region, with APDs shown in ms. Notice that in all experiments, the SB forms a portion of the line of separation between the short/long and long/short regions.

Because of its effect on the phase of RT alternans, the SB substantially affected spatial gradients of repolarization. Shown in Figure 4 are isochrone maps representing patterns of activation, APD, and repolarization for 2 consecutive beats in the absence and presence of a SB in the same heart and at an identical heart rate. The pattern of activation is similar from beat to beat both before and after the introduction of the SB. During control, there is a gradient of APD oriented in an apex (shortest APD)–to–base (longest APD) direction on both beats (Figure 4, left middle panel) as reported previously.²⁰ In contrast, the introduction of a SB produces large changes in the orientation and magnitude of APD gradients from beat to beat (Figure 4, right middle panel). The effect on APD accounts for the large differences in the orientation of repolarization gradients on consecutive beats (Figure 4, right bottom panel) and the larger spatial dispersion of repolarization as evidenced by the crowding of RT isochrones (Figure 4, compare repolarization maps on right with those on left).

View larger version (45K): [in this window] [in a new window]   Figure 4. Figure 4. Changes in ventricular repolarization caused by the SB. Ten-millisecond isochrone maps represent patterns of depolarization (DEPOL), APD, and repolarization (REPOL) for 2 consecutive beats in absence (ie, control) and presence of SB at an identical heart rate. Activation propagates as expected from site of stimulation (square pulse symbol) in all cases. SB produces large changes in magnitude and orientation of APD gradients (right middle panel) that were not present during control (left middle panel). This creates large spatial gradients of repolarization in presence of SB. These effects are evident on the ECG (top) as visible T-wave alternans is apparent with SB.

Role of SB on Alternans-Induced Arrhythmias
To determine the effect of the SB on alternans-induced reentry, we attempted to induce discordant alternans and arrhythmias in all 26 hearts. In 15 of these hearts, the arrhythmia-induction protocol was performed in the absence and presence of a SB, whereas in 11 hearts the protocol was performed only in the absence of a SB. In all 26 experiments, reentrant VF or SMVT was only initiated after discordant alternans formed; ie, reentrant excitation never occurred in the absence of discordant alternans. In the absence of a SB, VF was initiated in 93% of hearts, whereas SMVT was initiated in only 7%. In contrast, the presence of a SB resulted in reentrant SMVT in 70% of hearts and VF in 30%. Therefore, the SB was a significant and specific factor for the development of SMVT (²=11.1, P<0.001). Figure 5 illustrates representative examples of the induction of VF and SMVT in the same heart before and after the introduction of a SB. Therefore, discordant alternans occurring in the presence and absence of a SB formed the electrophysiological substrate for SMVT or VF, respectively.

View larger version (34K): [in this window] [in a new window]   Figure 5. Figure 5. Alternans-induced ventricular arrhythmias. Top, ECG lead showing initiation of VF with a 10-ms decrement in CL in a heart without a SB. Bottom, ECG tracing from same heart in which SMVT was initiated after a SB was introduced. Notice in both cases that visible T-wave alternans precedes the arrhythmia. T-wave alternans were associated with discordant action potential alternans between cells (not shown). Summary data shown in bar graphs demonstrate that despite the similar role of discordant alternans in the mechanism of unidirectional block and reentrant VF and SMVT, the SB was typically required for SMVT.

To ascertain the role of discordant alternans in forming an arrhythmogenic substrate, the maximum repolarization gradient within the mapping array was measured at baseline (CL=400) and during discordant alternans at the CL immediately preceding the initiation of either reentrant VF or SMVT. We found that discordant alternans created an electrophysiological substrate for unidirectional block by amplifying gradients of repolarization independent of underlying structure. Figure 6 shows the maximum gradient of repolarization measured within the mapping array at a baseline heart rate of 150 bpm as compared with during discordant alternans immediately preceding the initiation of VF (in hearts without a SB) and SMVT (in hearts with a SB). Notice that the maximum repolarization gradient is significantly larger preceding SMVT or VF compared with baseline, whereas the maximum repolarization gradients preceding SMVT and VF are similar. These data suggest that discordant alternans transforms minor gradients of repolarization into critical gradients sufficient for development of functional block that leads to both SMVT and VF. This is illustrated in the representative example shown in Figure 7 demonstrating the initiation of reentrant SMVT after the application of a single premature stimulus (S2) during discordant alternans. Reentry is immediately preceded by discordant alternans as shown by the action potential tracings from 2 selected sites, sites A and B, (Figure 7, bottom panel). Notice the large repolarization gradient between these 2 sites, which are only 5 mm apart. Although the wavefront propagates successfully along both sides of the SB on the S1 beat, the gradient caused by discordant alternans is of sufficient magnitude to cause unidirectional block of the premature (S2) beat. Notice that the wavefront fails to propagate against the repolarization gradient created by discordant alternans on the basal side of the SB but does propagate along the apical side. About 130 ms later, the wavefront reenters the area of functional block in a clockwise direction and propagates around the SB forming the first beat of reentry. Similarly, discordant alternans produced critical gradients of repolarization, which formed the substrate unidirectional block, which led to reentrant VF when no SB was available to stabilize the reentrant circuit. Therefore, in the presence of a SB, discordant alternans underlie the mechanism of SMVT because the SB served as an anchor around which a stable reentrant circuit could form.

View larger version (17K): [in this window] [in a new window]   Figure 6. Figure 6. Effect of discordant alternans on the maximum gradient of repolarization. Shown is the maximum repolarization gradient measured within the mapping array during baseline pacing and during discordant alternans in control hearts preceding the initiation of VF and in hearts with a SB preceding SMVT. Notice that the maximum gradient is significantly greater preceding a reentrant arrhythmia compared with baseline. However, the gradients preceding VF are similar to those preceding SMVT, suggesting that the conditions necessary for functional block are similar in the presence and absence of the SB.

View larger version (36K): [in this window] [in a new window]   Figure 7. Figure 7. Alternans-induced reentry in presence of SB. Top, ECG lead showing initiation of reentry by introduction of a premature stimulus (S2) in a heart with a SB (white box). Middle, 10-ms isochrone maps of depolarization for the beat preceding the S2 and during the initiation of the arrhythmia. Notice that discordant alternans (action potential tracings, bottom panel) between mapping sites A and B creates a large gradient of repolarization that produces functional block (hatched area, middle isochrone plot) necessary for reentry around the SB.

	Discussion

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For more than three-quarters of a century, T-wave alternans has been closely associated with susceptibility to ventricular arrhythmias in remarkably broad patient populations both with⁸ and without⁷ structural heart disease. Furthermore, T-wave alternans is now emerging as a clinical tool to screen for patients at risk for sudden cardiac death based on the principle that the heart rate threshold for T-wave alternans is significantly reduced in high-risk patients.^{8 23} Recently, we demonstrated that T-wave alternans is linked to the mechanism of reentry by discordant alternans that develops between cells possessing different electrophysiological properties.¹⁰ Importantly, discordant alternans greatly amplifies dispersion of repolarization, producing conditions necessary for functional block and reentry. We hypothesized that electrotonic uncoupling by a SB increases the propensity for discordant alternans, which may explain a mechanism of T-wave alternans and arrhythmias in patients with structural heart disease. In this report, we show conclusively that discordant alternans is a novel mechanism by which the electrophysiological substrate for block and reentry can form and that SBs greatly increase the propensity for discordant alternans to develop.

Although results obtained from the guinea pig pacing-induced model of T-wave alternans must be extrapolated cautiously to patients, the model offers several distinct advantages. As is the case in patients, electrical alternans in this model persists over time (ie, alternans are not transient), does not require ischemic damage, and principally involves the T-wave rather than the QRS complex.⁸ Although spontaneous initiation of VT or VF in patients is often not preceded by visible T-wave alternans as seen in these experimental studies, it is possible that microvolt-level T-wave alternans is present but unrecognized during clinical episodes of sudden cardiac death.⁸ Finally, cellular alternans elicited by steady-state pacing produces an electrophysiological substrate for reentrant VT and VF,¹⁰ which typically lasts indefinitely unless actively terminated by pacing or defibrillation, respectively.¹⁸

The effect of the SB on the development of T-wave alternans and arrhythmogenesis was ascertained using optical action potential mapping, which served to monitor the development of cellular alternans both in the absence (ie, control) and presence of a SB. This barrier was produced with a laser to create a well-defined anatomic obstacle that electrically insulated 2 regions of myocardium without altering the electrophysiological properties of cells surrounding the barrier.¹⁸ Therefore, our observation of profound changes in cellular repolarization and alternans caused by SBs emphasizes the importance of investigating cell physiology in the intact heart.

Previously, we have shown that the orientation of discordant alternans closely follows heterogeneities of restitution properties between cells¹⁰ that are known to exist across guinea pig epicardium,²⁰ suggesting that the reason neighboring regions of cells alternate with opposite phase is that the cells possess different ionic properties. One would predict that in the presence of reduced intercellular coupling, electrotonic forces that normally act to maintain synchronization of repolarization between cells are reduced, thereby enhancing dispersion of repolarization.²⁴ Conversely, several studies have shown that increased coupling attenuates repolarization gradients.^{12 13} The present investigation demonstrates an independent and important effect of cell-to-cell uncoupling on repolarization. In the presence of minor heterogeneities of repolarization properties between cells, electrotonic uncoupling promotes the development of discordant alternans, which in turn causes marked gradients of repolarization. Importantly, arrhythmias only occurred in the presence of steep repolarization gradients caused by discordant alternans. Discordant alternans has also been reported during ischemia,^{2 25} in which cell-to-cell coupling is impaired,^{26 27} further supporting a role of uncoupling in the mechanism of discordant alternans.

There are several lines of evidence indicating that a SB facilitates the development of discordant alternans. First, in structurally normal myocardium, concordant alternans always preceded discordant alternans, whereas in the presence of a SB, a direct transition from no alternans to discordant alternans occurred. Second, myocardial cells alternating with opposite phase were typically located in proximity to the SB (Figure 3). Third, the heart rate threshold required to elicit discordant alternans was reduced significantly by the SB (Figure 2B), which was also reflected in the ECG by a decrease in the T-wave alternans heart rate threshold. This corresponds well with recent clinical observations that suggest that patients in whom SMVT can be initiated during electrophysiological study have a significantly lower heart rate threshold for T-wave alternans compared with patients with negative electrophysiological tests.^{8 23} The findings in this study provide a possible mechanism explaining these clinical observations.

Our data suggest that discordant alternans can potentially form in the following 2 situations: (1) where large heterogeneities in cellular repolarization properties exist in structurally normal myocardium and (2) where minor heterogeneities of repolarization exist in the presence of uncoupling caused by SBs. For example, significant repolarization heterogeneities may exist in structurally normal myocardium in the congenital long-QT syndrome in which discordant alternans has been documented²⁸ and may explain the association of long-QT syndrome with torsade de pointes^{29 30} and VF.³¹ In contrast, hearts of patients with structural heart disease have reduced cell-to-cell coupling³² in part because of interdigitation of fibrous tissue between myocyte bundles.¹⁵ Even though these patients may have only minor heterogeneities of repolarization between cells, electrical uncoupling may promote the formation of discordant alternans. Moreover, the precise relation between the location of a SB and repolarization gradients may also explain why a scar can predispose to reentry in some cases but not others. Our data do not suggest that SBs of any type or having any orientation constitute a requisite condition for monomorphic and polymorphic reentry. For instance, if a barrier is made along rather than across the gradients of repolarization, reentry is not easily induced in this model.¹⁸ Biological variability in location of repolarization gradients relative to the SB and pacing site may explain why discordant patterns varied between experiments (Figure 3) and why, in turn, the site of block also varied (Figure 7).

The initiation of SMVT, rather than VF, in the presence of a SB was explained by the barrier serving as an anchor to stabilize a reentrant circuit (Figures 5 and 7). The concept is supported by studies of both atrial and ventricular reentry. During atrial flutter, the reentrant circuit can stabilize on structural discontinuities in atria such as the crista terminalis and eustachian ridge³³ or on the tricuspid annulus.³⁴ Similarly, in ventricle, a meandering core of a spiral wave anchors to structures such as small arteries, which stabilize the wave.³⁵ Following this reasoning, it is not surprising that discordant cellular alternans provided a common mechanism for VF and SMVT in our experiments. Such findings suggest a singular mechanism by which T-wave alternans forms a substrate for initiation of VF and polymorphic and monomorphic VT.

The structural alterations produced by heart disease are obviously much more complex than those of our experimental model. Although a requirement for unidirectional block in these studies, discordant alternans is clearly only one component of a more complex substrate for reentry that is undoubtedly present in patients and other models of reentry. For example, in these experiments a single SB served to promote both discordant alternans and an anchor for monomorphic reentry, whereas in patients it is more likely that multiple SBs produce these effects independently. In addition, our model does not account for potential disease-induced changes in the expression of ionic currents.^{36 37} However, the physical principles set forth by our experiments should apply to more complex disease states.

	Acknowledgments

 
This study was supported by NIH Grant HL-54807 and the American Heart Association. We are extremely grateful to Imad Libbus for his assistance with data analysis.

Received August 30, 2000; revision received October 6, 2000; accepted October 6, 2000.

	References

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