Entrainment of reentrant ventricular tachycardia in anisotropic rings of rabbit myocardium. Mechanisms of termination, changes in morphology, and acceleration

Optogenetic manipulation of cardiac electrical dynamics using sub-threshold illumination: dissecting the role of cardiac alternans in terminating rapid rhythms

Cardiac action potential (AP) shape and propagation are regulated by several key dynamic factors such as ion channel recovery and intracellular Ca²⁺ cycling. Experimental methods for manipulating AP electrical dynamics commonly use ion channel inhibitors that lack spatial and temporal specificity. In this work, we propose an approach based on optogenetics to manipulate cardiac electrical activity employing a light-modulated depolarizing current with intensities that are too low to elicit APs (sub-threshold illumination), but are sufficient to fine-tune AP electrical dynamics. We investigated the effects of sub-threshold illumination in isolated cardiomyocytes and whole hearts by using transgenic mice constitutively expressing a light-gated ion channel (channelrhodopsin-2, ChR2). We find that ChR2-mediated depolarizing current prolongs APs and reduces conduction velocity (CV) in a space-selective and reversible manner. Sub-threshold manipulation also affects the dynamics of cardiac electrical activity, increasing the magnitude of cardiac alternans. We used an optical system that uses real-time feedback control to generate re-entrant circuits with user-defined cycle lengths to explore the role of cardiac alternans in spontaneous termination of ventricular tachycardias (VTs). We demonstrate that VT stability significantly decreases during sub-threshold illumination primarily due to an increase in the amplitude of electrical oscillations, which implies that cardiac alternans may be beneficial in the context of self-termination of VT.

Double-wave reentry in excitable media

A monolayer of chick embryo cardiac cells grown in an annular geometry supports two simultaneous reentrant excitation waves that circulate as a doublet. We propose a mechanism that can lead to such behavior. The velocity restitution gives the instantaneous velocity of a wave as a function of the time since the passage of the previous wave at a given point in space. Nonmonotonic restitution relationships will lead to situations in which various spacings between circulating waves are possible. In cardiology, the situation in which two waves travel in an anatomically defined circuit is referred to as double-wave reentry. Since double-wave reentry may arise as a consequence of pacing during cardiac arrhythmias, understanding the dynamic features of double-wave reentry may be helpful in understanding the physiological properties of cardiac tissue and in the design of therapy.

Note on a possible proarrhythmic property of antiarrhythmic drugs aimed at improving gap-junction coupling

Reduced conduction velocity (CV) in the myocardium is well known to increase the probability of arrhythmia and can be caused by structural changes, reduced excitability of individual myocytes, or decreased electrical coupling in the tissue. Recently, investigators have developed antiarrhythmic drugs that target the connections between individual myocytes with the goal of restoring tissue CV, specifically through increasing gap-junction coupling. In a simple but qualitatively relevant mathematical model, we show here that the introduction of a drug that improves intercellular conductance will indeed increase the CV. However, conditions that would require such a drug, such as fibrotic remodeling, may also increase the load of fibroblasts. Fibroblasts may couple to myocytes in much the same way as myocytes couple to each other, and therefore the use of such an agent may also improve coupling between myocytes and fibroblasts. We present numerical examples illustrating that when the load of coupled fibroblasts on myocytes is low or nonexistent, the drug works as expected, i.e., the drug increases CV. On the other hand, when the fibroblast load is high, changes in CV are nonmonotonic, i.e., the CV first increases and then decreases with an increase in dosage. The existence of coupled fibroblasts may therefore impair the effect of the drug, and under unfortunate conditions may be proarrhythmic.

MEMORY AND BISTABILITY IN A ONE-DIMENSIONAL LOOP OF MODEL CARDIAC CELLS

Unidirectional propagation has been studied in a one-dimensional loop of model cardiac cells represented as a homogeneous and isotropic cable. Membrane ionic currents were represented by a modified Beeler-Reuter model. The time constants of the gate variables of the slow inward current acting during the plateau of the action potential were divided by a parameter K ≥1. In the space-clamped model, increasing K shortens the action potential duration, changes the shape of the restitution curve and adds a slow memory component to the dynamics. In a paced regime, it promotes bistability in which period-1 and period-2 patterns coexist over an interval of pacing frequencies. In the loop, bistability is created between periodic and aperiodic modes of sustained reentry for an interval of loop length. The bistability of the space-clamped and loop model are both related to the form of the restitution curve.

Pacing-induced spatiotemporal dynamics can be exploited to improve reentry termination efficacy

Some potentially fatal cardiac arrhythmias may be terminated by a series of premature stimuli. Monomorphic ventricular tachycardia, which may be modeled as an excitation wave traveling around in a ring, is one such arrhythmia. We investigated the mechanisms and requirements for termination of such reentry using an ionic cardiac ring model. Termination requires conduction block, which in turn is facilitated by spatial dispersion in repolarization and recovery time. When applying short series of two or three stimuli, we found that for conduction block to robustly occur, the magnitude of the spatial gradient in recovery time must exceed a critical value of 20 ms/cm. Importantly, the required spatial gradient can be induced in this homogeneous system by the dynamics of the stimulus-induced waves-we show analytically the necessary conditions. Finally, we introduce a type of pacing protocol, the "aggressive ramp," which increases the termination efficacy by exploiting such pacing-induced heterogeneities. This technique, which is straightforward to implement, may therefore have important clinical implications.

Dynamics of sustained reentry in a loop model with discrete gap junction resistances

The dynamics of reentry is studied in a one-dimensional loop of model cardiac cells with discrete intercellular gap junction resistance (R). Each cell is represented by a continuous cable with ionic current given by a modified Beeler-Reuter formulation. For R below a limiting value, propagation is found to change from period-1 to quasiperiodic (QP) at a critical loop length (L(crit)) that decreases with R. Quasiperiodic reentry exists from L(crit) to a minimum length (L(min)), which also shortens with R. The decrease of L(crit) (R) is not a simple scaling, but the bifurcation can still be predicted from the slope of the restitution curve giving the duration of the action potential as a function of the diastolic interval. However, the shape of the restitution curve changes with R. An increase of R does not seem to increase the number of possible QP solutions since, as in the continuous cable, only two QP modes of propagation were found despite an extensive search through alternative initial conditions.

Interactions between paced wavefronts and monomorphic ventricular tachycardia: implications for antitachycardia pacing

OBJECTIVES

Interactions between paced wavefronts and monomorphic ventricular tachycardia (VT) dictate antitachycardia pacing outcomes. We used optical mapping to assess those interactions during single and dual site pacing of rabbit ventricular epicardium.

METHODS AND RESULTS

Monomorphic VTs were initiated in six isolated rabbit hearts that were endocardially cryoablated to limit viable tissue to visible epicardium and establish apical tissue as the anatomic anchor. Preparations were optically mapped during single (n = 39) and dual (n = 43) site pacing at 50%-90% of VT cycle length (CL) with eight pulses per trial. Overall, we found six pulses that abruptly terminated VT. This occurred because the VT wavefront collided with the antidromic portion of the paced wavefront and the orthodromic portion of paced wavefront blocked in the VT's refractory region. When effective, dual site pacing that captured tissue at both leads simultaneously terminated the VT immediately, while single site pacing or dual site pacing that captured tissue at only one lead terminated the VT after resetting advanced the orthodromic wavefront. We found 12 pulses that induced polymorphic VT, with 11 of those pulses occurring during capture at only one lead. Expansion of the combined antidromic-VT wavefront around one or both ends of the arc of conduction block formed by the interaction of the orthodromic wavefront with the VT's refractory region initiated functional reentry. Six of these polymorphic VTs were nonsustained because the underlying wavefronts self-terminated. The wavefronts did persist for 4.2 +/- 3.5 cycles before self-terminating in these trials, and the post-pacing cycles presented a 146% increase in CL variability, compared with the variability prior to pacing. These temporal characteristics are similar to those of delayed termination in patients with ICDs.

CONCLUSIONS

The main difference between pulses that terminated abruptly and pulses that induced polymorphic VT was the effective separation of the antidromic and orthodromic portions of the paced wavefront from one another.

Comparison of conventional and biventricular antitachycardia pacing in a geometrically realistic model of the rabbit ventricle

INTRODUCTION

ICDs often are programmed with antitachycardia pacing (ATP) as the first response to ventricular tachycardia (VT). Many ICDs have an additional lead available for ventricular pacing. We hypothesized that using the additional lead for ATP would improve therapy by advancing the orthodromic wavefront, thereby reducing the size of the excitable gap and inducing block of all reentrant activity.

METHODS AND RESULTS

Monomorphic VT was initiated in a thin-walled model of rabbit ventricular myocardium that included an apical infarct and anatomically realistic dimensions. ATP with up to eight pulses was delivered at 90% of VT cycle length to one (conventional) or two (biventricular) stimulation areas. Stimulation areas were adjusted from 0.017 cm2 to 0.169 cm2 to modulate interactions between the antidromic and VT wavefronts, and between the orthodromic wavefront and the VT's refractory region. During conventional ATP, we found that larger stimulation areas terminated the VT in three pulses. Continued pacing after termination caused VT reinitiation in the reversed direction in some instances. With smaller stimulation areas, conventional ATP simply reset the circuit. During biventricular ATP, larger stimulation areas terminated VT in one pulse. There were no instances of reinitiation with reversal. However, with smaller stimulation areas, prolongation of refractoriness near the additional stimulation area facilitated induction of functional reentry with pathways modified by continued pacing.

CONCLUSION

Our modeling suggests that biventricular ATP is superior to conventional ATP under conditions where the additional ventricular lead effectively advances the orthodromic wavefront. Failure to achieve this advancement poses a risk of VT acceleration.

Basic mechanisms of cardiac impulse propagation and associated arrhythmias

Propagation of excitation in the heart involves action potential (AP) generation by cardiac cells and its propagation in the multicellular tissue. AP conduction is the outcome of complex interactions between cellular electrical activity, electrical cell-to-cell communication, and the cardiac tissue structure. As shown in this review, strong interactions occur among these determinants of electrical impulse propagation. A special form of conduction that underlies many cardiac arrhythmias involves circulating excitation. In this situation, the curvature of the propagating excitation wavefront and the interaction of the wavefront with the repolarization tail of the preceding wave are additional important determinants of impulse propagation. This review attempts to synthesize results from computer simulations and experimental preparations to define mechanisms and biophysical principles that govern normal and abnormal conduction in the heart.

Resetting and annihilation of reentrant activity in a model of a one-dimensional loop of ventricular tissue

Resetting and annihilation of reentrant activity by a single stimulus pulse (S1) or a pair (S1-S2) of coupled pulses are studied in a model of one-dimensional loop of cardiac tissue using a Beeler-Reuter-type ionic model. Different modes of reentry termination are described. The classical mode of termination by unidirectional block, in which a stimulus produces only a retrograde front that collides with the activation front of the reentry, can be obtained for both S1 and S1-S2 applied over a small vulnerable window. We demonstrate that another scenario of termination-that we term collision block-can also be induced by the S1-S2 protocol. This scenario is obtained over a much wider range of S1-S2 coupling intervals than the one leading to a unidirectional block. In the collision block, S1 produces a retrograde front, colliding with the activation front of the pre-existing reentry, and an antegrade front propagating in the same direction as the initial reentry. Then, S2 also produces an antegrade and a retrograde front. However, the propagation of these fronts in the spatial profile of repolarization left by S1 leads to a termination of the reentrant activity. More complex behaviors also occur in which the antegrade fronts produced by S1 and S2 both persist for several turns, displaying a growing alternation in action potential duration ("alternans amplification") that may lead to the termination of the reentrant activity. The hypothesis that both collision block and alternans amplification depend on the interaction between the action potential duration restitution curve and the recovery curve of conduction velocity is supported by the fact that the dynamical behaviors were reproduced using an integro-delay equation based on these two properties. We thus describe two new mechanisms (collision block and alternans amplification) whereby electrical stimulation can terminate reentrant activity. (c) 2002 American Institute of Physics.

Localized injury in cardiomyocyte network: a new experimental model of ischemia-reperfusion arrhythmias

We developed a new experimental approach to study the effects of local injury in a multicellular preparation and tested the ability of the method to induce reperfusion arrhythmias in cardiomyocyte monolayers. A small region of injury was created using geometrically defined flows of control and ischemia-like solutions. Calcium transients were acquired simultaneously from injured, control, and border zone cells using fluo 4. Superfusion with the injury solution rapidly diminished the amplitude of calcium transients within the injury zone, followed by cessation of cell beating. Reperfusion caused an immediate tachyarrhythmic response in approximately 17% of experiments, with a wave front propagating from a single cell or small cell cluster within the former injury zone. Inclusion of a gap junction uncoupler (1 mM heptanol) in the injury solution narrowed the functional border and sharply increased the number of ectopic foci and the incidence of reperfusion arrhythmias. The model holds a potential to reveal both micro- and macroscopic features of propagation, conduction, and cell coupling in the normal and diseased myocardium and to serve as a new tool to test antiarrhythmic protocols in vitro.

Abrupt loss of constant fusion during entrainment of ventricular tachycardia at a critical paced cycle length

Sustained monomorphic ventricular tachycardia (VT) can be frequently entrained and interrupted with rapid pacing and the mechanism of the pacing-induced interruption is considered to be due to orthodromic block. This study focused on the incidence of VT which was interrupted at a critical cycle length and was characterized by an abrupt loss of constant fusion in the surface electrocardiogram (ECG), and the role of orthodromic block as the cause of such characteristic change and interruption of VT was analyzed. Among 45 consecutive patients with symptomatic VT, rapid pacing was performed in 43 VTs of 39 patients. The exit was mapped as the earliest site of the activation during VT and an electrode catheter was located at the site. Rapid pacing was performed at progressively shorter cycle lengths in steps of 10 msec until VT was interrupted and the timing of the orthodromic and direct capture was compared at the exit. Abrupt loss of constant fusion was observed in 25 of 39 patients (64.1%): and the loss was invariably associated with interruption of VT. When the timings of the activation of the exit were compared, which were measured from the preceding (n-1) stimulus as the time reference, the direct capture was relatively delayed compared to that of the orthodromic capture. This finding suggests that orthodromic block is the cause of the direct capture as well as the pacing-induced interruption of VT. In the remaining 13 patients (35.9%), the surface ECG showed a gradual transition into the fully paced QRS morphology. The direct capture was confirmed in the non-fused beats, but it was not necessarily associated with interruption of VT. The interval from the stimulus to the entrained electrogram at the exit showed a gradual prolongation until the exit was finally captured directly from the pacing site. The confirmation of constant fusion followed by abrupt loss in ECG can be a reliable hallmark of orthodromic block as the cause of the interruption of VT during transient entrainment at a critical paced cycle length.

Antitachycardia burst pacing for pleomorphic reentrant ventricular tachycardias associated with non-coronary artery diseases: a morphology specific programming for ventricular tachycardias

To study the role of antitachycardia burst pacing in patients with reentrant pleomorphic ventricular tachycardia (VT) associated with non-coronary artery diseases, the efficacy of antitachycardia pacing and appropriate antitachycardia pacing cycle length were evaluated in each pleomorphic VT morphology of seven patients. Seven patients were included in this study. Clinically documented pleomorphic VTs were reproduced in an electrophysiologic study. For each VT, rapid ventricular pacing was attempted from the apex of the right ventricle at a cycle length which was 20 ms shorter than that of VT and repeated after a decrement of the cycle length in steps of 10 ms until the VT was terminated or accelerated. All 16 VTs could be entrained by the rapid pacing, and 13 of the 16 VTs (81%) were terminated, whereas pacing-induced acceleration was observed in the other 3 VTs of the 3 patients. VT cycle length (VTCL), block cycle length (BCL) which was defined as the longest VT interrupting paced cycle length, %BCL/VTCL and entrainment zone which was defined as VTCL minus BCL, varied in each VT morphology of each patient. In two patients, antitachycardia pacing was effective in all VT morphologies and the maximum difference of the %BCL/VTCL among the pleomorphic VTs was less than 10%. Thus, antitachycardia pacing seemed to be beneficial for these patients. In the other 5 patients, a difference of more than 10% in %BCL/VTCL was observed among the pleomorphic VT morphologies and/or at least one VT morphology showed pacing-induced acceleration. Compared to the 13 terminated VTs, three accelerated VTs had a wide entrainment zone [160 +/- 44 vs 90 +/- 48 ms, p < 0.04] and small %BCL/VTCL [61 +/- 6 vs 77 +/- 11%,p<0.03]. In pleomorphic VTs associated with non-coronary artery diseases, responses to rapid pacing was not uniform; VT might be terminable or accelerated even in the same patient. We need to pay close attention when programming antitachycardia pacing in patients with pleomorphic VT.

Acceleration of typical atrial flutter due to double-wave reentry induced by programmed electrical stimulation

BACKGROUND

Acceleration of reentrant tachycardia induced by programmed electrical stimulation is a well-documented phenomenon, but the mechanisms remain poorly understood.

METHODS AND RESULTS

Twelve patients with typical atrial flutter were studied. Activation sequence of the underlying reentrant circuit was recorded by multiple multipolar electrodes placed in the right atrium. In five patients, 27 episodes of atrial flutter acceleration were induced by single extrastimuli delivered in the isthmus between the tricuspid annulus and eustachian ridge (TA-ER isthmus) and one by rapid overdrive atrial pacing. Analyses of the activation sequences, intracardiac electrograms, and 12-lead surface ECG P-wave morphology indicated that the acceleration was caused by two successive activation wave fronts circulating in the same direction along the same reentrant circuit (double-wave reentry, DWR). DWR was induced only within a narrow range of coupling interval, from 2 to 45 ms beyond the effective refractory period, and was associated with unidirectional antidromic block of the paced impulse. Patients with DWR had a shorter effective refractory period (138.8+/-13.4 versus 163.8+/-12.2 ms, P<.015) and larger excitable gap (124.0+/-22.6 versus 83.2+/-13.2 ms, P<.009) compared with patients without inducible DWR. All of the DWR episodes were transient. Most (78.6%) terminated after one of the double wave fronts was blocked in the TA-ER isthmus.

CONCLUSIONS

DWR is one of the mechanisms responsible for programmed electrical stimulation-induced atrial flutter acceleration in human subjects. Its induction requires a sufficient excitable gap and antidromic unidirectional block of the paced impulse in the TA-ER isthmus. In addition, the TA-ER isthmus is the usual site of DWR termination.

Induction of ventricular fibrillation by T-wave field-shocks in the isolated perfused rabbit heart: role of nonuniform shock responses

OBJECTIVES

Single electrical field shocks are able to induce ventricular fibrillation (VF) if applied during the vulnerable period. During this period, a shock can either prolong the action potential duration or induce a new action potential. Whether the occurrence of different shock responses contributes to the induction of VF has not been investigated directly in the intact heart.

METHODS

In 12 isolated Langendorff-perfused rabbit hearts seven monophasic action potentials (MAPs) were recorded simultaneously during the application of 838 T-wave shocks. Post-shock repolarization was assessed by classifying the shock-induced response in each MAP recording either as a full action potential or an action potential prolongation. Heterogeneity of post-shock repolarization was defined if both response patterns were present in different MAP recordings at the same time. The heterogeneity of post-shock activation was measured as the dispersion of the post-shock activation time (PS-AT). The arrhythmogeneity of a shock was quantified as the number of rapid shock-induced repetitive responses.

RESULTS

Shocks inducing nonuniform repolarization were associated with greater arrhythmogeneity than shocks inducing uniform repolarization (17.6 +/- 30.0 versus 1.6 +/- 1.1 shock-induced repetitive responses, p < 0.001). The severity of the induced arrhythmia increased gradually with increasing nonuniformity of repolarization (p < 0.01 for a 10% increase), being maximal when the shock initiated near equal numbers of both full action potentials and action potential prolongations. The induction of severe arrhythmias by T-wave shocks was also associated with a higher dispersion of PS-AT (29 +/- 14 ms for the induction of VF, 19 +/- 12 ms for non-sustained arrhythmia, and 12 +/- 8 ms for no arrhythmic response, all p < 0.001). For VF inducing shocks, an increase in shock strength towards the upper limit of vulnerability decreased the dispersion of PS-AT from 34 +/- 15 ms to 23 +/- 11 ms (p < 0.001).

CONCLUSIONS

Nonuniform post-shock repolarization and dispersed post-shock activation contribute to the induction of VF by T-wave shocks. A decreasing dispersion of PS-AT towards higher shock strengths may contribute to the decreased or abolished inducibility by shocks above the upper limit of vulnerability.

Reversal of reentry and acceleration due to double-wave reentry: two mechanisms for failure to terminate tachycardias by rapid pacing

OBJECTIVES

We sought to demonstrate mechanisms by which rapid pacing can cause conduction block without terminating reentry.

BACKGROUND

Rapid pacing can fail to terminate or can accelerate tachycardias in patients. Mechanisms for these responses are poorly understood.

METHODS

We studied reentry in the canine atrial tricuspid ring and a left ventricular ring in vitro in 12 preparations. Activations were recorded from 10 sites around the ring, and monophasic action potentials were recorded from critical sites of block. Rapid pacing at cycle lengths that intermittently caused conduction block was performed at multiple sites.

RESULTS

Action potential alternans contributed to block of an orthodromic impulse during rapid pacing. When pacing continued for two stimuli after orthodromic block, a second episode of block could reverse the direction of tachycardia. Continued pacing at this site was likely to produce block of an antidromic impulse, which may initiate double-wave reentry. Double-wave reentry could be sustained or nonsustained. Its cycle length was 56% to 77% of the single-wave cycle length. The ratio of double-wave cycle length to single-wave cycle length was inversely correlated with the relative excitable gap (p < 0.01). Double-wave reentry can be a mechanism for persistent cycle length alternation during tachycardia.

CONCLUSIONS

Successful termination of reentry by rapid pacing required block of an othrodromic impulse and stopping pacing within one stimulus after orthodromic block. Reversal of reentry makes the circuit resistant to termination from this site of pacing. Antidromic block can cause acceleration due to double-wave reentry when there is a substantial excitable gap.

Myocardial vulnerability to T wave shocks: relation to shock strength, shock coupling interval, and dispersion of ventricular repolarization

INTRODUCTION

Induction of ventricular fibrillation (VF) by T wave shocks is of clinical interest due to the correlation between the upper limit of vulnerability (ULV) and the defibrillation threshold (DFT). However, the ULV has not yet been defined precisely in reference to the entire "area of vulnerability" (AOV), which is defined bifunctionally by both shock strengths and shock coupling intervals, nor has it been related to the dispersion of ventricular repolarization, considered to be an important determinant of vulnerability.

METHODS AND RESULTS

In 11 isolated perfused rabbit hearts immersed in a tissue bath containing a 3-lead ECG recording system and two opposite plate electrodes for field shock administration, 7 monophasic action potentials (MAPs) were recorded simultaneously from different epicardial and endocardial regions of the right and left ventricles. An average of 90 +/- 25 monophasic waveform shocks of varying shock strengths and coupling intervals were delivered to each heart to determine the horizontal and vertical boundaries of the AOV. The AOV approximated a rhomboid with homogenous VF inducibility. The ULV and lower limit of vulnerability (LLV) represented discrete corners of the AOV with significant changes in VF inducibility if either shock coupling intervals or shock strength were changed by only 10 msec or 10 V, respectively (P < 0.001). The ULV occurred at 7 +/- 10 msec shorter coupling intervals than the LLV (P < 0.05), and VF-inducing shock strengths at the left corner of the AOV were 50 +/- 67 V higher as compared to the right corner (P < 0.01). The maximal range of VF-inducing coupling intervals coincided (within < 2 msec) with the dispersion of MAPs at 70% repolarization, and the ULV coupling interval coincided (within < 4 msec) with the longest repolarization at 50%.

CONCLUSIONS

(1) VF vulnerability to monophasic T wave shocks is defined by an AOV that has the shape of a leftward tilted rhomboid. (2) Both the ULV and LLV are sharply defined upper and lower corners of the AOV rhomboid. (3) The width of the AOV corresponds to the dispersion of ventricular repolarization at the 70% level. (4) Considering the dispersion of ventricular repolarization may yield more precise ULV determinations and a better understanding of the correlation between the ULV and DFT.

Simulation of two-dimensional anisotropic cardiac reentry: effects of the wavelength on the reentry characteristics

A two-dimensional sheet model was used to study the dynamics of reentry around a zone of functional block. The sheet is a set of parallel, continuous, and uniform cables, transversely interconnected by a brick-wall arrangement of fixed resistors. In accord with experimental observations on cardiac tissue, longitudinal propagation is continuous, whereas transverse propagation exhibits discontinuous features. The width and length of the sheet are 1.5 and 5 cm, respectively, and the anisotropy ratio is fixed at approximately 4:1. The membrane model is a modified Beeler-Reuter formulation incorporating faster sodium current dynamics. We fixed the basic wavelength and action potential duration of the propagating impulse by dividing the time constants of the secondary inward current by an integer K. Reentry was initiated by a standard cross-shock protocol, and the rotating activity appeared as curling patterns around the point of junction (the q-point) of the activation (A) and recovery (R) fronts. The curling R front always precedes the A front and is separated from it by the excitable gap. In addition, the R front is occasionally shifted abruptly through a merging with a slow-moving triggered secondary recovery front that is dissociated from the A front and q-point. Sustained irregular reentry associated with substantial excitable gap variations was simulated with short wavelengths (K = 8 and K = 4). Unsustained reentry was obtained with a longer wavelength (K = 2), leading to a breakup of the q-point locus and the triggering of new activation fronts.

The dynamics of sustained reentry in a ring model of cardiac tissue

This paper describes the dynamics of circus movement around a fixed obstacle, using a one-dimensional continuous and uniform ring model of cardiac tissue to simulate sustained reentry. The membrane ionic current is simulated by a modified Beeler-Reuter formulation in which the kinetics of the fast sodium current were updated using more recent voltage-clamp data. Changes in the ring length are used to modify the dynamics of reentry. Reentry is stable if the ring length (X) exceeds a critical value (Xcrit) and complete block occurs if X is below a minimum (Xmin). Irregular sustained reentry is observed at intermediate ring lengths, as a narrow range of aperiodic reentry near Xcrit, and a larger range of quasi-periodic reentry at shorter ring lengths. The basic pattern of irregular reentry is an alternation between long and short cycle length, action potential duration (APD), diastolic interval (DIA), wavelength, and excitable gap. In aperiodic reentry cycle length variations are small, APD and DIA fluctuations are of medium amplitude, and conduction velocity over the whole pathway is essentially constant during successive turns. Much larger fluctuations in these various quantities occur during quasi-periodic reentry, and they increase in size as X approaches Xmin. The complexity of quasi-periodic reentry patterns is related to three factors: the slope of the APD versus DIA relation, which is greater than 1, the existence of a zone of slow conduction on the ring when the excitable gap becomes quite short, and the occurrence of triggered waves of secondary repolarization and excitability recovery. In the present model, quasi-periodic reentry with triggered secondary recovery covers most of the range of ring lengths, giving rise to sustained irregular reentry. There is very close agreement between our simulation results and experimental data obtained on rings of cardiac tissue.

Regional entrainment of atrial fibrillation studied by high-resolution mapping in open-chest dogs

BACKGROUND

It recently has been demonstrated that during atrial fibrillation, a short and variable excitable gap exists, allowing regional control of atrial fibrillation by local stimulation. In the present study, we visualized the process of excitation during regional entrainment of atrial fibrillation by rapid pacing.

METHODS AND RESULTS

In six open-chest dogs, the excitation of the left atrial free wall was mapped using a spoon-shaped mapping electrode (248 points). Episodes of atrial fibrillation were induced by burst pacing (50 Hz, 2 seconds). During atrial fibrillation, the electrograms showed rapid irregular activity with a median cycle length of 98 +/- 16 ms (mean +/- SD, n = 6). Rapid pacing in the center of the mapping electrode at intervals slightly shorter or longer than the median atrial fibrillation interval resulted in regional capture of atrial fibrillation. The window of entrainment was 16 +/- 5 ms. Mapping of atrial fibrillation showed that the left atrium was activated by fibrillatory wavelets coming from different directions. During entrainment, a relatively large area with a diameter of about 4 cm was activated by uniform wave fronts propagating away from the site of stimulation. The area of entrainment was limited by intra-atrial conduction block and by collision with fibrillation waves. Regional control of atrial fibrillation was lost by pacing either too slowly or too rapidly. In the first case, retrograde invasion of the area of entrainment by fibrillatory waves resulted in depolarization of the pacing site prior to the stimulus. Pacing too rapidly caused acceleration of atrial fibrillation by induction of local intra-atrial reentry circuits with a revolution time shorter than the pacing interval.

CONCLUSIONS

During atrial fibrillation, an area with a diameter of about 4 cm can be entrained by local pacing. The resulting reduction in fibrillating tissue mass was not sufficient to terminate atrial fibrillation. Extension of the area of entrainment was limited by intra-atrial conduction block, whereas entrainment at a too high rate resulted in acceleration of atrial fibrillation by induction of local microreentry.