If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
Extended regions of BZ tissue that penetrate through nonconducting necrotic core scar can give rise to anatomic isthmuses, which provide slow conducting reentrant pathways that can help sustain VT circuits.
Significant technical challenges involved in measuring 3-dimensional electrical behavior at the tissue/organ level have largely prevented detailed preclinical and clinical analysis of the proarrhythmic behavior of the BZ tissue. Specifically, comprehensive understanding of exactly how the complex structural and electrophysiological (EP) remodeling in this region provides the so important, yet elusive, arrhythmic trigger remains lacking.
VT is driven by the presence of triggers and substrate that promote initiation and sustenance of reentrant electrical wavefronts.
Reduced sodium (Na+) current (INa) and fibrosis have also been shown to provide a substrate, not only for block (by reducing excitability) but also for VT sustenance by slowing conduction and shortening the wavefront of electrical propagation.
At the cellular level, triggered activity has been shown to be caused by delayed afterdepolarizations (DADs) linked to spontaneous calcium (Ca2+) release (SCR) events from the sarcoplasmic reticulum (SR).
SCRs activate the Na+-Ca2+exchanger (NCX), an inward current that leads to depolarization of the diastolic membrane potential (Vm). DADs can be divided into subthreshold or suprathreshold depending on whether the resulting depolarization is below or above the threshold for action potential (AP) initiation, respectively. The conditions under which DADs can summate to form premature ventricular complexes (PVCs) are still a matter of debate and have been a topic of research in our group and others.
At the tissue scale, DADs can only capture the local tissue and trigger propagating PVCs if the generated net ionic current is sufficient to overcome the local electrotonic load imposed by the surrounding tissue.
In a computational study, we have demonstrated that this protective cardiac mechanism can be disrupted by fibrosis (which attenuates local electrotonic coupling), making DAD capture and PVCs more likely.
Less understood is the role played by subthreshold DADs in postinfarction VT initiation. Long-lasting subthreshold DADs have been shown to inactivate Na+ channels, leading to temporary conduction block
while not affecting overall activation wavelength as in the scenario of permanent changes in local repolarization. However, the role played by local infarct anatomy and altered intracellular coupling in facilitating this phenomenon is not well understood. The purpose of this study was to use in silico experiments to demonstrate a fundamental mechanism for arrhythmogenesis in the infarct BZ. Our hypothesis, as summarized in Figure 1, is that SCR-mediated subthreshold DADs occurring within the BZ can inactivate INa favoring conduction block in narrow isthmuses where electrotonic load is lessened by the nonconducting scar. This conduction block, occurring as it does at critical sites within the reentrant pathway, is responsible for directly initiating postinfarct VTs.
EP model and parameters
Figure 2A shows the magnetic resonance imaging (MRI)-based porcine models of the left ventricular (LV) anatomy and scar morphology used in this study. Animal studies were performed at the Institut de Chirurgie Guidée par l’image, Strasbourg, France. The experimental protocol was approved by the national institutional animal care and ethics committee.
Scar and BZ segmentation thresholds of 60% and 40% of the maximum pixel intensity in the late gadolinium enhancement MRI scans were used, respectively. Further details about model construction are given in our previous study.
In brief, Ca2+ overload was induced by increasing extracellular Ca2+ concentration, the NCX current was doubled, and the inward rectifier potassium current was decreased by 70%. Normal INa properties in the MSH model prevent subthreshold DADs to cause conduction block regardless of their amplitude. Thus, as performed in previous studies, the Na+ channel steady-state inactivation curve of the model was left-shifted by 5 mV to promote INa inactivation potentiating conduction failure.
SCR events were inhibited in healthy myocardium to investigate DAD formation only in cells in the BZ.
Anisotropic bulk conductivity values of 0.1361 S/m and 0.0176 S/m were assigned along and transverse to the fiber direction, respectively. Tissue conductivity in the BZ was adjusted by a scaling factor (1.0–0.25; steps of 0.25) to gauge the effects of gap junction uncoupling on DAD-mediated conduction block.
The MSH cell model was paced at a basic cycle length (BCL) of 500 ms for 100 cycles to stabilize. Single-cell model states at the end of the pacing protocol were stored and used as initial conditions for the LV model. Arrhythmia susceptibility was quantified by pacing the model for 3 S1 beats (BCL = 500 ms) from either apex or base (Figure 2B), followed by a 1500-ms pause, to see whether DADs would occur, followed by an S2 extrastimulus beat with coupling intervals (CIs) varying from 500 to 1000 ms. Due to the stochastic nature of the SCR-mediated DADs, 100 simulations were performed. Simulations with captured suprathreshold DADs (PVCs) were excluded to evaluate DADs only as substrate for conduction block rather than ectopy.
Subthreshold DADs within the postinfarction heart
Figure 3 shows the spatial distribution of Vm on the epicardial surface of the 3 LV models at the instant in time that DADs reached their highest amplitude inside the marked area. The endocardial DAD pattern (not shown) was similar to that of the epicardium. Note that for the same time instant after the paced beat, DADs with higher amplitudes were found to occur closer to the pacing site.
Figure 4 shows the maximum Vm elevation at all sites in the tissue caused by DADs over the entire duration of the pacing pause. DADs in cells within isthmus regions, where the presence of scar and fibrosis reduce cellular coupling and local electrotonic load, have a higher amplitude than DADs from regions surrounded by healthy, well-coupled myocardium. However, the direction of the wavefront had a minor effect on the max Vm elevation. Figure 4B illustrates subthreshold DADs from 2 different regions of the BZ of Pig 3. SCR events in cells from the isthmus were able to raise Vm to –55 mV (red trace) compared to –74 mV in the well-coupled region near the base. As shown in Figure 4C, DADs completely inactivated Na+ channels in the isthmus (red trace) and caused approximately 50% channel inactivation in well-coupled tissue (green trace).
DAD-mediated block and reentry
The pacing protocol described (Figure 2B) was used to investigate the occurrence of block and reentrant formation within the postinfarction models driven by subthreshold DADs. Figure 5 shows Vm maps of the S2 beat as well as the resulting sustained reentry in all 3 LV models. Conduction block depended on the specific S2 pacing location and subsequent conduction pathway. Particularly in Pig 3, VT was induced after an S2 beat (CI = 710 ms) had initially blocked at the isthmus mouth proximal to the stimulus site (t = 960 ms). Conduction block occurred in the isthmus as a direct result of the reduced excitability caused by the inactivation of the Na+ current (Figure 4C) by ongoing subthreshold DADs rather than prolonged AP duration. The wavefront then propagates around the scar, reentering the isthmus through the distal mouth, close to the apex leading to a sustained VT circuit.
Arrhythmogenic potential of tissue uncoupling in the BZ
Figures 6A and 6B show how reduced tissue conductivity affects the amplitude and spatial extent of DADs in Pig 3 model. The average Vm elevation due to DADs increases from about –72 mV to –69 mV when tissue conductivity is decreased from 1 (control) to 0.25 of the control values. However, the volume of tissue undergoing DADs correspondingly decreases with the decrease in tissue conductivity as uncoupling limits the dissipation of electrotonic currents from DAD sources.
The arrhythmogenic link between tissue uncoupling and subthreshold DADs is shown in Figure 6C. In control conditions (ie, scaling factor 1.0), an S2 beat with CI = 700 ms still was able to propagate through the isthmus. However, S2 beats within 700 ms < CI < 900 ms resulted in unidirectional block as DADs rendered the entire isthmus area refractory. S2 beats with CI ≥900 ms resulted in successful propagation. This window of conduction block is illustrated in Figure 7. Reductions in tissue conductivity led to a widening of the S2 CI window in which block was observed. As shown in Figure 6C, applying a scaling factor of 0.25 to tissue conductivity shifted the beginning of the window for conduction block from 700 to 710 ms and prolonged it by 60 ms (710 ms <CI < 960 ms).
Remodeling in the infarct BZ is thought to play a key role in facilitating the genesis of VT, but the exact mechanism behind arrhythmia formation and maintenance is not comprehensively characterized. Capture of DADs at the tissue scale (PVC formation) has been shown to be implicated in the generation of focal arrhythmias in the intact heart. However, the heart’s protective source-sink mismatch related to electrotonic loading between well-coupled cells make these unlikely. Here, we hypothesized that DADs occurring in the BZ of infarct tissue in fact do not need to capture themselves to be arrhythmogenic. Instead, subthreshold DADs can still provide a substrate for unidirectional conduction block and subsequent reentry in regions where local myocyte coupling is disrupted by the complex fibrotic structure of the infarct.
In silico experiments using high-resolution MRI-derived computational models have shown that narrow isthmuses within infarcted regions provide both the most likely and most critical sites of unidirectional block to facilitate VT onset by (1) reducing electrotonic loading as the depolarizing current is geometrically constrained by the surrounding scar, allowing DADs to inactivate the INa current, setting the stage for unidirectional conduction block; and (2) representing highly vulnerable areas for unidirectional block, being a crucial part of the reentrant circuit sustaining VT where conduction is “cablelike” or pseudo–1-dimensional. Furthermore, DADs did not change AP duration, making VT sustenance more likely as the cardiac wavelength is not altered, as in a scenario of long-lasting repolarization. Reduced tissue conductivity in the BZ enhanced DAD-mediated arrhythmogenicity by increasing the vulnerable window for conduction block.
DAD-mediated conduction block
VTs in postinfarction hearts are commonly sustained by anatomic isthmuses within regions of scar,
which provide a reentrant pathway. In order to set up such a reentrant circuit, a prerequisite is a substrate that provides the initial unidirectional conduction block, usually within or at the mouth of the isthmus. In the context of ischemic heart disease, nonuniform anisotropy (slow conduction), abnormal repolarization, and impedance mismatch in the BZ are some of the proposed mechanisms of VT onset.
As shown in Figure 4, DADs are observed throughout the BZ, but INa inactivation was more pronounced in regions where local diastolic Vm was elevated to approximately –60 mV. Across all models, narrow isthmuses seemed to be where depolarization is raised the most, providing both mostly likely and most critical sites of unidirectional conduction block (Figure 5). However, the timing and location of conduction block and subsequent reentry, and, consequently, the window of conduction block shown in Figures 6 and 7, were still significantly influenced by (1) the specific pacing location of the S2 beat, as this dictates the time and incidence of the wavefront at the critical region; and (2) the properties of the tissue and local anatomic milieu determining the velocity/pathway the S2 takes to reach the vulnerable region.
VT sustenance depends on the wavelength of the electrical impulse, which must be shorter than the anatomic pathlength formed by the conducting isthmus.
The wavelength is given by the mathematical product of the conduction velocity (CV) and the effective refractory period (closely related to AP duration). Thus, a “successful” arrhythmogenic substrate must have electrophysiological alterations that are proarrhythmic to both initiation (ie, formation of unidirectional block at the isthmus mouth) and reentry sustenance (ie, reducing wavelength). Prolonged APD in the BZ, although making unidirectional block likely upon rapid pacing at the BZ/healthy myocardium interface, is not conducive to reentry sustenance as it also prolongs wavelength, making termination through head–tail interactions more likely. Structural remodeling (fibrosis/tissue uncoupling) as well as reduced INa have also been linked to VT formation as they slow CV (shortening wavelength) and increase the likelihood of unidirectional block.
As shown in Figures 4 and 5, subthreshold DADs can also cause the unidirectional block required for VT initiation at critical isthmus sites within the infarct while also making VT sustenance more likely by not impacting activation wavelength, as in the scenario of long-lasting repolarization.
Incorporation of structural remodeling in our model by reducing tissue conductivity favored arrhythmogenesis by increasing the amplitude of subthreshold DADs (Figure 6A) enhancing INa inactivation, which in turn widened the vulnerable window for conduction block (Figure 6C). In this scenario, the reduced tissue coupling allows the local voltage near the site of DAD production to be driven higher, as local diffusive coupling is reduced. Furthermore, such reduced conductivity also slows CV, which also favors VT sustenance by shortening the wavelength.
Pharmacologic therapies aimed at severing the link between SCR and DADs could prevent arrhythmia formation and PVC burden. Although β-blockers are the first line of treatment by preventing adrenergic stimulation and Ca2+ loading, new compounds with more specific inhibition, such as stabilization of ryanodine receptors, may have potential benefits in arrhythmia/PVC prevention.
The amplitude of DADs has been demonstrated to be highly sensitive to the timing variability of SCR events between cardiomyocyte.s9 At the tissue level, electrotonic coupling between cells will decrease the effective rise of Vm in regions of unsynchronized DADs. Thus, any drug capable of widening the time distribution of SCR events would reduce the probability of neighboring cells to undergo DADs. Desynchronized DADs are less likely not only to render tissue refractory, as shown here, but also to capture and propagate in the form of a PVC.
Furthermore, as shown in Figure 6C, tissue uncoupling increased DAD amplitude, widening the time window for conduction block. Therefore, pharmacologic modulation of gap junctions to restore intercellular coupling as well as therapies aiming at reversing structural remodeling at the BZ would have the antiarrhythmic effects of narrowing down the vulnerable window and lengthening the wavelength by speeding up CV.
Finally, identification of entry or exit sites of the isthmus is of great clinical significance as they provide an important target for catheter ablation therapy.
Our results suggest that very narrow regions of surviving tissue, where subthreshold DADs could render all tissue inexcitable, could be specific ablation targets that not only may eliminate the substrate for conduction block but also interrupt a reentrant pathway. Because DAD-mediated VT initiation was often co-located near the mouth of narrow isthmus, a substrate-based ablation strategy might be preferable.
However, mapping of stochastic DADs is challenging, requiring intricate placement of the catheter in space and the time to detect them. Furthermore, reliable detection of DAD signatures on electrograms might be challenging for current mapping systems with sufficient signal-to-noise. Adequate cardiac imaging and segmentation may be more suitable to identify the structural anatomy of isthmuses prone to DAD-mediated block.
Future investigations should focus on understanding and mapping anatomic parameters promoting DAD-mediated VT formation.
In this study, conduction block occurred within 700–900 ms after the last S1, which is longer than the BCL. This is because DADs in the MSH model are generated by stochastic SCRs with a functional dependence on the SR Ca2+ concentration, which was adjusted here to prevent PVC formation.
this was not investigated here because we focused on the role played by local infarct anatomy. Future research into the effect of pacing cycle length on DAD formation and the role played by structural remodeling in reentry sustenance is needed. Finally, key parameters of the MSH model were modified to increase the probability of DADs because these are stochastic events, which are difficult to analyze without stressing the system.
The mechanisms behind initiation and maintenance of postinfarction VTs remain a matter of clinical interest. In this study we used in silico approaches to uncover a novel mechanism by which subthreshold DADs, when occurring in narrow isthmuses within the scar, can form a substrate for block and subsequent reentry. Simulation results showed that reduced electrotonic loading in those regions potentiates the ability of DADs to impair excitability by inactivating the INa current. Such substrate also makes VT sustenance more likely as the activation wavelength is not altered, as in a scenario of long-lasting repolarization. Moreover, tissue uncoupling, a hallmark of structural remodeling following MI, was shown to increase DAD amplitude, enhancing arrhythmogenic risk by increasing the time window of unidirectional block. This novel mechanistic insight represents a new perspective on arrhythmogenesis that cannot be investigated in current (pre)clinical settings due to technological constraints.
Funding Sources: This work was supported by the National Institute for Health Research (NIHR) Biomedical Research Centre at Guy’s and St. Thomas’ National Health Service (NHS) Foundation Trust and King’s College London (KCL). Open access for this article was funded by KCL. This research was funded in whole, or in part, by the Wellcome Trust [213342/Z/18/Z] and Wellcome Trust Innovator Award [213342/Z/18/Z]. Drs Campos and Bishop acknowledge the support of the British Heart Foundation through Project Grants PG/16/81/32441 and PG/18/74/34077. Dr Bishop acknowledges the support of the UK Medical Research Council through a New Investigator Research Grant number MR/N011007/1. Disclosures: The authors have no conflicts of interest to disclose.