Virtual electrode theory explains pacing threshold increase caused by cardiac tissue damage

An abrupt loss of capture during permanent His-bundle pacing: Assessment of mechanism underlying late capture threshold rise

A His-bundle (HB) capture threshold rise is still a significant concern in permanent His-bundle pacing (HBP). We present a case where an abrupt increase in HB threshold and loss of capture occurred even after 3.5 years of stable permanent HBP for an atrioventricular block. The development of local fibrosis around the lead helix and the change in an insertion angle of the lead might adversely affect the HB capture threshold in the chronic phase.

Value of high-output pace-mapping of the right phrenic nerve for enabling safe radiofrequency ablation of atrial fibrillation: insights from three-dimensional computed tomography segmentation

AIMS

Right phrenic nerve (RPN) injury is a disabling but uncommon complication of atrial fibrillation (AF) radiofrequency ablation. Pace-mapping is widely used to infer RPN's course, for limiting the risk of palsy by avoiding ablation at capture sites. However, information is lacking regarding the distance between the endocardial sites of capture and the actual anatomic RPN location. We aimed at determining the distance between endocardial sites of capture and anatomic CT location of the RPN, depending on the capture threshold.

METHODS AND RESULTS

In consecutive patients undergoing AF radiofrequency ablation, we defined the course of the RPN on the electroanatomical map with high-output pacing at up to 50 mA/2 ms, and assessed RPN capture threshold (RPN-t). The true anatomic course of the RPN was delineated and segmented using CT scan, then merged with the electroanatomical map. The distance between pacing sites and the RPN was assessed. In 45 patients, 1033 pacing sites were analysed. Distances from pacing sites to RPN ranged from 7.5 ± 3.0 mm (min 1) when RPN-t was ≤10 mA to 19.2 ± 6.5 mm (min 9.4) in cases of non-capture at 50 mA. A distance to the phrenic nerve > 10 mm was predicted by RPN-t with a ROC curve area of 0.846 [0.821-0.870] (P < 0.001), with Se = 80.8% and Sp = 77.5% if RPN-t > 20 mA, Se = 68.0% and Sp = 91.6% if RPN-t > 30 mA, and Se = 42.4% and Sp = 97.6% if non-capture at 50 mA.

CONCLUSION

These data emphasize the utility of high-output pace-mapping of the RPN. Non-capture at 50 mA/2 ms demonstrated very high specificity for predicting a distance to the RPN > 10 mm, ensuring safe radiofrequency delivery.

His bundle pacing in nodal versus infranodal atrioventricular block: a mid-term follow-up study

INTRODUCTION

This study evaluated the feasibility of His bundle pacing (HBP) in consecutive, unselected patients with advanced atrioventricular block (AVB) over a medium-term follow-up period, comparing procedural characteristics between nodal and infranodal sites of the conduction block.

MATERIALS AND METHODS

Seventy-five consecutive patients with second-degree or third-degree AVB in which HBP was attempted were prospectively included in this study. The clinical and procedural-related characteristics of the patients were recorded at baseline and over a mid-term follow-up.

RESULTS

72% of the patients had normal QRS duration at baseline. Intracardiac electrograms revealed nodal AVB in 46 patients (61.3%). The permanent HBP procedural success was significantly higher in nodal AVB (84.8%) vs infranodal AVB (31%). There was no statistical difference between paced QRS duration, impedance, pacing and sensing thresholds and fluoroscopy time in the two groups. Infranodal block, baseline QRS duration, left bundle branch block morphology and ejection fraction were significantly associated with HBP procedural failure. The patients were followed for a period of 627.71±160.93 days. There were no significant differences in parameters at follow-up. An increase of >1 V in the His bundle (HB) capture threshold was encountered in one patient with infranodal AVB (11.1 %) and in four patients with nodal AVB (10.25%).

CONCLUSION

Permanent HBP is a feasible pacing technique in nodal AVB with a high success rate and stable thresholds in the medium term. Most infranodal blocks are located within the HB, so there is still the possibility to capture the conduction system, although with lower success rates.

Utility of Very High-Output Pacing to Identify VT Circuits in Patients Manifesting Traditionally Inexcitable Scar

BACKGROUND

Entrainment and pace mapping are used to identify critical components (CCs) of ventricular tachycardia (VT) circuits. In patients with dense myocardial scarring, VT circuits may elude capture at standard high pacing outputs (up to 10 mA at a 2-millisecond pulse width).

OBJECTIVES

The purpose of this study was to assess the utility of very high-output pacing (V-HOP, 50 mA at 2 milliseconds) for identifying CCs of VT circuits after standard high pacing output failed to elicit capture in densely scarred myocardial tissue.

METHODS

Our standard VT ablation approach included electroanatomic mapping for substrate characterization and entrainment and/or pace mapping to identify CCs of VT circuits. Patients that required V-HOP to capture sites of interest comprised the study cohort. Ablation endpoints were VT termination and noninducibility.

RESULTS

Twenty-five patients (71 ± 10 years of age, all males) undergoing 26 VT ablations met the inclusion criteria. The mean left ventricular ejection fraction was 30% ± 14%, and 85% had ischemic cardiomyopathy. V-HOP was used to successfully entrain VT in 17 patients, yielding central isthmus sites in 10 and entrance/exit sites in 4. VT terminated with radiofrequency ablation at these sites in 15 patients. In 9 patients, V-HOP identified scar locations with a delayed exit. Acute procedural success was achieved in 24 patients without any adverse events. Over a follow-up period of 16 ± 21 months, 2 patients experienced VT recurrence requiring repeat ablation during which the same location was targeted successfully in 1 patient.

CONCLUSIONS

In VT patients with a dense scar that is traditionally inexcitable, V-HOP can identify CCs of the re-entrant circuit and guide successful ablation.

Injectable conductive hydrogel can reduce pacing threshold and enhance efficacy of cardiac pacemaker

Background: Pacemaker implantation is currently used in patients with symptomatic bradycardia. Since a pacemaker is a lifetime therapeutic device, its energy consumption contributes to battery exhaustion, along with its voltage stimulation resulting in local fibrosis and greater resistance, which are all detrimental to patients. The possible resolution for those clinical issues is an injection of a conductive hydrogel, poly-3-amino-4-methoxybenzoic acid-gelatin (PAMB-G), to reduce the myocardial threshold voltage for pacemaker stimulation. Methods: PAMB-G is synthesized by covalently linking PAMB to gelatin, and its conductivity is measured using two-point resistivity. Rat hearts are injected with gelatin or PAMB-G, and pacing threshold is evaluated using electrocardiogram and cardiac optical mapping. Results: PAMB-G conductivity is 13 times greater than in gelatin. The ex vivo model shows that PAMB-G significantly enhances cardiac tissue stimulation. Injection of PAMB-G into the stimulating electrode location at the myocardium has a 4 times greater reduction of pacing threshold voltage, compared with electrode-only or gelatin-injected tissues. Multi-electrode array mapping reveals that the cardiac conduction velocity of PAMB-G group is significantly faster than the non- or gelatin-injection groups. PAMB-G also reduces pacing threshold voltage in an adenosine-induced atrial-ventricular block rat model. Conclusion: PAMB-G hydrogel reduces cardiac pacing threshold voltage, which is able to enhance pacemaker efficacy.

Stimulation and propagation of activation in conduction tissue: Implications for left bundle branch area pacing

BACKGROUND

Characterizing wavefront generation and impulse conduction in left bundle (LB) has implications for left bundle branch area pacing (LBBAP).

OBJECTIVES

The purpose of this study was to describe the pacing characteristics of LB and to study the role of pacing pulse width (PW) in overcoming left bundle branch block.

METHODS

Twenty fresh ovine heart slabs containing well-developed and easily identifiable tissues of the conduction system were used for the study. LB stimulation, activation, and propagation were studied under baseline conditions, simulated conduction slowing, conduction block, and fascicular block.

RESULTS

The maximum radius of the LB early activation increased up to 13.4 ± 2.4 mm from the pacing stimulus, and the time from stimulus to evoked potential shortened when pacing PW was increased from 0.13 to 2 ms at baseline. Conduction slowing and block induced by cooling could be resolved by increasing pacing PW from 0.25 to 1.5 ms over a distance of 10 ± 1.5 mm from the pacing stimulus. The LB strength-duration (SD) curve was shifted to the left of the myocardial SD curve.

CONCLUSION

Increasing PW resolved conduction slowing and block and bypassed the experimental model of fascicular block in LB. Precise positioning of the LB lead in left ventricular subendocardium is not mandatory in LBBAP, as the SD curve of LB was shifted to the left of the myocardium SD curve and could be captured from a distance by optimizing PW.

Effect of His bundle pacing on right bundle branch block located distal to site of pacing

BACKGROUND

It is generally accepted that bundle branch block (BBB) may be corrected simply by selective His bundle pacing (S-HBP) distal to site of block. However, clinical HBP is often non-selective (NS), the site of block unknown and moving the HB lead is not always an option. Thus, while correction of right (R) BBB in NS-HBP has been reported, its mechanism is unknown.

METHODS

Threshold testing was done during His bundle pacing in 39 patients with RBBB. We compared NS-HBP and S-HBP and analyzed the role of pacing voltage and capture threshold of the parallel pathway, in partially or completely correcting RBBB.

RESULTS

During NS-HBP, higher pacing voltage completely corrected RBBB in 22/35 patients as compared to 5/18 patients at lower voltage. Remaining patients showed incomplete correction of RBBB. NS-HBP group with lower capture threshold of 1.3 ± 0.5 V completely resolved RBBB in 9/14 vs 3/11 patients in NS-S HBP group with higher capture threshold of 2.4 ± 0.8 V. QRS activation time in NSHBP was 91 ± 8 ms (baseline 97 ± 7 ms p = ns). S-HBP resolved RBBB in only 1/21 patients.

CONCLUSIONS

1.) Lack of correction with S-HBP suggests that RBBB was distal to site of HBP and yet was corrected with NS-HBP. 2.) Voltage dependent correction of RBBB in NS-HBP suggests that conduction via a specialized parallel pathway maintains normal ventricular activation time. 3.) Correction of RBBB in all patients with NS-HBP, suggests that conduction block was either bypassed or right ventricular free wall pre-excited by conduction via parallel pathway.

Effects of high-frequency biphasic shocks on ventricular vulnerability and defibrillation outcomes through synchronized virtual electrode responses

Electrical defibrillation is a well-established treatment for cardiac dysrhythmias. Studies have suggested that shock-induced spatial sawtooth patterns and virtual electrodes are responsible for defibrillation efficacy. We hypothesize that high-frequency shocks enhance defibrillation efficacy by generating temporal sawtooth patterns and using rapid virtual electrodes synchronized with shock frequency. High-speed optical mapping was performed on isolated rat hearts at 2000 frames/s. Two defibrillation electrodes were placed on opposite sides of the ventricles. An S1-S2 pacing protocol was used to induce ventricular tachyarrhythmia (VTA). High-frequency shocks of equal energy but varying frequencies of 125–1000 Hz were used to evaluate VTA vulnerability and defibrillation success rate. The 1000-Hz shock had the highest VTA induction rate in the shorter S1-S2 intervals (50 and 100 ms) and the highest VTA defibrillation rate (70%) among all frequencies. Temporal sawtooth patterns and synchronous shock-induced virtual electrode responses could be observed with frequencies of up to 1000 Hz. The improved defibrillation outcome with high-frequency shocks suggests a lower energy requirement than that of low-frequency shocks for successful ventricular defibrillation.

Ventricular Endocardial Tissue Geometry Affects Stimulus Threshold and Effective Refractory Period

BACKGROUND

Understanding the biophysical processes by which electrical stimuli applied to cardiac tissue may result in local activation is important in both the experimental and clinical electrophysiology laboratory environments, as well as for gaining a more in-depth knowledge of the mechanisms of focal-trigger-induced arrhythmias. Previous computational models have predicted that local myocardial tissue architecture alone may significantly modulate tissue excitability, affecting both the local stimulus current required to excite the tissue and the local effective refractory period (ERP). In this work, we present experimental validation of this structural modulation of local tissue excitability on the endocardial tissue surface, use computational models to provide mechanistic understanding of this phenomena in relation to localized changes in electrotonic loading, and demonstrate its implications for the capture of afterdepolarizations.

METHODS AND RESULTS

Experiments on rabbit ventricular wedge preparations showed that endocardial ridges (surfaces of negative mean curvature) had a stimulus capture threshold that was 0.21 ± 0.03 V less than endocardial grooves (surfaces of positive mean curvature) for pairwise comparison (24% reduction, corresponding to 56.2 ± 6.4% of the energy). When stimulated at the minimal stimulus strength for capture, ridge locations showed a shorter ERP than grooves (n = 6, mean pairwise difference 7.4 ± 4.2 ms). When each site was stimulated with identical-strength stimuli, the difference in ERP was further increased (mean pairwise difference 15.8 ± 5.3 ms). Computational bidomain models of highly idealized cylindrical endocardial structures qualitatively agreed with these findings, showing that such changes in excitability are driven by structural modulation in electrotonic loading, quantifying this relationship as a function of surface curvature. Simulations further showed that capture of delayed afterdepolarizations was more likely in trabecular ridges than grooves, driven by this difference in loading.

CONCLUSIONS

We have demonstrated experimentally and explained mechanistically in computer simulations that the ability to capture tissue on the endocardial surface depends upon the local tissue architecture. These findings have important implications for deepening our understanding of excitability differences related to anatomical structure during stimulus application that may have important applications in the translation of novel experimental optogenetics pacing strategies. The uncovered preferential vulnerability to capture of afterdepolarizations of endocardial ridges, compared to grooves, provides important insight for understanding the mechanisms of focal-trigger-induced arrhythmias.

Permanent His-Bundle pacing in a patient with advanced conduction system disease: What is the mechanism of QRS narrowing?

QRS narrowing during permanent His-bundle pacing is primarily thought to result from longitudinal dissociation within the His bundle. We present a case with an alternative mechanism, highlighting the likelihood that there are actually multiple explanations for this phenomenon. In addition, this case highlights the utility of His-bundle pacing even in the face of a wide QRS.

Cardiomyocyte lethality by multidirectional stimuli

Multidirectional defibrillation protocols have shown better efficiency than monodirectional; still, no testing was performed to assess cell lethality. We investigated lethality of multidirectional defibrillator-like shocks on isolated cardiomyocytes. Cells were isolated from adult male Wistar rats and plated into a perfusion chamber. Electrical field stimulation threshold (E_T) was obtained, and cells were paced with suprathreshold bipolar electrical field (E) pulses. Either one monodirectional high-intensity electrical field (HEF) pulse aligned at 0° (group Mono0) or 60° (group Mono60) to cell major axis or a multidirectional sequence of three HEF pulses aligned at 0°, 60°, and 120° each was applied. If cell recovered from shock, pacing was resumed, and a higher amplitude HEF, proportional to E_T, was applied. The sequence was repeated until cell death. Lethality curves were built by means of survival analysis from sub-lethal and lethal E. Non-linear fit was performed, and E values corresponding to 50% probability of lethality (E50) were compared. Multidirectional groups presented lethality curves similar to Mono0. Mono60 displayed the highest E50. The novel data endorse the idea of multidirectional stimuli being safer because their effects on lethality of individual cells were equal to a single monodirectional stimulus, while their defibrillatory threshold is lower. Graphical abstract Monodirectional and multidirectional lethality protocol comparison on isolated rat cardiomyocytes. The heart image is a derivative of "3D Heart in zBrush" ( https://vimeo.com/65568770 ) by Laloxl, used under CC BY 3.0 ( https://creativecommons.org/licenses/by/3.0/legalcode )/image extracted from original video.

Physiological mechanisms of QRS narrowing in bundle branch block patients undergoing permanent His bundle pacing

His bundle pacing is increasingly used to avoid chronic right ventricular pacing, and electrically resynchronize ventricular activation by narrowing or normalizing the QRS interval in left and right bundle branch block. The mechanisms by which this occurs remain poorly understood. In this review, the proposed mechanisms and evidence supporting them are discussed. Also discussed are aspects of mechanisms that are not completely supported by the evidence. We also review the differences and physiological bases for direct vs. indirect His bundle capture, and the physiological mechanisms for QRS narrowing vs. normalization following His bundle pacing.

Permanent His-Bundle Pacing: Case Studies

His bundle pacing (HBP) is a physiological alternative to right ventricular pacing. In addition to patients with normal His-Purkinje conduction, HBP can be valuable in patients with infranodal atrioventricular (AV) block and patients undergoing AV node ablation. The following case studies illustrate the challenges associated with HBP.

PDE constrained optimization of electrical defibrillation in a 3D ventricular slice geometry

A computational study of an optimal control approach for cardiac defibrillation in a 3D geometry is presented. The cardiac bioelectric activity at the tissue and bath volumes is modeled by the bidomain model equations. The model includes intramural fiber rotation, axially symmetric around the fiber direction, and anisotropic conductivity coefficients, which are extracted from a histological image. The dynamics of the ionic currents are based on the regularized Mitchell-Schaeffer model. The controls enter in the form of electrodes, which are placed at the boundary of the bath volume with the goal of dampening undesired arrhythmias. The numerical optimization is based on Newton techniques. We demonstrated the parallel architecture environment for the computation of potentials on multidomains and for the higher order optimization techniques.

Newer algorithms in bradycardia management

Permanent cardiac pacemakers (PPM) are effective in the treatment of bradycardia in a growing number of clinical scenarios. An appreciation of the capacity of PPMs to result in negative hemodynamic and proarrhythmic effects has grown alongside clinical experience with permanent pacing. Such experience has necessitated the development of algorithms aimed at optimizing device functionality across a broad spectrum of physiologic and pathologic conditions. This review highlights recent device-based algorithms used in automated threshold testing, reduction of right ventricular pacing, prevention and treatment of pacemaker-mediated tachycardia, mode switching for atrial tachyarrhythmias, rate-modulated pacing, and advances in arrhythmia storage and remote monitoring.

Strain-induced crack formations in PDMS/DXA drug collars

Drug-eluting systems are currently used in cardiac leads in order to reduce inflammation and fibrosis at the lead-tissue interface. Drug release from these drug delivery systems can be modulated by the manufacturing processes used to create the drug systems and assemble them onto the cardiac lead. In this study, scanning electron microscopy, atomic force microscopy and Raman microscopy are employed to explore the material characteristics of a polydimethylsiloxane-dexamethasone acetate drug collar used on cardiac leads when varying the strain during collar assembly on the lead. A novel test fixture was created in order to investigate these drug collars under simulated stresses. Measurements of the collar while fitted to a rod revealed microcracks that are hypothesized to affect the drug release performance, resulting in increased drug elution. It was found that the strain that occurs during assembly of the collar onto the lead is a key factor in the formation of these microcracks. Results also suggest that cracks tend to form in areas of high drug particle density, and propagate between drug particles.

Time course of current of injury is related to acute stability of active-fixation pacing leads in rabbits

Background

Magnitude of current of injury (COI) consequent to pacemaker lead fixation is recognized as a predictor of acute lead stability. It is unclear whether dynamic monitoring of COI after lead fixation provides additional information beyond a single assessment performed at the time of fixation.

Objectives

This study was aimed to test the hypothesis that the time course of COI is related to acute lead stability.

Methods and Results

Active fixation leads with fixed screw were anchored to either Langendorff-perfused rabbit hearts endocardially or in vivo hearts epicardially in manners of contact the helix with no rotation, half rotation and full rotation, respectively. Intracardiac electrogram (EGM) was monitored dynamically from onset to resolution of COI, and magnitudes of intrinsic R wave and COI, including ST-segment elevation, ST/R and intracardiac EGM duration (IED), were measured. A digital force gauge was applied to assess lead stability. In vitro, COI in contacted leads was significantly smaller than those in half rotated (p<0.05) and fully rotated leads (p<0.05), and presented most precipitous recovery to baseline (1.5±1.1 min, p<0.05). Half-rotated and fully rotated leads manifested the same magnitude of COI right after placement. However, the time course of COI was significantly longer in fully rotated leads than that in half rotated leads (26.5±2.8 min vs. 5.6±2.0 min, p<0.05). Similar findings were observed in vivo. The time course of COI was significantly correlated with the force needed to detach the lead from myocardium (r = 0. 72, n = 48, p<0.001).

Conclusions

Time course of COI is related to acute lead stability in rabbits. One might be misled by a single assessment of COI magnitude right after lead placement, whereas persistence of COI is likely to be a useful indicator of adequate lead stability.

ANALYSIS OF THE VIRTUAL ELECTRODE PHENOMENA USING BIDOMAIN MODEL: BASIC CHARACTERISTICS FOR PASSIVE MEMBRANE

The virtual electrode (VE) has been recognized as an important factor for success or failure of cardiac defibrillation. Many researches have been performed to study characteristics of the VE. However, there are some questions which remain unanswered. In this study, we developed a simulator to solve a three-dimensional bidomain model and performed several simulations to elucidate the basic characteristics of VE in a simplified cardiac tissue with passive membrane when a constant unipolar cathodal stimulus was applied. The results showed that for smaller electrodes, VE has a typical dog-bone shaped virtual cathode (VC) and two egg-shaped virtual anodes (VAs). The distributions both in intra- and extracellular potentials have concentric ellipsoidal isosurfaces, but their ellipticities are subtly different, producing VE. For larger electrodes, VC becomes larger and has a flat-dish shape rather than dog-bone, and VA becomes smaller and also flattens and collapses. The peak values of VE are larger for smaller electrodes, but their time courses show similar tendency among the different sized electrodes. The change of stimulus strength and polarity only affects the magnitude of VE in a linear manner and the distribution pattern is unchanged. These results provide us fundamental knowledge about VE.

Low-energy control of electrical turbulence in the heart

Controlling the complex spatio-temporal dynamics underlying life-threatening cardiac arrhythmias such as fibrillation is extremely difficult due to the nonlinear interaction of excitation waves within a heterogeneous anatomical substrate^1–4. Lacking a better strategy, strong, globally resetting electrical shocks remain the only reliable treatment for cardiac fibrillation^5–7. Here, we establish the relation between the response of the tissue to an electric field and the spatial distribution of heterogeneities of the scale-free coronary vascular structure. We show that in response to a pulsed electric field E, these heterogeneities serve as nucleation sites for the generation of intramural electrical waves with a source density ρ(E), and a characteristic time τ for tissue depolarization that obeys a power law τ∝E^α. These intramural wave sources permit targeting of electrical turbulence near the cores of the vortices of electrical activity that drive complex fibrillatory dynamics. We show in vitro that simultaneous and direct access to multiple vortex cores results in rapid synchronization of cardiac tissue and therefore efficient termination of fibrillation. Using this novel control strategy, we demonstrate, for the first time, low-energy termination of fibrillation in vivo. Our results give new insights into the mechanisms and dynamics underlying the control of spatio-temporal chaos in heterogeneous excitable media and at the same time provide new research perspectives towards alternative, life-saving low-energy defibrillation techniques.

Termination of atrial fibrillation using pulsed low-energy far-field stimulation

BACKGROUND

Electrically based therapies for terminating atrial fibrillation (AF) currently fall into 2 categories: antitachycardia pacing and cardioversion. Antitachycardia pacing uses low-intensity pacing stimuli delivered via a single electrode and is effective for terminating slower tachycardias but is less effective for treating AF. In contrast, cardioversion uses a single high-voltage shock to terminate AF reliably, but the voltages required produce undesirable side effects, including tissue damage and pain. We propose a new method to terminate AF called far-field antifibrillation pacing, which delivers a short train of low-intensity electric pulses at the frequency of antitachycardia pacing but from field electrodes. Prior theoretical work has suggested that this approach can create a large number of activation sites ("virtual" electrodes) that emit propagating waves within the tissue without implanting physical electrodes and thereby may be more effective than point-source stimulation.

METHODS AND RESULTS

Using optical mapping in isolated perfused canine atrial preparations, we show that a series of pulses at low field strength (0.9 to 1.4 V/cm) is sufficient to entrain and subsequently extinguish AF with a success rate of 93% (69 of 74 trials in 8 preparations). We further demonstrate that the mechanism behind far-field antifibrillation pacing success is the generation of wave emission sites within the tissue by the applied electric field, which entrains the tissue as the field is pulsed.

CONCLUSIONS

AF in our model can be terminated by far-field antifibrillation pacing with only 13% of the energy required for cardioversion. Further studies are needed to determine whether this marked reduction in energy can increase the effectiveness and safety of terminating atrial tachyarrhythmias clinically.

Low energy defibrillation in human cardiac tissue: a simulation study

We aim to assess the effectiveness of feedback-controlled resonant drift pacing as a method for low energy defibrillation. Antitachycardia pacing is the only low energy defibrillation approach to have gained clinical significance, but it is still suboptimal. Low energy defibrillation would avoid adverse side effects associated with high voltage shocks and allow the application of implantable cardioverter defibrillator (ICD) therapy, in cases where such therapy is not tolerated today. We present results of computer simulations of a bidomain model of cardiac tissue with human atrial ionic kinetics. Reentry was initiated and low energy shocks were applied with the same period as the reentry, using feedback to maintain resonance. We demonstrate that such stimulation can move the core of reentrant patterns, in the direction that depends on the location of the electrodes and the time delay in the feedback. Termination of reentry is achieved with shock strength one-order-of-magnitude weaker than in conventional single-shock defibrillation. We conclude that resonant drift pacing can terminate reentry at a fraction of the shock strength currently used for defibrillation and can potentially work where antitachycardia pacing fails, due to the feedback mechanisms. Success depends on a number of details that these numerical simulations have uncovered.

Spatial distribution and extent of electroporation by strong internal shock in intact structurally normal and chronically infarcted rabbit hearts

INTRODUCTION

Although life-saving, a strong internal defibrillation shock may temporarily or permanently damage the heart via disruption of cell membranes (electroporation). Spatial extent of electroporation in intact, normal, or infarcted hearts has not been investigated. In this study, shock-induced electroporation in intact rabbit hearts with and without chronic (>4 weeks) left ventricular myocardial infarction (MI) was characterized.

METHODS AND RESULTS

A coil shock electrode was inserted in the right ventricle of Langendorff-perfused hearts. One truncated exponential monophasic shock (+300 V, 8 ms) was delivered by a 150 microF capacitor clinical defibrillator while the heart was perfused with membrane-impermeant dye propidium iodide (PI). The heart was sectioned transversely, and uptake of PI into ventricular myocardium through electropores was quantified. Histological evaluation was performed via Masson's trichrome staining. PI accumulation was minimal in the control (n = 3) and MI (n = 3) hearts without shock. Following shock delivery, (1) in control (n = 5) and MI (n = 5) hearts, electroporation mostly occurred near the shock electrode and was longitudinally distributed along the active region of the shock electrode; (2) in MI group, electroporation was significantly increased (P < 0.05) in the surviving anterior epicardial layers of the infarcted region; and (3) between the control and MI groups, the overall extent of electroporation was similar.

CONCLUSION

Shock-induced electroporation was spatially dependent on the location and dimension of the active region of the shock electrode. The overall extent of electroporation in the MI heart was comparable with the control heart, but the surviving anterior epicardial layers in the infarcted region were more susceptible to electroporation.

Short-term pacing in the mouse alters cardiac expression of connexin43

Background

Cardiac insults such as ischemia, infarction, hypertrophy and dilatation are often accompanied by altered abundance and/or localization of the connexin43 gap junction protein, which may predispose towards arrhythmic complications. Models of chronic dyssynchronous cardiac activation have also been shown to result in redistribution of connexin43 in cardiomyocytes. We hypothesized that alterations in connexin43 expression and localization in the mouse heart might be induced by ventricular pacing over a short period of time.

Results

The subdiaphragmatic approach was used to pace a series of wild type mice for six hours before the hearts were removed for analysis. Mice were paced at 10–15% above their average anesthetized sinus rate and monitored to ensure 1:1 capture. Short-term pacing resulted in a significant reduction in connexin43 mRNA abundance, a partial redistribution of connexin43 from the sarcolemma to a non-sarcolemmal fraction, and accumulation of ubiquitinated connexin43 without a significant change in overall connexin43 protein levels. These early pacing-induced changes in connexin43 expression were not accompanied by decreased cardiac function, prolonged refractoriness or increased inducibility into sustained arrhythmias.

Conclusion

Our data suggest that short-term pacing is associated with incipient changes in the expression of the connexin43 gap junction, possibly including decreased production and a slowed rate of degradation. This murine model may facilitate the study of early molecular changes induced by pacing and may ultimately assist in the development of strategies to prevent gap junction remodeling and the associated arrhythmic complications of cardiac disease.

Atria are more susceptible to electroporation than ventricles: implications for atrial stunning, shock-induced arrhythmia and defibrillation failure

BACKGROUND

Defibrillation shock is known to induce atrial stunning, which is electrical and mechanical dysfunction.

OBJECTIVE

We hypothesized that atrial stunning is caused by higher atrial susceptibility to electroporation vs ventricles. We also hypothesize that electroporation may be responsible for early recurrence of atrial fibrillation.

METHODS

We investigated electroporation induced by 10-ms epicardial high-intensity shocks applied locally in atria and ventricles of Langendorff-perfused rabbit hearts (n = 12) using optical mapping.

RESULTS

Electroporation was centered at the electrode and was evident from transient diastolic depolarization and reduction of action potential amplitude and maximum upstroke derivative. Electroporation was voltage-dependent and polarity-dependent and was significantly more pronounced in the atria vs ventricles (P <.01), with a summary 50% of Effective Dose (ED50) for main measured parameters of 9.2 +/- 3.6 V/cm and 13.6 +/- 3.2 V/cm in the atria vs 37.4 +/- 1.5 V/cm and 48.4 +/- 2.8 V/cm in the ventricles, for anodal and cathodal stimuli, respectively. In atria (n = 5), shocks of both polarities (27.2 +/- 1.1 V/cm) transiently induced conduction block and reentry around the inexcitable area. Electroporation-induced ectopic activity was a possible trigger for reentry. However, in the thicker ventricles, electroporation and resulting conduction slowing and block were restricted to the surface only, preventing complete block and arrhythmia. The upstroke morphology revealed that the wave front dived below the electroporated region and resurfaced into unaffected epicardial tissue.

CONCLUSION

We showed that the atria are more vulnerable to electroporation and resulting block and arrhythmia than the ventricles.

Tissue damage by pulsed electrical stimulation

Repeated pulsed electrical stimulation is used in a multitude of neural interfaces; damage resulting from such stimulation was studied as a function of pulse duration, electrode size, and number of pulses using a fluorescent assay on chick chorioallontoic membrane (CAM) in vivo and chick retina in vitro. Data from the chick model were verified by repeating some measurements on porcine retina in-vitro. The electrode size varied from 100 microm to 1 mm, pulse duration from 6 micros to 6 ms, and the number of pulses from 1 to 7500. The threshold current density for damage was independent of electrode size for diameters greater than 300 microm, and scaled as 1/r2 for electrodes smaller than 200 microm. Damage threshold decreased with the number of pulses, dropping by a factor of 14 on the CAM and 7 on the retina as the number of pulses increased from 1 to 50, and remained constant for a higher numbers of pulses. The damage threshold current density on large electrodes scaled with pulse duration as approximately 1/t0.5, characteristic of electroporation. The threshold current density for repeated exposure on the retina varied between 0.061 A/cm2 at 6 ms to 1.3 A/cm2 at 6 micros. The highest ratio of the damage threshold to the stimulation threshold in retinal ganglion cells occurred at pulse durations near chronaxie-around 1.3 ms.

Modeling electroporation in a single cell

Electroporation uses electric pulses to promote delivery of DNA and drugs into cells. This study presents a model of electroporation in a spherical cell exposed to an electric field. The model determines transmembrane potential, number of pores, and distribution of pore radii as functions of time and position on the cell surface. For a 1-ms, 40 kV/m pulse, electroporation consists of three stages: charging of the cell membrane (0-0.51 micros), creation of pores (0.51-1.43 micros), and evolution of pore radii (1.43 micros to 1 ms). This pulse creates approximately 341,000 pores, of which 97.8% are small ( approximately 1 nm radius) and 2.2% are large. The average radius of large pores is 22.8 +/- 18.7 nm, although some pores grow to 419 nm. The highest pore density occurs on the depolarized and hyperpolarized poles but the largest pores are on the border of the electroporated regions of the cell. Despite their much smaller number, large pores comprise 95.3% of the total pore area and contribute 66% to the increased cell conductance. For stronger pulses, pore area and cell conductance increase, but these increases are due to the creation of small pores; the number and size of large pores do not increase.

Strategy for safe performance of extrathoracic magnetic resonance imaging at 1.5 tesla in the presence of cardiac pacemakers in non-pacemaker-dependent patients: a prospective study with 115 examinations

BACKGROUND

The purpose of the present study was to evaluate a strategy for safe performance of extrathoracic magnetic resonance imaging (MRI) in non-pacemaker-dependent patients with cardiac pacemakers.

METHODS AND RESULTS

Inclusion criteria were presence of a cardiac pacemaker and urgent clinical need for an MRI examination. Pacemaker-dependent patients and those requiring examinations of the thoracic region were excluded. The study group consisted of 82 pacemaker patients who underwent a total of 115 MRI examinations at 1.5T. To minimize radiofrequency-related lead heating, the specific absorption rate was limited to 1.5 W/kg. All pacemakers were reprogrammed before MRI: If heart rate was <60 bpm, the asynchronous mode was programmed to avoid magnetic resonance (MR)-induced inhibition; if heart rate was >60 bpm, sense-only mode was used to avoid MR-induced competitive pacing and potential proarrhythmia. Patients were monitored with ECG and pulse oximetry. All pacemakers were interrogated immediately before and after the MRI examination and after 3 months, including measurement of pacing capture threshold (PCT) and serum troponin I levels. All MR examinations were completed safely. Inhibition of pacemaker output or induction of arrhythmias was not observed. PCT increased significantly from pre- to post-MRI (P=0.017). In 2 of 195 leads, an increase in PCT was only detected at follow-up. In 4 of 114 examinations, troponin increased from a normal baseline value to above normal after MRI, and in 1 case (troponin pre-MRI 0.02 ng/mL, post-MRI 0.16 ng/mL), this increase was associated with a significant increase in PCT.

CONCLUSIONS

Extrathoracic MRI of non-pacemaker-dependent patients can be performed with an acceptable risk-benefit ratio under controlled conditions and by taking both MR- and pacemaker-related precautions.

Cardiac electrophysiology and tissue structure: bridging the scale gap with a joint measurement and modelling paradigm

Significant tissue structures exist in cardiac ventricular tissue that are of supracellular dimension. It is hypothesized that these tissue structures contribute to the discontinuous spread of electrical activation, may contribute to arrhymogenesis and also provide a substrate for effective cardioversion. However, the influences of these mesoscale tissue structures in intact ventricular tissue are difficult to understand solely on the basis of experimental measurement. Current measurement technology is able to record at both the macroscale tissue level and the microscale cellular or subcellular level, but to date it has not been possible to obtain large volume, direct measurements at the mesoscales. To bridge this scale gap in experimental measurements, we use tissue-specific structure and mathematical modelling. Our models have enabled us to consider key hypotheses regarding discontinuous activation. We also consider the future developments of our intact tissue experimental programme.

Cell and tissue responses to electric shocks

AIM

Existing models of myocardial membrane kinetics have not been able to reproduce the experimentally-observed negative bias in the asymmetry of transmembrane potential changes (DeltaV(m)) induced by strong electric shocks. The goals of this study are (1) to demonstrate that this negative bias could be reproduced by the addition, to the membrane model, of electroporation and an outward current, I(a), part of the K(+) flow through the L-type Ca(2+)-channel, and (2) to determine how such modifications in the membrane model affect shock-induced break excitation in a 2D preparation.

METHODS AND RESULTS

We conducted simulations of shocks in bidomain fibres and sheets with membrane dynamics represented by the Luo-Rudy dynamic model (LRd'2000), to which electroporation (LRd + EP model) and the outward current, I(a), activated upon strong shock-induced depolarization (aLRd model) was added. Assuming I(a) is a part of K(+) flow through the L-type Ca(2+)-channel enabled us to reproduce both the experimentally observed rectangularly-shaped positive DeltaV(m) and the value of near 2 of the negative-to-positive DeltaV(m) ratio. In the sheet, I(a) not only contributed to the negative bias in DeltaV(m) asymmetry at sites polarized by physical and virtual electrodes, but also restricted positive DeltaV(m). Electroporation, in its turn, was responsible for the decrease in cathode-break excitation threshold in the aLRd sheet, compared with the other two cases, as well as for the occurrence of the excitation after the shock-end rather than during the shock.

CONCLUSIONS

The incorporation of electroporation and I(a) in a membrane model ensures match between simulation results and experimental data. The use of the aLRd model results in a lower threshold for shock-induced break excitation.

Asymmetry in membrane responses to electric shocks: insights from bidomain simulations

Models of myocardial membrane dynamics have not been able to reproduce the experimentally observed negative bias in the asymmetry of transmembrane potential changes (DeltaVm) induced by strong electric shocks delivered during the action potential plateau. The goal of this study is to determine what membrane model modifications can bridge this gap between simulation and experiment. We conducted simulations of shocks in bidomain fibers and sheets with membrane dynamics represented by the LRd'2000 model. We found that in the fiber, the negative bias in DeltaVm asymmetry could not be reproduced by addition of electroporation only, but by further addition of hypothetical outward current, Ia, activated upon strong shock-induced depolarization. Furthermore, the experimentally observed rectangularly shaped positive DeltaVm, negative-to-positive DeltaVm ratio (asymmetry ratio) = approximately 2, electroporation occurring at the anode only, and the increase in positive DeltaVm caused by L-type Ca2+-channel blockade were reproduced in the strand only if Ia was assumed to be a part of K+ flow through the L-type Ca2+-channel. In the sheet, Ia not only contributed to the negative bias in DeltaVm asymmetry at sites polarized by physical and virtual electrodes, but also restricted positive DeltaVm. Inclusion of Ia and electroporation is thus the bridge between experiment and simulation.