Influence of postshock epicardial activation patterns on initiation of ventricular fibrillation by upper limit of vulnerability shocks

New insights into defibrillation of the heart from realistic simulation studies

Cardiac defibrillation, as accomplished nowadays by automatic, implantable devices, constitutes the most important means of combating sudden cardiac death. Advancing our understanding towards a full appreciation of the mechanisms by which a shock interacts with the heart, particularly under diseased conditions, is a promising approach to achieve an optimal therapy. The aim of this article is to assess the current state-of-the-art in whole-heart defibrillation modelling, focusing on major insights that have been obtained using defibrillation models, primarily those of realistic heart geometry and disease remodelling. The article showcases the contributions that modelling and simulation have made to our understanding of the defibrillation process. The review thus provides an example of biophysically based computational modelling of the heart (i.e. cardiac defibrillation) that has advanced the understanding of cardiac electrophysiological interaction at the organ level, and has the potential to contribute to the betterment of the clinical practice of defibrillation.

Modeling defibrillation of the heart: approaches and insights

Cardiac defibrillation, as accomplished nowadays by automatic, implantable devices (ICDs), constitutes the most important means of combating sudden cardiac death. While ICD therapy has proved to be efficient and reliable, defibrillation is a traumatic experience. Thus, research on defibrillation mechanisms, particularly aimed at lowering defibrillation voltage, remains an important topic. Advancing our understanding towards a full appreciation of the mechanisms by which a shock interacts with the heart is the most promising approach to achieve this goal. The aim of this paper is to assess the current state-of-the-art in ventricular defibrillation modeling, focusing on both numerical modeling approaches and major insights that have been obtained using defibrillation models, primarily those of realistic ventricular geometry. The paper showcases the contributions that modeling and simulation have made to our understanding of the defibrillation process. The review thus provides an example of biophysically based computational modeling of the heart (i.e., cardiac defibrillation) that has advanced the understanding of cardiac electrophysiological interaction at the organ level and has the potential to contribute to the betterment of the clinical practice of defibrillation.

Granulocyte colony-stimulating factor stabilizes cardiac electrophysiology and decreases infarct size during cardiac ischaemic/reperfusion in swine

AIM

Effects of granulocyte colony-stimulating factor (G-CSF) on cardiac electrophysiology during ischaemic/reperfusion (I/R) period are unclear. We hypothesized that G-CSF stabilizes cardiac electrophysiology during I/R injury by prolonging the effective refractory period (ERP), increasing the ventricular fibrillation threshold (VFT) and decreasing the defibrillation threshold (DFT), and that the cardioprotection of G-CSF is via preventing cardiac mitochondrial dysfunction.

METHODS

In intact-heart protocol, pigs were infused with either G-CSF or vehicle (n = 7 each group) without I/R induction. In I/R protocol, pigs were infused with G-CSF (0.33 μg kg(-1 ) min(-1) ) or vehicle (n = 8 each group) for 30 min prior to a 45-min left anterior descending artery occlusion and at reperfusion. Diastolic pacing threshold (DPT), ERP, VFT and DFT were determined in all pigs before and during I/R period. Rat's isolated cardiac mitochondria were used to test the protective effect of G-CSF (100 nm) in H(2) O(2) -induced mitochondrial oxidative damage.

RESULTS

Neither G-CSF nor vehicle altered any parameter in intact-heart pigs. During ischaemic period, G-CSF significantly increased the DPT, ERP and VFT without altering the DFT. During reperfusion, G-CSF continued to increase the DPT without altering other parameters. The infarct size was significantly decreased in the G-CSF group, compared to the vehicle. G-CSF could also prevent cardiac mitochondrial swelling, decrease ROS production, and prevent mitochondrial membrane depolarization.

CONCLUSION

G-CSF increases the DPT, ERP and VFT and reduces the infarct size, thus stabilizing the myocardial electrophysiology, and preventing fatal arrhythmia during I/R. The protective mechanism could be via its effect in preventing cardiac mitochondrial dysfunction.

Cilostazol attenuates ventricular arrhythmia induction and improves defibrillation efficacy in swine

Previous reports demonstrated that cilostazol, a phosphodiesterase 3 inhibitor, affected cellular electrophysiology and reduced episodes of ventricular fibrillation (VF) in patients with Brugada syndrome. However, its effects on VF induction and defibrillation efficacy have never been investigated. We tested the hypothesis that cilostazol increases the VF threshold (VFT) and decreases the upper limit of vulnerability (ULV) and the defibrillation threshold (DFT). A total of 48 pigs were randomly assigned to defibrillation and VF induction studies. The diastolic pacing threshold (DPT), VFT, ULV, DFT, and effective refractory period were determined before and after the infusion of cilostazol at 6 mg/kg, 3 mg/kg, or vehicle. The DPT was significantly increased after administration of 3 and 6 mg/kg cilostazol. The ULV and DFT were significantly decreased after administration of 6 mg/kg cilostazol only. The ULV in the 6 mg/kg group had 12% lower peak voltage and 25% lower total energy, and the DFT had 13% lower peak voltage and 25% lower total energy. The VFT was not altered in any experimental group. This study shows that cilostazol administration significantly increased the DPT, which was associated with significantly reduced DFT and ULV.

Mechanisms of defibrillation

Electrical shock has been the one effective treatment for ventricular fibrillation for several decades. With the advancement of electrical and optical mapping techniques, histology, and computer modeling, the mechanisms responsible for defibrillation are now coming to light. In this review, we discuss recent work that demonstrates the various mechanisms responsible for defibrillation. On the cellular level, membrane depolarization and electroporation affect defibrillation outcome. Cell bundles and collagenous septae are secondary sources and cause virtual electrodes at sites far from shocking electrodes. On the whole-heart level, shock field gradient and critical points determine whether a shock is successful or whether reentry causes initiation and continuation of fibrillation.

Towards predictive modelling of the electrophysiology of the heart

The simulation of cardiac electrical function is an example of a successful integrative multiscale modelling approach that is directly relevant to human disease. Today we stand at the threshold of a new era, in which anatomically detailed, tomographically reconstructed models are being developed that integrate from the ion channel to the electromechanical interactions in the intact heart. Such models hold high promise for interpretation of clinical and physiological measurements, for improving the basic understanding of the mechanisms of dysfunction in disease, such as arrhythmias, myocardial ischaemia and heart failure, and for the development and performance optimization of medical devices. The goal of this article is to present an overview of current state-of-art advances towards predictive computational modelling of the heart as developed recently by the authors of this article. We first outline the methodology for constructing electrophysiological models of the heart. We then provide three examples that demonstrate the use of these models, focusing specifically on the mechanisms for arrhythmogenesis and defibrillation in the heart. These include: (1) uncovering the role of ventricular structure in defibrillation; (2) examining the contribution of Purkinje fibres to the failure of the shock; and (3) using magnetic resonance imaging reconstructed heart models to investigate the re-entrant circuits formed in the presence of an infarct scar.

Tunnel propagation of postshock activations as a hypothesis for fibrillation induction and isoelectric window

Comprehensive understanding of the ventricular response to shocks is the approach most likely to succeed in reducing defibrillation threshold. We propose a new theory of shock-induced arrhythmogenesis that unifies all known aspects of the response of the heart to monophasic (MS) and biphasic (BS) shocks. The central hypothesis is that submerged "tunnel" propagation of postshock activations through shock-induced intramural excitable areas underlies fibrillation induction and the existence of isoelectric window. We conducted simulations of fibrillation induction using a realistic bidomain model of rabbit ventricles. Following pacing, MS and BS of various strengths/timings were delivered. The results demonstrated that, during the isoelectric window, an activation originated deep within the ventricular wall, arising from virtual electrodes; it then propagated fully intramurally through an excitable tunnel induced by the shock, until it emerged onto the epicardium, becoming the earliest-propagated postshock activation. Differences in shock outcomes for MS and BS were found to stem from the narrower BS intramural postshock excitable area, often resulting in conduction block, and the difference in the mechanisms of origin of the postshock activations, namely intramural virtual electrode-induced phase singularity for MS and virtual electrode-induced propagated graded response for BS. This study provides a novel analysis of the 3D mechanisms underlying the origin of postshock activations in the process of fibrillation induction by MS and BS and the existence of isoelectric window. The tunnel propagation hypothesis could open a new avenue for interventions exploration to achieve significantly lower defibrillation threshold.

Mechanistic enquiry into the effect of increased pacing rate on the upper limit of vulnerability

The goal of this study is to investigate the mechanisms responsible for the increase in the upper limit of vulnerability (ULV; highest shock strength that induces arrhythmia) following the increase in pacing rate. To accomplish this goal, the study employs a three-dimensional bidomain finite element model of a slice through the canine ventricles. The preparation was paced eight times at a basic cycle length (BCL) of either 80 or 150ms followed by delivery of shocks of various strengths and timings. Our results demonstrate that the shock strength, which induced an arrhythmia 50% of the time, increased 20% for the faster pacing compared to the slower pacing. Analysis of the mechanisms underlying the increased vulnerability revealed that delayed post-shock activations originating in the tissue depths appear as breakthrough activations on the surfaces of the preparation following an isoelectric window (IW). However, the IW duration was consistently shorter in the faster-paced preparation. Consequently, breakthrough activations appeared on the surfaces of this preparation earlier, when the tissue was less recovered, resulting in higher probability of unidirectional block and reentry. This explains why shocks of the same strength were more likely to result in arrhythmia induction when delivered to a preparation that was rapidly paced.

Effects of n-3 polyunsaturated fatty acid on upper limit of vulnerability shocks

BACKGROUND

Ventricular fibrillation (VF) can be induced when a strong shock is delivered during the vulnerable period of a cardiac cycle. VF, however, cannot be induced if the shock strength is increased to the "upper limit of vulnerability" (ULV) level. Docosahexaenoic acid (DHA) has been shown to prevent the occurrence of VF after coronary occlusion. However, its effects on the ULV have not been verified. We tested the hypothesis that ULV shock strength is decreased after DHA administration.

METHODS

In 10 pigs, 10 S1s (square, 5-ms) were delivered from the RV apex electrode at 300 ms cycle length. Shocks (S2, biphasic) were delivered from the RV-SVC electrodes after the last S1. The ULV was determined using an up/down protocol. In group 1 (n = 5), after the control ULV was determined at the beginning of the study, a solution containing 1.0 gm of DHA was infused intravenously within 90 min. The ULV (DHA-ULV) was determined again after the end of infusion. In group 2 (n = 5), the vehicle for DHA was infused instead of DHA to confirm that the vehicle did not have an effect on the ULV.

RESULTS

DHA-ULV (412 +/- 58 V, 12 +/- 3 J) was significantly decreased (P < 0.04) compared to the control ULV (478 +/- 32 V, 16 +/- 3 J). The ULV before (483 +/- 28 V, 16 +/- 1 J) and after (463 +/- 28 V, 15 +/- 2 J) the vehicle infusion was not different (P = 0.4). There was no change in the systolic blood pressure as well as heart rate in both groups.

CONCLUSION

DHA significantly decreases the ULV (13% by voltage and 25% by energy), suggesting that DHA can help to prevent VF induced by a strong stimulus delivered during the vulnerable period.

Characterization of the relationship between preshock state and virtual electrode polarization-induced propagated graded responses resulting in arrhythmia induction

BACKGROUND

Studies have demonstrated that failed defibrillation shocks often are followed by an electrically quiescent period (isoelectric window); however, the underlying mechanisms remain incompletely understood. We recently suggested a new mechanism termed "virtual electrode polarization-induced propagated graded responses" (VEPiPGRs) that might play a role in the origin of the global postshock activation following the isoelectric window.

OBJECTIVES

The purpose of this study to elucidate the circumstances under which VEPiPGR activations originate for shocks given to paced right ventricular preparations. Specifically, we examined the dependence of VEPiPGRs on coupling interval (CI) and shock polarity and whether VEPiPGRs emerge preferentially on the epicardium or the endocardium.

METHODS

Simultaneous endocardial and epicardial activity in isolated right ventricular preparations (n = 4) was imaged optically following shocks of strength +/-5A. All VEPiPGRs were analyzed, and the time T from shock end to activation onset was recorded (isoelectric window is the smallest T among activations that propagated globally).

RESULTS

VEPiPGR activations occurred for CIs in the range from 80 to 150 ms. Average duration of T was 64.5 +/- 18.15 ms, with T decreasing as CI increased (Tmax = 82 ms, Tmin = 46 ms, linear-fit slope = -0.675). The average earliest CI at which cathodal (+5A) shocks resulted in VEPiPGRs was 87 ms compared with 116 ms for anodal (-5A) shocks. All VEPiPGR activations emerged first on the epicardium in a focal pattern, and all induced ventricular fibrillation.

CONCLUSION

The global activation that terminates the isoelectric window could result from VEPiPGRs that find an exit pathway. VEPiPGRs originate at the sites of maximum action potential abbreviation by the shock, always on the epicardium for the preparation used here.

Effect of sildenafil citrate on the cardiovascular system

Sildenafil citrate is a drug commonly used to manage erectile dysfunction. It is designated chemically as 1-[[3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H -pyrazolo[4,3-d]pyrimidin-5-yl)-4 ethoxyphenyl] sulfonyl]-4-methylpiperazine citrate (C22H30N6(O4)S). It is a highly selective inhibitor of cyclic guanine monophosphate-specific phosphodiesterase type 5. In late March through mid-November 1998, the US Food and Drug Administration (FDA) published a report on 130 confirmed deaths among men (mean age, 64 years) who received prescriptions for sildenafil citrate, a period during which >6 million outpatient prescriptions (representing about 50 million tablets) were dispensed. The US FDA recently reported that significant cardiovascular events, including sudden cardiac death, have occurred in men with erectile dysfunction who were taking sildenafil citrate. These reports have raised concerns that sildenafil citrate may increase the risk of cardiovascular events, particularly fatal arrhythmias, in patients with cardiovascular disease. In the past few years, the cardiac electrophysiological effects of sildenafil citrate have been investigated extensively in both animal and clinical studies. According to extensive data available to date, sildenafil citrate has been shown to pose minimal cardiovascular risks to healthy people taking this drug. Some precautions are needed for patients with cardiovascular diseases. However, the only absolute contraindication for sildenafil citrate is the concurrent use of nitrates. This article is intended to review sildenafil citrate's cardiovascular effects, as well as current debates about its arrhythmogenic effects.

Development of an optrode for intramural multisite optical recordings of Vm in the heart

INTRODUCTION

Optical mapping of transmembrane potential (Vm) is an important tool in the investigation of impulse propagation in the heart. It provides valuable information about spatiotemporal changes of Vm that cannot be obtained by other techniques, but it presently is limited to measurements from the heart surfaces. Therefore, the goal of this work was to develop a technique for intramural multisite optical measurements of Vm using fiberoptic technology.

METHODS AND RESULTS

An optrode, a bundle of thin optical fibers, was developed for measuring intramural optical signals at multiple sites in the heart. The optrode consisted of seven fibers with diameter of 225 microm arranged in a hexagonal pattern that were used to deliver excitation light to the myocardium, to collect the emitted fluorescence, and to project the light onto a 16 x 16 array of photodiode detectors. Rabbit hearts were stained with the Vm-sensitive dye RH-237. Fluorescence was excited using a 100-W Hg lamp. Intramural action potentials were recorded at multiple sites separated by 2 mm inside the left ventricle. Signal-to-noise (RMS) ratio was 21.2 +/- 12 (n = 7) without averaging or ratiometry and with negligible cross-talk (<1.9%) between the neighboring photodiodes. The size of the recording area for an individual fiber was estimated at approximately 0.8 mm.

CONCLUSION

These data demonstrate feasibility of multisite transmural measurements of Vm without signal averaging and ratiometry. This technique might become useful in studies of transmural impulse conduction during arrhythmias and defibrillation.

Restitution Dynamics During Pacing and Arrhythmias in Isolated Pig Hearts

INTRODUCTION

The dependence of action potential duration (APD) on the preceding diastolic interval (DI), i.e., restitution, has been purported to predict the development of alternans and reentrant arrhythmias. However, restitution depends on the history of activation (i.e., memory), and its relevance to arrhythmia induction and maintenance is unknown.

METHODS AND RESULTS

Using a dual-camera video imaging system, we recorded action potentials from thousands of sites on the surface of the isolated pig heart. A steady-state pacing (SSP) protocol was performed to generate the SSP APD restitution curve. During SSP, the minimum DI and APD were 57 +/- 6 ms and 107 +/- 6 ms, respectively. The restitution slope was >1 for DIs <85 +/- 5 ms; however, alternans were not observed. Abrupt decreases in cycle length (CL) resulted in a rapid (<5 beats) decrease in APD followed by a slower decrease to "steady state." DI, APD pairs for the initial beats following these rate changes were significantly above the SSP restitution curve. DI, APD pairs measured during sustained ventricular fibrillation clustered significantly below the SSP restitution curve, at significantly shorter APDs (57 +/- 4 ms) and DIs (49 +/- 6 ms) than could be achieved during SSP. In addition, abrupt increases in CL following SSP resulted in APDs significantly shorter than those predicted from the SSP restitution curve.

CONCLUSION

Our results indicate that the responses of APD and DI to sudden rate changes and during arrhythmias are not predicted by the SSP restitution relationship. Acute dynamics act to damp out the proarrhythmic oscillations predicted from the SSP restitution curve.

Vortex cordis as a mechanism of postshock activation: arrhythmia induction study using a bidomain model

INTRODUCTION

The ventricular apex has a helical arrangement of myocardial fibers called the "vortex cordis." Experimental studies have demonstrated that the first postshock activation originates from the ventricular apex, regardless of the electrical shock outcome; however, the related underlying mechanism is unclear. We hypothesized that the vortex cordis contributes to the initiation of postshock activation. To clarify this issue, we numerically studied the transmembrane potential distribution produced by various electrical shocks.

METHODS AND RESULTS

Using an active membrane model, we simulated a two-dimensional bidomain myocardial tissue incorporating a typical fiber orientation of the vortex cordis. Monophasic or biphasic shock was delivered via two line electrodes located at opposite tissue borders. Transmembrane potential distribution during the monophasic shock at the center of the vortex cordis showed a gradient high enough to initiate postshock activation. The postshock activation from the center of the vortex cordis was not suppressed, regardless of the initiation of spiral wave reentry. Spiral wave reentry was induced by the monophasic shock when the center area of the vortex cordis was partially excited by the nonuniform virtual electrode polarization. Postshock activation following the biphasic shock also originated from the center of the vortex cordis, but it tended to be suppressed due to the narrower excitable gap around the center of the vortex cordis. The electroporation effect, which was maximal at the center of the vortex cordis, is another possible mechanism of postshock activation.

CONCLUSION

Our simulations suggest that the vortex cordis may cause postshock activation.

Pacing following shocks stronger than the defibrillation threshold: impact on defibrillation outcome

INTRODUCTION

A recent study of shocks near defibrillation threshold (DFT) strength demonstrated that at least three rapid cycles always occur after failed shocks but not after successful shocks, suggesting that the number and rapidity of postshock cycles are important in determining defibrillation success. To test this hypothesis, rapid pacing was performed following a shock stronger than the DFT that by itself did not induce rapid cycles and ventricular fibrillation (VF).

METHODS AND RESULTS

Epicardial activation was mapped in six pigs using a 504-electrode sock. The DFT was determined by an up/down protocol with S1 shocks (right ventricle-superior vena cava, biphasic). Ten shocks that were 100 to 200 V above the DFT (aDFT) were delivered after 10 seconds of VF to confirm they always defibrillated. Then, S2, S3, etc., pacing at 5 to 10 times diastolic threshold was performed from the left ventricular apex after aDFT shocks during VF. First, the postshock interval after aDFT shocks was scanned with an S2 stimulus to find the shortest S1-S2 coupling interval (CI) that captured. This was repeated for S3, S4, etc., until VF was induced. To induce VF after aDFT shocks, three pacing stimuli (S2, S3, S4) with progressively shorter CIs were always required; S2 or S2,S3 never induced VF. For the S2-S4 cycles, the intercycle interval was shorter (P < 0.01), and the wavefront conduction time was longer (P < 0.01) for episodes in which VF was induced (n = 57) than for episodes in which it was not (n = 60). Following the S4 cycle that induced VF, two types of spontaneous activation patterns appeared: focal (88%) and reentrant (12%).

CONCLUSION

VF induction after aDFT shocks always required at least three rapid successive paced-induced cycles. Thus, the number and rapidity of the first several postshock cycles rather than just the first postshock cycle may be determining factors for defibrillation outcome.

Prediction of defibrillation outcome by epicardial activation patterns following shocks near the defibrillation threshold

INTRODUCTION

Ventricular defibrillation is probabilistic and shock strength dependent. We investigated the relationship between defibrillation outcome and postshock activation patterns for shocks of the same strength (approximately 50% probability of success for defibrillation [ED50] to yield an equal number of successful and failed shocks).

METHODS AND RESULTS

In five pigs, 10 shocks of approximately ED50 strength (right ventricle-superior vena cava, biphasic, 6/4 msec) were delivered after 10 seconds of ventricular fibrillation (VF). Epicardial activation sequences following shocks were mapped with a 504-electrode shock and analyzed by animating dV/dt of the electrograms. Intercycle interval (ICI, time between the onset of successive postshock cycles), wavefront conduction time (WCT, time between the earliest and latest activation of a cycle), and overlapping index (WCT of cycle[n]/ICI of cycle[n+1]) were determined for the first five postshock cycles. An overlapping index >1 indicates overlap between successive cycles. Of 50 defibrillation attempts, 25 were successes. There was no difference between successful and failed episodes for both ICI (68 +/- 9 msec vs 62 +/- 10 msec) and WCT (97 +/- 24 msec vs 100 +/- 14 msec) of cycle 1. However, starting at cycle 2, the ICI was longer, and the WCT was shorter for successful than failed episodes (P < 0.01). Overlapping cycles (index > 1) were found during the transition from cycles 2 through 5 in all failed (index >1) and in no successful episodes.

CONCLUSIONS

(1) Defibrillation outcome cannot be determined during the first postshock cycle. (2) At least three rapid successive cycles with overlap of cycles 2 and 3 are present in all failed and in no successful episodes. (3) The overlapping index is a marker to predict defibrillation outcome.