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Background Shocks near defibrillation threshold (nDFT) strength commonly extinguish all ventricular fibrillation (VF) wavefronts, but a train of rapid, well-organized postshock activations (PAs) typically appears before sinus rhythm ensues. If one of the PA waves undergoes partial propagation block (wavebreak), reentry may be induced, causing VF to reinitiate and the shock to fail. Objective The purpose of this study was to determine whether wavebreak leading to VF reinititation following nDFT shocks occurs preferentially at the right ventricular insertion (RVI), which previous studies have identified as a key site for wavebreak. Methods We used panoramic optical mapping to image the ventricular epicardium of 6 isolated swine hearts during nDFT defibrillation episodes. After each experiment, the hearts were fixed and their geometry scanned with magnetic resonance imaging (MRI). The MRI and mapping datasets were spatially coregistered. For failed shocks, we identified the site of the first wavebreak of a PA wave during VF reinitiation. Results We recorded 59 nDFT failures. In 31 of these, the first wavebreak event occurred within 1 cm of the RVI centerline, most commonly on the anterior side of the right ventricular insertion (aRVI) (23/31). The aRVI region occupies 16.8% ± 2.5% of the epicardial surface and would be expected to account for only 10 wavebreaks if they were uniformly distributed. By χ2 analysis, aRVI wavebreaks were significantly overrepresented. Conclusion The anterior RVI is a key site in promoting nDFT failure. Targeting this site to prevent wavebreak could convert defibrillation failure to success and improve defibrillation efficacy.

Optical mapping has become a widely used and important method in cardiac electrophysiology. The method typically uses voltage-sensitive fluorescent dyes and high-speed cameras to image propagation of electrical waves. However, signals are highly susceptible to artifact caused by motion of the target organ. Consequently, cardiac optical mapping is traditionally performed in isolated, perfused organs whose contraction has been pharmacologically arrested. This has prevented optical mapping from being used to study interactions between electrical and mechanical motion. However, recently, a number of groups have developed methods to implement cardiac optical mapping in the presence of motion. These methods employ two basic strategies: (1) compensate for motion by measuring it or (2) ratiometry. In ratiometry, two signals are recorded from each site. The signals have differing sensitivity to membrane potential, but common motion artifact, which can be cancelled by taking the ratio of the two signals. Some methods use both of these strategies. Methods that measure motion have the additional advantage that this information can be used to quantify the organ’s mechanical function. Doing so enables combined “electromechanical mapping,” which allows optical study of electromechanical interactions. By allowing recording in the presence of motion, the new methods open the door to optical recording in in-vivo preparations. In addition, it is possible to implement electromechanical optical mapping techniques in organ systems other than the heart. For example, it was recently shown that optical mapping of slow wave propagation in the swine stomach is feasible. Such studies have the potential to uncover new information on the role of dysrhythmic slow wave propagation in gastric motility disorders. Impact statement Electrical and mechanical functions in the heart are bidirectionally coupled, yet are usually studied separately because of the different instrumentation technologies that are used in the two areas. Optical mapping is a powerful and widespread tool for imaging electrical propagation, but has traditionally required mechanical function to be arrested. Recently new methods have been devised that enable optical mapping to be performed in beating hearts and also to simultaneously quantify mechanical function. These new technologies promise to yield new information about electromechanical interactions in normal and pathological settings. They are also beginning to find application in other organ systems such as the gastrointestinal tract where they may provide new insight into motility disorders.

Introduction: Optical mapping is a powerful method for imaging cardiac propagation but is traditionally limited to motion-arrested, ex vivo preparations. Extending the method to beating, in situ preparations will enable study of bidirectional electromechanical interactions and physiological coupling between the heart and the animal. Challenges in adapting optical mapping to this setting include motion artifact, loading of voltage sensitive dye (VSD), and absorption of excitation and emission light by blood. Here, we outline a system to simultaneously image electrical propagation and contraction of intact beating hearts in an in vivo swine model. Methods: Fifteen pigs weighting 25-35 kg were anesthetized. Hearts were exposed via median sternotomy. We glued 20-30 fiducial markers (2 mm, ~1 cm spacing) to the anterior LV epicardium. We injected 14 ml of 50 uM di-4-ANEQ(F)PTEA into a catheter placed in the LAD. Blood flow in the artery was blocked during and for 1 min following the injection. This VSD works at longer wavelengths than commonly used VSDs and is therefore less affected by blood. Fluorescence was excited with alternating amber (602 nm) and green (526 nm) LEDs. Emission was long pass filtered (>700 nm) and recorded at 750 Hz with a pair of video cameras (SciMedia MiCAM03-N256) fitted with 35 mm, f/1.4 lenses . We analyzed marker motion to (1) quantify contraction and (2) partially correct motion artifact in membrane potential ( Vm ) signals. We also computed the ratio of green- to amber-elicited fluorescence on alternate frames to further correct motion artifact. Results and Conclusions: The method successfully images Vm and deformation. The figure shows an isochronal activation map of a paced beat (A), bars aligned with systolic principal strain (B), and Vm and shortening strain signals from selected sites (C). The method can be used to investigate coupled electromechanics and arrhythmogenesis in a more physiologically realistic setting than previous techniques.

On March 1 and 2, 2018, the National Institutes of Health 2018 Progenitor Cell Translational Consortium (PCTC) and Cardiovascular Bioengineering Symposium (CVBE) was held at the University of Alabama at Birmingham. Convergence of life sciences and engineering to advance the understanding and treatment of heart failure was the theme of the meeting. Over 150 attendees were present for more than 40 scientists presenting their latest works on engineering human functional myocardium for disease modeling, drug development, and heart failure research. The scientists, engineers and physicians in the field of cardiovascular sciences, met and discussed on the most recent advances in their works and propose future strategies in overcoming the major roadblocks of cardiovascular bioengineering and therapy. Particular emphasis was given for manipulation and using of stem/progenitor cells, biomaterials, and methods to provide molecular, chemical and mechanical cues to cells in order to influence their identity and fate in vitro and in vivo. Collectively, these works are profoundly impacting and progressing toward deciphering the mechanisms and developing novel treatments for left ventricular dysfunction of failing hearts. Here we present some important perspectives that emerged from this meeting.

Slow waves play a central role in coordinating gastric motor activity. High-resolution mapping of extracellular potentials from the stomach provides spatiotemporal detail on normal and dysrhythmic slow-wave patterns. All mapping studies to date have focused exclusively on tissue activation; however, the recovery phase contains vital information on repolarization heterogeneity, the excitable gap, and refractory tail interactions but has not been investigated. Here, we report a method to identify the recovery phase in slow-wave mapping data. We first developed a mathematical model of unipolar extracellular potentials that result from slow-wave propagation. These simulations showed that tissue repolarization in such a signal is defined by the steepest upstroke beyond the activation phase (activation was defined by accepted convention as the steepest downstroke). Next, we mapped slow-wave propagation in anesthetized pigs by recording unipolar extracellular potentials from a high-resolution array of electrodes on the serosal surface. Following the simulation result, a wavelet transform technique was applied to detect repolarization in each signal by finding the maximum positive slope beyond activation. Activation-recovery (ARi) and recovery-activation (RAi) intervals were then computed. We hypothesized that these measurements of recovery profile would differ for slow waves recorded during normal and spatially dysrhythmic propagation. We found that the ARi of normal activity was greater than dysrhythmic activity (5.1 ± 0.8 vs. 3.8 ± 0.7 s; P < 0.05), whereas RAi was lower (9.7 ± 1.3 vs. 12.2 ± 2.5 s; P < 0.05). During normal propagation, RAi and ARi were linearly related with negative unit slope indicating entrainment of the entire mapped region. This relationship was weakened during dysrhythmia (slope: −0.96 ± 0.2 vs −0.71 ± 0.3; P < 0.05). NEW & NOTEWORTHY The theoretical basis of the extracellular gastric slow-wave recovery phase was defined using mathematical modeling. A novel technique utilizing the wavelet transform was developed and validated to detect the extracellular slow-wave recovery phase. In dysrhythmic wavefronts, the activation-to-recovery interval (ARi) was shorter and recovery-to-activation interval (RAi) was longer compared with normal wavefronts. During normal activation, RAi vs. ARi had a slope of −1, whereas the weakening of the slope indicated a dysrhythmic propagation.

Key points Heart function is critically dependent upon the balance of energy production and utilization. Sarcolemmal ATP-sensitive potassium channels (KATP channels) in cardiac myocytes adjust contractile function to compensate for the level of available energy. Understanding the activation of KATP channels in working myocardium during high-stress situations is crucial to the treatment of cardiovascular disease, especially ischaemic heart disease. Using a new optical mapping approach, we measured action potentials from the surface of excised contracting rabbit hearts to assess when sarcolemmal KATP channels were activated during physiologically relevant workloads and during gradual reductions in myocardial oxygenation. We demonstrate that left ventricular pressure is closely linked to KATP channel activation and that KATP channel inhibition with a low concentration of tolbutamide prevents electromechanical decline when oxygen availability is reduced. As a result, KATP channel inhibition probably exacerbates a mismatch between energy demand and energy production when myocardial oxygenation is low. Abstract Sarcolemmal ATP-sensitive potassium channel (KATP channel) activation in isolated cells is generally understood, although the relationship between myocardial oxygenation and KATP activation in excised working rabbit hearts remains unknown. We optically mapped action potentials (APs) in excised rabbit hearts to test the hypothesis that hypoxic changes would be more severe in left ventricular (LV) working hearts (LWHs) than Langendorff (LANG) perfused hearts. We further hypothesized that KATP inhibition would prevent those changes. Optical APs were mapped when measuring LV developed pressure (LVDP), coronary flow rate and oxygen consumption in LANG and LWHs. Hearts were paced to increase workload and perfusate was deoxygenated to study the effects of myocardial hypoxia. A subset of hearts was perfused with 1 μm tolbutamide (TOLB) to identify the level of AP duration (APD) shortening attributed to KATP channel activation. During sinus rhythm, APD was shorter in LWHs compared to LANG hearts. APD in both LWHs and LANG hearts dropped steadily during deoxygenation. With TOLB, APDs in LWHs were longer at all workloads and APD reductions during deoxygenation were blunted in both LWHs and LANG hearts. At 50% perfusate oxygenation, APD and LVDP were significantly higher in LWHs perfused with TOLB (199 ± 16 ms; 92 ± 5.3 mmHg) than in LWHs without TOLB (109 ± 14 ms, P = 0.005; 65 ± 6.5 mmHg, P = 0.01). Our results indicate that KATP channels are activated to a greater extent in perfused hearts when the LV performs pressure–volume work. The results of the present study demonstrate the critical role of KATP channels in modulating myocardial function over a wide range of physiological conditions.

BACKGROUND: Cardiomyocytes that have been differentiated from CCND2-overexpressing human induced-pluripotent stem cells (hiPSC-CCND2(OE) CMs) can proliferate when transplanted into mouse hearts after myocardial infarction (MI). However, it is unknown whether remuscularization can replace the thin LV scar and if the large muscle graft can electrophysiologically synchronize to the recipient myocardium. Our objectives are to evaluate the structural and functional potential of hiPSC-CCND2(OE) CMs in replacing the LV thin scar. METHODS: NOD/SCID mice were treated with hiPSC-CCND2(OE) CMs (i.e., the CCND2(OE) group), hiPSC-CCND2(WT) CMs (the CCND2(WT) group), or an equal volume of PBS immediately after experimentally-induced myocardial infarction. The treatments were administered to one site in the infarcted zone (IZ), two sites in the border zone (BZ), and a fourth group of animals underwent Sham surgery. RESULTS: Six months later, engrafted cells occupied more than 50% of the scarred region in CCND2(OE) animals, and exceeded the number of engrafted cells in CCND2(WT) animals by ~8-fold. Engrafted cells were also more common in the IZ than in the BZ for both cell-treatment groups. Measurements of cardiac function, infarct size, wall thickness, and cardiomyocyte hypertrophy were significantly improved in CCND2(OE) animals compared to animals from the CCND2(WT) or PBS-treatment groups. Measurements in the CCND2(WT) and PBS groups were similar, and markers for cell cycle activation and proliferation were significantly higher in hiPSC-CCND2(OE) CMs than in hiPSC-CCND2(WT) CMs. Optical mapping of action potential propagation indicated that the engrafted hiPSC-CCND2(OE) CMs were electrically coupled to each other and to the cells of the native myocardium. No evidence of tumor formation was observed in any animals. CONCLUSIONS: Six months after the transplantation, CCND2-overexpressing hiPSC-CMs proliferated and replaced more than 50% of the myocardial scar tissue. The large graft hiPSC-CCND2(OE) CMs also electrically integrated with the host myocardium, which was accompanied by a significant improvement in LV function.

Background: Acute regional ischemia is a leading cause of death. Ectopic beats may initiate ventricular arrhythmias in the first hour after coronary occlusion; however, the origin of these beats remains poorly understood. Some studies suggest that abnormal mechanical stretch at the ischemic border zone may contribute to the initiation of ectopy. If mechanical stretch plays a major role in triggering ectopic beats, we therefore expect: I , left ventricular working (LVW) hearts will have higher frequency of post-occlusion ectopic activity than non-beating (NB) hearts; II , in LVW hearts, ectopic beats will arise from ventricular sites undergoing mechanical stretch. Methods and Results: Isolated pig hearts were assigned to NB or LVW groups (8 per group) and prepared for Langendorff or LVW perfusion, respectively. Contraction of NB hearts was suppressed with 10 μM blebbistatin. Acute regional ischemia was induced by LAD occlusion. Using a newly developed optical mapping system, electrical activity and mechanical deformation were simultaneously recorded from 0 to 60 minutes after LAD occlusion. Mechanical stretch (positive strain in 2 principal directions) was observed in the ischemic and border zones of LVW but not NB hearts (Figure, Panel A). There was no significant difference in the number of ectopic events in LVW hearts compared to NB hearts (58±35 vs. 32±36, p=0.17). Of the 45 ectopic events that originated from the LV mapping region in LVW hearts, Conclusion: Mechanical stretch does not play a major role in triggering ectopic beats in early acute regional ischemia in this model.

Mechanical stimulation can elicit electrical activation of the heart. This mechanosensitivity can start life-threatening arrhythmias (commotio cordis) or terminate them (precordial thump). Mechanosensitivity may also be involved in arrhythmogenesis in other settings. Stretch-activated ion channels (SACs) are thought to be important in mechanosensitivity and a number of agents that block them have been identified. Such agents could potentially be used as tools in experimental investigation of mechanosensitivity. However, studies using them in intact-heart preparations have yielded inconsistent results. In the present study, we used isolated, perfused hearts from 25–35 kg pigs and a computer-controlled device that repeatably delivered focal mechanical stimuli. The concentration-dependent ability of the SAC blocker gadolinium to suppress mechanical activation was assessed by the success rate of mechanical stimulation and by the delay between successful mechanical stimulation and electrical activation. In six hearts, perfusate was recirculated. In an additional six hearts, perfusate was not recirculated to prevent gadolinium from forming complexes with metabolic waste and possibly precipitating. Gadolinium did not suppress mechanically-induced activation. Although gadolinium has been shown to be an effective SAC blocker in isolated cells, using it to probe the role of mechanical stimulation in whole heart preparations should be done with great caution.

Background Improved understanding of the details of gastric slow wave propagation could potentially inform new diagnosis and treatment options for stomach motility disorders. Optical mapping has been used extensively in cardiac electrophysiology. Although optical mapping has a number of advantages relative to electrical mapping, optical signals are highly sensitive to motion artifact. We recently introduced a novel cardiac optical mapping method that corrects motion artifact and enables optical mapping to be performed in beating hearts. Here, we reengineer the method as an experimental tool to map gastric slow waves. Methods The method was developed and tested in 12 domestic farm pigs. Stomachs were exposed by laparotomy and stained with the voltage-sensitive fluorescence dye di-4-ANEPPS through a catheter placed in the gastroepiploic artery. Fiducial markers for motion tracking were attached to the serosa. The dye was excited by 450 or 505 nm light on alternate frames of an imaging camera running at 300 Hz. Emitted fluorescence was imaged between 607 and 695 nm. The optical slow wave signal was reconstructed using a combination of motion tracking and excitation ratiometry to suppress motion artifact. Optical slow wave signals were compared with simultaneously recorded bipolar electrograms and suction electrode signals, which approximate membrane potential. Key results The morphology of optical slow waves was consistent with previously published microelectrode recordings and simultaneously recorded suction electrode signals. The timing of the optical slow wave signals was consistent with the bipolar electrograms. Conclusions and inferences Optical mapping of slow wave propagation in the stomach is feasible.

Fluorescence optical imaging techniques have revolutionized the field of cardiac electrophysiology and advanced our understanding of complex electrical activities such as arrhythmias. However, traditional monocular optical mapping systems, despite having high spatial resolution, are restricted to a two-dimensional (2D) field of view. Consequently, tracking complex three-dimensional (3D) electrical waves such as during ventricular fibrillation is challenging as the waves rapidly move in and out of the field of view. This problem has been solved by panoramic imaging which uses multiple cameras to measure the electrical activity from the entire epicardial surface. However, the diverse engineering skill set and substantial resource cost required to design and implement this solution have made it largely inaccessible to the biomedical research community at large. To address this barrier to entry, we present an open source toolkit for building panoramic optical mapping systems which includes the 3D printing of perfusion and imaging hardware, as well as software for data processing and analysis. In this paper, we describe the toolkit and demonstrate it on different mammalian hearts: mouse, rat, and rabbit.

The characteristics of reentrant circuits during short duration ventricular fibrillation (SDVF; 20 s in duration) and the role of Ca++and rapid-activating delayed rectifier potassium currents during long duration ventricular fibrillation (LDVF; up to 10 min in duration) were investigated using verapamil and sotalol. Activation mapping of the LV epicardium with a 21 × 24 electrode plaque was performed in 12 open-chest pigs. Pigs were given either verapamil (0.136 mg/kg) or sotalol (1.5 mg/kg) and verapamil. Reentry patterns were quantified for SDVF, and, for LDVF, activation patterns were compared with our previously reported control LDVF data. Verapamil significantly increased conduction velocity around the reentrant core by 10% and reduced the reentrant cycle length by 15%, with a net reduction in reentry incidence of 70%. Sotolol had an opposite effect of decreasing the conduction velocity around the core by 6% but increasing the reentrant cycle length by 13%, with a net reduction of reentry incidence of 50%. After 200 s of VF, verapamil significantly slowed wavefront conduction velocity and activation rate compared with control data. Verapamil decreased the incidence of reentry in SDVF by accelerating conduction velocity to increase the likelihood of conduction block, possibly through increased sympathetic tone. The drug slowed activation rate and conduction velocity after 200 s of VF, suggesting that L-type Ca++channels remain active and may be important in the maintenance of LDVF. Sotalol in addition to verapamil caused no additional antiarrhythmic effect.

asystole, identified as a flatline in all ECG leads, is a nonshockable rhythm that is associated with high mortality. Patients with coronary artery disease who suffer a sudden occlusion may progress to asystole through a sequence of events initiated at the onset of the occlusion: local ischemia, ventricular tachycardia (VT), ventricular fibrillation (VF), global ischemia, and asystole. Defibrillation has the highest efficacy when applied during VF, well before asystole. Prognosis is dim for patients suffering from out-of-hospital cardiac arrest (OHCA) who are found in asystole. An analysis of the Swedish OHCA registry has shown that patients who were found in a nonshockable rhythm had an average one-month survival rate of 1.3% (8). Several retrospective studies have analyzed the incidence of lethal arrhythmias in OHCA and found that the incidence of arrest with VT/VF as the first recorded rhythm has steadily declined while the incidence of non-VF arrest, including asystole, has markedly increased (10, 13). The decline in VF arrest could be the result of reduced incidence of VT/VF as the cause of OHCA, likely because of increased use of implantable cardioverter defibrillators and β-blockers. Interestingly, the incidence of asystole as the first recorded rhythm increased substantially from 1992-2002 in Milwaukee, WI, with the increase offsetting the decline in VT/VF arrests. Overall, the total incidence of OHCA remained constant over the period of that study (13). Observed increases in non-VF OHCA in the United States and Europe have lead to speculation that VF in OHCA is quickly deteriorating into another rhythm, primarily asystole (10). There is further speculation that β-blockers could be the culprit in the accelerated deterioration of VF to asystole (10). In this issue of the American Journal of Physiology-Heart and Circulatory Physiology, Garg and colleagues (6a) present the results of investigations into factors that influence electrical function's degeneration to asystole during global ischemia. They hypothesized that both rapid heart rate and β-adrenergic stimulation would accelerate electrical failure. They studied isolated rabbit hearts in which contraction had been pharmacologically arrested. The hearts were paced at cycle lengths of either 300 or 200 ms and either were or were not exposed to 30 nM of the β-adrenergic agonist isoproterenol. Global no-flow ischemia was initiated, and the resulting progression of electrical failure was monitored with optical mapping. They found that a rapid heart rate and β-adrenergic activation synergistically accelerated the development of regional loss of excitability, which was tightly correlated with the onset of VF and, ultimately, the complete loss of excitability (asystole). Although measured during global ischemia, the activation rate data of Garg et al. may provide new insight into clinical observations that elevated basal heart rate is associated with increased cardiovascular risk factors (2, 5, 12), including sudden myocardial infarction (7) and VF (1). These data support clinical efforts to maintain lower heart rates in patients with coronary artery disease. It is reasonable to hypothesize that acute local ischemia within the context of high heart rate and increased β-adrenergic activity would be correlated with increased risk of VF and possibly a fast progression to asystole. Thus, during a coronary event, reduced basal heart rate may lengthen the time to asystole, thereby increasing the likelihood of observing a shockable rhythm in OHCA. β-Blockers may further lengthen the time to asystole; however, this point could be debated (10). In previous animal studies, slowing of sinus rate and blocking β-adrenergic activity delayed the time to onset of VF during ischemic events, presumably by preserving myocardial energy status (21). Studies by Vaillant and colleagues (21) measured the time to VF after acute myocardial ischemia in pigs. From the onset of ischemia, the time to VF was prolonged when a pacemaker current (IKf) inhibitor (ivabradine) was administered to reduce heart rate. When a β-blocker (propranolol) was administered, time to VF was also prolonged, but not nearly as much as that with IKf inhibition. The isoproterenol results of Garg et al. corroborate these earlier findings. Phosphocreatine-to-ATP ratios measured by Vaillant et al. (21) at the onset of VF were also higher during IKf inhibition, indicating improved preservation of myocardial energy stores. In the clinic, heart rate reduction in patients with angina pectoris increases the time to ST-segment depression and prolongs the onset of angina symptoms (15, 20). These beneficial effects are attributed to improved myocardial energy status, including reductions in myocardial oxygen consumption and improved coronary perfusion secondary to a longer diastolic interval (2). Garg and colleagues provide several thought-provoking interpretations of their optical mapping data. During ischemia, diminished excitability, conduction block, and asystole are typically attributed to elevated extracellular potassium that depolarizes resting membrane potential, thereby inactivating sodium channels (3, 18). However, Garg and colleagues argue against this mechanism by suggesting that the enhanced electrical depression that they observed was caused by increased calcium loading, not the accumulation of extracellular potassium. They further suggest that the initiation of VF in their experiments was not due to early or delayed afterdepolarizations, which are usually attributed to altered intracellular calcium homeostasis (16) and are typically invoked to explain arrhythmogenesis during β-adrenergic stimulation in nonischemic myocardium (11). Instead, they explain that the development of a spatially heterogeneous mosaic pattern of electrical depression was the mechanism of VF. Such a pattern would be expected to promote unidirectional block and reentry. As the authors speculate, the Purkinje network could play an important role in initiating the observed VF. Rabbits, the species used in this study, are similar to dogs and humans in that the Purkinje network is primarily subendocardial, making it nearly impossible to definitively associate epicardially mapped activation fronts with endocardial Purkinje activations. However, a number of studies (4, 9) have indicated that the Purkinje network is active during long duration VF, in which the heart is globally ischemic and is likely important in maintaining VF activation through a variety of focal and reentrant mechanisms. Similar mechanisms could very well be at play in the initiation of VF during global ischemia. A benefit of optical mapping in the studies of Garg et al. was that action potential duration heterogeneity could be analyzed. This revealed differences in the time course of action potential duration shortening between the right and left ventricles, with faster shortening occurring in the left ventricle for three out of four combinations of heart rate and isoproterenol administration. This result implies a shortening of reentrant path length as well as increased dispersion of repolarization, both of which are important VF mechanisms. An important limitation of the optical mapping studies was the requirement of electromechanical uncoupling to reduce motion artifact in the optical action potentials. Interpretation of data when cross-bridge cycling is inhibited must be done cautiously because actomyosin ATPases comprise 75% of total myocardial energy consumption (23). The remaining energy consumption is attributed to the Ca2+ ATPases (15%) and the Na+/K+ ATPase (9%) (17). It is likely that ATP use during both nonischemic and ischemic conditions in the studies of Garg et al. was only 24% of what it might be in an in vivo working heart. As such, it is likely that the time course of electrical events could be significantly shorter in vivo. This limitation is especially important if energy preservation is a mechanism by which lower heart rate and absence of β-adrenergic stimulation protect against the onset of VF. Inhibition of actomyosin ATPases has a large effect on physiological processes that are modulated by the balance of energy production and use, such as during increases in cardiac work and during ischemia (22). It is also worth noting that in their experiments, Garg et al. paced globally ischemia hearts at normal heart rates until VF spontaneously occurred. While this is a reasonable way to assess the effects of activation rate on the degeneration of electrical function, it does not parallel the typical progression of OHCA, which is thought to begin as an event of acute local ischemia and progress as stated in the introductory paragraph. Future optical mapping studies of local ischemia in fully contracting hearts could provide deeper insight into the initiation of VF and the progression of electrical failure. In light of clinical data that indicates increased incidence of asystole as the first recorded rhythm in OHCA (13), there is a clear need to understand the physiological mechanisms of asystole. Garg and colleagues have provided important new insights showing that rapid heart rate and β-adrenergic stimulation accelerate heterogeneous electrical depression and quicken the pace to asystole. Many questions remain, several of which are related to the role of β-adrenergic activity. When is the best time to administer exogeneous epinephrine during cardiac arrest (6, 19)? Do β-blockers prolong the maintenance of adequate levels of metabolic reserve during ischemia to maintain VF while also reducing defibrillation threshold? Could β-blockers alter mechanisms that maintain VF during ischemia and shorten the time to asystole (10) or increase defibrillation threshold (14)? Answers to these questions, and many others, will likely have significant impact on improving therapy for OHCA.

Quantitative analysis has demonstrated five temporal stages of activation during the first 10 min of ventricular fibrillation (VF) in dogs. To determine whether these stages exist in another species, we applied the same analysis to the first 10 min of VF recorded in vivo from two 504-electrode arrays, one each on left anterior and posterior ventricular epicardium in six anesthetized pigs. The following descriptors were continuously quantified: 1) number of wavefronts, 2) wavefront fractionations, 3) wavefront collisions, 4) repeatability, 5) multiplicity index, 6) wavefront conduction velocity, 7) activation rate, 8) mean area activated by the wavefronts, 9) negative peak rate of voltage change, 10) incidence of breakthrough/foci, 11) incidence of block, and 12) incidence of reentry. Cluster analysis of these descriptors divided VF into four stages ( stages i-iv). The values of most descriptors increased during stage i (1–22 s after VF induction), changed quickly to values indicating greater organization during stage ii (23–39 s), decreased steadily during stage iii (40–187 s), and remained relatively unchanged during stage iv (188–600 s). The epicardium still activated during stage iv instead of becoming silent as in dogs. In conclusion, during the first 10 min, VF activation can be divided into four stages in pigs instead of five stages as in dogs. Following a 16-s period during the first minute of VF when activation became more organized, all parameters exhibited progressive decreased organization. Further studies are warranted to determine whether these changes, particularly the increased organization of stage ii, have clinical consequences, such as alteration in defibrillation efficacy.

Aims The first seconds of ventricular fibrillation (VF) are well organized and can consist of just one to two rotating waves (rotors). New rotors are spawned when local propagation block causes wave fragmentation. We hypothesized that this process, which leads to fully developed VF, begins at a consistent anatomic site. Methods and results We initiated VF with a stimulus timed to the local T-wave in 10 isolated pig hearts. Hearts were stained with a voltage-sensitive dye and four video cameras recorded electrical propagation panoramically across the epicardium. In each VF episode, we identified the position of the first wavebreak event that produced new rotor(s) that persisted for at least one cycle. The first such wavebreak occurred along the anterior right ventricular insertion (ARVI) in 26 of 32 VF episodes. In these episodes, wavebreak sites were 6 ± 4 mm from the midline of the ARVI. In the remaining 6 episodes, wavebreak sites were 24 ± 5 mm from the midline on either the LV or RV. During rapid pacing, conduction speed was locally depressed at the ARVI when waves crossed parallel to the midline. Action potential duration (APD) was slightly longer (2.2 ± 2.1 ms) at the ARVI compared with other sites ( P < 0.01). Temporal APD alternans were small and not unique to the break site, suggesting that dynamic APD properties were not the cause of wavebreak. Conclusion The ARVI is the dominant site for wavebreak at the onset of VF in normal myocardium. This may be due to the anatomic complexity of the region.

We developed a new method for ratiometric optical mapping of transmembrane potential (V(m)) in cardiac preparations stained with di-4-ANEPPS. V(m)-dependent shifts of excitation and emission spectra establish two excitation bands (481 and481 nm) that produce fluorescence changes of opposite polarity within a single emission band (575-620 nm). The ratio of these positive and negative fluorescence signals (excitation ratiometry) increases V(m) sensitivity and removes artifacts common to both signals. We pulsed blue (450 ± 10 nm) and cyan (505 ± 15 nm) light emitting diodes (LEDs) at 375 Hz in alternating phase synchronized to a camera (750 frames-per-second). Fluorescence was bandpass filtered (585 ± 20 nm). This produced signals with upright (blue) and inverted (cyan) action potentials (APs) interleaved in sequential frames. In four whole swine hearts with motion chemically arrested, fractional fluorescence for blue, cyan, and ratio signals was 1.2 ± 0.3%, 1.2 ± 0.3%, and 2.4 ± 0.6%, respectively. Signal-to-noise ratios were 4.3 ± 1.4, 4.0 ± 1.2, and 5.8 ± 1.9, respectively. After washing out the electromechanical uncoupling agent, we characterized motion artifact by cross-correlating blue, cyan, and ratio signals with a signal with normal AP morphology. Ratiometry improved cross-correlation coefficients from 0.50 ± 0.48 to 0.81 ± 0.25, but did not cancel all motion artifacts. These findings demonstrate the feasibility of pulsed LED excitation ratiometry in myocardium.

INTRODUCTION Sotalol and verapamil alone reduce reentry incidence during ventricular fibrillation (VF). We tested whether the combination of these two drugs had a synergistic effect on initiation, maintenance, and termination of VF. METHODS Six open-chest pigs received intravenous sotalol (1.5 mg/kg) followed by verapamil (0.136 mg/kg). VF threshold (VFT) was determined by a burst pacing protocol. Two 20 seconds episodes of VF were recorded from a 21 × 24 unipolar electrode plaque on the lateral posterior left ventricular epicardium before and after each drug. VF activation patterns were quantified. The duration of long duration VF (LDVF) maintenance was compared to our previously published data. RESULTS Sotalol alone and combined with verapamil significantly increased the VFT from 12.3 ± 4.1 to 20.3 ± 7.1 and 26.7 ± 8.6 mA compared with baseline (P

Although it is well accepted that ventricular arrhythmias can arise by reentry, several mapping studies have reported focal activation sequences during arrhythmias in which a wavefront spreads away in all directions from the site of earliest recorded activation. If the maps are twodimensional, obtained by mapping the epicardial or endocardial surface, this activation sequence may not indicate a true focus but rather an intramural wavefront breaking through to the surface. However, this focal activation pattern has also been observed in three-dimensional mapping studies with plunge needles inserted through the ventricular wall.1, 2 These studies include the disclaimer that the focal activation pattern may arise from a microreentrant circuit that is so small it cannot be detected by the plunge needle electrodes. This Viewpoint examines the basis for this disclaimer and estimates the electrode spacing needed to detect the smallest microreentrant circuits.

Quantifying the regional frequency variation in ventricular fibrillation (VF) may lead to focal strategies in treating human VF. We hypothesized that during human VF there are quantifiable regional frequency variations in the ventricles and they relate to underlying fixed myocardial substrate. In eight myopathic human hearts, we studied 35 VF episodes. The electrograms during VF were acquired simultaneously from the epicardium and endocardium using 2 electrode arrays each consisting of 112 electrodes. Regional characterization was performed using a ratio parameter derived from the dominant frequency analysis of the electrograms. The findings were related to the anatomical substrate using bipolar voltage maps. The results of the analysis indicate that LV had a larger dominant frequency (DF) span than RV (p = 0.0111) while there was no significant difference (p = 0.1488) in the DF span between LV freewall (FW) and septum (SE). Correlation of areas of abnormal myocardium with the dominant frequency feature matched only in 50% of the cases indicating that ion channel heterogeneity and time-varying physiological factors may play an important role in maintaining VF.

Computer modeling of cardiac propagation suggests that curvature of muscle fibers modulates conduction velocity (CV). The effect could be involved in arrhythmogenesis by altering the dynamics of reentrant wavefronts or by causing propagation block. To verify the existence of this effect experimentally, we measured CV in anisotropic neonatal rat ventricular myocyte monolayers. The orientation of the cells was directed by scratches machined into plastic coverslips. Each substrate contained a region in which scratch radius of curvature varied from 0.25 to 1.0 cm. The CV anisotropy ratio (longitudinal CV/transverse CV in straight fiber regions) was 2.3 plusmn 0.3 (n = 38 ). We initiated wavefronts transverse to fibers with the fibers either curving toward or away from the wavefronts. Action potentials were recorded using a potentiometric dye and a video camera. Propagation was faster ( p = 0.0003) when fibers curved toward wavefronts than when fibers curved in the opposite direction. The mean CV difference was 0.38 plusmn 0.44 cm/s (n = 24), which is 3.5% of nominal straight fiber transverse CV (11.0 plusmn 3.2 cm/s). The effect was also present (p = 0.07) when pacing was slowed from 350 to 500 ms (n = 6). In a control group ( n = 8) with uncurved fibers, CV was the same in both directions (p = NS). We conclude that fiber curvature is a factor in modulating cardiac propagation.

Our objective was to establish a novel model for the study of ventricular fibrillation (VF) in humans. We adopted the established techniques of optical mapping to human ventricles for the first time to determine whether human VF is the result of wave breaks and singularity point formation and is maintained by high-frequency rotors and fibrillatory conduction. We describe the technique of acquiring optical signals in human hearts during VF, their characteristics, and the feasibility of possible analyses that could be performed to elucidate mechanisms of human VF. We used explanted hearts from five cardiomyopathic patients who underwent transplantation. The hearts were Langendorff perfused with Tyrode solution (95% O2-5% CO2), and the potentiometric dye di-4-ANEPPS was injected as a bolus into the coronary circulation. Fluorescence was excited at 531 ± 20 nm with a 150-W halogen light source; the emission signal was long-pass filtered at 610 nm and recorded with a mapping camera. Fractional change of fluorescence varied between 2% and 12%. Average signal-to-noise ratio was 40 dB. The mean velocity of VF wave fronts was 0.25 ± 0.04 m/s. Submillimetric spatial resolution (0.65–0.85 mm), activation mapping, and transformation of the data to phase-based analysis revealed reentrant, colliding, and fractionating wave fronts in human VF. On many occasions the VF wave fronts were as large as the entire vertical length (8 cm) of the mapping field, suggesting that there are a limited number of wave fronts on the human heart during VF. Phase transformation of the optical signals allowed the first demonstration ever of phase singularity point, wave breaks, and rotor formation in human VF. This method provides opportunities for potential analyses toward elucidation of the mechanisms of VF and defibrillation in humans.

During ventricular fibrillation (VF), activation waves are fragmented and the heart cannot contract synchronously. It has been proposed that VF waves emanate from stable sources (“mother rotors”). Previously, we used new optical mapping technology to image VF wavefronts from nearly the entire epicardial surface of six isolated swine hearts. We found that VF was not driven by epicardial rotors, but could not exclude the presence of stable rotors hidden within the ventricular walls. Here, we use graph theoretic analysis to show that, in all 17 VF episodes we analyzed, it was always possible to trace sequences of wavefronts through series of fragmentation and collision events from the beginning to the end of the episode. The set of wavefronts that were so related (the dominant component) consisted of 92%±1% of epicardial wavefronts. Because each such wavefront sequence constitutes a continuous activation front, this finding shows that complete reentrant pathways were always present on the epicardial surface and therefore, that wavefront infusion from nonepicardial sources was not strictly necessary for VF maintenance. These data suggest that VF in this model is not driven by localized sources; thus, new anti-VF treatments designed to target such sources may be less effective than global interventions.

During ventricular fibrillation (VF), electrical activation waves are fragmented, and the heart cannot contract in synchrony. It has been proposed that VF waves emanate from stable periodic sources (often called “mother rotors”). The objective of the present study was to determine if stable rotors are consistently present on the epicardial surface of hearts comparable in size to human hearts. Using new optical mapping technology, we imaged VF from nearly the entire ventricular surface of six isolated swine hearts. Using newly developed pattern analysis algorithms, we identified and tracked VF wave fronts and phase singularities (PS; the pivot point of a reentrant wave front). We introduce the notion of a compound rotor in which the rotor's central PS can change and describe an algorithm for automatically identifying such patterns. This prevents rotor lifetimes from being inappropriately abbreviated by wave front fragmentation and collision events near the PS. We found that stable epicardial rotors were not consistently present during VF: only 1 of 17 VF episodes contained a compound rotor that lasted for the entire mapped interval of 4 s. However, shorter-lived rotors were common; 12.2 (SD 3.3) compound rotors with lifetime >200 ms were visible on the epicardium at any given instant. We conclude that epicardial mother rotors do not drive VF in this experimental model; if mother rotors do exist, they are intramural or septal. This paucity of persistent rotors suggests that individual rotors will eventually terminate by themselves and therefore that the continual formation of new rotors is critical for VF maintenance.

The objective of this study was to test the hypothesis that on the epicardium of the in vivo human heart, ventricular fibrillation (VF) consists of chaotic small wavefronts that constantly change paths.Despite the significance of VF to cardiovascular mortality, little is known about the wavefronts that constitute VF in humans.In 9 patients undergoing cardiac surgery, a single VF episode was induced by rapid pacing immediately after institution of cardiopulmonary bypass while recordings were made from 504 electrodes spaced 2 mm apart in a 20 cm(2) plaque held against the anterior left ventricle epicardium. A total of 26 segments of VF, each 2 s long, were analyzed. A computer algorithm identified individual wavefronts and classified them into groups that followed similar activation sequences.The mean activation rate was 5.8 +/- 1.8 (mean +/- SD) cycles/s. The wavefronts during each epoch were grouped into 9.4 +/- 7.1 different activation pathways, and 8.3 +/- 2.3 wavefronts followed each pathway. Individual wavefronts spread to activate an area of 5.1 +/- 3.0 cm(2) in the mapped region. The majority of the wavefronts propagated into the mapped region and/or propagated out of the mapped region into adjacent tissue, suggesting that the wavefronts were larger than 5.1 cm(2). Reentry was identified in only 16 of the 26 (62%) 2-s segments, always completed2 cycles, and lasted for 9.5 +/- 6.6% of these 16 epochs, which is 5.8% of the total duration of all the segments analyzed.VF wavefronts on the human epicardium are usually large, repeatedly follow distinct pathways, and only occasionally reenter. If these results for the left ventricular epicardium are representative of those for the entire ventricular mass, they do not support the hypothesis that human VF consists of small, constantly changing wavefronts, but rather suggest that there is significant organization of human VF.

Although resuscitation for sudden cardiac arrest attempts are frequently not instituted for several minutes after the onset of ventricular fibrillation (VF), previous mapping studies have examined only the first 40 s of VF or have involved isolated perfused hearts that did not become ischemic during VF. We applied quantitative pattern analysis to mapping data throughout the first 10 min of VF acquired from a 21 x 24 unipolar electrode array located on the ventricular epicardium of six open-chest dogs. The following twelve descriptors were continuously quantified: 1) number of wavefronts, 2) incidence of reentry, 3) wavefront propagation velocity, 4) incidence of breakthrough/focus, 5) incidence of block, 6) mean area activated by the wavefronts, 7) wavefront fractionations, 8) wavefront collisions, 9) multiplicity index, 10) repeatability, 11) negative peak rate of voltage change, and 12) peak frequency of activation. Cluster analysis of these descriptors divided VF into five stages (stages i-v). The values of most descriptors (except block and breakthrough incidence) increased during stage i (1-11 s after VF induction) and maintained high values with rapid dynamic fluctuations during stage ii (12-62 s). Descriptors changed quickly to values indicating greater organization during stage iii (63-86 s), decreased steadily during stage iv (87-310 s), and approached zero during stage v (311-600 s). There was a high incidence of reentry just before, during, and after stage iii. In conclusion, during the first 10 min, VF can be divided into five stages according to the evolution of electrophysiological characteristics. All of the parameters show a rapid deterioration during VF, except for a temporary reversal approximately 1 min after induction when activation briefly became more organized. Thus a quantitative description of activation does not uniformly decrease as VF progresses, but undergo rapid changes and exhibit a brief interval of increased organization after approximately 1 min of VF. Further studies are warranted to determine whether these changes, particularly the increased organization of stage iii, have clinical consequences, such as an alteration in defibrillation efficacy.

We tested whether the interventions typically required for optical mapping affect activation patterns during ventricular fibrillation (VF). A 21 × 24 unipolar electrode array (1.5 mm spacing) was sutured to the left ventricular epicardium of 16 anesthetized pigs, and four episodes of electrically induced VF (30-s duration) were recorded. The hearts were then rapidly excised and connected to a Langendorff perfusion apparatus. Four of the hearts were controls, in which 24 additional VF episodes were then mapped. In the remaining 12 hearts, four VF episodes were mapped after isolation, four more episodes were mapped after exposure to the voltage-sensitive dye di-4-ANEPPS, and six more episodes were mapped after exposure to the electromechanical uncoupling agents diacetyl monoxime (DAM; 20 mmol/l, n = 6) or cytochalasin D (CytoD; 10 μmol/l, n = 6). VF episodes were separated by 4 min. VF activation patterns were quantified using custom pattern analysis algorithms. From comparisons with time-corrected control data, all interventions significantly changed VF patterns. Most changes were broadly consistent with slowing and regularization due to loss of excitability. Heart isolation had the largest effect on VF patterns, followed by CytoD, DAM, and dye.

It has been hypothesized that during ventricular fibrillation (VF), the fastest activating region, the dominant domain, contains a stable reentrant circuit called a mother rotor. This hypothesis postulates that the mother rotor spawns wavefronts that propagate to maintain VF elsewhere and implies that the ratio of wavefronts propagating off a region to those propagating onto it (propoff/propon) should be >1 for the dominant domain but P P

Background The mechanisms that maintain long duration ventricular fibrillation ( LDVF ) are unclear. The difference in distribution of the Purkinje system in dogs and pigs was explored to determine if Purkinje activation propagates to stimulate working myocardium ( WM ) during LDVF and WM pacing. Methods and Results In‐vivo extracellular recordings were made from 1044 intramural plunge and epicardial plaque electrodes in 6 pig and 6 dog hearts. Sinus activation propagated sequentially from the endocardium to the epicardium in dogs but not pigs. During epicardial pacing, activation propagated along the endocardium and traversed the LV wall almost parallel to the epicardium in dogs, but in pigs propagated away from the pacing site approximately perpendicular to the epicardium. After 1 minute of VF , activation rate near the endocardium was significantly faster than near the epicardium in dogs ( P P >0.05). From 2 to 10 minutes of LDVF , recordings exhibiting Purkinje activations were near the endocardium in dogs ( P WM activation rate in recordings in which Purkinje activations were present was significantly faster than the WM activation rate in recordings in which Purkinje activations were absent ( P LV endocardium was exposed and 2 microelectrodes were inserted into Purkinje and adjacent myocardial cells. After 5 minutes of LDVF , mean Purkinje activation rate was significantly faster than mean WM activation rate ( P Conclusion These extracellular and intracellular findings about activation support the hypothesis that Purkinje activation propagates to stimulate WM during sinus rhythm, pacing, and LDVF .

Objective: Controlled reperfusion and secondary cardioplegia are used to minimize reperfusion injury. The mechanisms for their benefit are incompletely defined and may include attenuation of myocyte sodium uptake. Methods: Pigs had 1 hour of cardioplegic arrest followed by reperfusion with blood (control) or warm cardioplegic solution followed by blood (test). Reperfusion injury in the control and test groups was quantified by measuring changes of intramyocyte ion content with atomic absorption spectrometry and by analyzing electrophysiologic recovery from recordings of reperfusion arrhythmias. Results: Control animals had an increase in intramyocyte sodium content at 5 minutes after initiating reperfusion (+20.2 μmol/g dry weight, P < .04), whereas the test group had an insignificant decrease (–14.0 μmol/g dry weight, P = .33). The first rhythm after initiating reperfusion was more often ventricular fibrillation in the control group (100% vs 50%, P < .02), and the control group required more defibrillations to establish a nonfibrillating rhythm (4.5 ± 1.2 vs 1.1 ± 0.3, P < .03). Conclusions: Controlled reperfusion eliminated the increase in intramyocyte sodium that was observed in the control group at 5 minutes after cardioplegic arrest. This improvement in myocyte ion homeostasis during postcardioplegia reperfusion was associated with fewer reperfusion arrhythmias. These data support the hypothesis that attenuation of myocyte sodium gain during postischemic reperfusion is a mechanism by which controlled reperfusion and secondary cardioplegia are beneficial. (J Thorac Cardiovasc Surg 2000;119:1093-101)

Examines relationships between transmembrane potential (V/sub m/), [Ca/sup 2+/]/sub i/ dependent membrane ionic currents, and [Ca/sup 2+/]/sub i/ handling by the sarcoplasmic reticulum (SR) in a two-dimensional model of cardiac tissue. Luo-Rudy dynamic (LRd) membrane equations were used because they include detailed formulations for triggered SR Ca/sup 2+/ release dependent on membrane Ca/sup 2+/ influx (CICR) and for spontaneous SR Ca/sup 2+/ release following calsequestrin buffer overload (SCR). Reentry's rapid rate (110-ms cycle length) elevated [Ca/sup 2+/]; and limited CICR, which in turn promoted SCR that occurred at intervals of 320-350 ms, was preferential at sites located inside the functional center, and destabilized the reentrant activation sequence. Although adjustment of LRd parameters for SR Ca/sup 2+/ modified SCR interval and peak [Ca/sup 2+/]/sub i/ in voltage clamp simulations with a command waveform representing V/sub m/ time course within the functional center, SCR persisted. Using the same command waveform, SCR also occurred with an alternate SR Ca/sup 2+/ formulation that represented subcellular details underlying CICR. LRd parameter adjustments to promote CICR and limit SCR in subsequent reentry simulations failed to eliminate SCR completely, as they modulated SCR intervals in a manner consistent with the voltage clamp simulations. Taken together, the authors' findings support a destabilizing influence of functional reentry on [Ca/sup 2+/]/sub i/ handling. However, [Ca/sup 2+/]/sub i/ instabilities did not always fractionate depolarization wavefronts during reentry. Fractionation depended, in part, upon CICR and SCR parameters in the LRd formulation for SR Ca/sup 2+/ release.

It is commonly thought that lethal tachyarrhythmias, such as ventricular fibrillation (VF), are perpetuated by functional reentry, which occurs when an activation wave blocks and rotates around tissue that is excitable (i.e., functional block). Electrograms recorded near these regions typically contain two sequential deflections representing activation on either side of the block. By detecting these "double potentials", we hypothesize that functional block can be detected by a single electrode. Unipolar electrograms were recorded from a 24/spl times/21 mapping array on the intact ventricular epicardium of five pigs during electrically-induced VF. The short time Fourier transform (STFT) of each electrogram was analyzed to identify double potentials. To evaluate the performance of the STFT algorithm, conduction block was located in activation maps using a minimum conduction velocity criterion (10 cm/s) and then compared to the results of the STFT algorithm. The STFT algorithm detected conduction block with a sensitivity of 0.74/spl plusmn/0.12 and a specificity of 0.99/spl plusmn/0.00. We have developed an automated algorithm that can detect functional block during VF from a single electrode recording. Possible applications include fast, objective identification of block in mapping data and real-time localization of reentrant substrates using mapping catheters.

d e t s v b b d e n g a r w r s d t p For more than a century, ventricular fibrillation (VF) has een considered a disorganized, uncoordinated tachyarhythmia. However, recent electrical and optical mapping tudies have demonstrated a certain level of organization uring VF in which some activation fronts are large and ctivate large portions of the ventricular myocardium. uestions about VF currently being investigated by maping studies include (1) how large are the activation fronts, 2) how repeatable are the activation sequences, (3) do onduction block and reentry occur more commonly in ome anatomic regions of the ventricles than others, and (4) hat are the mechanisms causing conduction block during F? The article by Choi et al in this issue of Heart Rhythm rovides new information about all of these questions using ptical mapping with the highest spatial and temporal samling yet reported. Because of the huge amount of data ollected by recording simultaneously from a large number f sites during VF, these authors developed computer algoithms for detecting different types of conduction block imilar to those developed by Rogers et al for analyzing ata obtained from electrical mapping of VF. Choi et al observed that activation fronts terminated requently during VF, with a mean number of wavefront erminations and or fractionations of 34/cm/s of VF. In pproximately half of these cases, the activation fronts terinated by decremental conduction in which the upstrokes f the action potentials became small before the front terinated. In one third of the cases, the activation fronts erminated by colliding with another activation front. In the emaining one sixth of cases, the activation fronts blocked at ites of discordant alternans. At these sites, block occurred t the node between two regions with alternans of the action otentials and in which the two regions were out of phase ith each other so that one region had a small action

It has been known for 85 years that ventricular fibrillation (VF) is a dynamic process. In a foundational study, Wiggers [(1)][1] used high-speed photography to show that VF progresses through a series of stages. Much more has been learned as cardiac mapping technology has progressed. In cardiac

An automated method to estimate vector fields of propagation velocity from observed epicardial extracellular potentials is introduced. The method relies on fitting polynomial surfaces T(x,y) to the space-time (x,y,t) coordinates of activity, Both speed and direction of propagation are computed from the gradient of the local polynomial surface. The components of velocity, which are total derivatives, are expressed in terms of the partial derivatives which comprise the gradient of T. The method was validated on two-dimensional (2-D) simulations of propagation and then applied to cardiac mapping data. Conduction velocity was estimated at multiple epicardial locations during sinus rhythm, pacing, and ventricular fibrillation (VF) in pigs. Data were obtained via a 528-channel mapping system from 23/spl times/22 and 24/spl times/21 arrays of unipolar electrodes sutured to the right ventricular epicardium. Velocity estimates are displayed as vector fields and are used to characterize propagation qualitatively and quantitatively during both simple and complex rhythms.

Background —Electrical mapping studies indicate an interval of 40 to 100 ms between a defibrillation shock and the earliest activation that propagates globally over the ventricles (globally propagated activation, GPA). This study determined whether activation occurs during this interval but propagates only locally before being blocked (locally propagated activation, LPA). Methods and Results —In five anesthetized pigs, the heart was exposed and a 504-electrode sock with 4-mm interelectrode spacing was pulled over the ventricles. Ten biphasic shocks of a strength near the defibrillation threshold (DFT) were delivered via intracardiac catheter electrodes, and epicardial activation sequences were mapped before and after attempted defibrillation. Local activation was defined as dV/dt ≤−0.5 V/s. Postshock activation times and wave-front interaction patterns were determined with an animated display of dV/dt at each electrode in a computer representation of the ventricular epicardium. LPAs were observed after 40 of the 50 shocks. A total of 173 LPA regions were observed, each of which involved 2±2 (mean±SD) electrodes. LPAs were observed after both successful and failed shocks but occurred earlier ( P P Conclusions —LPAs exist after successful and failed shocks near the DFT. Thus, the time from the shock to the GPA is not totally electrically silent.

A major controversy in cardiac electrophysiology is whether ventricular fibrillation (VF) is maintained by wandering wavelets or mother rotors. On the basis of a computer model of atrial fibrillation by Moe1 developed in 1962, fibrillation was hypothesized to be maintained by multiple small, wandering wavelets, in which the activation pattern is constantly changing. According to the wandering wavelet hypothesis, fibrillation is maintained by constantly changing reentrant circuits formed by some of the wandering wavelets. Because this random reentry can occur anywhere within the cardiac muscle, this hypothesis suggests that all parts of the ventricular myocardium are equally important for the maintenance of VF. Article p 536 The mother rotor hypothesis was put forward for the maintenance of atrial fibrillation first by Lewis2 and later by Gurvich.3 Recently, this hypothesis has been applied to VF by Jalife’s group.4 This group reported that in isolated guinea pig hearts and in isolated right ventricular slabs of sheep, a long-lived, stationary, stable reentrant circuit that they called a “mother rotor” was present.5,6 The mother rotor gave rise to daughter wavefronts that propagated into the remainder of the myocardium. As opposed to the mother rotor, these daughter wavefronts did not have regular, repeating activation sequences. Instead, activation of the daughter wavefronts was less organized, with changing activation sequences, conduction block, and fractionation. It was these unstable daughter wavefronts that gave rise to the disorganized pattern in the ECG. The region containing the mother rotor had a shorter refractory period than the remainder of the myocardium. Jalife’s group postulated that the reason activation was less organized in the regions outside of the mother rotor was that the longer refractory periods in these regions did not allow them to keep up with the mother rotor, whose cycle length was shorter than their …

Few techniques have been developed for deriving quantitative measures of activation patterns during ventricular fibrillation (VF). Such measures have many potential applications, for example, assessing the effects of time, drugs, or electrical interventions. We have developed a new framework for quantifying VF patterns as mapped from an array of approximately 500 unipolar electrodes. Individual activation wavefronts are isolated from one another using an algorithm that groups together adjacent active electrogram samples (dV/dt-0.5 V/sec). Contacts between wavefronts are detected: these include fractionations, in which a single wavefront breaks into multiple wavefronts, and collisions, in which multiple wavefronts coalesce to form a new wavefront. The timing and contact relationships between wavefronts are summarized as a directed graph. From this model of the VF episode, we derive several parameters: number of wavefronts, number of fractionations, number of collisions, mean wavefront size, mean area swept out, and mean duration. As an example of this analysis, we computed these parameters in six open-chest pigs at 5, 10, 15, and 20 sec after electrical induction of VF. The number of wavefronts and the number of collisions decreased, whereas the mean wavefront size and mean area swept out increased during this period. These results are consistent with previous studies showing a recovery of organization during the first minute of VF.

We developed a new method for fabricating plunge needle electrodes for use in cardiac mapping. The needles have 12 electrodes with 1-mm spacing, are 0.5 mm in diameter, and are fabricated from fiberglass reinforced epoxy. They are stiff enough to be easily inserted into beating hearts and durable enough to be reused many times. We found that these new needles elicit smaller, more quickly resolving injury potentials, and when inserted in a row with 2-mm spacing, disrupt ventricular fibrillation activation patterns less than traditional steel needles.

Optical mapping of cardiac propagation has traditionally been hampered by motion artifact, chiefly due to changes in photodetector-to-tissue registration as the heart moves. We have developed an optical mapping technique to simultaneously record electrical waves and mechanical contraction in isolated hearts. This allows removal of motion artifact from transmembrane potential (V(m)) recordings without the use of electromechanical uncoupling agents and allows the interplay of electrical and mechanical events to be studied at the whole organ level. Hearts are stained with the voltage-sensitive dye di-4-ANEPPS and ring-shaped markers are attached to the epicardium. Fluorescence, elicited on alternate frames by 450 and 505 nm light-emitting diodes, is recorded at 700 frames∕ per second by a camera fitted with a 605 ± 25 nm emission filter. Marker positions are tracked in software. A signal, consisting of the temporally interlaced 450 and 505 nm fluorescence, is collected from the pixels enclosed by each moving ring. After deinterlacing, the 505 nm signal consists of V(m) with motion artifact, while the 450 nm signal is minimally voltage-sensitive and contains primarily artifacts. The ratio of the two signals estimates V(m). Deformation of the tissue enclosed by each set of 3 rings is quantified using homogeneous finite strain.

Received January 25, 2009; accepted October 6, 2009. Phase is a descriptor that tracks the progression of a defined region of myocardium through the action potential and has been demonstrated to be an effective parameter in analyzing spatiotemporal changes during fibrillation. In this review, the basic principles behind phase mapping are presented mainly in the context of ventricular fibrillation (VF), atrial fibrillation (AF), and fibrillation from experimental monolayer data. During fibrillation, the phase distribution changes over time, depending on activation patterns. Analyzing these phase patterns provides us insight into the fibrillatory dynamics and helps clarify the mechanisms of cardiac fibrillation and modulation by interventions. Winfree1 introduced the phase analysis to study cardiac fibrillation in the late eighties. This time-encoding technique deals with a scenario where the activation periods are the same over the surface being mapped. To deal with the scenario of varying activation period over the mapped surface (common in animal and human fibrillation models), Gray et al2,3⇓ introduced the state-space encoding concept from nonlinear dynamics. In analyzing spatiotemporal phase maps constructed from electric or optical mapping of the surface of heart during VF, points around which the phase progresses through a complete cycle from −π to +π are of great interest. At these points, the phase becomes indeterminate and the activation wave fronts hinge to these points and rotate around them in an organized fashion. These points in the phase map are called phase singularity (PS) points. Bray et al4 developed a procedure to locate PS points in a phase map. Nash et al5 used phase mapping to study the entire ventricular epicardium of human hearts with a sock containing 256 unipolar contact electrodes. The development of this phase mapping tool has led to better understanding of fibrillation dynamics as evidenced by the …

Endocardial mapping has suggested that Purkinje fibers may play a role in the maintenance of long-duration ventricular fibrillation (LDVF). To determine the influence of Purkinje fibers on LDVF, we chemically ablated the Purkinje system with Lugol solution and recorded endocardial and transmural activation during LDVF. Dog hearts were isolated and perfused, and the ventricular endocardium was exposed and treated with Lugol solution ( n = 6) or normal Tyrode solution as a control ( n = 6). The left anterior papillary muscle endocardium was mapped with a 504-electrode (21 × 24) plaque with electrodes spaced 1 mm apart. Transmural activation was recorded with a six-electrode plunge needle on each side of the plaque. Ventricular fibrillation (VF) was induced, and perfusion was halted. LDVF spontaneously terminated sooner in Lugol-ablated hearts than in control hearts (4.9 ± 1.5 vs. 9.2 ± 3.2 min, P = 0.01). After termination of VF, both the control and Lugol hearts were typically excitable, but only short episodes of VF could be reinduced. Endocardial activation rates were similar during the first 2 min of LDVF for Lugol-ablated and control hearts but were significantly slower in Lugol hearts by 3 min. In control hearts, the endocardium activated more rapidly than the epicardium after 4 min of LDVF with wave fronts propagating most often from the endocardium to epicardium. No difference in transmural activation rate or wave front direction was observed in Lugol hearts. Ablation of the subendocardium hastens VF spontaneous termination and alters VF activation sequences, suggesting that Purkinje fibers are important in the maintenance of LDVF.

Recent studies suggest that during ventricular fibrillation (VF) epicardial vessels may be a site of conduction block and the posterior papillary muscle (PPM) in the left ventricle (LV) may be the location of a "mother rotor." The goal of this study was to obtain evidence to support or refute these possibilities.Epicardial activation over the posterior LV and right ventricle (RV) was mapped during the first 20 s of electrically induced VF in six open-chest pigs with a 504 electrode plaque covering a 20 cm(2) area centered over the posterior descending artery (PDA).The locations of epicardial breakthrough as well as reentry clustered in time and space during VF. Spatially, reentry occurred significantly more frequently over the LV than the RV in all 48 episodes, and breakthrough clustered near the PPM (p0.001). Significant temporal clustering occurred in 79% of breakthrough episodes and 100% of reentry episodes. These temporal clusters occurred at different times so that there was significantly less breakthrough when reentry was present (p0.0001). Conduction block occurred significantly more frequently near the PDA than elsewhere.The PDA is a site of epicardial block which may contribute to VF maintenance. Epicardial breakthrough clusters near the PPM. Reentry also clusters in space but at a separate site. The fact that breakthrough and reentry cluster at different locations and at different times supports the possibility of a drifting filament at the PPM so that at times reentry is present on the surface but at other times the reentrant wavefront breaks through to the epicardium.