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Cardiac experimental electrophysiology is in need of a well-defined Minimum Information Standard for recording, annotating, and reporting experimental data. As a step towards establishing this, we present a draft standard, called Minimum Information about a Cardiac Electrophysiology Experiment (MICEE). The ultimate goal is to develop a useful tool for cardiac electrophysiologists which facilitates and improves dissemination of the minimum information necessary for reproduction of cardiac electrophysiology research, allowing for easier comparison and utilisation of findings by others. It is hoped that this will enhance the integration of individual results into experimental, computational, and conceptual models. In its present form, this draft is intended for assessment and development by the research community. We invite the reader to join this effort, and, if deemed productive, implement the Minimum Information about a Cardiac Electrophysiology Experiment standard in their own work.

Objectives: Although the majority of sudden cardiac arrests occur in patients with ischemic heart disease, the effect of therapeutic hypothermia on arrhythmia susceptibility during acute global ischemia is not well understood. While both ischemia and severe hypothermia are arrhythmogenic, patients undergoing therapeutic hypothermia do not have an increase in arrhythmias, despite the fact that most sudden cardiac arrest occur in the setting of ischemia. We hypothesized that mild hypothermia induced prior to myocardial ischemia and reperfusion will have a beneficial effect on ischemia-related arrhythmia substrates.Design: We developed a model of global ischemia and reperfusion in the canine wedge preparation to study the transmural electrophysiologic effects of ischemia at different temperatures.Setting: Animal study.Subjects: Male mongrel dogs.Interventions: Canine left ventricle wedge preparations at 1) control (36°C) or 2) mild hypothermia, to simulate temperatures used in therapeutic hypothermia (32°C), were subjected to 15 mins of no-flow ischemia and subsequently reperfused.Measurements and Main Results: Optical action potentials were recorded spanning the transmural wall of left ventricle. Action potential duration for epicardial, mid-myocardial, and epicardial cells was measured. Transmural dispersion of repolarization and conduction velocity were measured at baseline, during ischemia, and during reperfusion. No difference was seen at baseline for conduction velocity or dispersion of repolarization between groups. Conduction velocity decreased from 0.46 ± 0.02 m/sec to 0.23 ± 0.07 m/sec, and dispersion of repolarization increased from 30 ± 5 msecs to 57 ± 4 msecs in the control group at 15 mins of ischemia. Mild hypothermia attenuated both the ischemia-induced conduction velocity slowing (decreasing from 0.44 ± 0.02 m/sec to 0.35 ± 0.03 m/sec; p = .019) and the ischemia-induced increase in dispersion of repolarization (25 ± 3 msecs to 37 ± 7 msecs; p = .037). Epicardial conduction block was observed in six of seven preparations of the control group, but no preparations in the mild hypothermia group developed conduction block (0/6).Conclusions: Mild hypothermia attenuated ischemia-induced increase in dispersion of repolarization, conduction slowing, and block, which are known mechanisms of arrhythmogenesis in ischemia. These data suggest that therapeutic hypothermia may decrease arrhythmogenesis during myocardial ischemia. [ABSTRACT FROM AUTHOR]

Background: Computer simulations have predicted that the balance of various electrogenic sarcolemmal ion currents may control the amplitude and phase of beat-to-beat alternans of membrane potential (V(m)). However, experimental evidence for the mechanism by which alternans of calcium transients produces alternation of V(m) (V(m)-ALT) is lacking.Objective: To provide experimental evidence that Ca-to-V(m) coupling during alternans is determined by the balanced influence of 2 Ca-sensitive electrogenic sarcolemmal ionic currents: I(NCX) and I(Ca).Methods and Results: V(m)-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n = 41) by using perforated patch and Indo-1(AM) fluorescence, respectively. There were 3 study groups: (1) control, (2) I(NCX) predominance created by adenoviral-induced NCX overexpression, and (3) I(Ca) predominance created by I(NCX) inhibition (SEA-0400) or enhanced I(Ca) (As(2)O(3)). During alternans, 14 of 14 control myocytes demonstrated positive Ca-to-V(m) coupling, consistent with I(NCX), but not I(Ca), as the major electrogenic current in modulating action potential duration. Positive Ca-to-V(m) coupling was maintained during I(NCX) predominance in 8 of 8 experiments with concurrent increase in Ca-to-V(m) gain (P <.05), reaffirming the role of increased forward-mode electrogenic I(NCX). Conversely, I(Ca) predominance produced negative Ca-to-V(m) coupling in 14 of 19 myocytes (P < .05) and decreased Ca-to-V(m) gain compared with control (P <.05). Furthermore, computer simulation demonstrated that Ca-to-V(m) coupling changes from negative to positive because of a shift from I(Ca) to I(NCX) predominance with increasing pacing rate.Conclusions: These data provide the first direct experimental evidence that coupling in phase and magnitude of Ca-ALT to V(m)-ALT is strongly determined by the relative balance of the prominence of I(NCX) vs I(Ca) currents. [ABSTRACT FROM AUTHOR]

This consensus guideline was prepared on behalf of the International Society for Holter and Noninvasive Electrocardiology and is cosponsored by the Japanese Circulation Society, the Computers in Cardiology Working Group on e-Cardiology of the European Society of Cardiology, and the European Cardiac Arrhythmia Society. It discusses the electrocardiographic phenomenon of T-wave alternans (TWA) (i.e., a beat-to-beat alternation in the morphology and amplitude of the ST-segment or T-wave). This statement focuses on its physiological basis and measurement technologies and its clinical utility in stratifying risk for life-threatening ventricular arrhythmias. Signal processing techniques including the frequency-domain Spectral Method and the time-domain Modified Moving Average method have demonstrated the utility of TWA in arrhythmia risk stratification in prospective studies in >12,000 patients. The majority of exercise-based studies using both methods have reported high relative risks for cardiovascular mortality and for sudden cardiac death in patients with preserved as well as depressed left ventricular ejection fraction. Studies with ambulatory electrocardiogram-based TWA analysis with Modified Moving Average method have yielded significant predictive capacity. However, negative studies with the Spectral Method have also appeared, including 2 interventional studies in patients with implantable defibrillators. Meta-analyses have been performed to gain insights into this issue. Frontiers of TWA research include use in arrhythmia risk stratification of individuals with preserved ejection fraction, improvements in predictivity with quantitative analysis, and utility in guiding medical as well as device-based therapy. Overall, although TWA appears to be a useful marker of risk for arrhythmic and cardiovascular death, there is as yet no definitive evidence that it can guide therapy. [ABSTRACT FROM AUTHOR]

Background: Cardiac alternans is promoted by heart failure (HF)-induced calcium (Ca 2+ ) cycling abnormalities. Late sodium current (I Na,L ) is enhanced in HF and promotes Ca 2+ overload; however, mechanisms underlying an antiarrhythmic effect of I Na,L blockade in HF remain unclear., Objective: The purpose of this study was to determine whether ranolazine suppresses cardiac alternans in HF by normalizing Ca 2+ cycling., Methods: Transmural dual optical mapping of Ca 2+ transients and action potentials was performed in wedge preparations from 8 HF and 8 control (normal) dogs. Susceptibility to action potential duration alternans (APD-ALT) and Ca 2+ transient alternans (Ca-ALT) was compared at baseline and with ranolazine (5-10 μM)., Results: HF increased APD- and Ca-ALT compared to normal (both P <.05), and ranolazine suppressed APD- and Ca-ALT in both groups (P <.05). The incidence of spatially discordant alternans (DIS-ALT) was increased by HF (8/8) compared to normal (4/8; P <.05), and ranolazine decreased DIS-ALT in HF (4/8; P <.05).Not only did ranolazine mitigate HF-induced Ca 2+ overload, it also attenuated APD-ALT to Ca-ALT gain (amount of APD-ALT produced by Ca-ALT). In HF, APD-ALT to Ca-ALT gain was significantly increased (0.55 ± 0.02) compared to normal (0.44 ± 0.02; P <.05) and was normalized by ranolazine (0.36 ± 0.05; P <.05), representing a complementary mechanism by which I Na,L blockade suppressed cardiac alternans., Conclusion: Ranolazine attenuated arrhythmogenic cardiac alternans in HF, both by suppressing Ca-ALT and decreasing the coupling gain of APD-ALT to Ca-ALT. Blockade of I Na,L may reverse impaired Ca 2+ cycling to mitigate cardiac alternans, representing a mechanism underlying the antiarrhythmic benefit of I Na,L blockade in HF., (Copyright © 2018 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.)

Connexin43 (Cx43) phosphorylation alters gap junction localization and function. In particular, phosphorylation at serine-368 (S368) has been suggested to alter gap junctional conductance, but previous reports have shown inconsistent results for both timing and functional effects of S368 phosphorylation. The objective of this study was to determine the functional effects of isolated S368 phosphorylation. We evaluated wild-type Cx43 (AdCx43) and mutations simulating permanent phosphorylation (Ad368E) or preventing phosphorylation (Ad368A) at S368. Function was assessed by optical mapping of electrical conduction in patterned cultures of neonatal rat ventricular myocytes, under baseline and metabolic stress (MS) conditions. Baseline conduction velocity (CV) was similar for all groups. In the AdCx43 and Ad368E groups, MS moderately decreased CV. Ad368A caused complete conduction block during MS. Triton-X solubility assessment showed no change in Cx43 location during conduction impairment. Western blot analysis showed that Cx43-S368 phosphorylation was present at baseline, and that it decreased during MS. Our data indicate that phosphorylation at S368 does not affect CV under baseline conditions, and that preventing S368 phosphorylation makes Cx43 hypersensitive to MS. These results show the critical role of S368 phosphorylation during stress conditions., (© 2015 Wiley Periodicals, Inc.)

Introduction: Mesenchymal stem cells (MSCs) have been associated with reduced arrhythmias; however, the mechanism of this action is unknown. In addition, limited retention and survival of MSCs can significantly reduce efficacy. We hypothesized that MSCs can improve impulse conduction and that alginate hydrogel will enhance retention of MSCs in a model of healed myocardial infarction (MI)., Methods and Results: Four weeks after temporary occlusion of the left anterior descending artery (LAD), pigs (n = 13) underwent a sternotomy to access the infarct and then were divided into two studies. In study 1, designed to investigate impulse conduction, animals were administered, by border zone injection, 9-15 million MSCs (n = 7) or phosphate-buffered saline (PBS) (control MI, n = 5). Electrogram width measured in the border zone 2 weeks after injections was significantly decreased with MSCs (-30 ± 8 ms, p < 0.008) but not in shams (4 ± 10 ms, p = NS). Optical mapping from border zone tissue demonstrated that conduction velocity was higher in regions with MSCs (0.49 ± 0.03 m/s) compared to regions without MSCs (0.39 ± 0.03 m/s, p < 0.03). In study 2, designed to investigate MSC retention, animals were administered an equal number of MSCs suspended in either alginate (2 or 1 % w/v) or PBS (n = 6/group) by border zone injection. Greater MSC retention and survival were observed with 2% alginate compared to PBS or 1% alginate. Confocal immunofluorescence demonstrated that MSCs survive and are associated with expression of connexin-43 (Cx43) for either PBS (control), 1%, or 2% alginate., Conclusions: For the first time, we are able to directly associate MSCs with improved impulse conduction and increased retention and survival using an alginate scaffold in a clinically relevant model of healed MI.

Background: Cardiac syncope is associated with poor outcomes and may result in traumatic injuries. In patients presenting to the emergency department (ED) with trauma, recognizing the cause of syncope is particularly challenging. Also, clinical markers to identify cardiac syncope are not well established., Study Objectives: We sought to evaluate clinical markers that could identify cardiac syncope in patients with traumatic falls derived from a large urban trauma database., Methods: All patients presenting to the ED during a 10-year study period with a traumatic fall were identified retrospectively. The subset of patients with syncope was ascertained by chart review and defined as cardiac syncope (e.g., presence of dysrhythmia, valvular abnormality), non-cardiac syncope (e.g., vasovagal, neurological), or syncope of unknown cause., Results: Of the 5420 patients with traumatic falls, 180 (3.3%) patients with syncope were identified. Among the 180 patients with syncope, the cause was identified as cardiac in 24 (13%), noncardiac in 58 (32%), and unknown in 98 (54%). Three independent predictors (i.e., risk factors) of cardiac syncope were identified: age >65 years, presence of coronary artery disease, and pathological Q waves. Presence of at least one risk factor accurately predicted cardiac syncope in this population, with a sensitivity of 100%, a specificity of 43%, and a negative predictive value of 100% (area under the receiver operating characteristic curve: 0.80 ± 0.04)., Conclusion: In patients with traumatic falls and syncope, simple clinical and electrocardiographical variables may identify patients with cardiac causes of syncope. Proper identification of cardiac syncope in this population can potentially prevent recurrence of life-threatening traumatic injury., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Emerging evidence suggests that ventricular electrical remodeling (VER) is triggered by regional myocardial strain via mechanoelectrical feedback mechanisms; however, the ionic mechanisms underlying strain-induced VER are poorly understood. To determine its ionic basis, VER induced by altered electrical activation in dogs undergoing left ventricular pacing (n = 6) were compared with unpaced controls (n = 4). Action potential (AP) durations (APDs), ionic currents, and Ca(2+) transients were measured from canine epicardial myocytes isolated from early-activated (low strain) and late-activated (high strain) left ventricular regions. VER in the early-activated region was characterized by minimal APD prolongation, but marked attenuation of the AP phase 1 notch attributed to reduced transient outward K(+) current. In contrast, VER in the late-activated region was characterized by significant APD prolongation. Despite marked APD prolongation, there was surprisingly minimal change in ion channel densities but a twofold increase in diastolic Ca(2+). Computer simulations demonstrated that changes in sarcolemmal ion channel density could only account for attenuation of the AP notch observed in the early-activated region but failed to account for APD remodeling in the late-activated region. Furthermore, these simulations identified that cytosolic Ca(2+) accounted for APD prolongation in the late-activated region by enhancing forward-mode Na(+)/Ca(2+) exchanger activity, corroborated by increased Na(+)/Ca(2+) exchanger protein expression. Finally, assessment of skinned fibers after VER identified altered myofilament Ca(2+) sensitivity in late-activated regions to be associated with increased diastolic levels of Ca(2+). In conclusion, we identified two distinct ionic mechanisms that underlie VER: 1) strain-independent changes in early-activated regions due to remodeling of sarcolemmal ion channels with no changes in Ca(2+) handling and 2) a novel and unexpected mechanism for strain-induced VER in late-activated regions in the canine arising from remodeling of sarcomeric Ca(2+) handling rather than sarcolemmal ion channels.

Mutations in cardiac myosin binding protein C (MyBP-C) are a common cause of familial hypertrophic cardiomyopathy (FHC). The majority of MyBP-C mutations are expected to reduce MyBP-C expression; however, the consequences of MyBP-C deficiency on the regulation of myofilament function, Ca²⁺ homeostasis, and in vivo cardiac function are unknown. To elucidate the effects of decreased MyBP-C expression on cardiac function, we employed MyBP-C heterozygous null (MyBP-C+/-) mice presenting decreases in MyBP-C expression (32%) similar to those of FHC patients carrying MyBP-C mutations. The levels of MyBP-C phosphorylation were reduced 53% in MyBP-C+/- hearts compared with wild-type hearts. Skinned myocardium isolated from MyBP-C+/- hearts displayed decreased cross-bridge stiffness at half-maximal Ca²⁺ activations, increased steady-state force generation, and accelerated rates of cross-bridge recruitment at low Ca²⁺ activations (<15% and <25% of maximum, respectively). Protein kinase A treatment abolished basal differences in rates of cross-bridge recruitment between MyBP-C+/- and wild-type myocardium. Intact ventricular myocytes from MyBP-C+/- hearts displayed abnormal sarcomere shortening but unchanged Ca²⁺ transient kinetics. Despite a lack of left ventricular hypertrophy, MyBP-C+/- hearts exhibited elevated end-diastolic pressure and decreased peak rate of LV pressure rise, which was normalized following dobutamine infusion. Furthermore, electrocardiogram recordings in conscious MyBP-C+/- mice revealed prolonged QRS and QT intervals, which are known risk factors for cardiac arrhythmia. Collectively, our data show that reduced MyBP-C expression and phosphorylation in the sarcomere result in myofilament dysfunction, contributing to contractile dysfunction that precedes compensatory adaptations in Ca²⁺ handling, and chamber remodeling. Perturbations in mechanical and electrical activity in MyBP-C+/- mice could increase their susceptibility to cardiac dysfunction and arrhythmia.

Nitric oxide (NO) derived from the activity of neuronal nitric oxide synthase (NOS1) is involved in S-nitrosylation of key sarcoplasmic reticulum (SR) Ca(2+) handling proteins. Deficient S-nitrosylation of the cardiac ryanodine receptor (RyR2) has a variable effect on SR Ca(2+) leak/sparks in isolated myocytes, likely dependent on the underlying physiological state. It remains unknown, however, whether such molecular aberrancies are causally related to arrhythmogenesis in the intact heart. Here we show in the intact heart, reduced NOS1 activity increased Ca(2+)-mediated ventricular arrhythmias only in the setting of elevated myocardial [Ca(2+)](i). These arrhythmias arose from increased spontaneous SR Ca(2+) release, resulting from a combination of decreased RyR2 S-nitrosylation (RyR2-SNO) and increased RyR2 oxidation (RyR-SOx) (i.e., increased reactive oxygen species (ROS) from xanthine oxidoreductase activity) and could be suppressed with xanthine oxidoreductase (XOR) inhibition (i.e., allopurinol) or nitric oxide donors (i.e., S-nitrosoglutathione, GSNO). Surprisingly, we found evidence of NOS1 down-regulation of RyR2 phosphorylation at the Ca(2+)/calmodulin-dependent protein kinase (CaMKII) site (S2814), suggesting molecular cross-talk between nitrosylation and phosphorylation of RyR2. Finally, we show that nitroso-redox imbalance due to decreased NOS1 activity sensitizes RyR2 to a severe arrhythmic phenotype by oxidative stress. Our findings suggest that nitroso-redox imbalance is an important mechanism of ventricular arrhythmias in the intact heart under disease conditions (i.e., elevated [Ca(2+)](i) and oxidative stress), and that therapies restoring nitroso-redox balance in the heart could prevent sudden arrhythmic death.

Altered mechanical loading of the heart leads to hypertrophy, decompensated heart failure and fatal arrhythmias. However, the molecular mechanisms that link mechanical and electrical dysfunction remain poorly understood. Growing evidence suggest that ventricular electrical remodeling (VER) is a process that can be induced by altered mechanical stress, creating persistent electrophysiological changes that predispose the heart to life-threatening arrhythmias. While VER is clearly a physiological property of the human heart, as evidenced by "T wave memory", it is also thought to occur in a variety of pathological states associated with altered ventricular activation such as bundle branch block, myocardial infarction, and cardiac pacing. Animal models that are currently being used for investigating stretch-induced VER have significant limitations. The zebrafish has recently emerged as an attractive animal model for studying cardiovascular disease and could overcome some of these limitations. Owing to its extensively sequenced genome, high conservation of gene function, and the comprehensive genetic resources that are available in this model, the zebrafish may provide new insights into the molecular mechanisms that drive detrimental electrical remodeling in response to stretch. Here, we have established a zebrafish model to study mechano-electrical feedback in the heart, which combines efficient genetic manipulation with high-precision stretch and high-resolution electrophysiology. In this model, only 90 min of ventricular stretch caused VER and recapitulated key features of VER found previously in the mammalian heart. Our data suggest that the zebrafish model is a powerful platform for investigating the molecular mechanisms underlying mechano-electrical feedback and VER in the heart., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Objectives: The aim of this study was to evaluate the links between connexin43 (Cx43) expression, myocardial conduction velocity, and ventricular tachycardia in a model of healed myocardial infarction., Background: Post-infarction ventricular arrhythmias frequently cause sudden death. Impaired myocardial conduction has previously been linked to ventricular arrhythmias. Altered connexin expression is a potential source of conduction slowing identified in healed scar border tissues. The functional effect of increasing border-zone Cx43 has not been previously evaluated., Methods: Twenty-five Yorkshire pigs underwent anterior infarction by transient left anterior descending coronary artery occlusion, followed by weekly testing for arrhythmia inducibility. Twenty animals with reproducibly inducible sustained monomorphic ventricular tachycardia were randomized 2:1:1 to receive AdCx43, Adβgal, or no gene transfer. One week later, animals underwent follow-up electrophysiologic study and tissue assessment for several functional and molecular measures., Results: Animals receiving AdCx43 had less electrogram fractionation and faster conduction velocity in the anterior-septal border zone. Only 40% of AdCx43 animals remained inducible for ventricular tachycardia, while 100% of controls were inducible after gene transfer. AdCx43 animals had 2-fold higher Cx43 protein levels in the anterior-septal infarct border, with similar percents of phosphorylated and intercalated disk-localized Cx43 compared with controls., Conclusions: These data mechanistically link Cx43 expression to slow conduction and arrhythmia susceptibility in the healed scar border zone. Targeted manipulation of Cx43 levels improved conduction velocity and reduced ventricular tachycardia susceptibility. Cx43 gene transfer represents a novel treatment strategy for post-infarction arrhythmias., (Copyright © 2012 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.)

Torsade de Pointes (TdP) proarrhythmia is a major complication of therapeutic drugs that block the delayed rectifier current. QT interval prolongation, the principal marker used to screen drugs for proarrhythmia, is both insensitive and nonspecific. Consequently, better screening methods are needed. Drug-induced transmural dispersion of repolarization (TDR) is mechanistically linked to TdP. Therefore, we hypothesized that drug-induced enhancement of TDR is more predictive of proarrhythmia than QT interval. High-resolution transmural optical action potential mapping was performed in canine wedge preparations (n = 19) at baseline and after perfusion with 4 different QT prolonging drugs at clinically relevant concentrations. Two proarrhythmic drugs in patients (bepridil and E4031) were compared with 2 nonproarrhythmic drugs (risperidone and verapamil). Both groups prolonged the QT (all P < 0.02), least with the proarrhythmic drug bepridil, reaffirming that QT is a poor predictor of TdP. In contrast, TDR was enhanced only by proarrhythmic drugs (P < 0.03). Increased TDR was due to a preferential prolongation of midmyocardial cell, relative to epicardial cell, APD, whereas nonproarrhythmic drugs similarly prolonged both cell types. In contrast to QT prolongation, augmentation of TDR was induced by proarrhythmic but not nonproarrhythmic drugs, suggesting TDR is a superior preclinical marker of proarrhythmic risk during drug development.

In many patients with left ventricular (LV) systolic dysfunction, the LV ejection fraction (LVEF)-a surrogate for reverse remodeling-fails to improve despite optimal medical therapy. The early identification of such patients would allow instituting aggressive treatment, including early therapy with implantable cardioverter defibrillators. We sought to establish the predictors of reverse remodeling in patients with LV systolic dysfunction receiving optimal medical therapy. Patients (n = 568) with newly documented LVEF of ≤0.35, who had ≥1 follow-up echocardiogram after ≥3 months, were evaluated. Reverse remodeling was defined as improvement in LVEF to >0.35. The clinical, laboratory, and echocardiographic data were compared between patients with (n = 263) and without (n = 305) reverse remodeling. The mean follow-up was 27 ± 16 months. Patients who demonstrated reverse remodeling had a significantly greater mean follow-up LVEF (0.51 ± 0.09 vs 0.25 ± 0.08; p <0.001). On multivariate analysis, the baseline LV end-systolic diameter index was the strongest predictor of reverse remodeling (odds ratio 5.79; 95% confidence interval 1.82 to 18.46; p <0.001). Other independent predictors of reverse remodeling were female gender (odds ratio 1.88; 95% confidence interval 1.19 to 2.98; p = 0.007), and nonischemic cardiomyopathy (odds ratio 1.65; 95% confidence interval 1.05 to 2.58; p = 0.03). Baseline LVEF was not an independent predictor of reverse remodeling. In conclusion, among patients with newly diagnosed LV systolic dysfunction, the LV end-systolic diameter index, but not the LVEF, at diagnosis, was a strong predictor of reverse remodeling. Patients with a low likelihood of reverse remodeling might benefit from more aggressive heart failure therapy, including the possible early use of implantable cardioverter defibrillators., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Sudden cardiac death exhibits diurnal variation in both acquired and hereditary forms of heart disease, but the molecular basis of this variation is unknown. A common mechanism that underlies susceptibility to ventricular arrhythmias is abnormalities in the duration (for example, short or long QT syndromes and heart failure) or pattern (for example, Brugada's syndrome) of myocardial repolarization. Here we provide molecular evidence that links circadian rhythms to vulnerability in ventricular arrhythmias in mice. Specifically, we show that cardiac ion-channel expression and QT-interval duration (an index of myocardial repolarization) exhibit endogenous circadian rhythmicity under the control of a clock-dependent oscillator, krüppel-like factor 15 (Klf15). Klf15 transcriptionally controls rhythmic expression of Kv channel-interacting protein 2 (KChIP2), a critical subunit required for generating the transient outward potassium current. Deficiency or excess of Klf15 causes loss of rhythmic QT variation, abnormal repolarization and enhanced susceptibility to ventricular arrhythmias. These findings identify circadian transcription of ion channels as a mechanism for cardiac arrhythmogenesis.

Rationale: Abnormal calcium release from sarcoplasmic reticulum (SR) is considered an important trigger of atrial fibrillation (AF). Whereas increased Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity has been proposed to contribute to SR leak and AF induction, downstream targets of CaMKII remain controversial., Objective: To test the hypothesis that inhibition of CaMKII-phosphorylated type-2 ryanodine receptors (RyR2) prevents AF initiation in FKBP12.6-deficient (-/-) mice., Methods and Results: Mice lacking RyR2-stabilizing subunit FKBP12.6 had a higher incidence of spontaneous and pacing-induced AF compared with wild-type mice. Atrial myocytes from FKBP12.6-/- mice exhibited spontaneous Ca(2+) waves (SCaWs) leading to Na(+)/Ca(2+)-exchanger activation and delayed afterdepolarizations (DADs). Mutation S2814A in RyR2, which inhibits CaMKII phosphorylation, reduced Ca(2+) spark frequency, SR Ca(2+) leak, and DADs in atrial myocytes from FKBP12.6-/-:S2814A mice compared with FKBP12.6-/- mice. Moreover, FKBP12.6-/-:S2814A mice exhibited a reduced susceptibility to inducible AF, whereas FKBP12.6-/-:S2808A mice were not protected from AF., Conclusions: FKBP12.6 mice exhibit AF caused by SR Ca(2+) leak, Na(+)/Ca(2+)-exchanger activation, and DADs, which promote triggered activity. Genetic inhibition of RyR2-S2814 phosphorylation prevents AF induction in FKBP12.6-/- mice by suppressing SR Ca(2+) leak and DADs. These results suggest suppression of RyR2-S2814 phosphorylation as a potential anti-AF therapeutic target.

Background: Several lines of evidence have suggested that maintenance of atrial fibrillation (AF) depends on reentrant mechanisms. Maintenance of reentry necessitates a sufficiently short refractory period and/or delayed conduction, and AF has been associated with both alterations. Fibrosis, cellular dysfunction, and gap junction protein alterations occur in AF and cause conduction delay. We performed this study to test the hypothesis that gap junction protein overexpression would improve conduction and prevent AF., Methods and Results: Thirty Yorkshire swine were randomized into 2 groups (sinus rhythm and AF), and each group into 3 subgroups: sham-operated control, gene therapy with adenovirus expressing connexin (Cx) 40, and gene therapy with adenovirus expressing Cx43 (n=5 per subgroup). All animals had epicardial gene painting; the AF group had burst atrial pacing. All animals underwent terminal study 7 days after gene transfer. Sinus rhythm animals had strong transgene expression but no atrial conduction changes. In AF animals, controls had reduced and lateralized Cx43 expression, and Cx43 gene transfer restored expression and cellular location to sinus rhythm control levels. In the AF group, both Cx40 and Cx43 gene transfer improved conduction and reduced AF relative to controls., Conclusions: Connexin gene therapy preserved atrial conduction and prevented AF., (© 2011 American Heart Association, Inc.)

Electrical remodeling of the heart takes place in response to both functional (altered electrical activation) and structural (including heart failure and myocardial infarction) stressors. These electrophysiological changes produce a substrate that is prone to malignant ventricular arrhythmias. Understanding the cellular and molecular mechanisms of electrical remodeling is important in elucidating potential therapeutic targets designed to alter maladaptive electrical remodeling. For example, altered patterns of electrical activation lead primarily to electrical remodeling, without significant structural remodeling. By contrast, secondary remodeling arises in response to a structural insult. In this article we review cardiac electrical remodeling (predominantly in the ventricle) with an emphasis on the mechanisms causing these adaptations. These mechanisms suggest novel therapeutic targets for the management or prevention of the most devastating manifestation of heart disease, sudden cardiac death (SCD)., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Background: Hypothermia is proarrhythmic, and, as the use of therapeutic hypothermia (TH) increases, it is critically important to understand the electrophysiological effects of hypothermia on cardiac myocytes and arrhythmia substrates. We tested the hypothesis that hypothermia-enhanced transmural dispersion of repolarization (DOR) is a mechanism of arrhythmogenesis in hypothermia. In addition, we investigated whether the degree of hypothermia, the rate of temperature change, and cooling versus rewarming would alter hypothermia-induced arrhythmia substrates., Methods and Results: Optical action potentials were recorded from cells spanning the transmural wall of canine left ventricular wedge preparations at baseline (36°C), during cooling and during rewarming. Electrophysiological parameters were examined while varying the depth of hypothermia. On cooling to 26°C, DOR increased from 26±4 ms to 93±18 ms (P=0.021); conduction velocity decreased from 35±5 cm/s to 22±5 cm/s (P=0.010). On rewarming to 36°C, DOR remained prolonged, whereas conduction velocity returned to baseline. Conduction block and reentry was observed in all severe hypothermia preparations. Ventricular fibrillation/ventricular tachycardia was seen more during rewarming (4/5) versus cooling (2/6). In TH (n=7), cooling to 32°C mildly increased DOR (31±6 to 50±9, P=0.012), with return to baseline on rewarming and was associated with decreased arrhythmia susceptibility. Increased rate of cooling did not further enhance DOR or arrhythmogenesis., Conclusions: Hypothermia amplifies DOR and is a mechanism for arrhythmogenesis. DOR is directly dependent on the depth of cooling and rewarming. This provides insight into the clinical observation of a low incidence of arrhythmias in TH and has implications for protocols for the clinical application of TH.

Background: Better risk stratification of patients receiving an implantable cardioverter-defibrillator (ICD) for primary prevention of sudden cardiac death (SCD) is needed. Although microvolt T-wave alternans (MTWA) and electrophysiologic study (EPS) are independent markers for SCD, the Alternans Before Cardioverter Defibrillator (ABCD) trial found the combination to be more predictive than either test alone., Objective: The purpose of this study was to test the hypothesis that EPS and MTWA measure different elements of the arrhythmogenic substrate and, therefore, predict distinct arrhythmia outcomes., Methods: The ABCD trial enrolled 566 patients with ischemic cardiomyopathy, left ventricular ejection fraction (LVEF) 0.30., Conclusion: The study data suggest that EPS and MTWA identify distinct arrhythmogenic substrates and, when used in combination, may better predict the complex electroanatomic substrates that underlie the risk for SCD., (Copyright (c) 2010. Published by Elsevier Inc.)

Altering the pattern of activation of the ventricle causes remodeling of the mechanical and electrical properties of the myocardium. The electrical remodeling is evident on the surface electrocardiogram as significant change in T-wave polarity following altered activation; this phenomenon is ascribed to as "T-wave memory" or "cardiac memory." The electrophysiological remodeling following altered activation is characterized by distinct changes in regions proximal (early-activated) versus distal (late-activated) to the site of altered activation. The early-activated region exhibits marked attenuation of epicardial phase 1 notch due to reduced expression of the transient outward potassium current (I(to)). This is attributed to electrotonic changes during altered activation, and angiotensin-mediated regulation of Kv4.3 (the pore-forming alpha subunit responsible for I(to)). The late-activated region exhibits the most significant action potential prolongation due to markedly increased mechanical strain through a mechano-electrical feedback mechanism. Consequently, regionally heterogeneous action potential remodeling occurs following altered activation. This enhances regional repolarization gradients that underlie the electrophysiological basis for T-wave memory. Further, recent clinical studies highlight detrimental consequences of altered activation including worsening mechanical function and increased susceptibility to arrhythmias. Future studies to identify molecular mechanisms that link electrotonic and mechanical strain-induced changes to cellular electrophysiology will provide important insights into the role of altered activation in regulating cardiac repolarization and arrhythmogenesis.

Gap junctions are critical to maintaining synchronized impulse propagation and repolarization. Heterogeneous expression of the principal ventricular gap junction protein connexin43 (Cx43) is associated with action potential duration (APD) dispersion across the anterior ventricular wall. Little is known about Cx43 expression patterns and their disparate impact on regional electrophysiology throughout the heart. We aimed to determine whether the anterior and posterior regions of the heart are electrophysiologically distinct. Multisegment, high-resolution optical mapping was performed in canine wedge preparations harvested separately from the anterior left ventricle (aLV; n = 8) and posterior left ventricle (pLV; n = 8). Transmural APD dispersion was significantly greater on the aLV than the pLV (45 +/- 13 vs. 26 +/- 8.0 ms; P < 0.05). Conduction velocity dispersion was also significantly higher (P < 0.05) across the aLV (39 +/- 7%) than the pLV (16 +/- 3%). Carbenoxolone perfusion significantly enhanced APD and conduction velocity dispersion on the aLV (by 1.53-fold and 1.36-fold, respectively), but not the pLV (by 1.27-fold and 1.2-fold, respectively), and produced a 4.2-fold increase in susceptibility to inducible arrhythmias in the aLV. Confocal immunofluorescence microscopy revealed significantly (P < 0.05) greater transmural dispersion of Cx43 expression on the aLV (44 +/- 10%) compared with the pLV wall (8.3 +/- 0.7%), suggesting that regional expression of Cx43 expression patterns may account for regional electrophysiological differences. Computer simulations affirmed that localized uncoupling at the epicardial-midmyocardial interface is sufficient to produce APD gradients observed on the aLV. These data demonstrate that the aLV and pLV differ importantly with respect to their electrophysiological properties and Cx43 expression patterns. Furthermore, local underexpression of Cx43 is closely associated with transmural electrophysiological heterogeneity on the aLV. Therefore, regional and transmural heterogeneous Cx43 expression patterns may be an important mechanism underlying arrhythmia susceptibility, particularly in disease states where gap junction expression is altered.

Background: Beat-to-beat alternans of cellular repolarization is closely linked to ventricular arrhythmias in humans. We hypothesized that sarcoplasmic reticulum calcium reuptake by SERCA2a plays a central role in the mechanism of cellular alternans and that increasing SERCA2a gene expression will retard the development of cellular alternans., Methods and Results: In vivo gene transfer of a recombinant adenoviral vector with the transgene for SERCA2a (Ad.SERCA2a) was performed in young guinea pigs. Isolated myocytes transduced with Ad.SERCA2a exhibited improved sarcoplasmic reticulum Ca(2+) reuptake (P<0.05) and were markedly resistant to cytosolic calcium alternans (P<0.05) under repetitive constant action potential clamp conditions (ie, when alternation of action potential duration was prevented), proving that sarcoplasmic reticulum Ca(2+) cycling is an important mechanism in the development of cellular alternans. Similarly, SERCA2a overexpression in the intact heart demonstrated significant resistance to alternation of action potential duration when compared with control hearts (heart rate threshold, 484+/-25 bpm versus 396+/-11 bpm, P<0.01), with no change in action potential duration restitution slope. Importantly, SERCA2a overexpression produced a 4-fold reduction in susceptibility to alternans-mediated ventricular arrhythmias (P<0.05)., Conclusions: These data provide new evidence that sarcoplasmic reticulum Ca(2+) reuptake directly modulates susceptibility to cellular alternans. Moreover, SERCA2a overexpression suppresses cellular alternans, interrupting an important pathway to cardiac fibrillation in the intact heart.

The proportion of cardiovascular deaths attributable to sudden cardiac death (SCD) is on the rise. Herein lies the rationale for developing risk stratification strategies to predict who will benefit from prophylactic implantable cardioverter-defibrillator (ICD) implantation. Current guidelines recommend prophylactic ICD therapy in patients with reduced left ventricular ejection fraction (LVEF). However, there are clear limitations in using LVEF alone to decide who should receive an ICD. There is mounting evidence that microvolt-level T-wave alternans (TWA) is an important marker of arrhythmic risk. TWA is appealing because it noninvasively probes the underlying electrophysiological substrate and has been linked to cellular mechanisms for arrhythmias. This review considers the clinical role of TWA for risk stratification of SCD.

The mechanisms underlying sudden cardiac death (SCD) are complex and diverse. Therefore, correct application of any marker to risk stratify patients for appropriate therapy requires knowledge regarding how the marker is reflective of a particular electroanatomical substrate for arrhythmias. Noninvasive measurement of beat-to-beat alternation of the electrocardiographic T-wave, referred to as T-wave alternans (TWA), is an important marker of risk for SCD. Is this relationship a mere association, or is TWA mechanistically linked to SCD? Recent experimental evidence strongly supports a mechanistic relationship between TWA and SCD. This review considers the underlying mechanisms of TWA derived from experimental studies, as they relate to clinical observations of TWA in humans, addressing the following questions derived from common clinical observations: (1) Where does TWA on the surface electrocardiogram come from? (2) Why is controlled heart rate elevation required to elicit TWA? (3) Why is TWA associated with risk for SCD? (4) Why is TWA associated with a broad range of ventricular arrhythmias? (5) How do commonly used medications affect TWA?

Objectives: Because risk stratification with electrophysiological study (EPS) improves efficiency but is invasive, we sought to determine whether noninvasive microvolt T-wave alternans (MTWA) testing could identify patients who benefit from implantable cardioverter-defibrillators (ICDs) as well as EPS., Background: Prevention of sudden cardiac death on the basis of left ventricular ejection fraction (LVEF) alone is inefficient, because most ICDs never deliver therapy., Methods: The ABCD (Alternans Before Cardioverter Defibrillator) trial is a multicenter prospective study that enrolled patients with ischemic cardiomyopathy (LVEF < or =0.40) and nonsustained ventricular tachycardia. All patients underwent MTWA and EPS. ICDs were mandated if either test was positive., Results: Of 566 patients followed for a median of 1.9 years, 39 (7.5%) met the primary end point of appropriate ICD discharge or sudden death at 1 year. As hypothesized, primary analysis showed that MTWA achieved 1-year positive (9%) and negative (95%) predictive values that were comparable to EPS (11% and 95%, respectively). In addition, secondary analysis showed that at the pre-specified 1-year end point, event rates were significantly higher in patients with both a positive MTWA-directed strategy (hazard ratio: 2.1, p = 0.03) and a positive EPS-directed strategy (hazard ratio: 2.4, p = 0.007). Moreover, the event rate in patients with both negative MTWA test and EPS was lower than in those with 2 positive tests (2% vs. 12%; p = 0.017)., Conclusions: The ABCD study is the first trial to use MTWA to guide prophylactic ICD insertion. Risk stratification strategies using noninvasive MTWA versus invasive EPS are comparable at 1 year and complementary when applied in combination. Strategies employing MTWA, EPS, or both might identify subsets of patients least likely to benefit from ICD insertion. (Study to Compare TWA Test and EPS Test for Predicting Patients at Risk for Life-Threatening Heart Rhythms [ABCD Study]; NCT00187291).

Background: Although heart failure (HF) is closely associated with susceptibility to sudden cardiac death (SCD), the mechanisms linking contractile dysfunction to cardiac electrical instability are poorly understood. Cardiac alternans has also been closely associated with SCD, and has been linked to a mechanism for amplifying electrical heterogeneities in the heart. However, previous studies have focused on alternans in normal rather than failing myocardium., Objective: This study sought to investigate the hypothesis that HF enhances susceptibility to arrhythmogenic cardiac alternans., Methods: High-resolution transmural optical mapping was performed in canine wedge preparations from normal (n = 8) and HF (n = 8) hearts produced by rapid ventricular pacing., Results: HF significantly (P < .004) lowered the heart rate (HR) threshold for action potential duration alternans (APD-ALT) from 236 +/- 25 beats/min to 185 +/- 25 beats/min. In dual optical mapping of action potentials and intracellular Ca experiments (n = 16), HF lowered the HR threshold for Ca-ALT (beat-to-beat alternations of cellular Ca cycling) from 238 +/- 35 to 177 +/- 26 beats/min (P < .005). Importantly: (1) Ca-ALT always either developed at slower HR or simultaneously with APD-ALT in the same cells, and (2) the magnitude of Ca-ALT and APD-ALT were closely correlated (P < .05). HF similarly lowered the HR threshold for Ca-ALT in isolated myocytes under nonalternating action potential clamp, indicating that HF enhances susceptibility to cellular alternans independent of HF-associated changes in repolarization. Importantly, HF significantly (P < .02) lowered the HR threshold for spatially discordant arrhythmogenic alternans (different regions of cells alternating in opposite phase, DIS-ALT). Ventricular fibrillation (VF) was induced in 88% of HF preparations, but only 12% of normal preparations (P < .003) and was uniformly preceded by development of DIS-ALT., Conclusion: Heart failure increases the susceptibility to arrhythmogenic cardiac alternans, which arises from HF-induced impairment in calcium cycling.

Animal models of atrial fibrillation have taught us about mechanisms of this common disease. A variety of animal models exist, including models of lone atrial fibrillation and models of atrial fibrillation in the setting of heart failure, aging, or pericardial inflammation. This article reviews these various models.

Background: Microvolt T-wave alternans (MTWA) measured from the surface electrocardiogram (ECG) is a marker of sudden cardiac death (SCD). Recently, it has been suggested that intracardiac alternans (ICA) detected from the endocardium underlies MTWA and is a marker of electrical instability leading to ventricular arrhythmias. As such, ICA may be used in future implantable cardioverter-defibrillators (ICDs) to monitor periods of electrical instability before ICD therapy. We examined whether electrical instability, as measured by MTWA, can be detected by ICA located from the right ventricle and if ICA can predict ventricular arrhythmias in patients with ICDs., Methods: Both MTWA and ICA were measured simultaneously during atrial pacing in 68 patients undergoing electrophysiology study (EPS). ICA was measured from unipolar electrograms acquired from a catheter at the apical, mid, and basal regions of the right ventricle in 48 patients and at the apical region alone in 20 patients. Indications for EPS included nonsustained ventricular tachycardia, cardiomyopathy, syncope, near syncope, or palpitation., Results: Fifty-six of 68 patients had cardiomyopathy with left ventricular ejection fraction (LVEF)

Despite the strong association between mechanical dysfunction of the heart and sudden death due to arrhythmias, the causal relationship is not well understood. Cardiac alternans has been linked to arrhythmogenesis and can be mediated by intracellular calcium handling. Given the integral role intracellular calcium plays in contractile function, calcium-mediated alternans may represent an important mechanistic link between mechanical dysfunction and electrical instability. This relationship, however, is not well understood due to complex feedback between membrane currents, intracellular calcium, and contraction. This manuscript describes the cellular mechanisms of cardiac alternans. Through several pathways, calcium transient alternans is coupled to repolarization alternans that can form a substrate for reentrant excitation. Abnormal intracellular calcium cycling, either impaired release or impaired reuptake of sarcoplasmic reticulum calcium, is a cellular mechanism of calcium transient alternans. Thus, cardiac alternans is an important mechanistic link between mechanical dysfunction and sudden cardiac death.