Variability Of Action Potential Duration Research Articles

Circadian rhythm in pro-arrhythmic activity of pulmonary vein cardiomyocytes

Abstract Introduction Cardiomyocytes in the pulmonary vein (PV) sleeves are a major source of ectopic activity driving atrial fibrillation (AF) and episodes of AF in patients are more prevalent at night. While it is known that circadian clocks within the heart regionally control 24-hour variation in pacemaking and ventricular repolarisation, the mechanisms underlying the nighttime preponderance in AF are unknown. Purpose This study addresses the hypothesis that circadian rhythms exist in the pro-arrhythmic activity of PV cardiomyocytes. Methods Male Wistar rats maintained in a 24-hour cycle of 12-hr light/dark (lights-on at Zeitgeber time, ZT=0; lights-off, ZT12) were subject to terminal general anaesthesia (140 mg/kg Na pentobarbital i.p.) at ZT=0, 6, 12 or 18 hr and the hearts removed. RNA was extracted from left atrial (LA) appendage (LAA) and proximal PV at the LA/PV junction (3 rats per ZT) and RNA sequencing conducted. Reads were mapped to the rat genome, counts normalised and models in which ZT was or was not included as a factor compared (Likelihood Ratio Test, adjusted-P<0.01). Whole-cell current clamp recordings were made from cardiomyocytes isolated from the proximal PV (n=308) and the LAA (n=264) (N=74 rats). The effects of noradrenaline (NA, 1 µM) and acetylcholine (ACh, 1 µM) were examined. Data were plotted against the ZT of the time of recording and fitted to a sine wave to establish circadian rhythmicity (P<0.05, extra-sum-of-squares F-test). The effect of ≥24 hr constant dark in the period immediately before experiment was examined. Data are reported as mean ± standard error. Results The expression of 1368 genes varied significantly with ZT, including circadian clock components (e.g. Bmal1, Per1). PV cells were larger than LAA cells (73±1.6 pF vs 53±1.3 pF, P<0.0001) and had more depolarised resting membrane potential (-69±0.2 mV vs -72±0.1 mV, P<0.0001). Both cell types showed circadian variation in action potential duration at 90% repolarisation (APD90) and frequency of pro-arrhythmic activity. Pro-arrhythmic activity was greatest in PV cells and the frequency was greater during the rest phase (ZT0-12) in both cell types. In contrast, the circadian rhythm in APD90 differed between cell type, with the longest APD­­90 recorded during the active phase in PV cells (ZT12-24) but during the rest phase in LAA cells. Pro-arrhythmic activity was increased by NA and decreased by ACh in both cell types with maximal effect during the rest phase. Conclusion Circadian variation in APD90 and proarrhythmic-activity has been demonstrated in isolated proximal PV and LAA cardiomyocytes, with differences in rhythm between the two cell types. RNA sequencing suggests the presence of peripheral clocks in LAA and PV cardiomyocytes. Understanding these mechanisms is important in development of future therapeutic options for AF.

Differential sex-dependent susceptibility to diastolic dysfunction and arrhythmia in cardiomyocytes from obese diabetic HFpEF model.

Sex-differences in heart failure with preserved ejection fraction (HFpEF) are important, but key mechanisms involved are incompletely understood. While animal models can inform about sex-dependent cellular and molecular changes, many previous preclinical HFpEF models have failed to recapitulate sex-dependent characteristics of human HFpEF. We tested for sex-differences in HFpEF using a two-hit mouse model (leptin receptor-deficient db/db mice plus aldosterone infusion for 4 weeks; db/db+Aldo). We performed echocardiography, electrophysiology, intracellular Ca2+ imaging, and protein analysis. Female HFpEF mice exhibited more severe diastolic dysfunction in line with increased titin N2B isoform expression and PEVK element phosphorylation, and reduced troponin-I phosphorylation. Female HFpEF mice had lower BNP levels than males despite similar comorbidity burden (obesity, diabetes) and cardiac hypertrophy in both sexes. Male HFpEF mice were more susceptible to cardiac alternans. Male HFpEF cardiomyocytes (versus female) exhibited higher diastolic [Ca2+], slower Ca2+ transient decay, reduced L-type Ca2+ current, more pronounced enhancement of the late Na+ current, and increased short-term variability of action potential duration (APD). However, male and female HFpEF myocytes showed similar downregulation of inward rectifier and transient outward K+ currents, APD prolongation, and frequency of delayed afterdepolarizations. Inhibition of Ca2+/calmodulin-dependent protein kinase II (CaMKII) reversed all pathological APD changes in HFpEF in both sexes, and empagliflozin pretreatment mimicked these effects of CaMKII inhibition. Vericiguat had only slight benefits, and these effects were larger in HFpEF females. We conclude that the db/db+Aldo preclinical HFpEF murine model recapitulates key sex-specific mechanisms in HFpEF and provides mechanistic insights into impaired excitation-contraction coupling and sex-dependent differential arrhythmia susceptibility in HFpEF with potential therapeutic implications. In male HFpEF myocytes, altered Ca2+ handling and electrophysiology aligned with diastolic dysfunction and arrhythmias, while worse diastolic dysfunction in females may depend more on altered myofilaments properties.

Abstract 11663: T Wave Alternans: A Clue in Plain Sight

Introduction: T-wave alternans (TWA) refers to the beat-to-beat variability in T-wave amplitudes or polarity on surface ECGs, and can signify susceptibility to malignant ventricular arrhythmias (VA). Aims: We present a case to emphasize the pathophysiology and clinical significance of this ECG finding. Case description: A 52 year-old female with history of HIV presents with shortness of breath and is treated for heart failure. In the ED, she has witnessed syncope and is found to have VT that progresses to VF. Resuscitation efforts are successful and include amiodarone loading. Figure 1A demonstrates baseline ECG on admission with a QTc of 495 ms. A few days later, an ECG reveals QTc of 696 ms with TWA (figure 1B). Shortly after, she develops polymorphic VT in the setting of long-short sequences and an R on T trigger (figure 1C) that is terminated with cardioversion. Review of pre-admission ECGs reveals that QTc prolongation occurred after initiation of a retroviral regimen that included rilpivirine and worsened with amiodarone. Both medications are discontinued, and she is eventually discharged with a QTc of 450 ms. Early theories for TWA's mechanism hypothesized that dispersion of refractoriness resulted in depolarization through alternating pathways and thus this phenomenon. Recent understanding further suggests that TWA occurs as a result of cellular alternans and variation in action potential duration due to dysregulation of repolarization. If discordant repolarization alternans ensues, whereby neighboring cells repolarize in opposite phases (long-short, short-long), the resultant repolarization gradient and heterogeneity in cellular repolarization serves as the substrate for VA; if a premature beat encounters refractory myocardium and unidirectional block occurs, a re-entrant excitation tachycardia can be initiated. Conclusions: TWA on ECG suggests myocardial electrical instability as a result of cellular repolarization inhomogeneity.

Ventricular Repolarization and Calcium Transient Show Resonant Behavior under Oscillatory Pacing Rate.

Cardiac EC coupling is triggered by rhythmic depolarizing current fronts originating from the sino-atrial node, and the way variability in rhythm is associated with variability in action potential duration (APD) and, in turn, in the variability of calcium transient amplitude (CTA) and contraction is a key determinant of beating stability. Sinusoidal-varying pacing rate is adopted here in order to establish whether APD and CTA oscillations, elicited in a human ventricular AP model (OR) under oscillatory pacing, are consistent with the dynamics of two coupled harmonic oscillators, e.g., a two-degree-of-freedom system of mass and springs (MS model). I show evidence that this is the case, and that the MS model, preliminarily fitted to OR behavior, retains key features of the physiological system, such as the dependence of APD and CTA oscillation amplitudes from average value and from beat-to-beat changes in pacing rate, and the phase relationship between them. The bi-directionality of coupling between APD and CTA makes it difficult to discriminate which one leads EC coupling dynamics under variable pacing. The MS model suggests that the calcium cycling, with its greater inertia chiefly determined by the SR calcium release, is the leading mechanism. I propose the present approach to also be relevant at the whole organ level, where the need of compact representations of electromechanical interaction, particularly in clinical practice, remains urgent.

Repolarization heterogeneity within the myocardial infarction border zone correlates with variability of myocyte remodeling

Abstract Background Myocardial infarction (MI) results in a regional scar, with a border zone (BZ) of surviving myocytes interspersed with fibrosis providing an anatomical substrate for re-entry. Heterogeneous repolarization within the BZ may add a functional component aggravating re-entrant arrhythmias. Purpose We studied BZ heterogeneity and developed novel methodology for high resolution mapping of local in vivo activation-repolarization intervals (ARI) within the BZ and for studying the relation to cellular action potential (AP) profiles of cells isolated from the BZ. Methods Anterior-septal myocardial infarction was induced in 5 domestic pigs by 120-minute occlusion of the left anterior descending artery followed by reperfusion (18.9±4.7% of the left ventricle). After 1-month, electro-anatomical mapping was performed. Contact mapping was used to define the BZ (bipolar voltage 0.5–1.5mV). A non-contact recording of a 64-electrode array was translated to 2048 non-contact electrograms distributed over the LV. The non-contact electrograms were processed to determine the ARIs using a custom-made algorithm, validated against monophasic action potential recordings. After 2–4 days recovery, single cardiomyocytes were enzymatically isolated from the anterior-septal BZs and remote regions. Cardiomyocytes were field stimulated at 1Hz at 37°C and cellular AP duration (APD) was optically recorded (fluorescent voltage-sensitive dye Di-8-Annepps). Results In vivo, regional ARIs tended to be longer in the BZs than remote. ARI heterogeneity, quantified as the standard deviation of ARIs in a neighborhood of 1cm radius, was increased in the BZ (anterior BZ: 3.4±1.0 ms, P=0.052, septal BZ: 3.6±1.7 ms, P=0.027 vs remote: 2.0±0.5 ms). Cellular APD was measured in large population samples (>100 cells per region in each pig) and was longer in BZ myocytes compared to the remote region. Cellular APD heterogeneity, measured as the standard deviation within cell population samples pooled by region per animal, was increased in the BZ (anterior BZ: 105.9±17.0 ms, P=0.0010; septal BZ: 98.1±20.8 ms, P=0.0127 vs remote: 73.9±8.6 ms). Cell APD correlated to in vivo ARI (R2=0.34, P=0.021) and cellular heterogeneity correlated strongly with in vivo heterogeneity (R2=0.67, P=0.002). Conclusion In the BZ of MI, in vivo regional heterogeneity adds a functional substrate for re-entry that may result from heterogeneous cellular remodeling and increased cell-cell APD variability. Funding Acknowledgement Type of funding sources: Public Institution(s). Main funding source(s): KU Leuven BOF-C1 “Blood pressure induced premature ventricular beats as triggers for ventricular arrhythmia in ischemic cardiomyopathy”

Beneficial Electrophysiological Effects of Rotigaptide Are Unable to Suppress Therapeutic Hypothermia-Provoked Ventricular Fibrillation in Failing Rabbit Hearts With Acute Ischemia-Reperfusion Injury.

Aims: Whether therapeutic hypothermia (TH) is proarrhythmic in preexisting failing hearts with acute ischemia–reperfusion (IR) injury is unknown. Additionally, the effectiveness of rotigaptide on improving conduction slowing in hearts with IR injury is ambiguous. We investigated the electrophysiological effects of TH and rotigaptide in failing rabbit hearts with acute IR injury and determined the underlying molecular mechanisms.Methods and Results: Heart failure was induced by right ventricular pacing (320 beats/min, 4 weeks). Rabbits with pacing-induced heart failure were randomly divided into TH (n = 14) and non-TH (n = 7) groups. The IR rabbit model was created by ligating the coronary artery for 60 min, followed by reperfusion for 15 min in vivo. Then, the hearts were excised quickly and Langendorff-perfused for simultaneous voltage and intracellular Ca2+ (Cai) optical mapping. Electrophysiological studies were conducted, and vulnerability to ventricular fibrillation (VF) was evaluated using pacing protocols. TH (33°C) was instituted after baseline studies, and electrophysiological studies were repeated. Rotigaptide (300 nM) was infused for 20 min, and electrophysiological studies were repeated under TH. Cardiac tissues were sampled for Western blotting. TH increased the dispersion and beat-to-beat variability of action potential duration (APD), aggravated conduction slowing, and prolonged Cai decay to facilitate spatially discordant alternans (SDA) and VF induction. Rotigaptide reduced the dispersion and beat-to-beat variability of APD and improved slowed conduction to defer the onset of arrhythmogenic SDA by dynamic pacing and elevate the pacing threshold of VF during TH. However, the effect of rotigaptide on TH-enhanced VF inducibility was statistically insignificant. TH attenuated IR-induced dysregulation of protein expression, but its functional role remained uncertain.Conclusion: Therapeutic hypothermia is proarrhythmic in failing hearts with acute IR injury. Rotigaptide improves TH-induced APD dispersion and beat-to-beat variability and conduction disturbance to defer the onset of arrhythmogenic SDA and elevate the VF threshold by dynamic pacing, but these beneficial electrophysiological effects are unable to suppress TH-enhanced VF inducibility significantly.

Is selective late sodium current inhibition different from class I/B antiarrhythmic action? Comparison of the effects of GS967 to mexiletine in canine ventricular myocardium

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – EU funding. Main funding source(s): National Research, Development and Innovation Office New National Excellence Programme Enhancement of the late Na+ current (INa,late) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS-458967 (GS) is an agent considered to be a selective blocker of INa,late. In the present study, effects of GS967 on INa,late, on L-type calcium current (ICaL), and on action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. These effects of GS were compared to tetrodotoxin (TTX) and to the class I/B antiarrhythmic compound mexiletine. GS (1 μM), mexiletine (40 μM) and TTX (10 μM) dissected largely similarly shaped inward currents under action potential voltage clamp conditions. In case of GS and mexiletine, the amplitude and integral of this inward current was significantly smaller when measured in the presence of 1 μM nisoldipine, while no difference was observed in case of TTX. Under conventional voltage clamp conditions, INa,late was significantly reduced by 1 μM GS and 40 μM mexiletine (about 79% and 63% reduction of current integrals, respectively). The integral of ICa,L was moderately but significantly decreased by both drugs (reduction of 9% and 14%, respectively). These changes were associated with a faster inactivation of ICa,L. Drug effects on early Na+ current (INa,early) were assessed by analyzing the maximal rate of depolarization (V + max) in multicellular preparations. Both GS and mexiletine showed fast onset and offset kinetics: 110 ms and 289 ms offset time constants, respectively, as determined from V + max measurements in right ventricular papillary muscles, while the onset kinetics was characterized by 5.3 AP and 2.6 AP lengths, respectively, at 2.5 Hz. Effects on beat-to-beat variability of AP duration (APD) was studied in isolated myocytes. Short-term variability was significantly decreased by both GS and mexiletine (average reduction of 42% and 24%, respectively) while they caused similar shortening of the APD. The electrophysiological effects of GS are similar to those of mexiletine, but with a somewhat faster offset kinetics of V + max block. However, since GS reduced V+ max and INa,late in the same concentration, the currently accepted view that GS that selectively blocks INa,late has to be questioned and it is suggested that GS should be classified as a class I/B (or I/B + IV) antiarrhythmic agent.

Mexiletine-like cellular electrophysiological effects of GS967 in canine ventricular myocardium

Enhancement of the late Na+ current (INaL) increases arrhythmia propensity in the heart, while suppression of the current is antiarrhythmic. GS967 is an agent considered as a selective blocker of INaL. In the present study, effects of GS967 on INaL and action potential (AP) morphology were studied in canine ventricular myocytes by using conventional voltage clamp, action potential voltage clamp and sharp microelectrode techniques. The effects of GS967 (1 µM) were compared to those of the class I/B antiarrhythmic compound mexiletine (40 µM). Under conventional voltage clamp conditions, INaL was significantly suppressed by GS967 and mexiletine, causing 80.4 ± 2.2% and 59.1 ± 1.8% reduction of the densities of INaL measured at 50 ms of depolarization, and 79.0 ± 3.1% and 63.3 ± 2.7% reduction of the corresponding current integrals, respectively. Both drugs shifted the voltage dependence of the steady-state inactivation curve of INaL towards negative potentials. GS967 and mexiletine dissected inward INaL profiles under AP voltage clamp conditions having densities, measured at 50% of AP duration (APD), of −0.37 ± 0.07 and −0.28 ± 0.03 A/F, and current integrals of −56.7 ± 9.1 and −46.6 ± 5.5 mC/F, respectively. Drug effects on peak Na+ current (INaP) were assessed by recording the maximum velocity of AP upstroke (V+max) in multicellular preparations. The offset time constant was threefold faster for GS967 than mexiletine (110 ms versus 289 ms), while the onset of the rate-dependent block was slower in the case of GS967. Effects on beat-to-beat variability of APD was studied in isolated myocytes. Beat-to-beat variability was significantly decreased by both GS967 and mexiletine (reduction of 42.1 ± 6.5% and 24.6 ± 12.8%, respectively) while their shortening effect on APD was comparable. It is concluded that the electrophysiological effects of GS967 are similar to those of mexiletine, but with somewhat faster offset kinetics of V+max block. However, since GS967 depressed V+max and INaL at the same concentration, the current view that GS967 represents a new class of drugs that selectively block INaL has to be questioned and it is suggested that GS967 should be classified as a class I/B antiarrhythmic agent.

Mechanisms underlying pro-arrhythmic abnormalities arising from Pitx2-induced electrical remodelling: an in silico intersubject variability study.

BackgroundElectrical remodelling as a result of the homeodomain transcription factor 2 (Pitx2)-dependent gene regulation induces atrial fibrillation (AF) with different mechanisms. The purpose of this study was to identify Pitx2-induced changes in ionic currents that cause action potential (AP) shortening and lead to triggered activity.MethodsPopulations of computational atrial AP models were developed based on AP recordings from sinus rhythm (SR) and AF patients. Models in the AF population were divided into triggered and untriggered AP groups to evaluate the relationship between each ion current regulated by Pitx2 and triggered APs. Untriggered AP models were then divided into shortened and unshortened AP groups to determine which Pitx2-dependent ion currents contribute to AP shortening.ResultsAccording to the physiological range of AP biomarkers measured experimentally, populations of 2,885 SR and 4,781 AF models out of the initial pool of 30,000 models were selected. Models in the AF population predicted AP shortening and triggered activity observed in experiments in Pitx2-induced remodelling conditions. The AF models included 925 triggered AP models, 1,412 shortened AP models and 2,444 unshortened AP models. Intersubject variability in IKs and ICaL primarily modulated variability in AP duration (APD) in all shortened and unshortened AP models, whereas intersubject variability in IK1 and SERCA mainly contributed to the variability in AP morphology in all triggered and untriggered AP models. The incidence of shortened AP was positively correlated with IKs and IK1 and was negatively correlated with INa, ICaL and SERCA, whereas the incidence of triggered AP was negatively correlated with IKs and IK1 and was positively correlated with INa, ICaL and SERCA.ConclusionsElectrical remodelling due to Pitx2 upregulation may increase the incidence of shortened AP, whereas electrical remodelling arising from Pitx2 downregulation may favor to the genesis of triggered AP.

Co-expression of calcium and hERG potassium channels reduces the incidence of proarrhythmic events.

Cardiac electrical activity is extraordinarily robust. However, when it goes wrong it can have fatal consequences. Electrical activity in the heart is controlled by the carefully orchestrated activity of more than a dozen different ion conductances. While there is considerable variability in cardiac ion channel expression levels between individuals, studies in rodents have indicated that there are modules of ion channels whose expression co-vary. The aim of this study was to investigate whether meta-analytic co-expression analysis of large-scale gene expression datasets could identify modules of co-expressed cardiac ion channel genes in human hearts that are of functional importance. Meta-analysis of 3653 public human RNA-seq datasets identified a strong correlation between expression of CACNA1C (L-type calcium current, ICaL) and KCNH2 (rapid delayed rectifier K+ current, IKr), which was also observed in human adult heart tissue samples. In silico modelling suggested that co-expression of CACNA1C and KCNH2 would limit the variability in action potential duration seen with variations in expression of ion channel genes and reduce susceptibility to early afterdepolarizations, a surrogate marker for proarrhythmia. We also found that levels of KCNH2 and CACNA1C expression are correlated in human-induced pluripotent stem cell-derived cardiac myocytes and the levels of CACNA1C and KCNH2 expression were inversely correlated with the magnitude of changes in repolarization duration following inhibition of IKr. Meta-analytic approaches of multiple independent human gene expression datasets can be used to identify gene modules that are important for regulating heart function. Specifically, we have verified that there is co-expression of CACNA1C and KCNH2 ion channel genes in human heart tissue, and in silico analyses suggest that CACNA1C-KCNH2 co-expression increases the robustness of cardiac electrical activity.

Complex Interaction Between Low-Frequency APD Oscillations and Beat-to-Beat APD Variability in Humans Is Governed by the Sympathetic Nervous System.

BackgroundRecent clinical, experimental and modeling studies link oscillations of ventricular repolarization in the low frequency (LF) (approx. 0.1 Hz) to arrhythmogenesis. Sympathetic provocation has been shown to enhance both LF oscillations of action potential duration (APD) and beat-to-beat variability (BVR) in humans. We hypothesized that beta-adrenergic blockade would reduce LF oscillations of APD and BVR of APD in humans and that the two processes might be linked.Methods and ResultsTwelve patients with normal ventricles were studied during routine electrophysiological procedures. Activation-recovery intervals (ARI) as a conventional surrogate for APD were recorded from 10 left and 10 right ventricular endocardial sites before and after acute beta-adrenergic adrenergic blockade. Cycle length was maintained constant with right ventricular pacing. Oscillatory behavior of ARI was quantified by spectral analysis and BVR as the short-term variability. Beta-adrenergic blockade reduced LF ARI oscillations (8.6 ± 4.5 ms2 vs. 5.5 ± 3.5 ms2, p = 0.027). A significant correlation was present between the initial control values and reduction seen following beta-adrenergic blockade in LF ARI (rs = 0.62, p = 0.037) such that when initial values are high the effect is greater. A similar relationship was also seen in the beat-to beat variability of ARI (rs = 0.74, p = 0.008). There was a significant correlation between the beta-adrenergic blockade induced reduction in LF power of ARI and the witnessed reduction of beat-to-beat variability of ARI (rs = 0.74, p = 0.01). These clinical results accord with recent computational modeling studies which provide mechanistic insight into the interactions of LF oscillations and beat-to-beat variability of APD at the cellular level.ConclusionBeta-adrenergic blockade reduces LF oscillatory behavior of APD (ARI) in humans in vivo. Our results support the importance of LF oscillations in modulating the response of BVR to beta-adrenergic blockers, suggesting that LF oscillations may play role in modulating beta-adrenergic mechanisms underlying BVR.

Sensitivity of Ablation Targets Prediction to Electrophysiological Parameter Variability in Image-Based Computational Models of Ventricular Tachycardia in Post-infarction Patients.

Ventricular tachycardia (VT), which could lead to sudden cardiac death, occurs frequently in patients with myocardial infarction. Computational modeling has emerged as a powerful platform for the non-invasive investigation of lethal heart rhythm disorders in post-infarction patients and for guiding patient VT ablation. However, it remains unclear how VT dynamics and predicted ablation targets are influenced by inter-patient variability in action potential duration (APD) and conduction velocity (CV). The goal of this study was to systematically assess the effect of changes in the electrophysiological parameters on the induced VTs and predicted ablation targets in personalized models of post-infarction hearts. Simulations were conducted in 5 patient-specific left ventricular models reconstructed from late gadolinium-enhanced magnetic resonance imaging scans. We comprehensively characterized all possible pre-ablation and post-ablation VTs in simulations conducted with either an “average human VT”-based electrophysiological representation (i.e., EPavg) or with ±10% APD or CV (i.e., EPvar); additional simulations were also executed in some models for an extended range of these parameters. The results showed that: (1) a subset of reentries (76.2–100%, depending on EP parameter set) conducted with ±10% APD/CV was observed in approximately the same locations as reentries observed in EPavg cases; (2) emergent VTs could be induced sometimes after ablation in EPavg models, and these emergent VTs often corresponded to the pre-ablation reentries in simulations with EPvar parameter sets. These findings demonstrate that the VT ablation target uncertainty in patient-specific ventricular models with an average representation of VT-remodeled electrophysiology is relatively low and the ablation targets stable, as the localization of the induced VTs was primarily driven by the remodeled structural substrate. Thus, personalized ventricular modeling with an average representation of infarct-remodeled electrophysiology may uncover most targets for VT ablation.

Left ventricular activation-recovery interval variability predicts spontaneous ventricular tachyarrhythmia in patients with heart failure

Enhanced beat-to-beat variability of repolarization is strongly linked to arrhythmogenesis and is largely due to variation in ventricular action potential duration (APD). Previous studies in humans have relied on QT interval measurements; however, a direct relationship between beat-to-beat variability of APD and arrhythmogenesis in humans has yet to be demonstrated. This study aimed to explore the beat-to-beat repolarization dynamics in patients with heart failure at the level of ventricular APD. Forty-three patients with heart failure and implanted cardiac resynchronization therapy - defibrillator devices were studied. Activation-recovery intervals as a surrogate for APD were recorded from the left ventricular epicardial lead while pacing from the right ventricular lead to maintain a constant cycle length. During a mean follow-up of 23.6±13.6 months, 11 patients sustained ventricular fibrillation/ventricular tachycardia (VT/VF) and received appropriate implantable cardioverter-defibrillator therapies (antitachycardia pacing or shock therapy). Activation-recovery interval variability (ARIV) was significantly greater in patients with subsequent VT/VF than in those without VT/VF (3.55±1.3 ms vs 2.77±1.09 ms; P=.047). Receiver operating characteristic curve analysis (area under the curve 0.71; P=.046) suggested high- and low-risk ARIV groups for VT/VF. Kaplan-Meier survival analysis demonstrated that the time until first appropriate therapy for VT/VF was significantly shorter in the high-risk ARIV group (P=.028). ARIV was a predictor for VT/VF in the multivariate Cox model (hazard ratio 1.623; 95% confidence interval 1.1-2.393; P=.015). Increased left ventricular ARIV is associated with an increased risk of VT/VF in patients with heart failure.

Quantification of Beat-To-Beat Variability of Action Potential Durations in Langendorff-Perfused Mouse Hearts.

Background: Beat-to-beat variability in action potential duration (APD) is an intrinsic property of cardiac tissue and is altered in pro-arrhythmic states. However, it has never been examined in mice.Methods: Left atrial or ventricular monophasic action potentials (MAPs) were recorded from Langendorff-perfused mouse hearts during regular 8 Hz pacing. Time-domain, frequency-domain and non-linear analyses were used to quantify APD variability.Results: Mean atrial APD (90% repolarization) was 23.5 ± 6.3 ms and standard deviation (SD) was 0.9 ± 0.5 ms (n = 6 hearts). Coefficient of variation (CoV) was 4.0 ± 1.9% and root mean square (RMS) of successive differences in APDs was 0.3 ± 0.2 ms. The peaks for low- and high-frequency were 0.7 ± 0.5 and 2.7 ± 0.9 Hz, respectively, with percentage powers of 39.0 ± 20.5 and 59.3 ± 22.9%. Poincaré plots of APDn+1 against APDn revealed ellipsoid shapes. The ratio of the SD along the line-of-identity (SD2) to the SD perpendicular to the line-of-identity (SD1) was 8.28 ± 4.78. Approximate and sample entropy were 0.57 ± 0.12 and 0.57 ± 0.15, respectively. Detrended fluctuation analysis revealed short- and long-term fluctuation slopes of 1.80 ± 0.15 and 0.85 ± 0.29, respectively. When compared to atrial APDs, ventricular APDs were longer (ANOVA, P < 0.05), showed lower mean SD and CoV but similar RMS of successive differences in APDs and showed lower SD2 (P < 0.05). No difference in the remaining parameters was observed.Conclusion: Beat-to-beat variability in APD is observed in mouse hearts during regular pacing. Atrial MAPs showed greater degree of variability than ventricular MAPs. Non-linear techniques offer further insights on short-term and long-term variability and signal complexity.

P793Ventricular action potential duration variability is enhanced in heart failure patients with spontaneous ventricular tachycardia or fibrillation

P793Ventricular action potential duration variability is enhanced in heart failure patients with spontaneous ventricular tachycardia or fibrillation

Altered Repolarization Reserve in Failing Rabbit Ventricular Myocytes: Calcium and β-Adrenergic Effects on Delayed- and Inward-Rectifier Potassium Currents.

Electrophysiological remodeling and increased susceptibility for cardiac arrhythmias are hallmarks of heart failure (HF). Ventricular action potential duration (APD) is typically prolonged in HF, with reduced repolarization reserve. However, underlying K+ current changes are often measured in nonphysiological conditions (voltage clamp, low pacing rates, cytosolic Ca2+ buffers). We measured the major K+ currents (IKr, IKs, and IK1) and their Ca2+- and β-adrenergic dependence in rabbit ventricular myocytes in chronic pressure/volume overload-induced HF (versus age-matched controls). APD was significantly prolonged only at lower pacing rates (0.2-1 Hz) in HF under physiological ionic conditions and temperature. However, when cytosolic Ca2+ was buffered, APD prolongation in HF was also significant at higher pacing rates. Beat-to-beat variability of APD was also significantly increased in HF. Both IKr and IKs were significantly upregulated in HF under action potential clamp, but only when cytosolic Ca2+ was not buffered. CaMKII (Ca2+/calmodulin-dependent protein kinase II) inhibition abolished IKs upregulation in HF, but it did not affect IKr. IKs response to β-adrenergic stimulation was also significantly diminished in HF. IK1 was also decreased in HF regardless of Ca2+ buffering, CaMKII inhibition, or β-adrenergic stimulation. At baseline Ca2+-dependent upregulation of IKr and IKs in HF counterbalances the reduced IK1, maintaining repolarization reserve (especially at higher heart rates) in physiological conditions, unlike conditions of strong cytosolic Ca2+ buffering. However, under β-adrenergic stimulation, reduced IKs responsiveness severely limits integrated repolarizing K+ current and repolarization reserve in HF. This would increase arrhythmia propensity in HF, especially during adrenergic stress.

Slow Delayed Rectifier K+ Current Stabilizes Ventricular Action Potentials across Species

The slow (IKs) and rapid (IKr) delayed rectifier K+ currents are responsible for action potential repolarization in ventricular myocytes. Although substantial differences in the voltage-dependent gating kinetics of the two currents have been carefully documented, differences in the physiological roles of IKr and IKs are less established. A recent study comparing simulations with experimental recordings in guinea pig myocytes proposed the hypothesis that IKs might be superior to IKr at stabilizing the ventricular action potential (AP) and protecting cells against arrhythmias. We sought to determine whether IKs is uniformly protective across species and to uncover mechanisms underlying this protective effect. To address this, we performed simulations using 10 ventricular myocyte models describing electrophysiology of human, rabbit, canine, and guinea pig cells. We assessed models using several methods including simulations of heterogeneous populations, susceptibility to action potential prolongation, and simulations of action potential clamps. We found that: (1) models with higher baseline IKs exhibited less cell-to-cell variability in action potential duration; (2) models with higher baseline IKs were less susceptible to early afterdepolarizations (EADs) induced by depolarizing perturbations; (3) as action potential duration (APD) is extended, IKs increases more profoundly than IKr, thereby providing negative feedback that resists excessive APD prolongation; and (4) the increase in IKs that occurs during beta-adrenergic stimulation is critical for protecting cardiac myocytes from EADs under these conditions. Overall, the results confirm a uniformly protective role of IKs across a variety of cell types and support the idea that augmentation of IKs could potentially be an effective antiarrhythmic strategy.

Heart rate variability alters cardiac repolarization and electromechanical dynamics

Heart rate continuously varies due to autonomic regulation, stochasticity in pacemaking, and circadian rhythm, collectively termed heart rate variability (HRV), during normal physiological conditions. Low HRV is clinically associated with an elevated risk of cardiac arrhythmias. Alternans, a beat-to-beat alternation in action potential duration (APD) and/or intracellular calcium (Ca) transient, is a well-known risk factor associated with cardiac arrhythmias that is typically studied under conditions of a constant pacing rate, i.e., the absence of HRV. In this study, we investigate the effects of HRV on the interplay between APD, Ca, and electromechanical properties, employing a nonlinear discrete-time map model that governs APD and intracellular Ca cycling with a stochastic pacing period. We find that HRV can decrease variation in APD and peak Ca at fast pacing rates for which alternans is present. Further, increased HRV typically disrupts the alternating pattern for both APD and peak Ca and weakens the correlation between APD and peak Ca, thus decoupling Ca-mediated instabilities from repolarization alternation. We find that the efficacy of these effects is regulated by the sarcoplasmic reticulum Ca uptake rate. Overall, these results demonstrate that HRV disrupts arrhythmogenic alternans and suggests that HRV may be a significant factor in preventing life-threatening arrhythmias.

Cardiac action potential repolarization revisited: early repolarization shows all-or-none behaviour.

In healthy mammalian hearts the action potential (AP) waveform initiates and modulates each contraction, or heartbeat. As a result, AP height and duration are key physiological variables. In addition, rate-dependent changes in ventricular AP duration (APD), and variations in APD at a fixed heart rate are both reliable biomarkers of electrophysiological stability. Present guidelines for the likelihood that candidate drugs will increase arrhythmias rely on small changes in APD and Q-T intervals as criteria for safety pharmacology decisions. However, both of these measurements correspond to the final repolarization of the AP. Emerging clinical evidence draws attention to the early repolarization phase of the action potential (and the J-wave of the ECG) as an additional important biomarker for arrhythmogenesis. Here we provide a mechanistic background to this early repolarization syndrome by summarizing the evidence that both the initial depolarization and repolarization phases of the cardiac action potential can exhibit distinct time- and voltage-dependent thresholds, and also demonstrating that both can show regenerative all-or-none behaviour. An important consequence of this is that not all of the dynamics of action potential repolarization in human ventricle can be captured by data from single myocytes when these results are expressed as 'repolarization reserve'. For example, the complex pattern of cell-to-cell current flow that is responsible for AP conduction (propagation) within the mammalian myocardium can change APD and the Q-T interval of the electrocardiogram alter APD stability, and modulate responsiveness to pharmacological agents (such as Class III anti-arrhythmic drugs).

Hysteretic Dynamics of Multi-Stable Early Afterdepolarisations with Repolarisation Reserve Attenuation: A Potential Dynamical Mechanism for Cardiac Arrhythmias

Some cardiovascular and non-cardiovascular drugs frequently cause excessive prolongation of the cardiac action potential (AP) and lead to the development of early afterdepolarisations (EADs), which trigger lethal ventricular arrhythmias. Combining computer simulations in APs with numerical calculations based on dynamical system theory, we investigated stability changes of APs observed in a paced human ventricular myocyte model by decreasing and/or increasing the rapid (IKr) and slow (IKs) components of delayed rectifying K+ current. Upon reducing IKr, the APs without EADs (no-EAD response) showed gradual prolongation of AP duration (APD), and were annihilated without AP configuration changes due to the occurrence of saddle-node bifurcations. This annihilation caused a transition to an AP with EADs as a new stable steady state. Furthermore, reducing repolarisation currents (repolarisation reserve attenuation) evoked multi-stable states consisting of APs with different APDs, and caused multiple hysteretic dynamics. Depending on initial ion circumstances within ventricular myocytes, these multi-stable AP states might increase the local/global heterogeneity of AP repolarisations in the ventricle. Thus, the EAD-induced arrhythmias with repolarisation reserve attenuation might be attributed to the APD variability caused by multi-stability in cardiac AP dynamics.