Beat-to-beat Variability Of Repolarization Research Articles

Regional beat-to-beat variability of repolarization increases during ischemia and predicts imminent arrhythmias in a pig model of myocardial infarction.

Ventricular arrhythmia (VT/VF) can complicate acute myocardial ischemia (AMI). Regional instability of repolarization during AMI contributes to the substrate for VT/VF. Beat-to-beat variability of repolarization (BVR), a measure of repolarization lability increases during AMI. We hypothesized that its surge precedes VT/VF. We studied the spatial and temporal changes in BVR in relation to VT/VF during AMI. In 24 pigs, BVR was quantified on 12-lead electrocardiogram recorded at a sampling rate of 1 kHz. AMI was induced in 16 pigs by percutaneous coronary artery occlusion (MI), whereas 8 underwent sham operation (sham). Changes in BVR were assessed at 5 min after occlusion, 5 and 1 min pre-VF in animals that developed VF, and matched time points in pigs without VF. Serum troponin and ST deviation were measured. After 1 mo, magnetic resonance imaging and VT induction by programmed electrical stimulation were performed. During AMI, BVR increased significantly in inferior-lateral leads correlating with ST deviation and troponin increase. BVR was maximal 1 min pre-VF (3.78 ± 1.36 vs. 5 min pre-VF, 1.67 ± 1.56, P < 0.0001). After 1 mo, BVR was higher in MI than in sham and correlated with the infarct size (1.43 ± 0.50 vs. 0.57 ± 0.30, P = 0.009). VT was inducible in all MI animals and the ease of induction correlated with BVR. BVR increased during AMI and temporal BVR changes predicted imminent VT/VF, supporting a possible role in monitoring and early warning systems. BVR correlated to arrhythmia vulnerability suggesting utility in risk stratification post-AMI.NEW & NOTEWORTHY The key finding of this study is that BVR increases during AMI and surges before ventricular arrhythmia onset. This suggests that monitoring BVR may be useful for monitoring the risk of VF during and after AMI in the coronary care unit settings. Beyond this, monitoring BVR may have value in cardiac implantable devices or wearables.

PO-02-216 SEGMENTAL INCREASE IN HETEROGENEITY OF REPOLARIZATION CORRESPONDS TO REGIONS OF DECREASED INNERVATION AFTER MYOCARDIAL INFARCTION

Myocardial infarction (MI) results in a dense scar region surrounded by a heterogeneous remodeled border zone. In this region denervation and nerve regeneration during healing could be region dependent and contribute to segmental differences in the functional substrate for arrhythmias. To examine regional electrophysiological remodeling in relation to perfusion and innervation changes after myocardial infarction. Anterio-septal MI was induced in 6 domestic pigs by a 120-minute occlusion of the left anterior descending artery (LAD) followed by reperfusion. After one month, PET-MR imaging was performed to evaluate myocardial perfusion (13N-ammonia) and innervation (11C-epinephrine), together with late gadolinium enhanced MR imaging for scar quantification. The next day electro-anatomical mapping was performed. The BZ was defined using contact mapping as the region with bipolar voltage between 0.5 and 1.5 mV. A non-contact recording of a 64-electrode array was translated to 2048 non-contact electrograms distributed over the LV (EnSite PrecisionTM, St. Jude/Abbott Medical). In each of the 2048 points non-contact electrograms over 25 consecutive beats were processed to determine the beat-to-beat variability of repolarization (BVR). Spatial heterogeneity of repolarization was quantified in each point as the repolarization time variability within a 1cm radius neighborhood. MI size was 14.2±3.2% of the left ventricle on gadolinium enhanced MR imaging. The perfusion and innervation defects were significantly larger (22.3±3.2%, P<0.01 and 28.1±4.3%, P<0.01 respectively). The innervation defect was also significantly larger than the perfusion defect (P=0.03). Regional analysis revealed less innervation in the anterior versus the septal border zone (14.2±2.9 %/min vs 18.6+3.1 %/min, P<0.01). In one animal the electrophysiology study was unsuccessful due to a high baseline heart rate (>120 BPM). In the remaining animals, the average spatial heterogeneity was higher in the anterior than septal border zone (5.8±3.0 ms vs 3.8±2.6 ms, P<0.01). Additionally, the maximum BVR tended to be higher in the anterior than septal border zone (2.6±1.3 ms vs 2.02±0.77 ms, P=0.08). MI resulting from LAD occlusion/reperfusion leads to increased denervation on the anterior side of the infarct, correlating with increased repolarization heterogeneity in this region.

Temporal Changes in Beat‐to‐Beat Variability of Repolarization Predict Imminent Nonsustained Ventricular Tachycardia in Patients With Ischemic and Nonischemic Dilated Cardiomyopathy

BackgroundAn increase in beat‐to‐beat variability of repolarization (BVR) predicts arrhythmia onset in experimental models, but its clinical translation is not well established. We investigated the temporal changes in BVR before nonsustained ventricular tachycardia (nsVT) in patients with implantable cardioverter defibrillator (ICD).Methods and ResultsPatients with nsVT on 24‐hour Holter before ICD implantation for ischemic cardiomyopathy (ischemic cardiomyopathy+nsVT, n=43) or dilated cardiomyopathy (dilated cardiomyopathy+nsVT, n=37), matched ICD candidates without nsVT (ischemic cardiomyopathy‐nsVT, n=29 and dilated cardiomyopathy‐nsVT, n=26), and patients without ICD without structural heart disease (n=50) were studied. Digital Holter recordings from these patients were analyzed using a modified fiducial segment averaging technique to detect the QT interval. The nsVT episodes were semi‐automatically identified and QT‐BVR was assessed 1‐, 5‐, and 30‐minutes before nsVT, and at rest (at 3:00 am). Resting BVR was higher in ICD patients compared with controls without structural heart disease. In ICD patients with nsVT, BVR increased significantly 1‐minute pre‐nsVT in ischemic cardiomyopathy (2.21±0.59 ms, versus 5 minutes pre‐nsVT: 1.78±0.50 ms, P<0.001) and dilated cardiomyopathy (2.09±0.57 ms, versus 5‐minutes pre‐nsVT: 1.58±0.51 ms, P<0.001), but not in patients without nsVT. In multivariable Cox regression analysis, pre‐nsVT BVR was a significant predictor for appropriate therapy during follow‐up.ConclusionsBaseline BVR is elevated and temporal changes in BVR predict imminent nsVT events in patients with ICD independent of underlying cause. Real‐time BVR monitoring could be used to predict impending ventricular arrhythmia and allow preventive therapy to be incorporated into ICDs.

Adrenergic stimulation amplifies the difference in beat-to-beat variability between the scar border zone and remote region

Abstract Funding Acknowledgements 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” Background Myocardial infarction (MI) results in a dense scar region surrounded by a heterogeneous region of fibrosis and remodeled myocytes called the border zone (BZ). Beta-adrenergic stimulation results in increased beat-to-beat variability of repolarization (BVR) which could increase spatial heterogeneity and arrhythmia vulnerability. Objective To examine the effect of adrenergic stimulation on the beat-to-beat variability in the BZ, compared to the remote region, using novel methodology for determining spatially dense activation-repolarization intervals. Methods Anterior-septal myocardial infarction (MI) was induced in 10 domestic pigs by 120-minute occlusion of the left anterior descending artery followed by reperfusion. Electro-anatomical mapping was performed after one month. The BZ was defined using contact mapping as the region with bipolar voltage between 0.5 and1.5mV. A non-contact recording of a 64-electrode array was translated to 2048 non-contact electrograms distributed over the LV (EnSite PrecisionTM, St. Jude/Abbott Medical). Electrophysiological recordings were made during baseline and during an isoproterenol (ISO) infusion (incremental doses of 0.01µg/kg until 0.04µg/kg). In each of the 2048 points non-contact electrograms over 25 consecutive beats were processed to determine the BVR using a custom-made algorithm, validated against monophasic action potential recordings. Results During baseline conditions the maximal BVR was increased in the BZ compared to the remote region (BZ: 3.28±0.90 ms vs remote: 2.61±0.67 ms, P=0.002). During ISO infusion the maximal BVR was also increased in the BZ (BZ: 3.55±0.74 ms vs remote: 2.21±0.60 ms, P&amp;lt;0.001). During baseline the BZ exhibited a larger spatial variance of BVR than the remote region (BZ: 0.20±0.11 ms2 vs remote: 0.087±0.055 ms2, P=0.002). During ISO infusion the spatial variance of BVR was larger in the BZ (BZ: 0.23±0.12 ms2 vs remote: 0.083±0.056 ms2, P=0.001). The maximal BVR was not significantly different during baseline and ISO in the BZ, nor the remote region (P&amp;gt;0.05). However, the difference of the maximal BVR between BZ and remote regions was significantly increased during ISO (baseline: 0.67±0.48 ms vs ISO: 1.34±0.49ms, P=0.001). Conclusion The MI BZ showed increased temporal heterogeneity in repolarization that could serve as functional substrate for re-entry. Adrenergic stimulation amplified this vulnerability by increasing the difference in maximal BVR between BZ and remote regions.

Structural and Electrophysiological Changes in a Model of Cardiotoxicity Induced by Anthracycline Combined With Trastuzumab.

BackgroundCombined treatment with anthracyclines (e.g., doxorubicin; Dox) and trastuzumab (Trz), a humanized anti-human epidermal growth factor receptor 2 (HER2; ErbB2) antibody, in patients with HER2-positive cancer is limited by cardiotoxicity, as manifested by contractile dysfunction and arrhythmia. The respective roles of the two agents in the cardiotoxicity of the combined therapy are incompletely understood.ObjectiveTo assess cardiac performance, T-tubule organization, electrophysiological changes and intracellular Ca2+ handling in cardiac myocytes (CMs) using an in vivo rat model of Dox/Trz-related cardiotoxicity.Methods and ResultsAdult rats received 6 doses of either Dox or Trz, or the two agents sequentially. Dox-mediated left ventricular (LV) dysfunction was aggravated by Trz administration. Dox treatment, but not Trz, induced T-tubule disarray. Moreover, Dox, but not Trz monotherapy, induced prolonged action potential duration (APD), increased incidence of delayed afterdepolarizations (DADs) and beat-to-beat variability of repolarization (BVR), and slower Ca2+ transient decay. Although APD, DADs, BVR and Ca2+ transient decay recovered over time after the cessation of Dox treatment, subsequent Trz administration exacerbated these abnormalities. Trz, but not Dox, reduced Ca2+ transient amplitude and SR Ca2+ content, although only Dox treatment was associated with SERCA downregulation. Finally, Dox treatment increased Ca2+ spark frequency, resting Ca2+ waves, sarcoplasmic reticulum (SR) Ca2+ leak, and long-lasting Ca2+ release events (so-called Ca2+ “embers”), partially reproduced by Trz treatment.ConclusionThese results suggest that in vivo Dox but not Trz administration causes T-tubule disarray and pronounced changes in electrical activity of CMs. While adaptive changes may account for normal AP shape and reduced DADs late after Dox administration, subsequent Trz administration interferes with such adaptive changes. Intracellular Ca2+ handling was differently affected by Dox and Trz treatment, leading to SR instability in both cases. These findings illustrate the specific roles of Dox and Trz, and their interactions in cardiotoxicity and arrhythmogenicity.

Electric Field-Based Dense Spatial Analysis of Unipolar Electrograms to Map Regional Activation-Repolarization Intervals

Objective: We wanted to develop a new method for processing dense spatially sampled unipolar electrograms, that yields physiological spatial distributions in activation-repolarization intervals.Background: Spatial heterogeneity in repolarization plays an important role in genesis and sustainment of cardiac arrhythmias. Reliable determination of repolarization times at high resolution on electrograms remains a difficult task. Methods: The new method describes the electrical activity on the heart's surface in terms of a physical electric field (E-field). Activation and repolarization times are calculated using an amplitude weighted average on QRS and T-waves. Activation-recovery intervals (ARI) obtained with this E-field method were validated against action potential durations (APD90) from monophasic action potential measurements. Relevance of the method was shown by calculating dense spatial distributions of beat-to-beat variability of repolarization (BVR) in myocardial infarction (MI).Results: E-field waveforms were more spatially homogeneous than the unipolar electrograms. The ARI strongly correlated with the APD90 (r2=0.94; p<0.001). Continuous activation and repolarization patterns were obtained, and the method removes non-physiological sharp spatial gradients obtained by currently adopted methods. Spatial BVR analysis revealed an increase in maximum BVR, spatial variance of BVR and spatial gradient of BVR in the border zone versus remote region of MI (all p<0.01). All BVR values were in pathophysiological range as described in other studies.Conclusion: The E-field method allows for a reliable determination of activation and repolarization times over the whole left ventricle. The new methodology confirmed that the border zone of myocardial infarction is a region with patches of increased variability of repolarization.

Altered adrenergic response in myocytes bordering a chronic myocardial infarction underlies in vivo triggered activity and repolarization instability.

Key points Ventricular arrhythmias are a major complication after myocardial infarction (MI), associated with sympathetic activation. The structurally heterogeneous peri‐infarct zone is a known substrate, but the functional role of the myocytes is less well known.Recordings of monophasic action potentials in vivo reveal that the peri‐infarct zone is a source of delayed afterdepolarizations (DADs) and has a high beat‐to‐beat variability of repolarization (BVR) during adrenergic stimulation (isoproterenol, ISO).Myocytes isolated from the peri‐infarct region have more DADs and spontaneous action potentials, with spontaneous Ca2+ release, under ISO. These myocytes also have reduced repolarization reserve and increased BVR. Other properties of post‐MI remodelling are present in both peri‐infarct and remote myocytes.These data highlight the importance of altered myocyte adrenergic responses in the peri‐infarct region as source and substrate of post‐MI arrhythmias. Ventricular arrhythmias are a major early complication after myocardial infarction (MI). The heterogeneous peri‐infarct zone forms a substrate for re‐entry while arrhythmia initiation is often associated with sympathetic activation. We studied the mechanisms triggering these post‐MI arrhythmias in vivo and their relation to regional myocyte remodelling. In pigs with chronic MI (6 weeks), in vivo monophasic action potentials were simultaneously recorded in the peri‐infarct and remote regions during adrenergic stimulation with isoproterenol (isoprenaline; ISO). Sham animals served as controls. During infusion of ISO in vivo, the incidence of delayed afterdepolarizations (DADs) and beat‐to‐beat variability of repolarization (BVR) was higher in the peri‐infarct than in the remote region. Myocytes isolated from the peri‐infarct region, in comparison to myocytes from the remote region, had more DADs, associated with spontaneous Ca2+ release, and a higher incidence of spontaneous action potentials (APs) when exposed to ISO (9.99 ± 4.2 vs. 0.16 ± 0.05 APs/min, p = 0.004); these were suppressed by CaMKII inhibition. Peri‐infarct myocytes also had reduced repolarization reserve and increased BVR (26 ± 10 ms vs. 9 ± 7 ms, P < 0.001), correlating with DAD activity. In contrast to these regional distinctions under ISO, alterations in Ca2+ handling at baseline and myocyte hypertrophy were present throughout the left ventricle (LV). Expression of some of the related genes was, however, different between the regions. In conclusion, altered myocyte adrenergic responses in the peri‐infarct but not the remote region provide a source of triggered activity in vivo and of repolarization instability amplifying the substrate for re‐entry. These findings stimulate further exploration of region‐specific therapies targeting myocytes and autonomic modulation.

Selective chemical ablation of transient receptor potential vanilloid 1 expressing neurons in the left stellate ganglion protects against ischemia-induced ventricular arrhythmias in dogs

ObjectivesFindings from prior investigations show that left stellate ganglion (LSG) inhibitory approaches protect the heart from ventricular arrhythmias (VAs) caused by acute myocardial infarction (AMI), which still remain many side effects. Targeted transient receptor potential vanilloid 1/tyrosine hydroxylase (TRPV-1/TH) expressing sympathetic neurons ablation is a novel neuro-ablative strategy. The aim of this investigation was to explore if targeted molecular neuro-ablative strategy by resiniferatoxin (RTX) stellate microinjection could protect against ischemia-induced VAs. MethodsTwenty-four anesthetized beagles were assigned to a control group (n = 12) and RTX group (n = 12) in a random manner. Targeted molecular neuro-ablative was produced by RTX stellate microinjection and DMSO was microinjected into LSG in the same way as control. Plasma norepinephrine (NE) level, heart rate variability (HRV), Tpeak-Tend interval (Tp-Te), LSG neural activity and function, ventricular effective refractory period (ERP), beat-to-beat variability of repolarization (BVR) and ventricular action potential duration (APD) were measured at baseline and 60 min after RTX or DMSO microinjection. AMI model was established by the ligation of left anterior descending coronary artery and 60-minute electrocardiography was continuously recorded for VAs analysis. Subsequently, HRV, Tp-Te, plasma NE level from jugular vein and coronary sinus, LSG neural activity and function, ventricular ERP, ventricular APD, BVR, action potential duration alternans (APDA) cycle length and ventricular fibrillation threshold (VFT) were evaluated after AMI. Finally, tissue collection of LSG was performed for examining the TRPV-1, nerve growth factor (NGF) protein and c-fos protein. ResultsTRPV-1 was highly expressed in the TH-expressing neurons and RTX injection significantly ablated TRPV-1/TH-positive neurons in LSG. Compared with baseline, RTX stellate microinjection significantly reduced plasma NE level, the sympathetic component of HRV, LSG neural activity and LSG function, shortened Tp-Te, prolonged ventricular ERP and APD, but there were no remarkable differences existed for control group. AMI resulted in the significant raise in plasma NE level from jugular vein and coronary sinus, the sympathetic component of HRV, LSG neural activity and LSG function, the marked prolongation in Tp-Te and BVR, the significant decrease in ERP and APD from ischemia area, and the increase in APDA cycle length in the ischemic region of the control group, which were remarkably attenuated in the RTX group. RTX pretreatment markedly rose the VFT in the RTX group. Furthermore, the AMI-triggered VAs was significantly prevented by RTX injection in the RTX group. RTX microinjection down-regulated significantly TRPV-1, NGF and c-fos expression in the LSG compared with the control group. ConclusionTargeted ablation of TRPV-1/TH positive sympathetic neurons induced by RTX stellate microinjection could suppress ischemia-induced cardiac autonomic imbalances and cardiac electrophysiology instability to protect against AMI-induced VAs.

P1622Cancer drugs induce functional and structural impairment in adult cardiomyocytes

Abstract Introduction The addition of anti-human epidermal growth factor receptor 2 (HER2; ErbB2) monoclonal antibody Trastuzumab (TRZ) to Doxorubicin (DOXO) chemotherapy is associated with a synergistic increase in cardiac toxicity. While previous studies have addressed the toxicity of both agents on isolated cardiomyocytes (CMs), little is known regarding this process in vivo, especially with respect to electrophysiological changes. Purpose To investigate electrical and structural changes in LV and RV CMs using an in vivo rat model of DOXO/TRZ cardiotoxicity. Methods Rats received 6 IP injections of either DOXO or TRZ over a 2-week period, or 6 doses of DOXO followed by 6 doses of TRZ (COMBO), or saline as a control. In-vivo echocardiography was performed. Electrical activity and Ca2+ handling were assessed in LV and RV CMs from rat hearts. Single cell patch-clamp and field stimulation experiments were performed. Spontaneous sarcoplasmic reticulum Ca2+ release events (Ca2+ sparks) were recorded at x100 magnification in line-scan mode (sampling rate 0.7 kHz) from 2 μM Fluo4-AM loaded CMs. To assess T-tubular disarray, CMs were incubated with di-3-ANEPPDHQ and periodic component was quantified by Fast Fourier Transform (FFT) analysis of confocal microscopy images. Results DOXO, and to a greater extent COMBO treatment was associated with significant increases in both LV end-systolic and end-diastolic volumes, and decreases in LVEF and fractional shortening. By contrast, TRZ alone merely increased LV end-systolic volume. Electrophysiological studies showed increases in action potential duration (APD), beat-to-beat variability of repolarization (BVR), delayed after depolarizations (DADs), and Ca2+-sparks in both DOXO and COMBO groups. Stimulated intracellular Ca2+ transients (1,2 and 4 Hz) showed significant changes with respect to time to peak, tau decay, amplitude, and fractional release in the DOXO group. These changes were associated with a significant downregulation of sarco/endoplasmic reticulum Ca2+ ATPase pump (SERCA) expression. From a structural viewpoint, these changes were associated with T-tubular disarray in the DOXO and COMBO groups. Conclusions DOXO, and to a greater extent COMBO treatment (but not TRZ alone) cause LV dysfunction in vivo. Moreover, both DOXO and COMBO treatments, but not TRZ alone, induce electrophysiological abnormalities and both structural and functional changes in the sarcoplasmic reticulum. These findings provide novel insights into the cellular mechanisms of CM dysfunction and arrhythmias associated with combined DOXO/TRZ therapy. Acknowledgement/Funding Swiss League against Cancer

Cardiac Sympathetic Afferent Denervation Protects Against Ventricular Arrhythmias by Modulating Cardiac Sympathetic Nerve Activity During Acute Myocardial Infarction.

BackgroundAugmented cardiac sympathetic afferent reflex (CSAR) plays a role in enhanced sympathetic activity. Given that a strategy for abolishing augmented CSAR-induced sympathetic activation may be beneficial for protecting against ventricular arrhythmias (VAs) triggered by acute myocardial infarction (AMI), we investigated whether cardiac sympathetic afferent denervation (CSAD) could protect against VAs by modulating cardiac sympathetic nerve activity in an AMI dog model.Material/MethodsTwenty-two anesthetized dogs were assigned to the CSAD group (n=9) and the sham group (n=13) randomly. CSAD was produced by epicardial application of resiniferatoxin. Heart rate variability (HRV), ventricular action potential duration (APD), APD dispersion, beat-to-beat variability of repolarization (BVR), effective refractory period (ERP) of ventricles, ERP dispersion, plasma norepinephrine (NE) concentration, and left stellate ganglion (LSG) neural activity were determined at baseline and after CSAD. We designed an AMI model by occluding the left anterior coronary artery, and performed analysis of VAs for 60 minutes using electrocardiography. Then, levels of c-fos and nerve growth factor (NGF) were determined.ResultsRelative to baseline values, CSAD prolonged ERP and APD of ventricles, increased HRV, decreased APD dispersion, BVR, ERP dispersion and serum NE concentration, and attenuated LSG activity in the CSAD group. AMI triggered a remarkable increase in LSG activity and function but decreased the HRV of the sham group animals relative to the CSAD group. Moreover, the CSAD group had higher levels of VAs relative to the sham group. This was accompanied by a corresponding decrease in proteins quantities of NGF and c-fos in the CSAD group in the LSG after AMI compared to the sham group.ConclusionsCSAD can suppress LSG neural activity, hence enhance the electrophysiological stability and protect the heart from AMI-triggered VAs.

Application of stochastic phenomenological modelling to cell-to-cell and beat-to-beat electrophysiological variability in cardiac tissue.

Variability in the action potential of isolated myocytes and tissue samples is observed in experimental studies. Variability is manifested as both differences in the action potential (AP) morphology between cells (extrinsic variability), and also ‘intrinsic’ or beat-to-beat variability of repolarization (BVR) in the AP duration of each cell. We studied the relative contributions of experimentally recorded intrinsic and extrinsic variability to dispersion of repolarization in tissue. We developed four cell-specific parameterizations of a phenomenological stochastic differential equation AP model exhibiting intrinsic variability using APs recorded from isolated guinea pig ventricular myocytes exhibiting BVR. We performed simulations in tissue using the four different model parameterizations in the presence and the absence of both intrinsic and extrinsic variability. We altered the coupling of the tissue to determine how inter-cellular coupling affected the dispersion of the AP duration in tissue. Both intrinsic and extrinsic variability were gradually revealed by reduction of tissue coupling. However, the recorded extrinsic variability between individual myocytes produced a greater degree of dispersion in repolarization in tissue than the intrinsic variability of each myocyte.

Chronic N-methyl-D-aspartate receptor activation induces cardiac electrical remodeling and increases susceptibility to ventricular arrhythmias.

N-Methyl-d-aspartate receptors, also known as NMDA Receptors or NMDAR, are glutamate receptors that control calcium ion channels and regulate synaptic plasticity. Acute NMDAR activation can induce ventricular arrhythmias (VAs). However, the influence of chronic NMDAR activation on cardiac electrophysiology remains unknown. Wistar rats were randomly administered 0.9% saline (CTL group), NMDA (N group), or NMDA plus MK801 (N+M group) for 14 days. Compared with the CTL group, the N group displayed elevated heart rate and prolonged QT, QTc, and TpTe intervals in the electrocardiogram (P < 0.05 for all). Then, the S1 S2 , S1 S1 , and Burst pacing were performed to assess the characteristics of repolarization; threshold of action potential duration (APD) alternans; beat-to-beat variability of repolarization (BVR); and VAs susceptibility in the left ventricular. The prolonged APD at 90% repolarization (APD90 ); decreased ERP/APD90 ; increased dispersion of APD90 , ERP, and ERP/APD90 ; decreased threshold of APD alternans; increased BVR; and incidence of VAs were showed in the N group compared with those of the CTL group (P < 0.01 for all). Moreover, chronic NMDA administration reduced the expression of Kv4.2, Kv4.3, KChIP2, and Kv11.1 proteins, and induced mild myocardial interstitial fibrosis (P < 0.01 for all). Importantly, these alterations induced by NMDA were normalized by co-treatment with MK801. Chronic NMDAR activation prolonged repolarization, induced electrical instability, and facilitated VAs, which may be associated with reduced Ito and IKr and myocardial fibrosis.

Rationale, objectives, and design of the EUTrigTreat clinical study: a prospective observational study for arrhythmia risk stratification and assessment of interrelationships among repolarization markers and genotype

AimsThe EUTrigTreat clinical study has been designed as a prospective multicentre observational study and aims to (i) risk stratify patients with an implantable cardioverter defibrillator (ICD) for mortality and shock risk using multiple novel and established risk markers, (ii) explore a link between repolarization biomarkers and genetics of ion (Ca2+, Na+, K+) metabolism, (iii) compare the results of invasive and non-invasive electrophysiological (EP) testing, (iv) assess changes of non-invasive risk stratification tests over time, and (v) associate arrythmogenomic risk through 19 candidate genes.Methods and resultsPatients with clinical ICD indication are eligible for the trial. Upon inclusion, patients will undergo non-invasive risk stratification, including beat-to-beat variability of repolarization (BVR), T-wave alternans, T-wave morphology variables, ambient arrhythmias from Holter, heart rate variability, and heart rate turbulence. Non-invasive or invasive programmed electrical stimulation will assess inducibility of ventricular arrhythmias, with the latter including recordings of monophasic action potentials and assessment of restitution properties. Established candidate genes are screened for variants. The primary endpoint is all-cause mortality, while one of the secondary endpoints is ICD shock risk. A mean follow-up of 3.3 years is anticipated. Non-invasive testing will be repeated annually during follow-up. It has been calculated that 700 patients are required to identify risk predictors of the primary endpoint, with a possible increase to 1000 patients based on interim risk analysis.ConclusionThe EUTrigTreat clinical study aims to overcome current shortcomings in sudden cardiac death risk stratification and to answer several related research questions. The initial patient recruitment is expected to be completed in July 2012, and follow-up is expected to end in September 2014.Clinicaltrials.gov identifier: NCT01209494.

Late Sodium Current Contributes to the Reverse Rate-Dependent Effect of I Kr Inhibition on Ventricular Repolarization

The reverse rate dependence (RRD) of actions of I(Kr)-blocking drugs to increase the action potential duration (APD) and beat-to-beat variability of repolarization (BVR) of APD is proarrhythmic. We determined whether inhibition of endogenous, physiological late Na(+) current (late I(Na)) attenuates the RRD and proarrhythmic effect of I(Kr) inhibition. Duration of the monophasic APD (MAPD) was measured from female rabbit hearts paced at cycle lengths from 400 to 2000 milliseconds, and BVR was calculated. In the absence of a drug, duration of monophasic action potential at 90% completion of repolarization (MAPD(90)) and BVR increased as the cycle length was increased from 400 to 2000 milliseconds (n=36 and 26; P<0.01). Both E-4031 (20 nmol/L) and d-sotalol (10 μmol/L) increased MAPD(90) and BVR at all stimulation rates, and the increase was greater at slower than at faster pacing rates (n=19, 11, 12 and 7, respectively; P<0.01). Tetrodotoxin (1 μmol/L) and ranolazine significantly attenuated the RRD of MAPD(90,) reduced BVR (P<0.01), and abolished torsade de pointes in hearts treated with either 20 nmol/L E-4031 or 10 μmol/L d-sotalol. Endogenous late I(Na) in cardiomyocytes stimulated at cycle lengths from 500 to 4000 milliseconds was greater at slower than at faster stimulation rates, and rapidly decreased during the first several beats at faster but not at slower rates (n=8; P<0.01). In a computational model, simulated RRD of APD caused by E-4031 and d-sotalol was attenuated when late I(Na) was inhibited. Endogenous late I(Na) contributes to the RRD of I(Kr) inhibitor-induced increases in APD and BVR and to bradycardia-related ventricular arrhythmias.

Microvolt T-wave alternans and beat-to-beat variability of repolarization during early postischemic remodeling in a pig heart

Repolarization variability is considered to predict sudden cardiac death. T-wave alternans (TWA) has been the subject of exhaustive research, whereas beat-to-beat variability of repolarization (BVR) is a new parameter that possibly predicts proarrhythmia. How these parameters interact has not been tested. The purpose of this study was to compare TWA and BVR as predictors of proarrhythmic substrate early after myocardial infarction (MI). In nine pigs, MI was induced by 1-hour occlusion of the left anterior descending coronary artery. Cardiac magnetic resonance imaging was performed at day 21. Six sham pigs served as control. Spectral TWA was tested during right atrial pacing before induction of MI and after 21 days. BVR was calculated from 60 consecutive QT intervals. Magnetic resonance imaging showed transmural MI. TWA was negative in all pigs at clinical threshold rate and equally present in MI versus sham pigs at higher rates (170 bpm: 55% vs 50% positive TWA). In MI pigs, BVR of QT intervals increased significantly during acute ischemia (2.44 ± 0.43 ms vs 3.55 ± 0.41 ms, P <.01) and even more on day 21 (5.80 ± 1.12 ms), but it differed significantly from sham (2.14 ± 0.54 ms, P <.01). A clinical ventricular tachycardia induction protocol was positive in 2 of 8 MI pigs and in none of 6 shams. In early remodeling after MI, BVR at intrinsic heart rate was a consistent phenomenon, whereas TWA during atrial pacing or baseline QT-interval changes were not. TWA and BVR could reflect different post-MI remodeling processes. BVR may be a new technique for predicting a potentially proarrhythmic substrate in the early postinfarction period.

Late Na+Current Inhibition by Ranolazine Reduces Torsades de Pointes in the Chronic Atrioventricular Block Dog Model

This study investigated whether ranolazine reduces dofetilide-induced torsades de pointes (TdP) in a model of long QT syndrome with down-regulated K(+) currents due to hypertrophic remodeling in the dog with chronic atrioventricular block (cAVB). Ranolazine inhibits the late Na(+) current (I(NaL)) and is effective against arrhythmias in long QT3 syndromes despite its blocking properties of the rapid component of delayed rectifying potassium current. Ranolazine was administered to cAVB dogs before or after TdP induction with dofetilide and electrophysiological parameters were determined including beat-to-beat variability of repolarization (BVR). In single ventricular myocytes, effects of ranolazine were studied on I(NaL), action potential duration, and dofetilide-induced BVR and early afterdepolarizations. After dofetilide, ranolazine reduced the number of TdP episodes from 10 +/- 3 to 3 +/- 1 (p < 0.05) and partially reversed the increase of BVR with no abbreviation of the dofetilide-induced QT prolongation. Likewise, pre-treatment with ranolazine, or using lidocaine as a specific Na(+) channel blocker, attenuated TdP, but failed to prevent dofetilide-induced increases in QT, BVR, and ectopic activity. In cAVB myocytes, ranolazine suppressed dofetilide-induced early afterdepolarizations in 25% of cells at 5 micromol/l, in 75% at 10 micromol/l, and in 100% at 15 micromol/l. At 5 micromol/l, ranolazine blocked 26 +/- 3% of tetrodotoxin-sensitive I(NaL), and 49 +/- 3% at 15 micromol/l. Despite a 54% reduction of I(NaL) amplitude in cAVB compared with control cells, I(NaL) inhibition by 5 micromol/l tetrodotoxin equally shortened relative action potential duration and completely abolished dofetilide-induced early afterdepolarizations. Despite down-regulation of I(NaL) in remodeled cAVB hearts, ranolazine is antiarrhythmic against drug-induced TdP. The antiarrhythmic effects are reflected in concomitant changes of BVR.

Literature-based evaluation of four 'hard endpoint' models for assessing drug-induced torsades de pointes liability.

In safety pharmacology, a number of preclinical models for detecting drug-induced proarrhythmia liability have been recently introduced that utilize hard endpoints: early after depolarziations (EADs), torsades de pointes (TdP) or both as the principal biomarker. To explore the validity of four of the most common of these models, (the isolated canine/rabbit left ventricular wedge preparation, the isolated rabbit heart, the methoxamine-pretreated anaesthetized rabbit and the complete, chronic AV-blocked (CAVB) dog (conscious and anaesthetized), the present article reviews published data sets for three drugs with recognized and different human TdP liabilities (cisparide, terfenadine and moxifloxacinin). Finally, this review considers the value of inclusion of analysis of beat-to-beat variability of repolarization (BVR) in TdP liability testing to improve sensitivity and specificity.

No proarrhythmic properties of the antibiotics Moxifloxacin or Azithromycin in anaesthetized dogs with chronic‐AV block

The therapeutically available quinolone antibiotic moxifloxacin has been used as a positive control for prolonging the QT interval in both clinical and non-clinical studies designed to assess the potential of new drugs to delay cardiac repolarization. Despite moxifloxacin prolonging QT, it has not been shown to cause torsades de pointes arrhythmias (TdP). Azithromycin is a macrolide antibiotic that has rarely been associated, clinically, with cases of proarrhythmia. As there is a lack of clinical data available, the cardiac safety of these drugs was assessed in a TdP-susceptible animal model by evaluating their repolarization and proarrhythmia effects. In transfected HEK cells, the IC(50)s for I (hERG) were 45+/-6 and 856+/-259 microg ml(-1) for moxifloxacin and azithromycin, respectively. Intravenous administration of 2 and 8 mg kg(-1) moxifloxacin (total peak-plasma concentrations 4.6+/-1.5 and 22.9+/-6.8 microg ml(-1)) prolonged the QT(c) in 6 anaesthetized dogs with chronic AV block by 7+/-3 and 21+/-19%, respectively. Similar intravenous doses of azithromycin (total peak-plasma concentrations 5.4+/-1.3 and 20.8+/-4.9 microg ml(-1)) had no electrophysiological effects in the same dogs. The reference compound, dofetilide (25 microg kg(-1) i.v.) caused QT(c) prolongation (29+/-15%) and TdP in all dogs. Beat-to-beat variability of repolarization (BVR), quantified as short-term variability of the left ventricular monophasic action potential duration, was only increased after dofetilide (1.8+/-0.7 to 3.8+/-1.5 ms; P<0.05). As neither moxifloxacin nor azithromycin caused TdP or an increase in the BVR, we conclude that both drugs can be used safely in clinical situations.

Beat-to-Beat Variability of Repolarization Determines Proarrhythmic Outcome in Dogs Susceptible to Drug-Induced Torsades de Pointes

We investigated whether increasing or decreasing beat-to-beat variability of repolarization (BVR) would change drug-induced proarrhythmic outcome accordingly. Increased variability of repolarization has been suggested as a prelude to proarrhythmic circumstances in experimental and clinical situations. The non-cardiovascular, I(Kr)-blocking drug sertindole was administered to anesthetized dogs with chronic atrioventricular block. Three interventions were used to prevent or suppress sertindole-induced torsades de pointes (TdP). Supratherapeutic doses of sertindole (1.0 mg/kg intravenously) induced TdP in 10 of 13 dogs whereas 0.2 mg/kg induced no TdP, despite increases in QT intervals by both doses. The BVR, quantified as short-term variability (STV) from Poincaré plots, was the only parameter that predicted TdP outcome (1.0 mg/kg sertindole: 2.3 +/- 0.7 ms to 5.1 +/- 2.1 ms, p < 0.05; 0.2 mg/kg sertindole: 2.3 +/- 0.8 ms to 3.2 +/- 1.1 ms, p= NS). 1) KCl, intravenous, reduced the incidence of sertindole-induced TdP from 6 of 7 to 1 of 7 dogs (p<0.05) and prevented sertindole-related increase of STV: 3.0 +/- 1.1 ms vs. 4.5 +/- 1.3 ms (p < 0.05); 2) levcromakalim (I(K,ATP) activator) reduced sertindole-induced TdP and decreased STV from 4.9 +/- 2.1 ms to 2.6 +/- 0.9 ms (p < 0.05); 3) steady-state ventricular pacing (60 beats/min) abolished sertindole-induced TdP and decreased STV from 4.9 +/- 1.5 to 3.2 +/- 1.0 (p < 0.05). Torsades de pointes reappeared upon return to non-paced idioventricular rhythm. None of the 3 interventions reduced the sertindole-induced prolonged QT interval. Proarrhythmic intervention is related to an increase in BVR, whereas antiarrhythmic treatment is associated with a decrease in BVR. The BVR is superior to QT interval prolongation in the prediction and prevention of drug-induced TdP in this experimental model.

Beat-to-beat variability of repolarization determines proarrhythmic outcome in dogs susceptible to drug-induced torsades de pointes

Background: Recent results in dogs show that increased beat-to-beat variability of repolarization (BVR) predicts drug-induced torsades de pointes (TdP) more accurately than conventional measures of repolarization prolongation. We investigated whether increasing or decreasing BVR would change proarrhythmic outcome accordingly. Methods and Results: The non-cardiovascular, IKr-blocking drug sertindole (1.0 mg/kg IV) induced TdP in 10 of 13 anesthetized dogs with chronic AV block. At a lower dose (0.2 mg/kg IV) no TdP was induced, despite similar increases in QT intervals (1.0 mg/kg: 400±60 to 495±65 ms, P<0.05 (⁎); 0.2 mg/kg: 400±60 to 475±85 ms⁎).