Dispersion Of Action Potential Duration Research Articles

Abstract 202: Liposome-Encapsulated Hemoglobin (HbV) Rescues Hemorrhagic Shock Heart Through Anti-Ischemic and Anti-Arrhythmogenic Effects on Myocardium in Repetitive 65% Bleeding Rat Model

Phase I clinical trial of Liposome-encapsulated hemoglobin vesicles (HbVs) was successfully accomplished. Since several ameliorating mechanisms are not fully understood on HbVs, to investigate its mechanisms of salutary effect on myocardium, optical mapping analysis (OMP) and electrophysiological study (EPS), immunostaining pathological examination for HIF1-alpha in heart and measurement of blood troponin I (TnI) were performed in 30% spontaneous arterial bleedings five times up to approximately total 65% hemorrhage in rats. Eighteen rats in each group were resuscitated by immediately after bleeding by same volume as bleedings each five times by transvenous infusion of 5%albumin solution (ALB-group), washed erythrocytes (wRBC-group) and HbVs (HbV-group). Survival effects over 24 hours were examined. After excising heart, OMP and EPS were performed. All rats died in ALB-group, whereas all survived in wRBC-group and HbV-group. OMP showed impaired (prolonged) action potential duration dispersion (APDd: which is arrhythmogenic index) in LV in ALB-group, whereas APDds were attenuated in HbV-group and wRBC-group (26±4 vs. 11±6, 14±5ms, P<0.05). Lethal arrhythmias (ventricular tachycardia [VT]/ventricular fibrillation [VF]; VT/VF) were provoked by EPS and/or spontaneously occurred in ALB-group whereas not in both wRBC-group and HbV-group. HIF1-alpha was positively stained and TnI levels were significantly elevated only in ALB-group (Peak TnI; 186±47 vs. 64±25, 91±24, 44±10pg/mL, P<0.05: Figure). Conclusions: HbVs rescue lethal hemorrhage through improving lethal arrhythmias and myocardial damages by reflected by inducing HIF1-alpha and elevated TnI. HbVs could be useful for resuscitation and saving life from gun-shot arterial bleeding on-site occurred in terrorisms.

Spatio-temporal repolarization dispersion before and after exercise in patients with long QT syndrome type 1 vs controls - probing into the arrhythmia substrate.

Congenital long QT syndrome (LQTS) carries an increased risk for syncope and sudden death. QT prolongation promotes ventricular extrasystoles which in the presence of an arrhythmia substrate might trigger ventricular tachycardia degenerating into fibrillation. Increased electrical heterogeneity (dispersion) is the suggested arrhythmia substrate in LQTS. In the most common subtype LQT1, physical exercise predisposes for arrhythmia and spatio-temporal dispersion was therefore studied in this context. Thirty-seven patients (57% on beta-blockers) and 37 healthy controls (mean age 31 vs 35, range 6-68 vs 6-72 years) performed an exercise test. Frank vectorcardiography was used to assess spatio-temporal dispersion as Tamplitude, Tarea, the ventricular gradient (VG), and the Tpeak-end interval from 10s-signal averages before and 7±2 min after exercise; during exercise too much signal disturbance excluded analysis. Baseline and maximum heart rates as well as estimated exercise intensity were similar, but heart rate recovery was slower in patients. At baseline QT and heart rate corrected QT (QTcB) were significantly longer in patients (as expected), while dispersion parameters were numerically larger in controls. After exercise QTpeakcB and Tpeak-endcB increased significantly more in patients (18±23 vs 7±10ms and 12±17 vs 2±6ms; p<0.001 and p<0.01). There was, however, no difference in the change in Tamplitude,Tarea and VG between groups. In conclusion, while temporal dispersion of repolarization increased significantly more after exercise in LQT1 patients, there were no signs of exercise-induced increase in global dispersion of action potential duration and morphology. The arrhythmia substrate/mechanism in LQT1 warrants further study.

Liposome-Encapsulated Hemoglobin Vesicle Improves Persistent Anti-arrhythmogenesis through Improving Myocardial Electrical Remodeling and Modulating Cardiac Autonomic Activity in a Hemorrhagic Shock-Induced Rat Heart Model

Shock heart syndrome (SHS) is associated with lethal arrhythmias (ventricular tachycardia/ventricular fibrillation, VT/VF). We investigated whether liposome-encapsulated human hemoglobin vesicles (HbVs) has comparable persistent efficacy to washed red blood cells (wRBCs) for improving arrhythmogenesis in the subacute to chronic phase of SHS. Optical mapping analysis (OMP), electrophysiological study (EPS), and pathological examinations were performed on blood samples from Sprague-Dawley rats following induction of hemorrhagic shock. After hemorrhagic shock, the rats were immediately resuscitated by transfusing 5% albumin (ALB), HbV, or wRBCs. All rats survived for 1 week. OMP and EPS were performed on Langendorff-perfused hearts. Spontaneous arrhythmias and heart rate variability (HRV) were evaluated using awake 24-h telemetry, cardiac function by echocardiography, and pathological examination of Connexin43. OMP showed significantly impaired action potential duration dispersion (APDd) in the left ventricle (LV) in the ALB group whereas APDd was substantially preserved in the HbV and wRBCs groups. Sustained VT/VF was easily provoked by EPS in the ALB group. No VT/VF was induced in the HbV and wRBCs groups. HRV, spontaneous arrhythmias, and cardiac function were preserved in the HbV and wRBCs groups. Pathology showed myocardial cell damage and Connexin43 degradation in the ALB group, all of which were attenuated in the HbV and wRBCs groups. LV remodeling after hemorrhagic shock caused VT/VF in the presence of impaired APDd. Similar to wRBCs, HbV persistently prevented VT/VF by inhibiting persistent electrical remodeling, preserving myocardial structures, and ameliorating arrhythmogenic modifying factors in the subacute to chronic phase of hemorrhagic shock-induced SHS.

Effects of Cardiac Stem Cell on Postinfarction Arrhythmogenic Substrate.

Clinical data suggest that cardiosphere-derived cells (CDCs) could modify post-infarction scar and ventricular remodeling and reduce the incidence of ventricular tachycardia (VT). This paper assesses the effect of CDCs on VT substrate in a pig model of postinfarction monomorphic VT. We studied the effect of CDCs on the electrophysiological properties and histological structure of dense scar and heterogeneous tissue (HT). Optical mapping and histological evaluation were performed 16 weeks after the induction of a myocardial infarction by transient occlusion of the left anterior descending (LAD) artery in 21 pigs. Four weeks after LAD occlusion, pigs were randomized to receive intracoronary plus trans-myocardial CDCs (IC+TM group, n: 10) or to a control group. Optical mapping (OM) showed an action potential duration (APD) gradient between HT and normal tissue in both groups. CDCs increased conduction velocity (53 ± 5 vs. 45 ± 6 cm/s, p < 0.01), prolonged APD (280 ± 30 ms vs. 220 ± 40 ms, p < 0.01) and decreased APD dispersion in the HT. During OM, a VT was induced in one and seven of the IC+TM and control hearts (p = 0.03), respectively; five of these VTs had their critical isthmus located in intra-scar HT found adjacent to the coronary arteries. Histological evaluation of HT revealed less fibrosis (p < 0.01), lower density of myofibroblasts (p = 0.001), and higher density of connexin-43 in the IC+TM group. Scar and left ventricular volumes did not show differences between groups. Allogeneic CDCs early after myocardial infarction can modify the structure and electrophysiology of post-infarction scar. These findings pave the way for novel therapeutic properties of CDCs.

Abstract 12847: Atrial Fibrillation and Atrial Flutter: What Are the Substrate Differences Seen in 3D Panoramic Optical Mapping?

Background: Panoramic optical mapping (POM) is an effective tool for 3D visualization and analysis of action potential duration (APD) heterogeneity, a key driver for initiation and persistence of reentry. POM may elucidate cardiac electrophysiological predisposition to develop chaotic arrhythmia such as atrial fibrillation (AF) or organized arrhythmia such as atrial flutter (AFL). Objective: To determine the APD differences in AF and AFL in a transgenic (TG) murine model. Methods: We performed high resolution POM with 4 simultaneous CMOS cameras on 14 Langendorff perfused TG murine hearts (SCN5A mutation at lidocaine binding site F1759A) with spontaneous AF and AFL. We generated serial 2D activation and APD maps in normal sinus rhythm (NSR), while phase movies were created in AF/AFL. All optical maps were then reconstructed in 3D. We averaged the APD80 and APD dispersion values over each atrial chamber and compared the mean differences: i) within AF and AFL across right atrium (RA) and left atrium (LA); and ii) within RA and LA across AF and AFL. Results: AF (n=7) and AFL (n=7) were present at baseline (8/14) or induced by 10Hz burst pacing (6/14). Mean APD80 in RA/LA was: 57.03 ± 4.66 / 50.93± 3.38ms (p&lt;0.05) in AF vs 77.18 ± 4.30 / 58.25 ± 3.88ms (p&lt; 0.01) in AFL, and mean dispersion: 6.23 ± 1.04/ 7.25± 1.71ms in AF vs 5.49 ± 0.80/ 8.42 ± 1.15ms (p&lt;0.01) in AFL. High mean APD80 in the RA with low intra-atrial dispersion in the RA lead to AFL rotational reentry cores, while lower overall APD80 values with high intra-atrial dispersion in the LA resulted in the presence of AF phase singularity points. Conclusion: Development of AF was associated with high intra-atrial APD dispersion in NSR, whereas AFL was associated with high inter-atrial APD dispersion in NSR (1.4-fold higher mean APD80 in the RA). Certain atrial APD dispersion patterns observed in NSR are associated with development of chaotic vs. organized atrial arrhythmias.

Multimodal imaging shows fibrosis architecture and action potential dispersion are predictors of arrhythmic risk in spontaneous hypertensive rats.

Hypertensive heart disease (HHD) increases risk of ventricular tachycardia (VT) and ventricular fibrillation (VF). The roles of structural vs. electrophysiological remodelling and age vs. disease progression are not fully understood. This cross‐sectional study of cardiac alterations through HHD investigates mechanistic contributions to VT/VF risk. Risk was electrically assessed in Langendorff‐perfused, spontaneously hypertensive rat hearts at 6, 12 and 18 months, and paced optical membrane voltage maps were acquired from the left ventricular (LV) free wall epicardium. Distributions of LV patchy fibrosis and 3D cellular architecture in representative anterior LV mid‐wall regions were quantified from macroscopic and microscopic fluorescence images of optically cleared tissue. Imaging showed increased fibrosis from 6 months, particularly in the inner LV free wall. Myocyte cross‐section increased at 12 months, while inter‐myocyte connections reduced markedly with fibrosis. Conduction velocity decreased from 12 months, especially transverse to the myofibre direction, with rate‐dependent anisotropy at 12 and 18 months, but not earlier. Action potential duration (APD) increased when clustered by age, as did APD dispersion at 12 and 18 months. Among 10 structural, functional and age variables, the most reliably linked were VT/VF risk, general LV fibrosis, a measure quantifying patchy fibrosis, and non‐age clustered APD dispersion. VT/VF risk related to a quantified measure of patchy fibrosis, but age did not factor strongly. The findings are consistent with the notion that VT/VF risk is associated with rate‐dependent repolarization heterogeneity caused by structural remodelling and reduced lateral electrical coupling between LV myocytes, providing a substrate for heterogeneous intramural activation as HHD progresses. Key points There is heightened arrhythmic risk with progression of hypertensive heart disease.Risk is related to increasing left ventricular fibrosis, but the nature of this relationship has not been quantified.This study is a novel systematic characterization of changes in active electrical properties and fibrotic remodelling during progression of hypertensive heart disease in a well‐established animal disease model.Arrhythmic risk is predicted by several left ventricular measures, in particular fibrosis quantity and structure, and epicardial action potential duration dispersion.Age alone is not a good predictor of risk.An improved understanding of links between arrhythmic risk and fibrotic architectures in progressive hypertensive heart disease aids better interpretation of late gadolinium‐enhanced cardiac magnetic resonance imaging and electrical mapping signals.

Neuronal nitric oxide synthase inhibition exacerbates atrial electrical remodeling via small-conductance Ca2+-activated K+ channel activation

Abstract Funding Acknowledgements Type of funding sources: Public Institution(s). Main funding source(s): The Japan Society for the Promotion of Science KAKENHI Background The presence of atrial fibrillation (AF) is associated with electrical remodeling processes that promote a substrate for the maintenance of AF itself. Small conductance Ca2+-activated K+ (SK) channels; K+-selective and voltage-independent ion channels are key factors in the atrial electrical remodeling (2, 3). However, the mechanism of its activation remains unclear. A recent study showed that neuronal nitric oxide synthase (nNOS) expression and activity are reduced in AF patients and that nNOS depletion causes the abbreviation of action potential duration (APD), leading to increased AF inducibility in animal experiments (4). Decreased NO production, especially driven by nNOS inhibition, might play a key role in the atrial electrical remodeling, and the downstream alteration of SK channels might result from this process. Purpose We aimed to evaluate the potential of SK channel blocking to mitigate abnormal electrophysiological properties and the inducibility of atrial tachyarrhythmia (ATA) which was induced by nNOS depletion, and to describe the related mechanism. Methods Atrial tachyarrhythmia induction and optical mapping were performed in perfused rat hearts. nNOS was pharmacologically inhibited by S-methylthiocitrulline (SMTC, 100 nM). The influence of the SK channel was examined by a specific channel inhibitor, apamin (100 nM). APD, conduction velocity, and calcium transient (CaT) parameters (CaTD, rise time, time to 50% decay, and tau) were evaluated by voltage and calcium dual optical mapping. Dominant frequency was evaluated to analyze the wave dynamics of AF. Results SMTC increased the inducibility of ATA and apamin mitigated the nNOS inhibition-induced arrhythmogenicity (0% [0/7] vs 62% [8/13] vs 15% [2/13] in control, SMTC and SMTC + apamin). SMTC caused the abbreviation and enhanced spatial dispersion of APD, which were reversed by apamin. In contrast, conduction velocity was not affected by SMTC or apamin. Moreover, apamin reduced the dominant frequency of SMTC-induced ATA. In voltage and calcium optical mapping, STMC and apamin did not alter the parameters associated with CaT, however, SMTC caused the abbreviation of APD, which was reversed by apamin (APD80: 48.4 ± 2.7 msec in control group, 30.8 ± 1.5 msec in SMTC group, p &amp;lt;0.0001 vs. control, 41.8 ± 1.5 msec in SMTC + apamin, p = 0.01 vs. SMTC by One-way ANOVA and Tukey’s multiple-comparison test) (Figure A-C). Conclusion Acute nNOS inhibition abbreviated APD via activating SK channels. A specific SK channel blocker mitigated APD abbreviation without the alteration of CaT, implying an underlying mechanism of post translational modification of SK channels.

PO-705-01 ACTION POTENTIAL RESTITUTION CURVES OBTAINED FROM FULL EXPLANTED HUMAN HEARTS

Much of dynamics and stability of electrical waves in the heart is attributed to action potential restitution curve. These curves have been used to investigate how changes in periods of stimulation can lead to the induction of alternans and the initiation of electrically driven arrhythmias. To date, there have been only obtained a few restitution curves from clinical measurements in human hearts using MAPs, or from single human cardiac cells. To obtain the first in tissue action potential duration (APD) restitution curves across large areas of human cardiac tissue. We studied three arterially-perfused whole explanted human hearts (two from recipient patient and one from a rejected donor) using simultaneous voltage and calcium optical mapping at high spatial and temporal resolutions (256x256 pixels, at 500 Hz) and a large field of view of 6-10 cm2. Voltage was measured across the anterior epicardial RV and LV tissue and the endocardium LV, paced at cycle lengths from 3 second down up to conduction block or induction of VF. We use optical mapping to obtain the voltage signal from a large region of the epicardium and quantify the action potential from each pixel (Panel A) to obtain their APD as a function of cycle length (Panel B) across space (as large as 10cm2). We quantify in detail, for the first time, the bifurcation to period doubling (the period of stimulation at which alternans develops) in human hearts across space which appear at or below 600ms (Panel C). For each pacing cycle length we quantify the values of APD dispersion across the tissue and how much they shift during alternans, between even and odd beats as in Panel D, which shows the shortest period of stimulation before VF induction (346ms for heart 1). APD dispersion across the tissue can vary by up to 100 ms and shift the majority of the values by up to 20ms. Optical mapping of explanted hearts is a powerful tool to quantify the dispersion of voltage and action potential duration across tissue. We present the first quantification of the action potential restitution curve for a large range of period of stimulation including the initiation and window of periods for alternans of action potential which is associated with the development of T-wave alternans.

PO-630-05 A NOVEL MECHANISM OF SMALL-CONDUCTANCE CA2+-ACTIVATED K+ CHANNEL ACTIVATION VIA NEURONAL NITRIC OXIDE SYNTHASE INHIBITION IN RAT ATRIA

The presence of atrial fibrillation (AF) is associated with electrical remodeling processes that promote a substrate for the maintenance of AF itself. Small-conductance Ca2+-activated K+ (SK) channel is a key factor in the atrial electrical remodeling. However, the mechanism of its activation is unclear. A recent study showed that neuronal nitric oxide synthase (nNOS) expression and activity are reduced in AF patients and that nNOS depletion causes the abbreviation of APD, leading to increased AF vulnerability in animals. Decreased NO production, especially driven by nNOS inhibition, might play a key role in the atrial electrical remodeling, and the downstream alteration of SK channels might result from this process. We aimed to evaluate the potential of SK channel blocking to mitigate abnormal electrophysiological properties and the inducibility of atrial tachyarrhythmia which was induced by neuronal nitric oxide synthase (nNOS) depletion, and to describe the related mechanism. Atrial tachyarrhythmia (ATA) induction and optical mapping were performed on perfused rat hearts. nNOS was pharmacologically inhibited by S-methylthiocitrulline (SMTC, 100nM). The influence of the SK channel was examined by a specific channel inhibitor, apamin (100nM). Action potential duration (APD), conduction velocity, and calcium transient (CaT) parameters (CaTD, Rise time, T 1/2, Tau) were evaluated by voltage and calcium optical mapping. Dominant frequency was examined in the analysis of AF dynamics. SMTC increased the inducibility of ATA and apamin mitigated the nNOS inhibition-induced arrhythmogenicity. SMTC caused the abbreviation and enhanced spatial dispersion of APD, which was reversed by apamin. In contrast, conduction velocity was not affected by SMTC or apamin. Moreover, apamin reduced the dominant frequency of SMTC-induced ATA. In voltage and calcium optical mapping, STMC and apamin does not altered parameters associated with CaT, however, SMTC caused abbreviation APD, which was reversed by apamin. Acute nNOS inhibition abbreviated APD via activating SK channels. A specific SK channel blocker mitigated APD abbreviation without alteration of CaT, implying an underlying mechanism of post translational modification of SK channels.

Methodology for Cross-Talk Elimination in Simultaneous Voltage and Calcium Optical Mapping Measurements With Semasbestic Wavelengths.

Most cardiac arrhythmias at the whole heart level result from alteration of cell membrane ionic channels and intracellular calcium concentration ([Ca2+]i) cycling with emerging spatiotemporal behavior through tissue-level coupling. For example, dynamically induced spatial dispersion of action potential duration, QT prolongation, and alternans are clinical markers for arrhythmia susceptibility in regular and heart-failure patients that originate due to changes of the transmembrane voltage (Vm) and [Ca2+]i. We present an optical-mapping methodology that permits simultaneous measurements of the Vm - [Ca2+]i signals using a single-camera without cross-talk, allowing quantitative characterization of favorable/adverse cell and tissue dynamical effects occurring from remodeling and/or drugs in heart failure. We demonstrate theoretically and experimentally in six different species the existence of a family of excitation wavelengths, we termed semasbestic, that give no change in signal for one dye, and thus can be used to record signals from another dye, guaranteeing zero cross-talk.

Rotors anchored by refractory islands drive torsades de pointes in an experimental model of electrical storm.

Electrical storm (ES) is a life-threatening emergency in patients at high risk of ventricular tachycardia/ventricular fibrillation (VF), but the pathophysiology and molecular basis are poorly understood. The purpose of this study was to explore the electrophysiological substrate for experimental ES. A model was created by inducing chronic complete atrioventricular block in defibrillator-implanted rabbits, which recapitulates QT prolongation, torsades des pointes (TdP), and VF episodes. Optical mapping revealed island-like regions with action potential duration (APD) prolongation in the left ventricle, leading to increased spatial APD dispersion, in rabbits with ES (defined as ≥3 VF episodes/24 h). The maximum APD and its dispersion correlated with the total number of VF episodes invivo. TdP was initiated by an ectopic beat that failed to enter the island and formed a reentrant wave and perpetuated by rotors whose centers swirled in the periphery of the island. Epinephrine exacerbated the island by prolonging APD and enhancing APD dispersion, which was less evident after late Na+ current blockade with 10 μM ranolazine. Nonsustained ventricular tachycardia in a non-ES rabbit heart with homogeneous APD prolongation resulted from multiple foci with an electrocardiographic morphology different from TdP driven by drifting rotors in ES rabbit hearts. The neuronal Na+-channel subunit NaV1.8 was upregulated in ES rabbit left ventricular tissues and expressed within the myocardium corresponding to the island location in optically mapped ES rabbit hearts. The NaV1.8 blocker A-803467 (10 mg/kg, intravenously) attenuated QT prolongation and suppressed epinephrine-evoked TdP. A tissue island with enhanced refractoriness contributes to the generation of drifting rotors that underlies ES in this model. NaV1.8-mediated late Na+ current merits further investigation as a contributor to the substrate for ES.

Intraosseous infusion of liposome-encapsulated hemoglobin (HbV) acutely prevents hemorrhagic anemia-induced lethal arrhythmias, and its efficacy persists with preventing proarrhythmic side effects in the subacute phase of severe hemodilution model.

Artificial oxygen carriers (HbV) can treat hemorrhagic shock with lethal arrhythmias (VT/VF). No reports exist on subacute HbV's effects. Acute and subacute resuscitation effects with anti-arrhythmogenesis of HbV were studied in 85% blood exchange rat model (85%-Model). Lethal 85%-Model was created by bone marrow transfusion and femoral artery bleeding in 80 SD rats in HbV-administered group (HbV-group), washed erythrocyte-administered group (wRBC-group), and 5% albumin-administered group (ALB-group). Survival rates, anti-arrhythmic efficacy by optical mapping system (OMP) with electrophysiological study (EPS) in Langendorff heart, cardiac autonomic activity by heart rate variability (HRV) and ventricular arrhythmias by 24-h electrocardiogram telemetry monitoring (24h-ECG) in awake, and left ventricular function by echocardiography (left ventricular ejection fraction [LVEF]) were measured. All rats in HbV- and wRBC-groups survived for 4 weeks, whereas no rats in ALB-group. HbV and wRBC acutely suppressed VT/VF in Langendorff heart through ameliorating action potential duration dispersion (APDd) analyzed by OMP with EPS. For subacute analysis, 50% blood exchange by 5% albumin was used (ALB-group 50). Subacute salutary effect on APDd and VT/VF inducibility was confirmed in HbV- and wRBC-groups. 24h-ECG showed that HbV and wRBC suppressed none-sustained ventricular tachycardia (NSVT) and sympathetic component of HRV (LF/HF) with preserved LVEF (HbV-group, wRBC-group vs. ALB-group 50; NSVT numbers/days, 0.5±0.3, 0.4±0.3 vs. 3.9±1.2*; LF/HF, 1.1±0.2, 0.8±0.2 vs. 3.5±1.0*; LVEF, 84±5, 83±4, vs. 77±4%*; *p<0.05). Collectively, HbV has sustained antiarrhythmic effect in subacute 85%-Model by ameliorating electrical remodeling and improving arrhythmogenic modifying factors (HRV and LVEF). These findings are useful in now continuing clinical trials of HbV.

Not all Long-QTs Are The Same, Proarrhytmic Quantification with Action Potential Triangulation and Alternans.

Long-QT is commonly associated with an increased risk of polymorphic ventricular tachycardia from drug therapy. However, not all drugs prolonging QT interval are proarrhythmic. This study aimed to characterize cellular and tissue mechanisms under which QT-interval prolonging drugs and their combination are proarrhythmic, examining arrhythmia susceptibility due to action potential (AP) triangulation and spatial dispersion of action potential duration (APD). Additionally, we aimed to elucidate that Torsades de Pointe (TdP) associated with long-QT are not necessarily caused by early-after-depolarization (EADs) but are related to the presence of AP alternans in both time and space. Isolated Guinea Pig hearts were Langendorff perfused, and optical mapping was done with a voltage dye-sensitive dye. Two commonly used drugs at the beginning of the COVID-19 pandemic, hydroxychloroquine (HCQ) and Azithromycin (AZM), were added to study the effects of QT interval prolongation. Alternans in time and space were characterized by performing restitution pacing protocols. Comparing APs, HCQ prolongs APD during phase-III repolarization, resulting in a higher triangulation ratio than AZM alone or AZM combined with HCQ. Lower triangulation ratios with AZM are associated with phase-II prolongation, lower arrhythmia, and lower incidence of spatially discordant alternans.

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.

B-011-05 PANORAMIC OPTICAL MAPPING AND MICRO COMPUTED TOMOGRAPHY FOR IN-DEPTH 3D CHARACTERIZATION OF THE ARRHYTHMOGENIC SUBSTRATE IN MURINE HEARTS

Optical mapping of murine hearts is currently limited to the epicardium. Action potential duration (APD) heterogeneity is a key driver for initiation and persistence of reentry. When combined with micro CT, optical mapping can provide data on anatomical alterations and structural remodeling contributing to arrhythmogenesis. In this study, we developed a 3D visualization tool combining panoramic optical mapping (POM) with a convolutional neural network automatically segmenting the heart anatomy on micro CT for improved computational modeling and arrhythmia risk stratification in mice ex vivo. We performed high resolution POM with 4 simultaneous CMOS cameras on 17 Langendorff murine hearts: 7 control, 4 transaortic constriction (TAC), 5 transgenic (TG) SCN5A mutation at lidocaine binding site F1759A with spontaneous VF and 1 post LAD ligation. Activation and APD maps were projected onto 3D reconstructed cardiac silhouettes. We measured APD dispersion and the % of ventricular tissue exhibiting dispersion > 10 ms was analyzed. We then coregistered and overlaid the functional maps onto the reconstructed structural meshes obtained with the deep learning algorithm. The mean % of ventricular tissue with APD dispersion > 10 ms was significantly different (ANOVA, p<0.0001) across control, TAC, TG and LAD mice (0.6 ± 0.4 %, 4.8 ± 0.5 %, 12.9 ± 1.4 % and 13.9 %). Myocardial thickness on the TAC micro CT depicted LV hypertrophy, correlating with increased dispersion on POM. POM merged with micro CT allows us to visualize how anatomical markers correlate with arrhythmia mechanisms and thus offers improved understanding of disease progression.

Liposome-encapsulated hemoglobin (HbV) transfusion rescues rats undergoing progressive lethal 85% hemorrhage as a result of an anti-arrhythmogenic effect on the myocardium.

Liposome-encapsulated hemoglobin vesicles (HbV) can serve as a blood substitute with oxygen-carrying capacity comparable to that of human blood and lethal hemorrhage is associated with lethal arrhythmias. To investigate the resuscitation effect of HbV on lethal hemorrhage and anti-arrhythmogenesis, we performed optical mapping analysis (OMP) and electrophysiological study (EPS) in graded blood exchange (85% blood loss) in the rat model. We also measured cardiac autonomic activity, as assessed by heart rate variability (HRV), and changes in plasma norepinephrine and left ventricle ejection fraction (LVEF) by echocardiography. Pathological study on Connexin43 was performed. A 5% albumin (ALB group), washed rat erythrocytes (wRBC group), and HbV (HbV group) were used as a resuscitation fluid. The survival effects over 24hours were examined. All rats died in the ALB group, whereas almost all survived for 24-hours period in wRBC and HbV groups. OMP showed impaired action potential duration dispersion (APDd) in the ALB group, whereas normal APDs in HbV and wRBC groups. Lethal arrhythmias were induced by EPS in the ALB group, but not in wRBC and HbV groups. HRV indices, LVEF, Connexin43 were preserved in HbV and wRBC groups. Lethal hemorrhage causes lethal arrhythmias in the presence of impaired APDd. HbV acutely rescues lethal hemorrhage by preventing lethal arrhythmias and preserving arrhythmogenic factors.

In silico investigation of pro-arrhythmic effects of azithromycin on the human ventricle

The macrolide antibiotic azithromycin (AZM) is widely used for respiratory infections and has been suggested to be a possible treatment for the Coronavirus Disease of 2019 (COVID-19). However, AZM-associated QT interval prolongation and arrhythmias have been reported. Integrated mechanistic information on AZM actions on human ventricular excitation and conduction is lacking. Therefore, this study was undertaken to investigate the actions of AZM on ventricular cell and tissue electrical activity. The O'Hara- Virag-Varro-Rudy dynamic (ORd) model of human ventricular cells was modified to incorporate experimental data on the concentration-dependent actions of AZM on multiple ion channels, including INa, ICaL, IKr, IKs, IK1 and INaL in both acute and chronic exposure conditions. In the single cell model, AZM prolonged the action potential duration (APD) in a concentration-dependent manner, which was predominantly attributable to IKr reduction in the acute condition and potentiated INaL in the chronic condition. High concentrations of AZM also increased action potential (AP) triangulation (determined as an increased difference between APD30 and APD90) which is a marker of arrhythmia risk. In the chronic condition, the potentiated INaL caused a modest intracellular Na + concentration accumulation at fast pacing rates. At the 1D tissue level, the AZM-prolonged APD at the cellular level was reflected by an increased QT interval in the simulated pseudo-ECG, consistent with clinical observations. Additionally, AZM reduced the conduction velocity (CV) of APs in the acute condition due to a reduced INa, and it augmented the transmural APD dispersion of the ventricular tissue, which is also pro-arrhythmic. Such actions were markedly augmented when the effects of chronic exposure of AZM were also considered, or with additional IKr block, as may occur with concurrent use of other medications. This study provides insights into the ionic mechanisms by which high concentrations of AZM may modulate ventricular electrophysiology and susceptibility to arrhythmia.

Parameter variations in personalized electrophysiological models of human heart ventricles.

The objectives of this study were to evaluate the accuracy of personalized numerical simulations of the electrical activity in human ventricles by comparing simulated electrocardiograms (ECGs) with real patients' ECGs and analyzing the sensitivity of the model output to variations in the model parameters. We used standard 12-lead ECGs and up to 224 unipolar body-surface ECGs to record three patients with cardiac resynchronization therapy devices and three patients with focal ventricular tachycardia. Patient-tailored geometrical models of the ventricles, atria, large vessels, liver, and spine were created using computed tomography data. Ten cases of focal ventricular activation were simulated using the bidomain model and the TNNP 2006 cellular model. The population-based values of electrical conductivities and other model parameters were used for accuracy analysis, and their variations were used for sensitivity analysis. The mean correlation coefficient between the simulated and real ECGs varied significantly (from r = 0.29 to r = 0.86) among the simulated cases. A strong mean correlation (r > 0.7) was found in eight of the ten model cases. The accuracy of the ECG simulation varied widely in the same patient depending on the localization of the excitation origin. The sensitivity analysis revealed that variations in the anisotropy ratio, blood conductivity, and cellular apicobasal heterogeneity had the strongest influence on transmembrane potential, while variation in lung conductivity had the greatest influence on body-surface ECGs. Futhermore, the anisotropy ratio predominantly affected the latest activation time and repolarization time dispersion, while the cellular apicobasal heterogeneity mainly affected the dispersion of action potential duration, and variation in lung conductivity mainly led to changes in the amplitudes of ECGs and cardiac electrograms. We also found that the effects of certain parameter variations had specific regional patterns on the cardiac and body surfaces. These observations are useful for further developing personalized cardiac models.

Calibration of single-cell model parameters based on membrane resistance improves the accuracy of cardiac tissue simulations

In the calibration process, replicating some cellular properties is the main focus, while the importance of membrane resistance (Rm) that is primal in the tissue-level modeling is often underestimated. Previously, we presented a framework in which Rm in addition to action potential (AP) waveform was considered in the cellular model fitting. In this paper, we test the hypothesis that this approach for tuning cellular model parameters improves the accuracy of simulations at the tissue level. In doing so, two different sets of single-cell models are generated via independent realizations of our multi-objective optimization approach. In the first set of calibration (Model I), root-mean-square error (RMSE) of AP, and absolute error (AE) of maximum upstroke velocity are included as optimization functions; however, in the second set of calibration (Model II), RMSE of Rm curve in the repolarization phase is also added to the optimization functions. The calibrated cell models are then used in several tissue configurations of physiological relevance. We adopt well-defined evaluation metrics to compare tissue models tuned using Models I and II. In the source-sink mismatch configuration, the average absolute relative error (ARE) of the critical transition border, defined as the smallest required window width between source and sink for AP propagation, is less than 4.7 % in Model II, while this error is increased to more than 8.9 % in Model I. In addition, in Model I, the average ARE of total time for activation of tissue is 3.3–6.3 %; however, in Model II, this error is reduced to 0.7–1.6 %. In the Purkinje-myocardium configuration, the average of RMSE of activation time map is reduced approximately 75 % in Model II. Finally, in the transmural APD heterogeneity configuration, the average AREs of AP duration (APD) and APD dispersion (i.e., the difference between maximum and minimum of APD) are about 13.2 % and 17.4 % in Model I and 5.8 % and 6.8 % in Model II, respectively. Overall, our results demonstrate that consideration of Rm in the single-cell optimization procedure yields a substantial improvement in the accuracy of tissue models.

Theoretical Models and Computational Analysis of Action Potential Dispersion for Cardiac Arrhythmia Risk Stratification.

Reentrant cardiac arrhythmias such as atrial fibrillation (AF) and ventricular fibrillation (VF) are common cardiac arrhythmias that account for substantial morbidity and mortality throughout the world. However, the mechanisms and optimal ablation treatment strategies for such arrhythmias are still unclear. Using 2D optical mapping of a mouse model with AF and VF, we have identified regional heterogeneity of the action potential duration (APD) in the atria and ventricles of the heart as key drivers for the initiation and persistence of reentry. The purpose of this paper is to discuss theoretical patterns of dispersion, demonstrate patterns of dispersion seen in our mouse model and discuss the computational analysis of APD dispersion patterns. These analyses and discussions may lead to better understanding of dispersion patterns in patients with these arrhythmias, as well as help comprehend whether and how reducing dispersion can lead to arrhythmia risk stratification and treatment strategies for arrhythmias.