β-adrenergic stimulation augments transmural dispersion of repolarization via modulation of delayed rectifier currents IKs and IKr in the human ventricle

Human sodium current voltage-dependence at physiological temperature measured by coupling a patch-clamp experiment to a mathematical model

Voltage-gated Na+ channels are crucial to action potential propagation in excitable tissues. Because of the high amplitude and rapid activation of the Na+ current, voltage-clamp measurements are very challenging and are usually performed at room temperature. In this study, we measured Na+ current voltage-dependence in stem cell-derived cardiomyocytes at physiological temperature. While the apparent activation and inactivation curves, measured as the dependence of current amplitude on voltage, fall within the range reported in previous studies, we identified a systematic error in our measurements. This error is caused by the deviation of the membrane potential from the command potential of the amplifier. We demonstrate that it is possible to account for this artifact using computer simulation of the patch-clamp experiment. We obtained surprising results through patch-clamp model optimization: a half-activation of -11.5 mV and a half-inactivation of -87 mV. Although the half-activation deviates from previous research, we demonstrate that this estimate reproduces the conduction velocity dependence on extracellular potassium concentration. KEY POINTS: Voltage-gated Na+ currents play a crucial role in excitable tissues including neurons, cardiac and skeletal muscle. Measurement of Na+ current is challenging because of its high amplitude and rapid kinetics, especially at physiological temperature. We have used the patch-clamp technique to measure human Na+ current voltage-dependence in human induced pluripotent stem cell-derived cardiomyocytes. The patch-clamp data were processed by optimization of the model accounting for voltage-clamp experiment artifacts, revealing a large difference between apparent parameters of Na+ current and the results of the optimization. We conclude that actual Na+ current activation is extremely depolarized in comparison to previous studies. The new Na+ current model provides a better understanding of action potential propagation; we demonstrate that it explains propagation in hyperkalaemic conditions.

Ionic current changes underlying action potential repolarization responses to physiological pacing and adrenergic stimulation in adult rat ventricular myocytes

This study aimed to simulate ventricular responses to elevations in myocyte pacing and adrenergic stimulation using a novel electrophysiological rat model and investigate ion channel responses underlying action potential (AP) modulations. Peak ion currents and AP repolarization to 50% and 90% of full repolarization (APD_50-90 ) were recorded during simulations at 1-10 Hz pacing under control and adrenergic stimulation conditions. Further simulations were performed with incremental ion current block (L-type calcium current, I_Ca ; transient outward current, I_to ; slow delayed rectifier potassium current, I_Ks ; rapid delayed rectifier potassium current, I_Kr ; inward rectifier potassium current, I_K1 ) to identify current influence on AP response to exercise. Simulated APD_50-90 closely resembled experimental findings. Rate-dependent increases in I_Ks (6%-101%), I_Kr (141%-1339%), and I_Ca (0%-15%) and reductions in I_to (11%-57%) and I_K1 (1%-9%) were observed. Meanwhile, adrenergic stimulation triggered moderate increases in all currents (23%-67%) except I_K1 . Further analyses suggest AP plateau is most sensitive to modulations in I_to and I_Ca while late repolarization is most sensitive to I_K1 , I_Ca , and I_Ks , with alterations in I_Ks predominantly stimulating the greatest magnitude of influence on late repolarization (35%-846% APD₉₀ prolongation). The modified Leeds rat model (mLR) is capable of accurately modeling APs during physiological stress. This study highlights the importance of I_Ca , I_to , I_K1, and I_Ks in controlling electrophysiological responses to exercise. This work will benefit the study of cardiac dysfunction, arrythmia, and disease, though future physiologically relevant experimental studies and model development are required.

Cardiac Repolarization in Health and Disease

Abnormal cardiac repolarization is at the basis of life-threatening arrhythmias in various congenital and acquired cardiac diseases. Dysfunction of ion channels involved in repolarization at the cellular level are often the underlying cause of the repolarization abnormality. The expression pattern of the gene encoding the affected ion channel dictates its impact on the shape of the T-wave and duration of the QT interval, thereby setting the stage for both the occurrence of the trigger and the substrate for maintenance of the arrhythmia. Here we discuss how research into the genetic and electrophysiological basis of repolarization has provided us with insights into cardiac repolarization in health and disease and how this in turn may provide the basis for future improved patient-specific management.

Remodelling of potassium currents underlies arrhythmic action potential prolongation under beta-adrenergic stimulation in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) patients often present an enhanced arrhythmogenicity that can lead to lethal arrhythmias, especially during exercise. Recent studies have indicated an abnormal response of HCM cardiomyocytes to β-adrenergic receptor stimulation (β-ARS), with prolongation of their action potential rather than shortening. The mechanisms underlying this aberrant response to sympathetic stimulation and its possible proarrhythmic role remain unknown. The aims of this study are to investigate the key ionic mechanisms underlying the HCM abnormal response to β-ARS and the resultant repolarisation abnormalities using human-based experimental and computational methodologies. We integrated and calibrated the latest models of human ventricular electrophysiology and β-ARS using experimental measurements of human adult cardiomyocytes from control and HCM patients. Our major findings include: (1) the developed in silico models of β-ARS capture the behaviour observed in the experimental data, including the aberrant response of HCM cardiomyocytes to β-ARS; (2) the reduced increase of potassium currents under β-ARS was identified as the main mechanism of action potential prolongation in HCM, rather than a more sustained inward calcium current; (3) action potential duration differences between healthy and HCM cardiomyocytes were increased upon β-ARS, while endocardial to epicardial differences in HCM cardiomyocytes were reduced; (4) models presenting repolarisation abnormalities were characterised by downregulation of the rapid delayed rectifier potassium current and the sodium‑potassium pump, while inward currents were upregulated. In conclusion, our results identify causal relationships between the HCM phenotype and its arrhythmogenic response to β-ARS through the downregulation of potassium currents.

IKs inhibitor JNJ303 prolongs the QT interval and perpetuates arrhythmia when combined with enhanced inotropy in the CAVB dog

INTRODUCTION

Impaired I_Ks induced by drugs or due to a KCNQ1 mutation, diagnosed as long QT syndrome type 1 (LQT1) prolongs the QT interval and predisposes the heart to Torsade de Pointes (TdP) arrhythmias. The anesthetized chronic AV block (CAVB) dog is inducible for TdP after remodeling and I_Kr inhibitor dofetilide. We tested the proarrhythmic effect of I_Ks inhibition in the CAVB dog, and the proarrhythmic role of increased contractility herein.

METHODS

Dofetilide-inducible animals were included to test the proarrhythmic effect of 1) I_Ks inhibition by JNJ303 (0.63 mg/kg/10min i.v.; n = 4), 2) I_Ks inhibition combined with enhanced inotropy (ouabain, 0.045 mg/kg/1min i.v.; n = 6), and 3) the washout period of the anesthetic regime (n = 10).

RESULTS

JNJ303 prolonged the QTc interval (from 477 ± 53 ms to 565 ± 14 ms, P < 0.02) resembling standardized dofetilide-induced QTc prolongation. Single ectopic beats (n = 4) and ventricular tachycardia (VT) (n = 3) were present, increasing the arrhythmia score (AS) from 1.0 ± 0 to 7.1 ± 6.5. JNJ303 combined with ouabain increased contractile parameters (LVdP/dt_max from 1725 ± 273 to 4147 ± 611 mmHg/s, P < 0.01). Moreover, TdP arrhythmias were induced in 4/6 dogs and AS increased from 1.0 ± 0 to 20.2 ± 19.0 after JNJ303 and ouabain (P < 0.05). Finally, TdP arrhythmias were induced in 4/10 dogs during the anesthesia washout period and the AS increased from 1.1 ± 0.3 to 9.2 ± 11.2.

CONCLUSION

Mimicking LQT1 using I_Ks inhibitor JNJ303 prolongs the QTc interval and triggers ectopic beats and non-sustained VT in the CAVB dog. Induction of the more severe arrhythmic events (TdP) demands a combination of I_Ks inhibition with enhanced inotropy or ending the anesthetic regime.

Effects of beta-blockers on ventricular repolarization documented by 24-h electrocardiography in long-QT syndrome type 2

BACKGROUND

Long QT syndrome (LQTS) is an inherited arrhythmia disorder characterized by ventricular repolarization abnormalities and a risk of sudden cardiac death. The electrophysiological components generating the high risk of arrhythmias in LQTS are prolonged repolarization, increased dispersion of repolarization, and early afterdepolarizations, which are clinically estimated as QT interval, T-wave peak to end (TPE) interval, and T2/T1-wave amplitude ratio, respectively. In experimental LQTS type 2 (LQT2) models, beta-blockers decrease dispersion of repolarization and prevent early afterdepolarizations. In clinical studies among LQT2 patients beta-blockers are more effective against exercise-induced than arousal-induced cardiac events.

OBJECTIVES AND METHODS

The aim of the study was to investigate the effects of beta-blocker therapy on QT and TPE intervals, and maximal T2/T1-wave amplitude ratios recorded by 24-h electrocardiograms (ECG) among 25 LQT2 patients.

RESULTS

Beta-blocker therapy decreased the maximal T2/T1-wave amplitude ratio from 2.9±1.1 to 1.8±0.7 (p<0.001), but did not change pause-induced T2/T1-wave amplitude ratio. Under medication abrupt maximal TPE intervals were shorter at heart rates of 75 beats/min or over, and maximal QT intervals were shorter at a heart rate of 100 beats/min.

CONCLUSIONS

Beta-blockers stabilize ventricular repolarization in LQT2 by reducing electrocardiographic early afterdepolarizations, and by reducing abrupt prolongation of electrocardiographic dispersion of repolarization and ventricular repolarization duration at elevated heart rates. The effect of beta-blockers on pause-induced electrocardiographic early afterdepolarizations is weak. The findings provide electrocardiographic explanation for the protective effects of beta-blockers against exercise-induced cardiac events in LQT2.

Action potential responses to changes in stimulation frequency and isoproterenol in rat ventricular myocytes

PURPOSE

Current understanding of ventricular action potential adaptation to physiological stress is generally based on protocols using non-physiological rates and conditions isolating rate effects from escalating adrenergic stimulation. To permit refined understanding, ventricular action potentials were assessed across physiological pacing frequencies in the presence and absence of adrenergic stimuli. Isolated and combined effects were analyzed to assess their ability to replicate in-vivo responses.

METHODS

Steady-state action potentials from ventricular myocytes isolated from male Wistar rats (3 months; N = 8 animals) were recorded at 37°C with steady-state pacing at 1, 2, 4, 6, 8 and 10 Hz using whole-cell patch-clamp. Action potential repolarization to 25, 50, 75, 90 and 100% of full repolarization (APD25-100 ) was compared before and after 5 nM, 100 nM and 1 µM isoproterenol doses.

RESULTS

A Repeated measures ANOVA found APD50-90  shortened with 5 nM isoproterenol infusion by 6-25% (but comparable across doses) (p ≤ 0.03). Pacing frequencies emulating a normal rat heart rate (6 Hz) prolonged APD50 23% compared with 1 Hz pacing. Frequencies emulating exercise or stress (10 Hz) shortened APD90 (29%).

CONCLUSION

These results demonstrate modest action potential shortening in response to adrenergic stimulation and elevations in pacing beyond physiological resting rates. Our findings indicate changes in action potential plateau and late repolarization predominantly underlie simulated exercise responses in the rat heart. This work provides novel action potential reference data and will help model cardiac responses to physiological stimuli in the rat heart via computational techniques.

Abnormal Cardiac Repolarization in Thyroid Diseases: Results of an Observational Study

Background: The relationship between thyroid function and cardiac disease is complex. Both hypothyroidism and thyrotoxicosis can predispose to ventricular arrhythmia and other major adverse cardiovascular events (MACE), so that a U-shaped relationship between thyroid signaling and the incidence of MACE has been postulated. Moreover, recently published data suggest an association between thyroid hormone concentration and the risk of sudden cardiac death (SCD) even in euthyroid populations with high-normal FT4 levels. In this study, we investigated markers of repolarization in ECGs, as predictors of cardiovascular events, in patients with a spectrum of subclinical and overt thyroid dysfunction.

Methods: Resting ECGs of 100 subjects, 90 patients (LV-EF > 45%) with thyroid disease (60 overt hyperthyroid, 11 overt hypothyroid and 19 L-T4-treated and biochemically euthyroid patients after thyroidectomy or with autoimmune thyroiditis) and 10 healthy volunteers were analyzed for Tp-e interval. The Tp-e interval was measured manually and was correlated to serum concentrations of thyroid stimulating hormone (TSH), free triiodothyronine (FT3) and thyroxine (FT4).

Results: The Tp-e interval significantly correlated to log-transformed concentrations of TSH (Spearman's rho = 0.30, p < 0.01), FT4 (rho = −0.26, p < 0.05), and FT3 (rho = −0.23, p < 0.05) as well as log-transformed thyroid's secretory capacity (SPINA-GT, rho = −0.33, p < 0.01). Spearman's rho of correlations of JT interval to log-transformed TSH, FT4, FT3, and SPINA-GT were 0.51 (p < 1e−7), −0.45 (p < 1e−5), −0.55 (p < 1e−8), and −0.43 (p < 1e−4), respectively. In minimal multivariable regression models, markers of thyroid homeostasis correlated to heart rate, QT, Tp-e, and JT intervals. Group-wise evaluation in hypothyroid, euthyroid and hyperthyroid subjects revealed similar correlations in all three groups.

Conclusion: We observed significant inverse correlations of Tp-e and JT intervals with FT4 and FT3 over the whole spectrum of thyroid function. Our data suggest a possible mechanism of SCD in hypothyroid state by prolongation of repolarization. We do not observe a U-shaped relationship, so that the mechanism of SCD in patients with high FT4 or hyperthyroidism seems not to be driven by abnormalities in repolarization.

Electrophysiological Remodeling: Cardiac T-Tubules and ß-Adrenoceptors

Beta-adrenoceptors (βAR) are often viewed as archetypal G-protein coupled receptors. Over the past fifteen years, investigations in cardiovascular biology have provided remarkable insights into this receptor family. These studies have shifted pharmacological dogma, from one which centralized the receptor to a new focus on structural micro-domains such as caveolae and t-tubules. Important studies have examined, separately, the structural compartmentation of ion channels and βAR. Despite links being assumed, relatively few studies have specifically examined the direct link between structural remodeling and electrical remodeling with a focus on βAR. In this review, we will examine the nature of receptor and ion channel dysfunction on a substrate of cardiomyocyte microdomain remodeling, as well as the likely ramifications for cardiac electrophysiology. We will then discuss the advances in methodologies in this area with a specific focus on super-resolution microscopy, fluorescent imaging, and new approaches involving microdomain specific, polymer-based agonists. The advent of powerful computational modelling approaches has allowed the science to shift from purely empirical work, and may allow future investigations based on prediction. Issues such as the cross-reactivity of receptors and cellular heterogeneity will also be discussed. Finally, we will speculate as to the potential developments within this field over the next ten years.

Suppression of amiodarone-induced torsade de pointes by landiolol in a patient with atrial fibrillation-mediated cardiomyopathy

An elderly Japanese woman developed acute decompensated heart failure caused by persistent atrial fibrillation (AF) and left ventricular systolic dysfunction. Approximately 6 days after starting intravenous administration of amiodarone (600 mg/day) for maintaining sinus rhythm after cardioversion of AF, electrocardiograms revealed a prolonged QT interval associated with torsade de pointes (TdP). The amiodarone-induced TdP disappeared after intravenous administration of landiolol plus magnesium and potassium, without discontinuation of amiodarone or overdrive cardiac pacing, although the prolonged QT interval persisted. To the best of our knowledge, this is the first report that landiolol could be effective for amiodarone-induced TdP.

Utility of Normalized TdP Score System in Drug Proarrhythmic Potential Assessment: A Blinded in vitro Study of CiPA Drugs

Drugs that prolong QT may cause torsade de pointes (TdP). However, translation of nonclinical assessment of QT prolongation or hERG channel, targeted by QT-prolonging drugs, into clinical TdP risk has been insufficient to date. In this blinded study, we confirmed the utility of a Normalized TdP Score System in predicting drug-induced TdP risks among 34 drugs, including 28 with low, intermediate, and high TdP risks under the Comprehensive In Vitro Proarrhythmia Assay (CiPA) initiative plus six compounds with names blinded to the investigators, using the rabbit ventricular wedge assay. Concentration-dependent TdP scores were determined by drug-induced changes in QT, T_p-e , and proarrhythmias. Disclosure of the names and testing concentrations was made after completion of the experiments and report to the sponsors. Drugs' normalized TdP scores were calculated thereafter based on their respective free clinical maximum concentration (C_max ). Drugs' normalized TdP scores were calculated and ranked for 33 drugs, excluding 1 investigational drug, and the TdP risks of the 28 CiPA drugs were correctly distinguished according to their respective categories of low, intermediate, and high TdP risks under the CiPA initiative. Accordingly, we are able to propose the cutoff values of the normalized TdP scores at 1 × C_max : ≤ 0, > 0 to < 0.65 and ≥ 0.65, respectively, for low, intermediate, and high risk. This blinded study supports utility of our Normalized TdP Score System in predicting drug-induced TdP risks in 33 drugs, including 28 used for characterization of other assays under the CiPA initiative. However, these results need to be replicated in other laboratories.

Insights into Cardiac IKs (KCNQ1/KCNE1) Channels Regulation

The delayed rectifier potassium IKs channel is an important regulator of the duration of the ventricular action potential. Hundreds of mutations in the genes (KCNQ1 and KCNE1) encoding the IKs channel cause long QT syndrome (LQTS). LQTS is a heart disorder that can lead to severe cardiac arrhythmias and sudden cardiac death. A better understanding of the IKs channel (here called the KCNQ1/KCNE1 channel) properties and activities is of great importance to find the causes of LQTS and thus potentially treat LQTS. The KCNQ1/KCNE1 channel belongs to the superfamily of voltage-gated potassium channels. The KCNQ1/KCNE1 channel consists of both the pore-forming subunit KCNQ1 and the modulatory subunit KCNE1. KCNE1 regulates the function of the KCNQ1 channel in several ways. This review aims to describe the current structural and functional knowledge about the cardiac KCNQ1/KCNE1 channel. In addition, we focus on the modulation of the KCNQ1/KCNE1 channel and its potential as a target therapeutic of LQTS.

Neurohormonal Regulation of IKs in Heart Failure: Implications for Ventricular Arrhythmogenesis and Sudden Cardiac Death

Heart failure (HF) results in sustained alterations in neurohormonal signaling, including enhanced signaling through the sympathetic nervous system and renin-angiotensin-aldosterone system pathways. While enhanced sympathetic nervous system and renin-angiotensin-aldosterone system activity initially help compensate for the failing myocardium, sustained signaling through these pathways ultimately contributes to HF pathophysiology. HF remains a leading cause of mortality, with arrhythmogenic sudden cardiac death comprising a common mechanism of HF-related death. The propensity for arrhythmia development in HF occurs secondary to cardiac electrical remodeling that involves pathological regulation of ventricular ion channels, including the slow component of the delayed rectifier potassium current, that contribute to action potential duration prolongation. To elucidate a mechanistic explanation for how HF-mediated electrical remodeling predisposes to arrhythmia development, a multitude of investigations have investigated the specific regulatory effects of HF-associated stimuli, including enhanced sympathetic nervous system and renin-angiotensin-aldosterone system signaling, on the slow component of the delayed rectifier potassium current. The objective of this review is to summarize the current knowledge related to the regulation of the slow component of the delayed rectifier potassium current in response to HF-associated stimuli, including the intracellular pathways involved and the specific regulatory mechanisms.

Cannabidiol inhibits multiple cardiac ion channels and shortens ventricular action potential duration in vitro

Cannabidiol (CBD) is a non-psychoactive component of Cannabis which has recently received regulatory consideration for the treatment of intractable forms of epilepsy such as the Dravet and the Lennox-Gastaut syndromes. The mechanisms of the antiepileptic effects of CBD are unclear, but several pre-clinical studies suggest the involvement of ion channels. Therefore, we have evaluated the effects of CBD on seven major cardiac currents shaping the human ventricular action potential and on Purkinje fibers isolated from rabbit hearts to assess the in vitro cardiac safety profile of CBD. We found that CBD inhibits with comparable micromolar potencies the peak and late components of the Na_V1.5 sodium current, the Ca_V1.2 mediated L-type calcium current, as well as all the repolarizing potassium currents examined except K_ir2.1. The most sensitive channels were K_V7.1 and the least sensitive were K_V11.1 (hERG), which underly the slow (I_Ks) and rapid (I_Kr) components, respectively, of the cardiac delayed-rectifier current. In the Purkinje fibers, CBD decreased the action potential (AP) duration more potently at half-maximal than at near complete repolarization, and slightly decreased the AP amplitude and its maximal upstroke velocity. CBD had no significant effects on the membrane resting potential except at the highest concentration tested under fast pacing rate. These data show that CBD impacts cardiac electrophysiology and suggest that caution should be exercised when prescribing CBD to carriers of cardiac channelopathies or in conjunction with other drugs known to affect heart rhythm or contractility.

Slow Delayed Rectifier Current Protects Ventricular Myocytes From Arrhythmic Dynamics Across Multiple Species: A Computational Study

BACKGROUND

The slow and rapid delayed rectifier K⁺ currents (I_Ks and I_Kr, respectively) are responsible for repolarizing the ventricular action potential (AP) and preventing abnormally long APs that may lead to arrhythmias. Although differences in biophysical properties of the 2 currents have been carefully documented, the respective physiological roles of I_Kr and I_Ks are less established. In this study, we sought to understand the individual roles of these currents and quantify how effectively each stabilizes the AP and protects cells against arrhythmias across multiple species.

METHODS

We compared 10 mathematical models describing ventricular myocytes from human, rabbit, dog, and guinea pig. We examined variability within heterogeneous cell populations, tested the susceptibility of cells to proarrhythmic behavior, and studied how I_Ks and I_Kr responded to changes in the AP.

RESULTS

We found that (1) models with higher baseline I_Ks exhibited less cell-to-cell variability in AP duration; (2) models with higher baseline I_Ks were less susceptible to early afterdepolarizations induced by depolarizing perturbations; (3) as AP duration is lengthened, I_Ks increases more profoundly than I_Kr, thereby providing negative feedback that resists excessive AP prolongation; and (4) the increase in I_Ks that occurs during β-adrenergic stimulation is critical for protecting cardiac myocytes from early afterdepolarizations under these conditions.

CONCLUSIONS

Slow delayed rectifier current is uniformly protective across a variety of cell types. These results suggest that I_Ks enhancement could potentially be an effective antiarrhythmic strategy.

Critical Volume of Human Myocardium Necessary to Maintain Ventricular Fibrillation

BACKGROUND

Abnormal QT intervals, long QT or short QT, have been epidemiologically linked with sudden cardiac death because of ventricular fibrillation (VF). Consequently, Food and Drug Administration recommends testing all pharmacological agents for QT toxicity as a risk factor for cardiac toxicity. Such tests assess QT/QTc interval, which represents ventricular depolarization and repolarization. However, the current QT toxicity analysis does not account for the well-known anisotropy in cardiac tissue conductivity. Mines demonstrated in 1913 that cardiac wavelength (λ) determines inducibility of reentrant arrhythmia, where both repolarization time or action potential duration and conduction velocity determine λ=action potential duration×conduction velocity. We aimed to determine the role of anisotropic wavelength in inducibility of VF in explanted human left ventricular preparations. We tested the hypothesis that 3-dimensional cardiac wavelength, which takes into account anisotropic cardiac tissue conductivity, can accurately predict VF sustainability.

METHODS

We conducted panoramic optical mapping of coronary perfused human left ventricular wedge preparations subjected to pharmacologically induced shortening and prolongation of action potential duration, by I_K,ATP agonist pinacidil and antagonist glybenclamide, respectively. This measured action potential duration, conduction velocity, and thus determined pacing cycle length-dependent wavelengths in longitudinal (λ_L), transverse (λ_TV), and transmural (λ_TM) directions using S1S1 pacing protocol, from which wavelength volume (V_λ) was determined, as V_λ=λ_L×λ_TV×λ_TM, and compared with tissue volume_. We tested a hypothesis that tissue volume/V_λ ratio can predict VF sustainability.

RESULTS

At baseline, at pacing rate of 240 beats per minute, the wavelengths were λ_L=9.6±0.6 cm, λ_TV=4.2±0.3 cm, and λ_TM=5.8±0.2 cm, respectively (n=7), and thus V_λ=246.4±42.1 cm³. Administration of pinacidil at escalating concentrations progressively decreased V_λ, and VF became sustained, when tissue volume/V_λ was above safety factor κ=4.4±0.6 (n=9) during rapid pacing. Treatment with glybenclamide decreased V_T/V_λ below κ at any pacing rate and prevented VF sustainability.

CONCLUSIONS

Sustained VF was only sustained in ventricular volume exceeding critical V_λ=λ_L×λ_TV×λ_TM.

Real-time interactive simulations of large-scale systems on personal computers and cell phones: Toward patient-specific heart modeling and other applications

Cardiac dynamics modeling has been useful for studying and treating arrhythmias. However, it is a multiscale problem requiring the solution of billions of differential equations describing the complex electrophysiology of interconnected cells. Therefore, large-scale cardiac modeling has been limited to groups with access to supercomputers and clusters. Many areas of computational science face similar problems where computational costs are too high for personal computers so that supercomputers or clusters currently are necessary. Here, we introduce a new approach that makes high-performance simulation of cardiac dynamics and other large-scale systems like fluid flow and crystal growth accessible to virtually anyone with a modest computer. For cardiac dynamics, this approach will allow not only scientists and students but also physicians to use physiologically accurate modeling and simulation tools that are interactive in real time, thereby making diagnostics, research, and education available to a broader audience and pushing the boundaries of cardiac science.

Differential Effects of Isoproterenol on Regional Myocardial Mechanics in Rat using 3D cine DENSE Cardiovascular Magnetic Resonance

The present study assessed the acute effects of isoproterenol on left ventricular (LV) mechanics in healthy rats with the hypothesis that ß-adrenergic stimulation influences the mechanics of different myocardial regions of the LV wall in different ways. To accomplish this, magnetic resonance images were obtained in the LV of healthy rats with or without isoproterenol infusion. The LV contours were divided into basal, mid-ventricular, and apical regions. Additionally, the mid-ventricular myocardium was divided into three transmural layers with each layer partitioned into four segments (i.e., septal, inferior, lateral, and anterior). Peak systolic strains and torsion were quantified for each region. Isoproterenol significantly increased peak systolic radial strain and circumferential-longitudinal shear strain, as well as ventricular torsion, throughout the basal, mid-ventricle, and apical regions. In the mid-ventricle, isoproterenol significantly increased peak systolic radial strain, and induced significant increases in peak systolic circumferential strain and longitudinal strain in the septum. Isoproterenol consistently increased peak systolic circumferential-longitudinal shear strain in all mid-ventricular segments. Ventricular torsion was significantly increased in nearly all segments except the inferior sub-endocardium. The effects of isoproterenol on LV systolic mechanics (i.e., 3D strains and torsion) in healthy rats depend on the region. This region-dependency is also strain component-specific. These results provide insight into the regional response of LV mechanics to ß-adrenergic stimulation in rats, and could act as a baseline for future studies on subclinical abnormalities associated with the inotropic response in heart disease.