Substrate size as a determinant of fibrillatory activity maintenance in a mathematical model of canine atrium

Determinants of myocardial conduction velocity: implications for arrhythmogenesis

Slowed myocardial conduction velocity (θ) is associated with an increased risk of re-entrant excitation, predisposing to cardiac arrhythmia. θ is determined by the ion channel and physical properties of cardiac myocytes and by their interconnections. Thus, θ is closely related to the maximum rate of action potential (AP) depolarization [(dV/dt)_max], as determined by the fast Na⁺ current (I_Na); the axial resistance (r_a) to local circuit current flow between cells; their membrane capacitances (c_m); and to the geometrical relationship between successive myocytes within cardiac tissue. These determinants are altered by a wide range of pathophysiological conditions. Firstly, I_Na is reduced by the impaired Na⁺ channel function that arises clinically during heart failure, ischemia, tachycardia, and following treatment with class I antiarrhythmic drugs. Such reductions also arise as a consequence of mutations in SCN5A such as those occurring in Lenègre disease, Brugada syndrome (BrS), sick sinus syndrome, and atrial fibrillation (AF). Secondly, r_a, may be increased due to gap junction decoupling following ischemia, ventricular hypertrophy, and heart failure, or as a result of mutations in CJA5 found in idiopathic AF and atrial standstill. Finally, either r_a or c_m could potentially be altered by fibrotic change through the resultant decoupling of myocyte–myocyte connections and coupling of myocytes with fibroblasts. Such changes are observed in myocardial infarction and cardiomyopathy or following mutations in MHC403 and SCN5A resulting in hypertrophic cardiomyopathy (HCM) or Lenègre disease, respectively. This review defines and quantifies the determinants of θ and summarizes experimental evidence that links changes in these determinants with reduced myocardial θ and arrhythmogenesis. It thereby identifies the diverse pathophysiological conditions in which abnormal θ may contribute to arrhythmia.

A transgenic zebrafish model of a human cardiac sodium channel mutation exhibits bradycardia, conduction-system abnormalities and early death

The recent exponential increase in human genetic studies due to the advances of next generation sequencing has generated unprecedented numbers of new gene variants. Determining which of these are causative of human disease is a major challenge. In-vitro studies and murine models have been used to study inherited cardiac arrhythmias but have several limitations. Zebrafish models provide an attractive alternative for modeling human heart disease due to similarities in cardiac electrophysiology and contraction, together with ease of genetic manipulation, external development and optical transparency. Although zebrafish cardiac mutants and morphants have been widely used to study loss and knockdown of zebrafish gene function, the phenotypic effects of human dominant-negative gene mutations expressed in transgenic zebrafish have not been evaluated. The aim of this study was to generate and characterize a transgenic zebrafish arrhythmia model harboring the pathogenic human cardiac sodium channel mutation SCN5A-D1275N, that has been robustly associated with a range of cardiac phenotypes, including conduction disease, sinus node dysfunction, atrial and ventricular arrhythmias, and dilated cardiomyopathy in humans and in mice. Stable transgenic fish with cardiac expression of human SCN5A were generated using Tol2-mediated transgenesis and cardiac phenotypes were analyzed using video microscopy and ECG. Here we show that transgenic zebrafish expressing the SCN5A-D1275N mutation, but not wild-type SCN5A, exhibit bradycardia, conduction-system abnormalities and premature death. We furthermore show that SCN5A-WT, and to a lesser degree SCN5A-D1275N, are able to compensate the loss of endogenous zebrafish cardiac sodium channels, indicating that the basic pathways, through which SCN5A acts, are conserved in teleosts. This proof-of-principle study suggests that zebrafish may be highly useful in vivo models to differentiate functional from benign human genetic variants in cardiac ion channel genes in a time- and cost-efficient manner. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Atrial arrhythmia, triggering events and conduction abnormalities in isolated murine RyR2-P2328S hearts

AIM

RyR2 mutations are associated with catecholaminergic polymorphic tachycardia, a condition characterized by ventricular and atrial arrhythmias. The present experiments investigate the atrial electrophysiology of homozygotic murine RyR2-P2328S (RyR2(S/S)) hearts for ectopic triggering events and for conduction abnormalities that might provide a re-entrant substrate.

METHODS

Electrocardiograph recordings were made from regularly stimulated RyR2(S/S) and wild type (WT) hearts, perfused using a novel modified Langendorff preparation. This permitted the simultaneous use of either floating intracellular microelectrodes to measure action potential (AP) parameters, or a multielectrode array to measure epicardial conduction velocity (CV).

RESULTS

RyR2(S/S) showed frequent sustained tachyarrhythmias, delayed afterdepolarizations and ectopic APs, increased interatrial conduction delays, reduced epicardial CVs and reduced maximum rates of AP depolarization ((dV/dt)(max)), despite similar effective refractory periods, AP durations and AP amplitudes. Effective interatrial CVs and (dV/dt)(max) values of APs following ectopic (S2) stimulation were lower than those of APs following regular stimulation and decreased with shortening S1S2 intervals. However, although RyR2(S/S) atria showed arrhythmias over a wider range of S1S2 intervals, the interatrial CV and (dV/dt)(max) of S2 APs provoking such arrhythmias were similar in RyR2(S/S) and WT.

CONCLUSIONS

These results suggest that abnormal intracellular Ca(2+) homoeostasis produces both arrhythmic triggers and a slow-conducting arrhythmic substrate in RyR2(S/S) atria. A similar mechanism might also contribute to arrhythmogenesis in other conditions, associated with diastolic Ca(2+) release, such as atrial fibrillation.

Importance of atrial surface area and refractory period in sustaining atrial fibrillation: testing the critical mass hypothesis

OBJECTIVE

The critical mass hypothesis for atrial fibrillation (AF) was proposed in 1914; however, there have been few studies defining the relationship between atrial surface area and AF. This study evaluated the effect of tissue area and effective refractory period (ERP) on the probability of sustaining AF in an in vivo model.

METHODS

Domestic pigs (n = 9) underwent median sternotomy. Epicardial activation maps were constructed from bipolar electrograms recorded from form-fitting electrode templates placed on the atria. Baseline ERPs were determined. ERP was lowered with a continuous infusion of acetylcholine (0.005-0.04 mg/Kg/min) until AF could be sustained after burst pacing. The atria were sequentially partitioned using bipolar radiofrequency ablation. ERPs were lowered using acetylcholine until AF could be sustained in each subdivision of atrial tissue. Each subdivision was further divided until AF was no longer inducible. At study completion, the heart was excised and the surface area of each section was measured.

RESULTS

Over a range of ERPs from 75 to 250 ms, the probability of AF was correlated with increasing tissue area (range, 19.5-105 cm(2)) and decreasing ERP. Logistic regression analysis identified shorter ERP (P < .001) and larger area (P = .006) as factors predictive of an increased probability of sustained AF (area under the curve of the receiver-operator characteristic = 0.878).

CONCLUSIONS

The probability of sustained AF was significantly associated with increasing tissue area and decreasing ERP. These data may lead to a greater understanding of the mechanism of AF and help to design better interventional procedures.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Modeling atrial arrhythmias: impact on clinical diagnosis and therapies

Atrial arrhythmias are the most frequent sustained rhythm disorders in humans and often lead to severe complications such as heart failure and stroke. Despite the important insights provided by animal models into the mechanisms of atrial arrhythmias, direct translation of experimental findings to new therapies in patients has not been straightforward. With the advances in computer technology, large-scale electroanatomical computer models of the atria that integrate information from the molecular to organ scale have reached a level of sophistication that they can be used to interpret the outcome of experimental and clinical studies and aid in the rational design of therapies. This paper reviews the state-of-the-art of computer models of the electrical dynamics of the atria and discusses the evolving role of simulation in assisting the clinical diagnosis and treatment of atrial arrhythmias.

Heart failure, diastolic dysfunction and atrial fibrillation; mechanistic insight of a complex inter-relationship

Atrial fibrillation (AF) and heart failure (HF) commonly coexist, and their co-presence is associated with adverse outcomes relating to thromboembolic events, HF progression, hospitalisation and death. Diastolic dysfunction (DD) is also frequently present in patients with HF and is an independent predictor of hospitalisation and mortality. The presence of DD is a strong predictor of incident AF in patients with HF. In this review, we provide mechanistic insight into pathophysiological processes that frequently promote the occurrence of AF, HF and DD and outline the yin-yang relationship between AF, DD and HF. More recently, invasive studies have also shown that asymptomatic paroxysmal atrial fibrillation (PAF) is a common phenomenon in HF patients. We examine complex inter-relationships between PAF, HF and DD and speculate upon the possible clinical influence of undiagnosed PAF in HF patients.

Pro-arrhythmogenic effects of the S140G KCNQ1 mutation in human atrial fibrillation - insights from modelling

Functional analysis has shown that the missense gain-in-function KCNQ1 S140G mutation associated with familial atrial fibrillation produces an increase of the slow delayed rectifier potassium current (I(Ks)). Through computer modelling, this study investigated mechanisms by which the KCNQ1 S140G mutation promotes and perpetuates atrial fibrillation. In simulations, Courtemanche et al.'s model of human atrial cell action potentials (APs) was modified to incorporate experimental data on changes of I(Ks) induced by the KCNQ1 S140G mutation. The cell models for wild type (WT) and mutant type (MT) I(Ks) were incorporated into homogeneous multicellular 2D and 3D tissue models. Effects of the mutation were quantified on AP profile, AP duration (APD) restitution, effective refractory period (ERP) restitution, and conduction velocity (CV) restitution.Temporal and spatial vulnerabilities of atrial tissue to genesis of re-entry were computed. Dynamic behaviours of re-entrant excitation waves (lifespan (LS), tip meandering patterns and dominant frequency) in 2D and 3D models were characterised. It was shown that the KCNQ1 S140G mutation abbreviated atrial APD and ERP and flattened APD and ERP restitution curves. It reduced atrial CV at low excitation rates, but increased it at high excitation rates that facilitated the conduction of high rate atrial excitation waves. Although it increased slightly tissue temporal vulnerability for initiating re-entry, it reduced markedly the minimal substrate size necessary for sustaining re-entry (increasing the tissue spatial vulnerability). In the 2D and 3D models, the mutation also stabilized and accelerated re-entrant excitation waves, leading to rapid and sustained re-entry. In the 3D model, scroll waves under the mutation condition MT conditions also degenerated into persistent and erratic wavelets, leading to fibrillation. In conclusion, increased I(Ks) due to the KCNQ1 S140G mutation increases atrial susceptibility to arrhythmia due to increased tissue vulnerability, shortened ERP and altered atrial conduction velocity, which, in combination, facilitate initiation and maintenance of re-entrant excitation waves.

Impact of tissue geometry on simulated cholinergic atrial fibrillation: a modeling study

Atrial fibrillation (AF), arising in the cardiac atria, is a common cardiac rhythm disorder that is incompletely understood. Numerous characteristics of the atrial tissue are thought to play a role in the maintenance of AF. Most traditional theoretical models of AF have considered the atrium to be a flat two-dimensional sheet. Here, we analyzed the relationship between atrial geometry, substrate size, and AF persistence, in a mathematical model involving heterogeneity. Spatially periodic properties were created by variations in times required for reactivation due to periodic acetylcholine concentration [ACh] distribution. The differences in AF maintenance between the sheet and the cylinder geometry are found for intermediate gradients of inexcitable time (intermediate [ACh]). The maximum difference in AF maintenance between geometry decreases with increasing tissue size, down to zero for a substrate of dimensions 20 × 10 cm. Generators have the tendency to be anchored to the regions of longer inexcitable period (low [ACh]). The differences in AF maintenance between geometries correlate with situations of moderate anchoring for which rotor-core drifts between low-[ACh] regions occur, favoring generator disappearance. The drift of generators increases their probability of disappearance at the tissue borders, resulting in a decreased maintenance rate in the sheet due to the higher number of no-flux boundaries. These interactions between biological variables and the role of geometry must be considered when selecting an appropriate model for AF in intact hearts.

Pathophysiological mechanisms of atrial fibrillation: a translational appraisal

Atrial fibrillation (AF) is an arrhythmia that can occur as the result of numerous different pathophysiological processes in the atria. Some aspects of the morphological and electrophysiological alterations promoting AF have been studied extensively in animal models. Atrial tachycardia or AF itself shortens atrial refractoriness and causes loss of atrial contractility. Aging, neurohumoral activation, and chronic atrial stretch due to structural heart disease activate a variety of signaling pathways leading to histological changes in the atria including myocyte hypertrophy, fibroblast proliferation, and complex alterations of the extracellular matrix including tissue fibrosis. These changes in electrical, contractile, and structural properties of the atria have been called "atrial remodeling." The resulting electrophysiological substrate is characterized by shortening of atrial refractoriness and reentrant wavelength or by local conduction heterogeneities caused by disruption of electrical interconnections between muscle bundles. Under these conditions, ectopic activity originating from the pulmonary veins or other sites is more likely to occur and to trigger longer episodes of AF. Many of these alterations also occur in patients with or at risk for AF, although the direct demonstration of these mechanisms is sometimes challenging. The diversity of etiological factors and electrophysiological mechanisms promoting AF in humans hampers the development of more effective therapy of AF. This review aims to give a translational overview on the biological basis of atrial remodeling and the proarrhythmic mechanisms involved in the fibrillation process. We pay attention to translation of pathophysiological insights gained from in vitro experiments and animal models to patients. Also, suggestions for future research objectives and therapeutical implications are discussed.

New antiarrhythmic drugs for treatment of atrial fibrillation

Inadequacies in current therapies for atrial fibrillation have made new drug development crucial. Conventional antiarrhythmic drugs increase the risk of ventricular proarrhythmia. In drug development, the focus has been on favourable multichannel-blocking profiles, atrial-specific ion-channels, and novel non-channel targets (upstream therapy). Molecular modification of the highly effective multichannel blocker, amiodarone, to improve safety and tolerability has produced promising analogues such as dronedarone, although this drug seems less effective than does amiodarone. Vernakalant, an atrial-selective drug with reduced proarrhythmic risk, might be useful for cardioversion in atrial fibrillation. Ranolazine, another atrial-selective agent initially developed as an antianginal, has efficacy for atrial fibrillation and is being tested in prospective clinical trials. So-called upstream therapy with angiotensin-converting enzyme and angiotensin-receptor inhibitors, statins, or omega-3 fatty acids and fish oil that target atrial remodelling could be effective, but need further clinical validation. We focus on the basic and clinical pharmacology of newly emerging antiarrhythmic drugs and non-traditional approaches such as upstream therapy for atrial fibrillation.

Animal models for atrial fibrillation: clinical insights and scientific opportunities

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. A variety of animal models have been used to study the pathophysiology of AF, including molecular basis, ion-current determinants, anatomical features, and macroscopic mechanisms. In addition, animal models play a key role in the development of new therapeutic approaches, whether drug-based, molecular therapeutics, or device-related. This article discusses the various types of animal models that have been used for AF research, reviews the principle mechanisms governing atrial arrhythmias in each model, and provides some guidelines for model selection for various purposes.

Determination of the vectorelectrogram in isolated rat atria: application to the study of arrhythmias

Atrial tachyarrhythmias, the most frequent type of cardiac arrhythmia, are associated with increased stroke risk. Reentry and focal activity are considered as the main mechanisms underlying this dysfunction. In this study, we describe determination of the vectorelectrogram in isolated rat atria as a means to distinguish different patterns of electrical propagation. In all studied right atria beating at sinus rhythm, the mean electric vector (MEV) trajectory was clockwise, and each cycle was preceded by electric diastole (null MEV), either in the absence or presence of muscarinic cholinergic or beta-adrenergic receptor stimulation. During cholinergic tachyarrhythmia (induced by high-rate electric stimulation in both atria, plus exposure to carbachol in left atria), vector loops were ellipsoidal and stable, with variable direction, and did not cross the origin, which is consistent with reentrant activation and with findings obtained in vivo by other authors. In contrast, during spontaneous activity induced by rapid pacing in isoproterenol-exposed left atria, vector loops were similar to those in right atria at sinus rhythm, thus suggestive of focal activity. It is concluded that the vectorelectrogram approach allows discrimination of different patterns of propagation during arrhythmia in isolated atria and may be useful for high-output tests of pro- and anti-arrhythmic compounds.

Differences in atrial fibrillation properties under vagal nerve stimulation versus atrial tachycardia remodeling

BACKGROUND

There are many similarities between atrial effects of atrial tachycardia remodeling (ATR) and vagal nerve stimulation (VS): both promote atrial fibrillation (AF), reduce atrial effective refractory period (AERP) and AERP rate accommodation, enhance AERP heterogeneity, and increase inward-rectifier K+ current.

OBJECTIVE

This study sought to compare the consequences of ATR and VS at similar levels of AERP abbreviation in dogs.

METHODS

ATR dogs (n = 6) were subjected to 7-day atrial tachypacing at 400 beats/min, with radiofrequency-induced atrioventricular block and ventricular demand pacing (80 beats/min) to control ventricular response. VS was applied in 6 matched dogs with stimulation parameters selected to produce similar mean AERP values to ATR dogs.

RESULTS

ATR and VS produced similarly short AERPs (79 +/- 12 and 80 +/- 12 ms, respectively), AERP rate-adaptation loss, and AERP heterogeneity increases. Although both ATR and VS increased AF duration, VS was significantly more effective in AF promotion, with mean AF duration of 992 +/- 134 seconds, versus 440 +/- 240 seconds (P <.05) under ATR. The greater AF-promoting effect of VS was associated with greater mean dominant frequency values during AF (11.7 +/- 1.8 versus 10.0 +/- 1.3 Hz ATR, P <.05). VS greatly enhanced the spatial dominant frequency variability, increasing the coefficient of variation to 15.2 +/- 1.9 Hz, versus 8.9 +/- 1.5 Hz for ATR (P <.05), primarily by increasing the per-dog maximum dominant frequency (15.4 +/- 0.6 Hz versus 12.5 +/- 0.6 for ATR, P <.01).

CONCLUSION

For matched AERP values, VS promotes AF more strongly than ATR. Despite similar AERP changes, VS produces considerably greater increases in dominant frequencies, particularly maximum values, consistent with previous suggestions that inward-rectifier K+ current enhancement is particularly effective at accelerating and stabilizing spiral wave rotors that maintain AF.

Atrial Remodeling And Atrial Fibrillation: Mechanistic Interactions And Clinical Implications

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. The prevalence of AF increases dramatically with age and is seen in as high as 9% of individuals by the age of 80 years. In high-risk patients, the thromboembolic stroke risk can be as high as 9% per year and is associated with a 2-fold increase in mortality. Although the pathophysiological mechanism underlying the genesis of AF has been the focus of many studies, it remains only partially understood. Conventional theories focused on the presence of multiple re-entrant circuits originating in the atria that are asynchronous and conducted at various velocities through tissues with various refractory periods. Recently, rapidly firing atrial activity in the muscular sleeves at the pulmonary veins ostia or inside the pulmonary veins have been described as potential mechanism,. AF results from a complex interaction between various initiating triggers and development of abnormal atrial tissue substrate. The development of AF leads to structural and electrical changes in the atria, a process known as remodeling. To have effective surgical or catheter ablation of AF good understanding of the possible mechanism(s) is crucial.Once initiated, AF alters atrial electrical and structural properties that promote its maintenance and recurrence. The role of atrial remodeling (AR) in the development and maintenance of AF has been the subject of many animal and human studies over the past 10-15 years. This review will discuss the mechanisms of AR, the structural, electrophysiologic, and neurohormonal changes associated with AR and it is role in initiating and maintaining AF. We will also discuss briefly the role of inflammation in AR and AF initiation and maintenance, as well as, the possible therapeutic interventions to prevent AR, and hence AF, based on the current understanding of the interaction between AF and AR.

Mother rotor anchoring in branching tissue with heterogeneous membrane properties

Abstract Current understanding of atrial fibrillation is based on the co-existence of multiple re-entrant waves propagating randomly throughout the tissue. However, recent experimental results indicate that in many cases one or a small number of periodic, high-frequency re-entrant sources (mother rotors) can drive the arrhythmia. Owing to the high activation rate, mother rotors seem to be located in regions of shortened action potential duration. In this study a computer model of cardiac propagation was applied to investigate mechanisms leading to the formation and maintenance of such mother rotors. For this purpose, a region of short action potential duration was generated by varying the acetylcholine concentration across the tissue. A mother rotor initiated in the center of this region drifts away, and the activation terminates. If an additional heterogeneity such as a bundle is included into the model, a further drift mechanism directed to the bundle is observed and the rotor can be stabilized. Therefore, bundle insertions may play an important role in the maintenance of mother rotors. The influence of the driving rotor on the activation pattern was studied in a three-dimensional model of rectangular shape and a monolayer model of anatomically correct atrial geometry.

Autonomic mechanism to explain complex fractionated atrial electrograms (CFAE)

OBJECTIVE

To simulate complex fractionated atrial electrograms (CFAE) during sustained atrial fibrillation (AF) in experimental animals.

BACKGROUND

The mechanism(s) underlying CFAE has not been fully elucidated.

METHODS

Twenty-two dogs were subjected to a right and/or left thoracotomy. A gauze patch soaked with acetylcholine (ACh) was placed on the right atrial appendage (RAA) to induce sustained AF. During AF, varying concentrations of ACh (1, 10, 100 mM) were "painted" on the RA where electrograms showed regular organized activity. In another six dogs, anterior right ganglionated plexi (ARGP) near the sino-atrial node and inferior right GP (IRGP) at the junction of inferior vena cava and atria were sequentially ablated. In five dogs, ACh was injected into ARGP to induce CFAE.

RESULTS

During sustained AF, local "painting" with ACh 1 mM and 10 mM induced intermittent CFAE in 1 of 11 and 10 of 11 dogs, respectively. With 100 mM ACh, all 11 showed CFAE (two intermittent, nine continuous). In six other dogs, continuous CFAE induced by topical application of 100 mM ACh were markedly attenuated by ARGP + IRGP ablation. In another five of five dogs, ACh injection into ARGP induced a gradient of CFAE with the continuous CFAE always occurring near the ARGP and CFAE also occurring at left pulmonary vein-atrial junctions. During ARGP ablation, AF was terminated in all five dogs immediately after regularization of the rotor-like electrograms or continuous CFAE.

CONCLUSIONS

This study demonstrates an autonomic basis for CFAE formation, suggesting that graded hyperactive states of the autonomic nervous system (ANS) may induce various types of CFAE observed clinically.

Spatially discordant voltage alternans cause wavebreaks in ventricular fibrillation

BACKGROUND

Ventricular fibrillation (VF) is characterized by complex ECG patterns emanating from multiple, short-lived, reentrant electrical waves. The incessant breakup and creation of new daughter waves (wavebreaks) perpetuate VF. Dispersion of refractoriness (static or dynamic) has been implicated as a mechanism underlying wavebreaks.

OBJECTIVE

The purpose of this study was to investigate the mechanisms underlying wavefront instability in VF by localizing wave fractionation sites (the appearance of multiple waves) and their relationship to local spatial dispersion of voltage (V(m)) oscillations.

METHODS

Wave fractionations were identified by tracking V(m) oscillations optically at unprecedented spatial (100 x 100 pixels) and temporal (2,000 frames per second) resolution using a CMOS camera viewing the surface (1 x 1 cm(2)) of perfused guinea pig hearts (n = 6). VF was induced by burst stimulation, and wavefront dynamics were highlighted using region-based image analysis to automatically detect wavebreaks. Direct detection of wavebreak locations by image analysis was more reliable than the phase reconstruction method because baseline noise obstructed the correct identification of phase singularities by detecting false-positives.

RESULTS

Wave fractionations (34 +/- 4 splits/s.cm(2)) fell into three categories: decremental conduction (49% +/- 7%), wave collisions (32% +/- 8%), and wavebreaks (17 +/- 2%). Wavebreaks occurred at a frequency of 5.8 +/- 1 splits/s.cm(2) and did not preferentially occur at anatomic obstacles (i.e., coronary vessels) but coincided with discordant alternans where V(m) amplitudes and durations shifted from high to low to from low to high on opposite sides of wavebreak sites.

CONCLUSION

Spatial discordant alternans cause wavebreaks most likely because they are sites of abrupt dispersion of refractoriness.

Innovative approaches to anti-arrhythmic drug therapy

Normal cardiac function requires an appropriate and regular beating rate (cardiac rhythm). When the heart rhythm is too fast or too slow, cardiac function can be impaired, with derangements that vary from mild symptoms to life-threatening complications. Irregularities, particularly those involving excessively fast or slow rates, constitute cardiac 'arrhythmias'. In the past, drug treatment of cardiac arrhythmias has proven difficult, both because of inadequate effectiveness and a risk of serious complications. However, a variety of recent advances have opened up exciting possibilities for the development of novel and superior approaches to arrhythmia therapy. This article will review recent progress and future prospects for treating two particularly important cardiac arrhythmias: atrial fibrillation and ventricular fibrillation.

Whole heart action potential duration restitution properties in cardiac patients: a combined clinical and modelling study

Steep action potential duration (APD) restitution has been shown to facilitate wavebreak and ventricular fibrillation. The global APD restitution properties in cardiac patients are unknown. We report a combined clinical electrophysiology and computer modelling study to: (1) determine global APD restitution properties in cardiac patients; and (2) examine the interaction of the observed APD restitution with known arrhythmia mechanisms. In 14 patients aged 52-85 years undergoing routine cardiac surgery, 256 electrode epicardial mapping was performed. Activation-recovery intervals (ARI; a surrogate for APD) were recorded over the entire ventricular surface. Mono-exponential restitution curves were constructed for each electrode site using a standard S1-S2 pacing protocol. The median maximum restitution slope was 0.91, with 27% of all electrode sites with slopes<0.5, 29% between 0.5 and 1.0, and 20% between 1.0 and 1.5. Eleven per cent of restitution curves maintained slope>1 over a range of diastolic intervals of at least 30 ms; and 0.3% for at least 50 ms. Activation-recovery interval restitution was spatially heterogeneous, showing regional organization with multiple discrete areas of steep and shallow slope. We used a simplified computer model of 2-D cardiac tissue to investigate how heterogeneous APD restitution can influence vulnerability to, and stability of re-entry. Our model showed that heterogeneity of restitution can act as a potent arrhythmogenic substrate, as well as influencing the stability of re-entrant arrhythmias. Global epicardial mapping in humans showed that APD restitution slopes were organized into regions of shallow and steep slopes. This heterogeneous organization of restitution may provide a substrate for arrhythmia.

Losartan and Prevention of Atrial Fibrillation Recurrence in Hypertensive Patients

The aim of the study was to evaluate the effect of losartan as compared with amlodipine, both associated with amiodarone, in preventing the recurrence of atrial fibrillation (AF) in hypertensive patients with a history of recent paroxysmal atrial fibrillation. Two hundred and fifty mild hypertensive (SBP > 140 mm Hg and/or DBP > 90 < 100 mm Hg) outpatients in sinus rhythm but with at least two ECG-documented episodes of symptomatic atrial fibrillation in the previous 6 months and in treatment with amiodarone were randomized to losartan or amlodipine and were followed up for 1 year. Clinic blood pressure (BP) and a 24-hour ECG was evaluated every month; the patients were asked to report any episode of symptomatic atrial fibrillation and to perform an ECG as early as possible. Two hundred and thirteen patients completed the study, 107 in the losartan group and 106 in the amlodipine group. After 12 months the SBP/DBP mean values were significantly reduced by both losartan (from 151.4/95.6 to 135.5/83.7 mm Hg, P < 0.001 versus baseline) and amlodipine (from 152.3/96.5 to 135.2/83.4 mm Hg, P < 0.001 versus baseline), with no difference between the two treatments. At least one ECG-documented episode of atrial fibrillation was reported in 13% of the patients treated with losartan and in 39% of the patients treated with amlodipine. The use of losartan in combination with amiodarone seems more effective than amlodipine/amiodarone combination in preventing new episodes of atrial fibrillation in hypertensive patients with recurrent atrial fibrillation. This might be related to possible favorable impact of angiotensin II receptor blockers (ARB) on the atrial electrical and structural remodeling in this type of patients.

The pathology of atrial fibrillation in dogs

Atrial fibrillation (AF) occurs in dogs in a number of settings, the most common of which is congestive heart failure. This paper reviews what is known about the pathology of AF in dogs, as well as its clinical relevance. We begin by discussing several experimental AF paradigms in dogs, the associated pathology, and its relevance to AF mechanisms. We then discuss clinical AF in dogs and its relationship to experimental pathology. Finally, we conclude by assessing the potential therapeutic relevance of understanding AF-related pathology in dogs, as well as its potential to have practical applications in the future.