Experimental models of atrial fibrillation/flutter

Postnatal undernutrition in mice causes cardiac arrhythmogenesis which is exacerbated when pharmacologically stressed

Growth restriction caused by postnatal undernutrition increases risk for cardiovascular disease in adulthood with the potential to induce arrhythmogenesis. Thus, the purpose was to determine if undernutrition during development produced arrhythmias at rest and when stressed with dobutamine in adulthood. Mouse dams were fed (CON: 20% protein), or low-protein (LP: 8%) diet before mating. A cross-fostering model was used where pups nursed by dams fed LP diet in early [EUN; postnatal day (PN) 1–10], late (LUN; PN11–21) and whole (PUN; 1–21) phases of postnatal life. Weaned pups were switched to CON diets for the remainder of the study (PN80). At PN80, body composition (magnetic resonance imaging), and quantitative electrocardiogram (ECG) measurements were obtained under 1% isoflurane anesthesia. After baseline ECG, an IP injection (1.5 µg/g body weight) of dobutamine was administered and ECG repeated. Undernutrition significantly (P<0.05) reduced body weight in LUN (22.68±0.88 g) and PUN (19.96±0.32 g) but not in CON (25.05±0.96 g) and EUN (25.28±0.9207 g). Fat mass decreased in all groups compared with controls (CON: 8.00±1.2 g, EUN: 6.32±0.65 g, LUN: 5.11±1.1 g, PUN: 3.90±0.25 g). Lean mass was only significantly reduced in PUN (CON: 17.99±0.26 g, EUN: 17.78±0.39 g, LUN: 17.34±0.33 g, PUN: 15.85±0.28 g). Absolute heart weights were significantly less from CON, with PUN having the smallest. ECG showed LUN had occurrences of atrial fibrillation; EUN had increases of 1st degree atrioventricular block upon stimulation, and PUN had increased risk for ventricular depolarization arrhythmias. CON did not display arrhythmias. Undernutrition in early life resulted in ventricular arrhythmias under stressed conditions, but undernutrition occurring in later postnatal life there is an increased incidence of atrial arrhythmias.

Prediction of new onset atrial fibrillation after cardiac revascularization surgery

The aim of this study was to create a simple risk index to predict new-onset atrial fibrillation (AF) after coronary artery bypass grafting in patients with histories of AF. AF after coronary artery bypass grafting (referred to here as AF) is associated with increased morbidity and mortality. Identifying patients at high risk for developing AF may help identify a group of patients who might benefit from strategies to prevent postoperative AF. A cohort of 18,517 patients enrolled from January 1, 1996, to December 31, 2009, was used to derive a risk index for AF prediction. A multivariate logistic regression model determined the independent predictive impact of clinical and demographic characteristics on the occurrence of AF. A subset of these variables was used to construct a risk index to predict AF. This risk index was validated in a sequential cohort of 1,378 consecutive patients who underwent coronary artery bypass grafting from January 1, 2010, to June 30, 2011. AF occurred in 3,486 patients in the calibration cohort (18.83%) and in 269 patients in the validation cohort (19.52%). After considering patients' demographics, co-morbid conditions, and severity of illness, advanced age appeared as the most powerful predictor of AF (odds ratio 1.059/year, 95% confidence interval 1.055 to 1.063). Age, height, weight, and the presence of peripheral vascular disease contributed most to the prediction model. An AF risk index including these variables had adequate discriminatory power, with a concordance index of 0.68. In conclusion, using a large cohort of patients, a simple risk index relying only on preoperative clinical variables was developed, which will help predict AF. This risk index can be used clinically to identify patients at high risk for the development of AF.

Ablation of sarcolipin results in atrial remodeling

Sarcolipin (SLN) is a key regulator of sarco(endo)plasmic reticulum (SR) Ca(2+)-ATPase (SERCA), and its expression is altered in diseased atrial myocardium. To determine the precise role of SLN in atrial Ca(2+) homeostasis, we developed a SLN knockout (sln-/-) mouse model and demonstrated that ablation of SLN enhances atrial SERCA pump activity. The present study is designed to determine the long-term effects of enhanced SERCA activity on atrial remodeling in the sln-/- mice. Calcium transient measurements show an increase in atrial SR Ca(2+) load and twitch Ca(2+) transients. Patch-clamping experiments demonstrate activation of the forward mode of sodium/calcium exchanger, increased L-type Ca(2+) channel activity, and prolongation of action potential duration at 90% repolarization in the atrial myocytes of sln-/- mice. Spontaneous Ca(2+) waves, delayed afterdepolarization, and triggered activities are frequent in the atrial myocytes of sln-/- mice. Furthermore, loss of SLN in atria is associated with increased interstitial fibrosis and altered expression of genes encoding collagen and other extracellular matrix proteins. Our results also show that the sln-/- mice are susceptible to atrial arrhythmias upon aging. Together, these findings indicate that ablation of SLN results in increased SERCA activity and SR Ca(2+) load, which, in turn, could cause abnormal intracellular Ca(2+) handling and atrial remodeling.

Age-related increase in atrial fibrillation induced by transvenous catheter-based atrial burst pacing: an in vivo rat model of inducible atrial fibrillation

INTRODUCTION

Large animal models of atrial fibrillation (AF) are well established, but limited experimental reports exist on small animal models. We sought to develop an in vivo rat model of AF using a transvenous catheter and to evaluate the model's underlying characteristics.

METHODS AND RESULTS

Echocardiogram, surface electrocardiogram (ECG), and atrial effective refractory period (AERP) were recorded at baseline in young (3 months) and middle-aged (9 months) Wistar rats. AF inducibility and duration were measured through transvenous electrode catheter in young (n = 11) and middle-aged rats (n = 11) and middle-aged rats treated with either pilsicainide (1 mg/kg iv, n = 7) or amiodarone (10 mg/kg iv, n = 9). Degrees of interstitial fibrosis and cellular hypertrophy in the atria were assessed histologically. The P-wave duration and AERP were significantly longer and echocardiographic left atrial dimension significantly larger in middle-aged versus young rats. AF was inducible in >90% of all procedures in both untreated rat groups, whereas AF inducibility was reduced by the antiarrhythmic drugs. The AF duration was significantly longer in middle-aged than in young rats and was significantly shortened by treatment with either pilsicainide or amiodarone. Histologic analysis revealed significant increases in atrial interstitial fibrosis and cellular diameter in middle-aged versus young rats.

CONCLUSIONS

Transvenous catheter-based AF is significantly longer in middle-aged than in young rats and is markedly reduced by treatment with antiarrhythmic drugs. This rat model of AF is simple, reproducible, and reliable for examining pharmacologic effects on AF and studying the process of atrial remodeling.

Physical activity, height, and left atrial size are independent risk factors for lone atrial fibrillation in middle-aged healthy individuals

AIMS

The aetiology of atrial fibrillation (AF) remains unknown in some patients. The aim of the study was to identify new risk factors for developing lone AF (LAF).

METHODS AND RESULTS

A series of 107 consecutive patients younger than 65, seen in the emergency room for an episode of LAF of <48 h duration were included in the study. A group of 107 healthy volunteers matched for age and sex were recruited as controls. All subjects answered a validated questionnaire concerning leisure and occupational activities performed throughout their lifetimes to estimate accumulated hours of physical effort, classified in four levels of intensity. Demographic and echocardiographic measurements were also recorded. There were 69% of males and mean age was 48 +/- 11 years. AF was paroxysmal in 57% and persistent in the remaining 43%. Patients with AF performed more hours of both moderate and heavy intensity physical activity. They also were taller, and had a larger left atria, ventricle, and body surface area. At the multivariable analysis, only moderate and heavy physical activity, height, and anteroposterior atrial diameter were independently associated with LAF.

CONCLUSIONS

Accumulated lifetime physical activity, height, and left atrial size are risk factors for LAF in healthy middle-aged individuals.

A pharmacological model for calcium overload-induced tachycardia in isolated rat left atria

Few experimental models produce spontaneous tachycardia in normal left atria to allow the study of the cellular mechanisms underlying this contributor to atrial fibrillation. We reported 2-aminoethoxydiphenyl borate (2-APB) that provokes sporadic spontaneous mechanical activity and calcium leak in isolated rat left atria. Since sarcoplasmic reticulum calcium leak in the presence of high calcium load may trigger tachyarrhythmias, we tested how conditions that increase calcium load affect 2-APB-induced ectopic activity. Exposing superfused rat left atria to (i) 30 nM isoproterenol, (ii) 3 microM forskolin, (iii) 300 nM (-)BayK 8644 ((4S)-1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluormethyl)phenyl]-3-pyridinecarboxylic acid methyl ester), (iv) 300 nM FPL-64176 (2,5-Dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester) or (v) 120 microM ouabain increases their force of contraction, evidence of calcium loading, but does not produce ectopic activity. Spontaneous mechanical activity occurs in left atria superfused with 20 microM 2-APB at 47+/-6 contractions/min in the absence of pacing. Any of these five agents increase rates of 2-APB-induced spontaneous mechanical activity to >200 contractions/min in the absence of pacing. Washing tachycardic left atria with superfusate lacking 2-APB restores normal function, demonstrating the reversibility of these effects. Decreasing superfusate sodium reverses this tachycardia and two hyperpolarization-activated current (I(f)) inhibitors blunt this ectopic activity. Thus conditions that increase atrial calcium load increase the frequency of spontaneous mechanical activity. Decreasing extracellular sodium and I(f) inhibitors suppress this spontaneous tachycardia suggesting forward-mode sodium-calcium exchange and I(f)-like activities underlie this activity. This model may help define cell pathways that trigger atrial tachyarrhythmias.

2-APB Induces Instability in Rat Left Atrial Mechanical Activity

Atrial contractile abnormalities are common clinical disorders but few pharmacological models can reliably produce such abnormalities in isolated atrial muscle. Since sarcoplasmic reticulum (SR) calcium leak may underlie these contractile irregularities, we investigated whether 2-aminoethoxydiphenyl borate (2-APB), a calcium leak-inducer, affects mechanical function in isolated, superfused rat left atria. Exposing left atria paced at 3 Hz to >10 microM 2-APB produced sporadic mechanical events that occurred in the absence of pacing stimulus. Prolonging atrial diastole in the presence of 2-APB produced spontaneous mechanical activity (SMA) defined as numerous mechanical events occurring in the absence of pacing stimulus. SMA depends on atrial sodium and chloride gradients as decreasing superfusate concentration of either ion suppressed SMA. Increasing superfusate potassium to produce an EK of approximately -74mV reversed SMA, revealing possible membrane potential sensitivity. Mechanical function decreased with time in left atria treated with 2-APB and low sodium or the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) compared with atria exposed to low sodium or DIDS alone, suggesting 2-APB may decrease left atrial SR activator calcium. Thus, 2-APB produces instability in regular left atrial mechanical activity that may require forward-mode sodium-calcium exchange and chloride channel activities. This data identify a new model for studying atrial contractile abnormalities.

The Relationship Between Stature and the Prevalence of Atrial Fibrillation in Patients With Left Ventricular Dysfunction

OBJECTIVES

This study sought to determine the influence of stature on atrial fibrillation (AF) in high-risk patients with reduced left ventricular (LV) systolic function.

BACKGROUND

Left atrial (LA) enlargement is a potent risk factor for AF. Because LA size is strongly associated with stature, we hypothesized that height and body surface area (BSA) are risk factors for AF, independent of other known associations.

METHODS

Data were obtained from ADVANCENT, a multicenter registry of patients with impaired LV function. Height and BSA were divided into quartiles by gender. Statistical analysis was done using the Cochran Mantel-Haenszel statistic, and multivariable logistic regressions were used to adjust for the effects of known confounders on the association between stature and AF.

RESULTS

A total of 25,268 patients were enrolled. The mean age was 66 years, and the cohort consisted mostly of white men (72%) and patients with ischemic cardiomyopathy (72%). The mean left ventricular ejection fraction was 31%. A history of AF was present in 7,027 patients (27.8%). The AF prevalence increased significantly between the lowest and highest quartiles for height (32% relative increase, p < 0.0001). In the multivariable analysis, the effect of height on AF risk persisted after adjusting for age, gender, race, left ventricular ejection fraction, heart failure class and etiology, hypertension, diabetes, and medication use (odds ratio 1.026/cm, 95% confidence interval [CI] 1.022 to 1.030). In the multivariable analysis, BSA was also an independent predictor of AF risk (odds ratio 4.221/m2, 95% CI 3.358 to 5.306).

CONCLUSIONS

In patients with LV dysfunction, increasing stature portends a higher risk of AF independent of other traditional risk factors for the arrhythmia. This association seems to account for the higher prevalence of AF in men and may be useful for identification of a high-risk population.

A Simple In Vivo Atrial Fibrillation Model of Rat Induced by Transesophageal Atrial Burst Pacing

A simple in vivo closed-chest atrial fibrillation (Af) model of rats was developed. Af was reproducibly induced by transesophageal atrial burst pacing for 30 s in each of the pentobarbital-anesthetized rats, whereas the cardiohemodynamic condition as well as the inducibility and duration of Af episode was stable over time. Moreover, the anti-Af effect of the class Ic drug pilsicainide was confirmed in this model, which was essentially the same as those reported previously in other Af animal models and clinical practice. Thus, this new model may become an alternative to current techniques.

Atrial Fibrillation in Rats Induced by Rapid Transesophageal Atrial Pacing during Brief Episodes of Asphyxia: A New In Vivo Model

Non-pharmacological in vivo models of atrial fibrillation (AF) have been developed in large animals only. We aimed to develop and characterize a new small animal non-pharmacological in vivo model of AF. AF was induced by transesophageal atrial burst pacing during 35 seconds periods of asphyxia in anesthetized male Sprague-Dawley rats. AF was reproducibly induced in 81% of the rats. The presence of AF was associated with an increased heart rate, and a decreased blood pressure. Treatment with amiodarone, D,L-sotalol, flecainide, and propranolol all reduced duration of AF, whereas verapamil treatment was associated with a marked profibrillatory effect. Increasing gap junction intracellular communication using the antiarrhythmic peptide analogue AAP10 did not affect AF duration. Basal plasma level of epinephrine and norepinephrine were increased 5- to 20-fold relative to values reported by others, but unchanged following 35 seconds of asphyxia. The results from our study demonstrate that the rat model shares several clinical key characteristics with human AF: (1) hemodynamic response to AF; (2) increased autonomic tone; (3) antiarrhythmic effects of clinically used drugs; (4) profibrillatory effect of verapamil. Relative to existing models of AF in larger animals, this model offers rapid, predictive, and inexpensive testing of antiarrhythmic/profibrillatory effects of new drugs.