Mechanisms of sinoatrial node dysfunction in a canine model of pacing-induced atrial fibrillation

Identification of miR-1 and miR-499 in chronic atrial fibrillation by bioinformatics analysis and experimental validation

Background

Atrial fibrillation (AF) is one of the most prevalent arrhythmias and is characterized by a high risk of heart failure and embolic stroke, yet its underlying mechanism is unclear. The primary goal of this study was to establish a miRNA-mRNA network and identify the miRNAs associated with chronic AF by bioinformatics and experimental validation.

Methods

The GSE79768 dataset was collected from the Gene Expression Omnibus(GEO) database to extract data from patients with or without persistent AF. Differentially expressed genes (DEGs) were identified in left atrial appendages (LAAs). The STRING platform was utilized for protein-protein interaction (PPI) network analysis. The target miRNAs for the top 20 hub genes were predicted by using the miRTarBase Web tool. The miRNA-mRNA network was established and visualized using Cytoscape software. The key miRNAs selected for verification in the animal experiment were confirmed by miRwalk Web tool. We used a classic animal model of rapid ventricular pacing for chronic AF. Two groups of animals were included in the experiment, namely, the ventricular pacing group (VP group), where ventricular pacing was maintained at 240-280 bpm for 2 weeks, and the control group was the sham-operated group (SO group). Finally, we performed reverse transcription-quantitative polymerase chain reaction (RT-qPCR) to validate the expression of miR-1 and miR-499 in LAA tissues of the VP group and the SO group. Left atrial fibrosis and apoptosis were evaluated by Masson staining and caspase-3 activity assays, respectively.

Results

The networks showed 48 miRNAs in LAA tissues. MiR-1 and miR-499 were validated using an animal model of chronic AF. The expression level of miR-1 was increased, and miR-499 was decreased in VP group tissues compared to SO group tissues in LAAs (P < 0.05), which were correlated with left atrial fibrosis and apoptosis in AF.

Conclusion

This study provides a better understanding of the alterations in miRNA-1 and miR-499 in chronic AF from the perspective of the miRNA-mRNA network and corroborates findings through experimental validation. These findings may offer novel potential therapeutic targets for AF in the future.

Prognostic cardiovascular cut-off values of dietary caffeine in a cohort of unselected men and women from general population

BACKGROUND AND AIMS

Among an unselected cohort of men and women from general population (n = 1.668), the prognostic effects of being over the cut-off of all-source dietary caffeine intake were studied.

METHODS AND RESULTS

Prognostic cut-off values for coronary events, incident heart failure (HF), cerebrovascular events (CBV) and arrhythmic events (ARR) were found by means of the receiver-operating-characteristic curves method. Those for HF (>230 mg/day), for CBV (>280 mg/day) and for ARR (>280 mg/day) were confirmed in multivariate Cox analysis adjusted for age, body mass index, circulating thyroid hormone, diabetes mellitus, arterial hypertension, smoking, dietary intake of ethanol, basal heart rate, low-density-lipoprotein cholesterol, forced expiratory volume in 1 s and β-blocking therapy. Being over these cut-off values was associated to a reduced hazard ratio during the follow-up in the whole cohort (HR 0.678, 95%CI 0.567-0.908, p = 0.009 for HF; 0.651, 95%CI 0.428-0.994, p = 0.018 for CBV; 0.395, 95%CI 0.395-0.933, p = 0.022 for ARR) and in men (0.652, 0.442-0.961, p = 0.029; 0.432, 0.201-0.927, p = 0.03; 0.553, 0.302-1.000, p = 0.05, respectively) but not in women. The caffeine-induced risk decrease observed in the whole cohort is therefore entirely attributable to men. In the case of HF, heart rate entered the risk equation in a positive manner without rejecting caffeine. The -163C>A polymorphism of the CYP1A2 gene, codifying for ability to metabolize caffeine, introduced in sensitivity analysis, did not alter the prognostic models.

CONCLUSION

Men introducing >230 mg/day caffeine show a reduced risk of HF, and those introducing >280 mg/day a reduced risk of CBV and ARR independent of genetic pattern.

Three-dimensional electroanatomic mapping characteristics of superior vena cava myocardial sleeve and sinoatrial node in patients with atrial fibrillation

Background

Three-dimensional activation mapping during sinus rhythm can demonstrate the earliest atrial activation (EAA) site, which could be the sinoatrial node (SAN). We aimed to compare the electroanatomical characteristics of superior vena cava (SVC), myocardial sleeve, and SAN between patients with atrial fibrillation (AF) and non-AF.

Materials and methods

In this study, 136 patients with AF were assigned to the study group, and 20 patients with premature ventricular contractions (PVCs) who had no history of AF were assigned to the control group. The right atrium (RA) and SVC anatomical activation models were constructed, and the EAA of SAN was delineated using the CARTO3 mapping system. The length of the SVC myocardial sleeve (LSVC) was measured.

Results

Of the 136 patients, 93 patients had paroxysmal AF (PAF), and 43 patients had persistent AF (PsAF). The LSVC was not significantly different among AF and non-AF, PAF, and PsAF. The LSVC in men was longer than in women (42.1 ± 9.4 mm vs. 35.4 ± 8.1 mm, p < 0.001). The LSVC was longer in patients with EAA of SAN above the RA-SVC junction than in those with below the RA-SVC junction (p < 0.001). The EAA of SAN was below the RA-SVC junction in 64/136 (47.1%) and was above the junction in 72/136 (52.9%) patients with AF. The spatial distribution of the EAA of SAN between PAF and PsAF was not different. There was a trend of statistical difference in the distribution of the EAA of SAN between PsAF and non-AF.

Conclusion

The EAA of SAN was located in the SVC in most of the patients, especially in patients with PsAF.

Risk of sick sinus syndrome in patients diagnosed with atrial fibrillation: A population-based cohort

BACKGROUND

Sinoatrial node dysfunction and atrial fibrillation (AF) frequently coexist and interact with each other, often to initiate and perpetuate each other.

OBJECTIVE

To determine the effect of AF on the incidence and risk of sick sinus syndrome (SSS).

METHODS

The association of incident AF with the development of incident SSS was assessed from 2004 to 2014 in 302 229 SSS- and pacemaker-free subjects aged ≥60 years in the Korea National Health Insurance Service-Senior cohort.

RESULTS

During an observation period of 1 854 800 person-years, incident AF was observed in a total of 12 797 subjects (0.69%/year). The incidence of SSS was 3.4 and 0.2 per 1000 person-years in the propensity score-matched incident AF and no-AF groups, respectively. After adjustment, the significantly increased risk of SSS was observed in the incident AF group, with a hazard ratio (HR) of 13.4 (95% confidence interval [CI]: 8.4-21.4). This finding was consistently observed after censoring for heart failure (HR: 16.0; 95% CI: 9.2-28.0) or heart failure/myocardial infarction (HR: 16.6; 95% CI: 9.3-29.7). Incident AF also was associated with an increased risk of pacemaker implantation related with both SSS (HR: 21.8; 95% CI: 8.7-18.4) and atrioventricular (AV) block (HR: 9.5; 95% CI: 4.9-18.4). These results were consistent regardless of sex and comorbidities.

CONCLUSION

Incident AF was associated with more than 10 times increased risk of SSS in an elderly population regardless of comorbidities. The risk of pacemaker implantations related with both sinus node dysfunction and AV block was increased in the elderly population with incident AF.

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

Molecular Signatures of Sinus Node Dysfunction Induce Structural Remodeling in the Right Atrial Tissue

The sinus node (SN) is located at the apex of the cardiac conduction system, and SN dysfunction (SND)-characterized by electrical remodeling-is generally attributed to idiopathic fibrosis or ischemic injuries in the SN. SND is associated with increased risk of cardiovascular disorders, including syncope, heart failure, and atrial arrhythmias, particularly atrial fibrillation. One of the histological SND hallmarks is degenerative atrial remodeling that is associated with conduction abnormalities and increased right atrial refractoriness. Although SND is frequently accompanied by increased fibrosis in the right atrium (RA), its molecular basis still remains elusive. Therefore, we investigated whether SND can induce significant molecular changes that account for the structural remodeling of RA. Towards this, we employed a rabbit model of experimental SND, and then compared the genome-wide RNA expression profiles in RA between SND-induced rabbits and sham-operated controls to identify the differentially expressed transcripts. The accompanying gene enrichment analysis revealed extensive pro-fibrotic changes within 7 days after the SN ablation, including activation of transforming growth factor-β (TGF-β) signaling and alterations in the levels of extracellular matrix components and their regulators. Importantly, our findings suggest that periostin, a matricellular factor that regulates the development of cardiac tissue, might play a key role in mediating TGF-β-signaling-induced aberrant atrial remodeling. In conclusion, the present study provides valuable information regarding the molecular signatures underlying SND-induced atrial remodeling, and indicates that periostin can be potentially used in the diagnosis of fibroproliferative cardiac dysfunctions.

In silico study of the effects of anti-arrhythmic drug treatment on sinoatrial node function for patients with atrial fibrillation

Sinus node dysfunction (SND) is often associated with atrial fibrillation (AF). Amiodarone is the most frequently used agent for maintaining sinus rhythm in patients with AF, but it impairs the sinoatrial node (SAN) function in one-third of AF patients. This study aims to gain mechanistic insights into the effects of the antiarrhythmic agents in the setting of AF-induced SND. We have adapted a human SAN model to characterize the SND conditions by incorporating experimental data on AF-induced electrical remodelling, and then integrated actions of drugs into the modified model to assess their efficacy. Reductions in pacing rate upon the implementation of AF-induced electrical remodelling associated with SND agreed with the clinical observations. And the simulated results showed the reduced funny current (If) in these remodelled targets mainly contributed to the heart rate reduction. Computational drug treatment simulations predicted a further reduction in heart rate during amiodarone administration, indicating that the reduction was the result of actions of amiodarone on INa, IKur, ICaL, ICaT, If and beta-adrenergic receptors. However, the heart rate was increased in the presence of disopyramide. We concluded that disopyramide may be a desirable choice in reversing the AF-induced SND phenotype.

TBX18 overexpression enhances pacemaker function in a rat subsidiary atrial pacemaker model of sick sinus syndrome

KEY POINTS

The sinoatrial node (SAN) is the primary pacemaker of the heart. SAN dysfunction, or 'sick sinus syndrome', can cause excessively slow heart rates and pauses, leading to exercise limitation and syncope, currently treated by implantation of an electronic pacemaker. 'Biopacemaking' utilises gene therapy to restore pacemaker activity by manipulating gene expression. Overexpressing the HCN pacemaker ion channel has been widely used with limited success. We utilised bradycardic rat subsidiary atrial pacemaker tissue to evaluate alternative gene targets: the Na+ /Ca2+ exchanger NCX1, and the transcription factors TBX3 and TBX18 known to be involved in SAN embryonic development. TBX18 overexpression restored normal SAN function, as assessed by increased rate, improved heart rate stability and restoration of isoprenaline response. TBX3 and NCX1 were not effective in accelerating the rate of subsidiary atrial pacemaker tissue. Gene therapy targeting TBX18 could therefore have the potential to restore pacemaker function in human sick sinus syndrome obviating electronic pacemakers.

ABSTRACT

The sinoatrial node (SAN) is the primary pacemaker of the heart. Disease of the SAN, sick sinus syndrome, causes heart rate instability in the form of bradycardia and pauses, leading to exercise limitation and syncope. Biopacemaking aims to restore pacemaker activity by manipulating gene expression, and approaches utilising HCN channel overexpression have been widely used. We evaluated alternative gene targets for biopacemaking to restore normal SAN pacemaker physiology within bradycardic subsidiary atrial pacemaker (SAP) tissue, using the Na+ /Ca2+ exchanger NCX1, and the transcription factors TBX3 and TBX18. TBX18 expression in SAP tissue restored normal SAN function, as assessed by increased rate (SAN 267.5 ± 13.6 bpm, SAP 144.1 ± 8.6 bpm, SAP-TBX18 214.4 ± 14.4 bpm; P < 0.001), improved heart rate stability (standard deviation of RR intervals fell from 39.3 ± 7.2 ms to 6.9 ± 0.8 ms, P < 0.01; root mean square of successive differences of RR intervals fell from 41.7 ± 8.2 ms to 6.1 ± 1.2 ms, P < 0.01; standard deviation of points perpendicular to the line of identity of Poincaré plots (SD1) fell from 29.5 ± 5.8 ms to 7.9 ± 2.0 ms, P < 0.05) and restoration of isoprenaline response (increases in rates of SAN 65.5 ± 1.3%, SAP 28.4 ± 3.4% and SAP-TBX18 103.3 ± 10.2%; P < 0.001). These changes were driven by a TBX18-induced switch in the dominant HCN isoform in SAP tissue, with a significant upregulation of HCN2 (from 1.01 × 10-5  ± 2.2 × 10-6 to 2.8 × 10-5  ± 4.3 × 10-6 arbitrary units, P < 0.001). Biophysically detailed computer modelling incorporating isoform-specific HCN channel electrophysiology confirmed that the measured changes in HCN abundance could account for the observed changes in beating rates. TBX3 and NCX1 were not effective in accelerating the rate of SAP tissue.

Caffeine intake reduces incident atrial fibrillation at a population level

Background

The general belief is that caffeine increases the risk of hyperkinetic arrhythmias, including atrial fibrillation. The aim of this study is to investigate the effect of chronic caffeine intake on incident atrial fibrillation in general population.

Design and methods

A population cohort of 1475 unselected men and women observed for 12 years and left free to intake food or beverages containing caffeine was studied. Subjects were stratified into tertiles of caffeine intake both in the whole cohort and after genotyping for the –163C > A polymorphism of the CYP1A2 gene, regulating caffeine metabolism.

Results

In the whole cohort, the 12-year incidence of atrial fibrillation was significantly lower in the third (2.2%) than in the first (10.2%) or second (5.7%) tertile of caffeine intake ( P < 0.001). The same trend was observed in all genotypes; the apparently steeper reduction of atrial fibrillation in slow caffeine metabolisers found at univariate analysis was proved wrong by multivariate Cox analysis. Age, chronic pulmonary disease, history of heart failure and of coronary artery disease, and systolic blood pressure − but not the genotype or the caffeine ×  CYP1A2 interaction term − were significant confounders of the association between incident atrial fibrillation and being in the third tertile of caffeine intake (hazard ratio 0.249, 95% confidence intervals 0.161–0.458, P < 0.01).

Conclusions

A higher caffeine intake (>165 mmol/day or > 320 mg/day) is associated with a lower incidence of atrial fibrillation in the 12-year epidemiological prospective setting based on the general population.

Oscillatory behavior of P wave duration and PR interval in experimental congestive heart failure: a preliminary study

OBJECTIVE

The relationship between the autonomic nervous system (ANS) modulation of the sinus node and heart rate variability has been extensively investigated. The current study sought to evaluate, in an animal experimental model of pacing-induced tachycardia congestive heart failure (CHF), a possible ANS influence on the P wave duration and PR interval oscillations.

APPROACH

Short-term (5 min) time and frequency domain analysis has been obtained in six dogs for the following electrocardiographic intervals: P wave duration (P), from the onset to peak of P wave (P _p), from the onset of P wave to the q onset (PR) and from the end of P wave to the onset of q wave (P _e R). Direct vagal nerve activity (VNA), stellate ganglion nerve activity (SGNA) and electrocardiogram (ECG) intervals have been evaluated contextually by implantation of three bipolar recording leads.

MAIN RESULTS

At the baseline, multiple regression analysis pointed out that VNA was strongly positively associated with the standard deviation of PP and P _e R intervals (r ²:0.997, p  <  0.05). The same variable was also positively associated with high-frequency (HF) of P expressed in normalized units, of P _p, and of P _e R (b: 0.001) (r ²: 0.993; p  <  0.05). During CHF, most of the time and frequency domain variability significantly decreased from 20% to 50% in comparison to the baseline values (p  <  0.05) and SGNA correlated inversely with the low frequency (LF) obtained from P _e R (p  <  0.05) and PR (p  <  0.05) (r ²:0.899, p  <  0.05). LF components, expressed in absolute and normalized power, obtained from all studied intervals, were reduced significantly during CHF. Any difference between the RR and PP spectral components was observed.

SIGNIFICANCE

The data showed a significant relationship between ANS and atrial ECG variables, independent of the cycle duration. In particular, the oscillations were vagal mediated at the baseline, while sympathetic mediated during CHF. Whereas P wave variability might have a clinical utility in CHF management, it needs to be addressed in specific studies.

The Absence of Atrial Contraction as a Predictor of Permanent Pacemaker Implantation after Maze Procedure with Cryoablation

BACKGROUND

The absence of atrial contraction (AC) after the maze procedure has been reported to cause subsequent annular dilatation and to increase the risk of embolic stroke. We hypothesized that the lack of AC could increase the risk of permanent pacemaker (PPM) implantation in patients undergoing the maze procedure.

METHODS

In 376 consecutive patients who had undergone a cryo-maze procedure and combined valve operation, recovery of AC was assessed at baseline and at immediate (≤2 weeks), early (≤1 year, 4.6±3.8 months), and late (>1 year, 3.5±1.1 years) postoperative stages.

RESULTS

With a median follow-up of 53 months, 10 patients underwent PPM implantation. Seven PPM implants were for sinus node dysfunction (pauses of 9.6±2.4 seconds), one was for marked sinus bradycardia, and two were for advanced/complete atrioventricular block. The median (interquartile range) time to PPM implantation was 13.8 (0.5-68.2) months. Our time-varying covariate Cox models showed that the absence of AC was a risk factor for PPM implantation (hazard ratio, 11.92; 95% confidence interval, 2.52 to 56.45; p=0.002).

CONCLUSION

The absence of AC may be associated with a subsequent risk of PPM implantation.

Sinus Node and Atrial Arrhythmias

Although sinus node dysfunction (SND) and atrial arrhythmias frequently coexist and interact, the putative mechanism linking the 2 remain unclear. Although SND is accompanied by atrial myocardial structural changes in the right atrium, atrial fibrillation (AF) is a disease of variable interactions between left atrial triggers and substrate most commonly of left atrial origin. Significant advances have been made in our understanding of the genetic and pathophysiologic mechanism underlying the development and progression of SND and AF. Although some patients manifest SND as a result of electric remodeling induced by periods of AF, others develop progressive atrial structural remodeling that gives rise to both conditions together. The treatment strategy will thus vary according to the predominant disease phenotype. Although catheter ablation will benefit patients with predominantly AF and secondary SND, cardiac pacing may be the mainstay of therapy for patients with predominant fibrotic atrial cardiomyopathy. This contemporary review summarizes current knowledge on sinus node pathophysiology with the broader goal of yielding insights into the complex relationship between sinus node disease and atrial arrhythmias.

Combined Algorithm Using a Poor Increase in Inferior P-Wave Amplitude During Sympathetic Stimulation and Sinus Node Recovery Time for the Diagnosis of Sick Sinus Syndrome

BACKGROUND

This study sought to evaluate whether a poor increase in inferior P-wave amplitude during sympathetic stimulation might be a helpful diagnostic tool for sick sinus syndrome (SSS).

METHODS AND RESULTS

Three-dimensional electroanatomic mapping of the right atrium, inferior P-wave amplitude and conventional corrected sinus node recovery time (CSNRT) were compared in 112 consecutive atrial fibrillation (AF) patients with (n=21) and without SSS (n=91). The significant cranial shift of earliest activation site (EAS) (the distance from the superior vena cava to the EAS: 11.1 vs. 5.9 mm, P<0.001) and the increases of inferior P-wave amplitudes during isoproterenol infusion (all P<0.001) were observed in patients without SSS. However, cranial shift of EAS (16.5 vs. 14.2 mm, P=0.375) and P-wave amplitude increases were not observed in those with SSS. Although CSNRT >550 ms showed a sensitivity of 50% and specificity of 84% for diagnosing SSS, poor increases of P-waves amplitude in lead aVF (<0.1 mV) during isoproterenol infusion showed an improved sensitivity of 71% and specificity of 89%. Finally, the combined algorithm using CSNRT >550 ms and poor increase of P-waves amplitude in lead aVF showed more improved diagnostic accuracy (sensitivity 89%, specificity 75%).

CONCLUSIONS

A combined algorithm using inferior P-wave amplitude showed improved performance for the diagnosis of SSS compared with CSNRT >550 ms alone.

Function and dysfunction of human sinoatrial node

Sinoatrial node (SAN) automaticity is jointly regulated by a voltage (cyclic activation and deactivation of membrane ion channels) and Ca²⁺ clocks (rhythmic spontaneous sarcoplasmic reticulum Ca²⁺ release). Using optical mapping in Langendorff-perfused canine right atrium, we previously demonstrated that the β-adrenergic stimulation pushes the leading pacemaker to the superior SAN, which has the fastest activation rate and the most robust late diastolic intracellular calcium (Ca_i) elevation. Dysfunction of the superior SAN is commonly observed in animal models of heart failure and atrial fibrillation (AF), which are known to be associated with abnormal SAN automaticity. Using the 3D electroanatomic mapping techniques, we demonstrated that superior SAN served as the earliest atrial activation site (EAS) during sympathetic stimulation in healthy humans. In contrast, unresponsiveness of superior SAN to sympathetic stimulation was a characteristic finding in patients with AF and SAN dysfunction, and the 3D electroanatomic mapping technique had better diagnostic sensitivity than corrected SAN recovery time testing. However, both tests have significant limitations in detecting patients with symptomatic sick sinus syndrome. Recently, we reported that the location of the EAS can be predicted by the amplitudes of P-wave in the inferior leads. The inferior P-wave amplitudes can also be used to assess the superior SAN responsiveness to sympathetic stimulation. Inverted or isoelectric P-waves at baseline that fail to normalize during isoproterenol infusion suggest SAN dysfunction. P-wave morphology analyses may be helpful in determining the SAN function in patients at risk of symptomatic sick sinus syndrome.

Atrial fibrillation complicated by heart failure induces distinct remodeling of calcium cycling proteins

Atrial fibrillation (AF) and heart failure (HF) are two of the most common cardiovascular diseases. They often coexist and account for significant morbidity and mortality. Alterations in cellular Ca²⁺ homeostasis play a critical role in AF initiation and maintenance. This study was designed to specifically elucidate AF-associated remodeling of atrial Ca²⁺ cycling in the presence of mild HF. AF was induced in domestic pigs by atrial burst pacing. The animals underwent electrophysiologic and echocardiographic examinations. Ca²⁺ handling proteins were analyzed in right atrial tissue obtained from pigs with AF (day 7; n = 5) and compared to sinus rhythm (SR) controls (n = 5). During AF, animals exhibited reduction of left ventricular ejection fraction (from 73% to 58%) and prolonged atrial refractory periods. AF and HF were associated with suppression of protein kinase A (PKA)_RII (-62%) and Ca²⁺-calmodulin-dependent kinase II (CaMKII) δ by 37%, without changes in CaMKIIδ autophosphorylation. We further detected downregulation of L-type calcium channel (LTCC) subunit α₂ (-75%), sarcoplasmic reticulum Ca²⁺-ATPase (Serca) 2a (-29%), phosphorylated phospholamban (Ser16, -92%; Thr17, -70%), and phospho-ryanodine receptor 2 (RyR2) (Ser2808, -62%). Na⁺-Ca²⁺ exchanger (NCX) levels were upregulated (+473%), whereas expression of Ser2814-phosphorylated RyR2 and LTCCα_1c subunits was not significantly altered. In conclusion, AF produced distinct arrhythmogenic remodeling of Ca²⁺ handling in the presence of tachycardia-induced mild HF that is different from AF without structural alterations. The changes may provide a starting point for personalized approaches to AF treatment.

Sick sinus syndrome and atrial fibrillation in older persons - A view from the sinoatrial nodal myocyte

Sick sinus syndrome remains a highly relevant clinical entity, being responsible for the implantation of the majority of electronic pacemakers worldwide. It is an infinitely more complex disease than it was believed when first described in the mid part of the 20th century. It not only involves the innate leading pacemaker region of the heart, the sinoatrial node, but also the atrial myocardium, predisposing to atrial tachydysrhythmias. It remains controversial as to whether the dysfunction of the sinoatrial node directly causes the dysfunction of the atrial myocardium, or vice versa, or indeed whether these two aspects of the condition arise through some related underlying pathological mechanism, such as extracellular matrix remodeling, i.e., fibrosis. This review aims to shed new light on the myriad possible contributing factors in the development of sick sinus syndrome, with a particular focus on the sinoatrial nodal myocyte. This article is part of a Special Issue entitled CV Aging.

Ca(2+) cycling properties are conserved despite bradycardic effects of heart failure in sinoatrial node cells

BACKGROUND

In animal models of heart failure (HF), heart rate decreases due to an increase in intrinsic cycle length (CL) of the sinoatrial node (SAN). Pacemaker activity of SAN cells is complex and modulated by the membrane clock, i.e., the ensemble of voltage gated ion channels and electrogenic pumps and exchangers, and the Ca(2+) clock, i.e., the ensemble of intracellular Ca(2+) ([Ca(2+)]i) dependent processes. HF in SAN cells results in remodeling of the membrane clock, but few studies have examined its effects on [Ca(2+)]i homeostasis.

METHODS

SAN cells were isolated from control rabbits and rabbits with volume and pressure overload-induced HF. [Ca(2+)]i concentrations, and action potentials (APs) and Na(+)-Ca(2+) exchange current (INCX) were measured using indo-1 and patch-clamp methodology, respectively.

RESULTS

The frequency of spontaneous [Ca(2+)]i transients was significantly lower in HF SAN cells (3.0 ± 0.1 (n = 40) vs. 3.4 ± 0.1 Hz (n = 45); mean ± SEM), indicating that intrinsic CL was prolonged. HF slowed the [Ca(2+)]i transient decay, which could be explained by the slower frequency and reduced sarcoplasmic reticulum (SR) dependent rate of Ca(2+) uptake. Other [Ca(2+)]i transient parameters, SR Ca(2+) content, INCX density, and INCX-[Ca(2+)]i relationship were all unaffected by HF. Combined AP and [Ca(2+)]i recordings demonstrated that the slower [Ca(2+)]i transient decay in HF SAN cells may result in increased INCX during the diastolic depolarization, but that this effect is likely counteracted by the HF-induced increase in intracellular Na(+). β-adrenergic and muscarinic stimulation were not changed in HF SAN cells, except that late diastolic [Ca(2+)]i rise, a prominent feature of the Ca(2+) clock, is lower during β-adrenergic stimulation.

CONCLUSIONS

HF SAN cells have a slower [Ca(2+)]i transient decay with limited effects on pacemaker activity. Reduced late diastolic [Ca(2+)]i rise during β-adrenergic stimulation may contribute to an impaired increase in intrinsic frequency in HF SAN cells.

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

Chronic amiodarone therapy impairs the function of the superior sinoatrial node in patients with atrial fibrillation

BACKGROUND

The mechanisms underlying amiodarone-induced sinoatrial node (SAN) dysfunction remain unclear, so we used 3-dimensional endocardial mapping of the right atrium (RA) to investigate.

METHODS AND RESULTS

In a matched-cohort design, 18 patients taking amiodarone before atrial fibrillation (AF) ablation (amiodarone group) were matched for age, sex and type of AF with 18 patients who had undergone AF ablation without taking amiodarone (no-amiodarone group). The amiodarone group had a slower heart rate than the no-amiodarone group at baseline and during isoproterenol infusion. Only the amiodarone group had sick sinus syndrome (n=4, 22%, P=0.03) and abnormal (>550ms) corrected SAN recovery time (n=5, 29%; P=0.02). The median distance from the junction of the superior vena cava (SVC) and RA to the most cranial earliest activation site (EAS) was longer in the amiodarone group than in the no-amiodarone group at baseline (20.5 vs. 10.6mm, P=0.04) and during isoproterenol infusion (12.8 vs. 6.3mm, P=0.03). The distance from the SVC-RA junction to the EAS negatively correlated with the P-wave amplitudes of leads II (r=-0.47), III (r=-0.60) and aVF (r=-0.56) (P<0.001 for all).

CONCLUSIONS

In a quarter of the AF patients, amiodarone causes superior SAN dysfunction, which results in a downward shift of the EAS and reduced P-wave amplitude in leads II, III and aVF at baseline and during isoproterenol infusion.

Sinus node revisited

PURPOSE OF REVIEW

Sinus node disease (SND) is a common clinical condition and is the most common indication for permanent pacemaker implantation. This review aims to revisit the complex sinus node anatomy, the evolving understanding of its pacemaking mechanisms, the atrial myopathy in SND and sinus node remodeling.

RECENT FINDINGS

Recent high-density noncontact mapping of the human sinus node showed multiple origins of sinus activation and exit sites with preferential pathways of conduction. Perhaps, a newly described discrete paranodal area containing a molecular mixture of nodal and atrial cells may account for this long recognized discrepancy between the anatomical and functional sinus node. The funny current (I(f)) driven 'membrane clock' is not solely responsible for sinus node automaticity, following recent recognition of the importance of the 'calcium clock'. Several molecular links to sinus node remodeling have recently been identified: loss of connexin-43 expression and down-regulation of I(ca,L) in aging; reduced I(f) and down-regulation of I(f) encoding HCN4 and HCN2 subunits in heart failure; and calcium clock malfunction with down-regulated HCN4, HCN2 and minK in atrial fibrillation.

SUMMARY

Ongoing research with improved technology and techniques continues to unravel new understandings and challenges to the century old discovery of the anatomical sinus node.

Abnormal response of superior sinoatrial node to sympathetic stimulation is a characteristic finding in patients with atrial fibrillation and symptomatic bradycardia

BACKGROUND

We hypothesized that unresponsiveness of superior sinoatrial node (SAN) to sympathetic stimulation is strongly associated with the development of symptomatic bradycardia in patients with atrial fibrillation (AF).

METHODS AND RESULTS

We performed 3D endocardial mapping in healthy controls (group 1, n=10) and patients with AF without (group 2, n=57) or with (group 3, n=15) symptomatic bradycardia at baseline and during isoproterenol infusion. Corrected SAN recovery time was abnormal in 0%, 11%, and 36% of groups 1, 2, and 3, respectively (P=0.02). At baseline, 90%, 26%, and 7% (P<0.001) of the patients had multicentric SAN activation patterns. For groups 1, 2, and 3, the median distance from the superior vena cava-right atrial junction to the most cranial earliest activation site (EAS) was 5.0 (25-75 percentile range, 3.5-21.3), 10.0 (4-20), and 17.5 (12-34) mm at baseline (P=0.01), respectively, and 4.0 (0-5), 5.0 (1-10), and 15.0 (5.4-33.3) mm, respectively, during isoproterenol infusion (P=0.01), suggesting an upward shift of EAS during isoproterenol infusion. However, although the EAS during isoproterenol infusion was at the upper one third of the crista terminalis in 100% of group 1 and 78% of group 2 patients, only 20% of group 3 patients showed a move of the EAS to that region (P<0.001).

CONCLUSIONS

Superior SAN serves as the EAS during sympathetic stimulation in patients without AF and in most patients with AF without symptomatic bradycardia. In contrast, unresponsiveness of superior SAN to sympathetic stimulation is a characteristic finding in patients with AF and symptomatic bradycardia.

The role of the calcium and the voltage clocks in sinoatrial node dysfunction

Recent evidence indicates that the voltage clock (cyclic activation and deactivation of membrane ion channels) and Ca(2+) clocks (rhythmic spontaneous sarcoplasmic reticulum Ca(2+) release) jointly regulate sinoatrial node (SAN) automaticity. However, the relative importance of the voltage clock and Ca(2+) clock for pacemaking was not revealed in sick sinus syndrome. Previously, we mapped the intracellular calcium (Ca(i)) and membrane potentials of the normal intact SAN simultaneously using optical mapping in Langendorff-perfused canine right atrium. We demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with robust late diastolic Ca(i) elevation (LDCAE) during β-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic sarcoplasmic reticulum (SR) Ca(2+) release and was closely related to heart rate changes. In contrast, in pacing induced canine atrial fibrillation and SAN dysfunction models, Ca(2+) clock of SAN was unresponsiveness to β-adrenergic stimulation and caffeine. Ryanodine receptor 2 (RyR2) in SAN was down-regulated. Using the prolonged low dose isoproterenol together with funny current block, we produced a tachybradycardia model. In this model, chronically elevated sympathetic tone results in abnormal pacemaking hierarchy in the right atrium, including suppression of the superior SAN and enhanced pacemaking from ectopic sites. Finally, if the LDCAE was too small to trigger an action potential, then it induced only delayed afterdepolarization (DAD)-like diastolic depolarization (DD). The failure of DAD-like DD to consistently trigger a sinus beat is a novel mechanism of atrial arrhythmogenesis. We conclude that dysfunction of both the Ca(2+) clock and the voltage clock are important in sick sinus syndrome.

Mapping cardiac pacemaker circuits: methodological puzzles of the sinoatrial node optical mapping

Historically, milestones in science are usually associated with methodological breakthroughs. Likewise, the advent of electrocardiography, microelectrode recordings and more recently optical mapping have ushered in new periods of significance of advancement in elucidating basic mechanisms in cardiac electrophysiology. As with any novel technique, however, data interpretation is challenging and should be approached with caution, as it cannot be simply extrapolated from previously used methodologies and with experience and time eventually becomes validated. A good example of this is the use of optical mapping in the sinoatrial node (SAN): when microelectrode and optical recordings are obtained from the same site in myocardium, significantly different results may be noted with respect to signal morphology and as a result have to be interpreted by a different set of principles. Given the rapid spread of the use of optical mapping, careful evaluation must be made in terms of methodology with respect to interpretation of data gathered by optical sensors from fluorescent potential-sensitive dyes. Different interpretations of experimental data may lead to different mechanistic conclusions. This review attempts to address the origin and interpretation of the "double component" morphology in the optical action potentials obtained from the SAN region. One view is that these 2 components represent distinctive signals from the SAN and atrial cells and can be fully separated with signal processing. A second view is that the first component preceding the phase 0 activation represents the membrane currents and intracellular calcium transients induced diastolic depolarization from the SAN. Although the consensus from both groups is that ionic mechanisms, namely the joint action of the membrane and calcium automaticity, are important in the SAN function, it is unresolved whether the double-component originates from the recording methodology or represents the underlying physiology. This overview aims to advance a common understanding of the basic principles of optical mapping in complex 3D anatomic structures.

The initiation of the heart beat

During a normal lifetime, the heart may beat over 2 billion times, but the mechanisms by which the heart beats are initiated remain a subject of intense investigation. Since the discovery of a pacemaker current (I(f)) in 1978, multiple studies have shown that rhythmic changes in membrane voltage (the "membrane voltage clock") underlie the mechanisms of automaticity. The I(f) is a depolarization current activated during hyperpolarization. Therefore, when the cardiac cells recover, the I(f) is activated and slowly depolarizes the cell membrane, leading to the onset of action potential. Recent studies, however, suggest that increased intracellular Ca (Ca(i)) induced by spontaneous rhythmic sarcoplasmic reticulum Ca release (the "calcium clock") is also jointly responsible for the initiation of the heart beat. Elevated Ca(i) activates another ionic current (the sodium-calcium exchanger current or I(NCX)), leading to spontaneous phase 4 depolarization. Under normal conditions, both clocks are needed to initiate the heart beat. Malfunction of the clocks is associated with sinus node dysfunction in heart failure and atrial fibrillation. More studies are needed to determine how both clocks work together to initiate heart beat under normal and disease conditions.