Sinus node dysfunction: current understanding and future directions

Bradycardias in Patients with Pulmonary Hypertension-Prevalence, Pathophysiology and Clinical Relevance

INTRODUCTION

Arrhythmias are a frequent complication of pulmonary hypertension (PH). Supraventricular tachycardias (SVT) are predominantly reported and are associated with clinical deterioration and an increased mortality. In contrast, the prevalence and clinical relevance of bradycardias is largely unclear. Therefore, the aim of the present study was to determine a prevalence of bradycardias in PH patients and to outline their clinical relevance.

MATERIAL AND METHODS

Between January 2000 and June 2013, consecutive PH patients were pro- and retrospectively enrolled in two cohorts. Patients received either a 24 h or 72 h Holter ECG.

RESULTS

A total of 314 patients (58% female, mean age: 63 years) from PH groups 1-5 (39%, 11%, 19%, 28%, 3%) were included. Basic heart rhythm was sinus rhythm in 87% of patients (9% atrial fibrillation, 2% atrial flutter and 2% paced rhythm). Further arrhythmias were detected in 34% of patients (SVT: 12%, non-sustained ventricular tachycardia: 16%) with a 6% prevalence of relevant bradycardias. Atrioventricular block was revealed in 5% of patients (seven first-degree, one and three second-degree Wenckebach and Mobitz type, respectively, four third-degree), and 1% revealed sinoatrial block (one second-degree, third-degree and unspecified each).

CONCLUSIONS

The prevalence of bradycardias appears to be about 5-10% in PH patients. Most of them are short and self-limiting. However, some patients experience syncope or clinical deterioration and, therefore, need specific treatment. To find these patients, long-term ECG monitoring combined with ECG-symptom correlation may be useful. Bradycardic medication should be excluded as a cause.

Causality of Childhood and Adult Body Mass Index on Sick Sinus Syndrome: A Mendelian Randomization Study

Background The relationship between body mass index (BMI) and the risk of sick sinus syndrome (SSS) remains unclear. Clarifying the impact of BMI on SSS at different life stages is essential for advancing precision medicine and implementing effective prevention strategies to reduce the burden of SSS. Methods The causalities of childhood and adult BMI with SSS were investigated by univariate and multivariate Mendelian randomization. Reverse causalities were also explored to improve the accuracy of causality findings. Different sources of exposure data were used for replication analysis, and the effects of sample overlap were investigated using MRlap. The stability of the results was further enhanced through meta-analysis. Results There was a positive correlation of adult BMI with the risk of SSS in both the FinnGen (odds ratio (OR) = 1.17, 95% confidence interval (CI) 1.01-1.35, P = 0.031) and Integrative Epidemiology Unit (IEU) open genome-wide association study (GWAS) project (OR = 1.18, 95% CI 1.04-1.34, P = 0.009) databases. The causality remained valid after the correction of telomere length. There was no causality detected between childhood BMI and SSS, as determined by independent studies of the Early Growth Genetics (EGG) 2020 (OR = 1.06, 95% CI 0.89-1.27, P = 0.513) and EGG2015 (OR = 1.02, 95% CI 0.97-1.09, P = 0.423). Meta-analysis results further confirmed the reliability of the causal inference. Conclusions The findings indicate that elevated BMI in adults, particularly among middle-aged and elderly populations, increases the risk of developing SSS. In contrast, no causal relationship was observed between childhood BMI and SSS, suggesting that the influence of BMI on SSS susceptibility may predominantly emerge during later life stages. These results highlight the need for targeted public health interventions to address adult obesity as a modifiable risk factor for SSS.

Lower Risk of New-Onset Atrial Fibrillation in Conduction System Pacing Compared With Right Ventricular Pacing

BACKGROUND

Conduction system pacing (CSP) has been reported to improve clinical outcomes in comparison of right ventricular pacing (RVP). However, the performance between CSP and RVP on the risk of new-onset atrial fibrillation (AF) remains elusive.

METHODS

Four online databases were systematically searched up to July 1, 2024. Studies comprising the rate/risk of new-onset AF between CSP and RVP group were included. Subgroup analysis was performed to screen the potential determinants for the new-onset AF risk for CSP therapy. The pooled risk of new-onset AF based on ventricular pacing burden (Vp) between CSP and RVP group were evaluated.

RESULTS

A total of six studies including 1577 patients requiring pacing therapy were eligible. The pooled new-onset AF rates for CSP and RVP group were 0.09 and 0.27, respectively. Compared with RVP group, CSP group showed a lower pooled risk (risk ratio [RR] 0.38, p = 0.000) and adjusted risk (hazard ratio [HR] 0.32, p = 0.000) of new-onset AF. Meanwhile, a significant intervention-covariate interaction for the adjusted risk of new-onset AF between CSP and RVP group was identified with Vp < 20% and Vp ≥ 20%.

CONCLUSIONS

Our study suggests that CSP is associate with a significantly lower occurrence of new-onset AF compared with RVP. The Vp ≥ 20% may be the key determinant on the lower risk of new-onset AF with CSP therapy.

TRIAL REGISTRATION

https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42023492551, identifier (CRD42023492551).

The Role of Electrophysiologic Study in Device Selection for Leadless Atrial Pacing

The AVEIR DR (Abbott Medical) was the first dual-chamber leadless pacing system approved by the US Food and Drug Administration. Many patients with sinus node dysfunction have physiologically normal atrioventricular conduction, so an atrial-only pacing system may be appropriate for them. Three patients with symptomatic sinus node dysfunction underwent device implantation with electrophysiologic study, including incremental atrial pacing until loss of 1:1 atrioventricular conduction (atrioventricular Wenckebach cycle length) and His-ventricular interval measurement. If the atrioventricular Wenckebach cycle length was no more than 400 milliseconds and the His-ventricular interval measurement was no more than 55 milliseconds, single-chamber atrial implantation was deemed appropriate. Each patient displayed a different response to atrioventricular conduction testing, demonstrating how electrophysiologic study before device implantation may identify patients for whom atrial-only pacing is not appropriate.

Genetic basis and causal relationship between atrial fibrillation and sinus node dysfunction: Evidence from comprehensive genetic analysis

BACKGROUND

Atrial fibrillation (AF) and sinus node dysfunction (SND) are commonly observed together clinically. However, little is known about the genetic background and causal relationship between the two.

METHODS

Firstly, we investigated the global and local genetic correlations between AF and SND using LDSC and HESS. Then, we identified shared "Novel SNPs" between AF and SND through two complementary cross-trait meta-analyses and mapped the "pleiotropic genes" behind these SNPs, validated by colocalization analysis. Additionally, we explored the degree of genetic enrichment of SNPs in specific tissues using LDSC-SEG and MAGMA, and identified potential functional genes in tissues using summary data-based Mendelian randomization (SMR). Finally, two-sample Mendelian randomization (TSMR) and multivariable Mendelian randomization (MVMR) were used to explore the causal relationship between AF and SND.

RESULTS

Both global and local genetic correlation analyses revealed a high positive genetic correlation between AF and SND. HESS identified 9 shared loci, with chr4(q25-q26) and chr11(p11.12-q11) being prominent. Cross-trait meta-analysis and colocalization analysis identified ENPEP and PITX2 as novel pleiotropic genes. MAGMA revealed genetic enrichment of SNPs for AF and SND in the "Heart Left Ventricle" and "Heart Atrial Appendage" tissues, with CEP68 and BEST3 identified as potential functional genes through SMR. MR analysis indicated that AF increases the risk of SND, even after adjusting for confounding factors.

CONCLUSION

This study provides genetic evidence for the increased risk of SND associated with AF, identifying multiple shared risk loci and enriched tissues, and discovering 2 novel pleiotropic genes and 2 new functional genes.

Ca2+/Calmodulin-Dependent Protein Kinase II (CaMKII) Regulates Basal Cardiac Pacemaker Function: Pros and Cons

The spontaneous firing of the sinoatrial (SA) node, the physiological pacemaker of the heart, is generated within sinoatrial nodal cells (SANCs) and is regulated by a "coupled-clock" pacemaker system, which integrates a "membrane clock", the ensemble of ion channel currents, and an intracellular "Ca2+ clock", sarcoplasmic reticulum-generated local submembrane Ca2+ releases via ryanodine receptors. The interactions within a "coupled-clock" system are modulated by phosphorylation of surface membrane and sarcoplasmic reticulum proteins. Though the essential role of a high basal cAMP level and PKA-dependent phosphorylation for basal spontaneous SANC firing is well recognized, the role of basal CaMKII-dependent phosphorylation remains uncertain. This is a critical issue with respect to how cardiac pacemaker cells fire spontaneous action potentials. This review aspires to explain and unite apparently contradictory results of pharmacological studies in the literature that have demonstrated a fundamental role of basal CaMKII activation for basal cardiac pacemaker function, as well as studies in mice with genetic CaMKII inhibition which have been interpreted to indicate that basal spontaneous SANC firing is independent of CaMKII activation. The assessment of supporting and opposing data regarding CaMKII effects on phosphorylation of Ca2+-cycling proteins and spontaneous firing of SANC in the basal state leads to the necessary conclusion that CaMKII activity and CaMKII-dependent phosphorylation do regulate basal cardiac pacemaker function.

iPSC-Derived Biological Pacemaker-From Bench to Bedside

Induced pluripotent stem cell (iPSC)-derived biological pacemakers have emerged as an alternative to traditional electronic pacemakers for managing cardiac arrhythmias. While effective, electronic pacemakers face challenges such as device failure, lead complications, and surgical risks, particularly in children. iPSC-derived pacemakers offer a promising solution by mimicking the sinoatrial node's natural pacemaking function, providing a more physiological approach to rhythm control. These cells can differentiate into cardiomyocytes capable of autonomous electrical activity, integrating into heart tissue. However, challenges such as achieving cellular maturity, long-term functionality, and immune response remain significant barriers to clinical translation. Future research should focus on refining gene-editing techniques, optimizing differentiation, and developing scalable production processes to enhance the safety and effectiveness of these biological pacemakers. With further advancements, iPSC-derived pacemakers could offer a patient-specific, durable alternative for cardiac rhythm management. This review discusses key advancements in differentiation protocols and preclinical studies, demonstrating their potential in treating dysrhythmias.

Mitochondrial dysfunction is a key link involved in the pathogenesis of sick sinus syndrome: a review

Sick sinus syndrome (SSS) is a grave medical condition that can precipitate sudden death. The pathogenesis of SSS remains incompletely understood. Existing research postulates that the fundamental mechanism involves increased fibrosis of the sinoatrial node and its surrounding tissues, as well as disturbances in the coupled-clock system, comprising the membrane clock and the Ca2+ clock. Mitochondrial dysfunction exacerbates regional tissue fibrosis and disrupts the functioning of both the membrane and calcium clocks. This plays a crucial role in the underlying pathophysiology of SSS, including mitochondrial energy metabolism disorders, mitochondrial oxidative stress damage, calcium overload, and mitochondrial quality control disorders. Elucidating the mitochondrial mechanisms involved in the pathophysiology of SSS and further investigating the disease's mechanisms is of great significance.

Brazilian Guideline for Exercise Testing in Children and Adolescents - 2024

CLASSES OF RECOMMENDATION

LEVELS OF EVIDENCE

Decreased connexin 40 expression of the sinoatrial node mediates ischemic stroke-induced arrhythmia in mice

BACKGROUND

Arrhythmia is the most common cardiac complication after ischemic stroke. Connexin 40 is the staple component of gap junctions, which influences the propagation of cardiac electrical signals in the sinoatrial node. However, the role of connexin 40 in post-stroke arrhythmia remains unclear.

METHODS

In this study, a permanent middle cerebral artery occlusion model was used to simulate the occurrence of an ischemic stroke. Subsequently, an electrocardiogram was utilized to record and assess variations in electrocardiogram measures. In addition, optical tissue clearing and whole-mount immunofluorescence staining were used to confirm the anatomical localization of the sinoatrial node, and the sinoatrial node tissue was collected for RNA sequencing to screen for potential pathological mechanisms. Lastly, the rAAV9-Gja5 virus was injected with ultrasound guidance into the heart to increase Cx40 expression in the sinoatrial node.

RESULTS

We demonstrated that the mice suffering from a permanent middle cerebral artery occlusion displayed significant arrhythmia, including atrial fibrillation, premature ventricular contractions, atrioventricular block, and abnormal electrocardiogram parameters. Of note, we observed a decrease in connexin 40 expression within the sinoatrial node after the ischemic stroke via RNA sequencing and western blot. Furthermore, rAAV9-Gja5 treatment ameliorated the occurrence of arrhythmia following stroke.

CONCLUSIONS

In conclusion, decreased connexin 40 expression in the sinoatrial node contributed to the ischemic stroke-induced cardiac arrhythmia. Therefore, enhancing connexin 40 expression holds promise as a potential therapeutic approach for ischemic stroke-induced arrhythmia.

Electrocardiographic abnormalities in patients with microtia

The main objective of this study was to investigate the incidence and characteristics of electrocardiographic abnormalities in patients with microtia, and to explore cardiac maldevelopment associated with microtia. This retrospective study analyzed a large cohort of microtia patients admitted to Plastic Surgery Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, from September 2017 to August 2022. The routine electrocardiographic reports of these patients were reviewed to assess the incidence and characteristics of abnormalities. The study included a total of 10,151 patients (5598 in the microtia group and 4553 in the control group) who were admitted to the Plastic Surgery Hospital of Peking Union Medical College. The microtia group had a significantly higher incidence of abnormal electrocardiographies compared to the control group (18.3% vs. 13.6%, P < 0.01), even when excluding sinus irregularity (6.1% vs. 4.4%, P < 0.01). Among the 1025 cases of abnormal electrocardiographies in the microtia group, 686 cases were reported with simple sinus irregularity. After excluding sinus irregularity as abnormal, the most prevalent abnormalities was right bundle branch block (37.5%), followed by sinus bradycardia (17.4%), ST-T wave abnormalities (13.3%), atrial rhythm (9.1%), sinus tachycardia (8.3%), and ventricular high voltage (4.7%). Less common ECG abnormalities included atrial tachycardia (2.1%), ventricular premature contraction (2.4%), and ectopic atrial rhythm (1.8%). atrioventricular block and junctional rhythm were present in 1.2% and 0.9% of the cases, respectively. Wolff Parkinson White syndrome and dextrocardia had a lower prevalence, at 0.6% and 0.9%, respectively. The occurrence of electrocardiographic abnormalities in microtia patients was found to be higher compared to the control group. These findings highlight the potential congenital defect in cardiac electrophysiology beyond the presence of congenital heart defect that coincide with microtia.

Cardiac conduction diseases: understanding the molecular mechanisms to uncover targets for future treatments

INTRODUCTION

The cardiac conduction system (CCS) is crucial for maintaining adequate cardiac frequency at rest and modulation during exercise. Furthermore, the atrioventricular node and His-Purkinje system are essential for maintaining atrioventricular and interventricular synchrony and consequently maintaining an adequate cardiac output.

AREAS COVERED

In this review article, we examine the anatomy, physiology, and pathophysiology of the CCS. We then discuss in detail the most common genetic mutations and the molecular mechanisms of cardiac conduction disease (CCD) and provide our perspectives on future research and therapeutic opportunities in this field.

EXPERT OPINION

Significant advancement has been made in understanding the molecular mechanisms of CCD, including the recognition of the heterogeneous signaling at the subcellular levels of sinoatrial node, the involvement of inflammatory and autoimmune mechanisms, and the potential impact of epigenetic regulations on CCD. However, the current treatment of CCD manifested as bradycardia still relies primarily on cardiovascular implantable electronic devices (CIEDs). On the other hand, an If specific inhibitor was developed to treat inappropriate sinus tachycardia and sinus tachycardia in heart failure patients with reduced ejection fraction. More work is needed to translate current knowledge into pharmacologic or genetic interventions for the management of CCDs.

Use of machine learning and Poincaré density grid in the diagnosis of sinus node dysfunction caused by sinoatrial conduction block in dogs

BACKGROUND

Sinus node dysfunction because of abnormal impulse generation or sinoatrial conduction block causes bradycardia that can be difficult to differentiate from high parasympathetic/low sympathetic modulation (HP/LSM).

HYPOTHESIS

Beat-to-beat relationships of sinus node dysfunction are quantifiably distinguishable by Poincaré plots, machine learning, and 3-dimensional density grid analysis. Moreover, computer modeling establishes sinoatrial conduction block as a mechanism.

ANIMALS

Three groups of dogs were studied with a diagnosis of: (1) balanced autonomic modulation (n = 26), (2) HP/LSM (n = 26), and (3) sinus node dysfunction (n = 21).

METHODS

Heart rate parameters and Poincaré plot data were determined [median (25%-75%)]. Recordings were randomly assigned to training or testing. Supervised machine learning of the training data was evaluated with the testing data. The computer model included impulse rate, exit block probability, and HP/LSM.

RESULTS

Confusion matrices illustrated the effectiveness in diagnosing by both machine learning and Poincaré density grid. Sinus pauses >2 s differentiated (P < .0001) HP/LSM (2340; 583-3947 s) from sinus node dysfunction (8503; 7078-10 050 s), but average heart rate did not. The shortest linear intervals were longer with sinus node dysfunction (315; 278-323 ms) vs HP/LSM (260; 251-292 ms; P = .008), but the longest linear intervals were shorter with sinus node dysfunction (620; 565-698 ms) vs HP/LSM (843; 799-888 ms; P < .0001).

CONCLUSIONS

Number and duration of pauses, not heart rate, differentiated sinus node dysfunction from HP/LSM. Machine learning and Poincaré density grid can accurately identify sinus node dysfunction. Computer modeling supports sinoatrial conduction block as a mechanism of sinus node dysfunction.

Cardiovascular aging: spotlight on mitochondria

Mitochondria are cellular organelles critical for ATP production and are particularly relevant to cardiovascular diseases including heart failure, atherosclerosis, ischemia-reperfusion injury, and cardiomyopathies. With advancing age, even in the absence of clinical disease, mitochondrial homeostasis becomes disrupted (e.g., redox balance, mitochondrial DNA damage, oxidative metabolism, and mitochondrial quality control). Mitochondrial dysregulation leads to the accumulation of damaged and dysfunctional mitochondria, producing excessive reactive oxygen species and perpetuating mitochondrial dysfunction. In addition, mitochondrial DNA, cardiolipin, and N-formyl peptides are potent activators of cell-intrinsic and -extrinsic inflammatory pathways. These age-related mitochondrial changes contribute to the development of cardiovascular diseases. This review covers the impact of aging on mitochondria and links these mechanisms to therapeutic implications for age-associated cardiovascular diseases.

Case Report: SCN5A mutations in three young patients with sick sinus syndrome

Background

Sick Sinus Syndrome (SSS) is generally regarded as a degenerative disease with aging; however, genetic mutations have been confirmed to be associated with SSS. Among them, mutations in SCN5A are common in patients with SSS. We report three young SSS patients with SCN5A mutations at different sites that have not been previously reported in Asian patients.

Case presentation

The three patients were all young females who presented with symptoms of severe bradycardia and paroxysmal atrial flutter, for which two patients received ablation therapy. However, after ablation, Holter monitoring indicated a significant long cardiac arrest; therefore, the patients received pacemaker implantation. The three patients had familial SSS, and genetic testing was performed. Mutations were found in SCN5A at different sites in the three families. All three patients received pacemaker implantation, resulting in the symptoms of severe bradycardia disappearing.

Conclusion

SCN5A heterozygous mutations are common among patients clinically affected by SSS. Their causative role is confirmed by our data and by the co-occurrence of genetic arrhythmias among our patients. Genetic testing for SSS cannot be performed as a single gene panel because of feasible literature results, but in presence of familial and personal history of SSS in association with arrhythmias can provide clinically useful information.

The W101C KCNJ5 Mutation Induces Slower Pacing by Constitutively Active GIRK Channels in hiPSC-Derived Cardiomyocytes

Mutations in the KCNJ5 gene, encoding one of the major subunits of cardiac G-protein-gated inwardly rectifying K+ (GIRK) channels, have been recently linked to inherited forms of sinus node dysfunction. Here, the pathogenic mechanism of the W101C KCNJ5 mutation underlying sinus bradycardia in a patient-derived cellular disease model of sinus node dysfunction (SND) was investigated. A human-induced pluripotent stem cell (hiPSCs) line of a mutation carrier was generated, and CRISPR/Cas9-based gene targeting was used to correct the familial mutation as a control line. Both cell lines were further differentiated into cardiomyocytes (hiPSC-CMs) that robustly expressed GIRK channels which underly the acetylcholine-regulated K+ current (IK,ACh). hiPSC-CMs with the W101C KCNJ5 mutation (hiPSCW101C-CM) had a constitutively active IK,ACh under baseline conditions; the application of carbachol was able to increase IK,ACh, further indicating that not all available cardiac GIRK channels were open at baseline. Additionally, hiPSCW101C-CM had a more negative maximal diastolic potential (MDP) and a slower pacing frequency confirming the bradycardic phenotype. Of note, the blockade of the constitutively active GIRK channel with XAF-1407 rescued the phenotype. These results provide further mechanistic insights and may pave the way for the treatment of SND patients with GIRK channel dysfunction.

Selection of patients with symptomatic vagal-induced sinus node dysfunction: Who will be the best candidate for cardioneuroablation?

Sinus node dysfunction is a multifaceted disorder with variable manifestations, the prevalence of which increases with age. In a specific group of patients, excessive vagal activity may be the sole cause for this condition. These patients are characterized as having recurrent daytime symptoms attributed to bradyarrhythmia, no evidence of organic sinus node lesions, cardiac vagal overactivation, and are non-elderly. For sinus node dysfunction patients, a permanent pacemaker implantation appears to be the ultimate solution, although it is not an etiological treatment. Cardioneuroablation is a promising emerging therapy that can fundamentally eliminate symptoms in a highly selective sub-set of sinus node dysfunction patients by cardiac vagal nerve denervation. Denervation with ablation for vagal-induced sinus node dysfunction can effectively improve sinus bradycardia and reduce syncope. To date, guidelines for selection of suitable candidates for cardioneuroablation remain lacking. The primary objective of this study was to distinguish the nature of abnormal sinus node function and to find methods for quantifying vagal tone. Clear selection criteria could help physicians in identification of patients with autonomic imbalance, thereby maximizing patient benefits and the success rate of cardioneuroablations.