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Patients carrying the heterozygous A414G mutation in the HCN4 gene, which encodes the HCN4 protein, demonstrate moderate to severe bradycardia of the heart. Tetramers of HCN4 subunits compose the ion channels in the sinus node that carry the hyperpolarization-activated ‘funny’ current (If ), also named the ‘pacemaker current’. If plays an essential modulating role in sinus node pacemaker activity. To assess the mechanism by which the A414G mutation results in sinus bradycardia, we first performed voltage clamp measurements on wild-type (WT) and heterozygous mutant HCN4 channels expressed in Chinese hamster ovary (CHO) cells. These experiments were performed at physiological temperature using the amphotericin-perforated patch-clamp technique. Next, we applied the experimentally observed mutation-induced changes in the HCN4 current of the CHO cells to If of the single human sinus node cell model developed by Fabbri and coworkers. The half-maximal activation voltage V1/2 of the heterozygous mutant HCN4 current was 19.9 mV more negative than that of the WT HCN4 current (p < 0.001). In addition, the voltage dependence of the heterozygous mutant HCN4 current (de)activation time constant showed a −11.9 mV shift (p < 0.001) compared to the WT HCN4 current. The fully-activated current density, the slope factor of the activation curve, and the reversal potential were not significantly affected by the heterozygous A414G mutation. In the human sinus node computer model, the cycle length was substantially increased, almost entirely due to the shift in the voltage dependence of steady-state activation, and this increase was more prominent under vagal tone. The introduction of a passive atrial load into the model sinus node cell further reduced the beating rate, demonstrating that the bradycardia of the sinus node was even more pronounced by interactions between the sinus node and atria. In conclusion, the experimentally identified A414G-induced changes in If can explain the clinically observed sinus bradycardia in patients carrying the A414G HCN4 gene mutation. [ABSTRACT FROM AUTHOR]

Two groups of investigators investigated the heart pacemaker and its morphological basis in the early twentieth century. The first group was formed by Henrich Ewald Hering (physiologist), Sunao Tawara and Ludwig Aschoff (morphologists). The second group was composed of James Mackenzie (general practitioner and clinical investigator), Arthur Keith and Martin Flack (morphologists). These groups were formed almost at the same time in 1903. Their work resulted in the discovery of the atrioventricular node and Purkinje network (Sunao Tawara, in 1906), heart pacemaker (H E Hering, in 1907) and sinoatrial node (Keith and Flack, in 1907). Here, it is shown how the interconnections of the concurrent works of these groups resulted in the discovery not only of the function, but also of the structure of the sinoatrial node. [ABSTRACT FROM AUTHOR]

Suppression of atrial fibrillation (AF) arising within the superior vena cava (SVC) requires SVC electric isolation (SVCI) without sinus node (SN) injury. If an ectopic rhythm and AF trigger coexist within the SVC, the intra‐SVC ectopic rhythm complicates the pre‐SVCI search for the SN. This coexistence is without precedent; however, it is important to carefully locate the SN to prevent injury during SVCI. This case shows a paroxysmal AF with both phenomena coexisting in the SVC. Moreover, outpatient electrocardiographic assessment for tall P‐waves in inferior leads before catheter ablation could predict these phenomena and enable safer SVCI. [ABSTRACT FROM AUTHOR]

Objectives: Patients suffering from supraventricular tachycardia (SVT) require diagnostic or therapeutic intervention in a cardiac electrophysiology (EP) laboratory. Some anesthetic medications may adversely affect cardiac EP and conduction, altering the ability to induce the arrhythmia, and may have a negative impact on the ablation treatment. This prospective, randomized, pilot study was conducted in the cardiac EP laboratory of a tertiary care hospital with the aim to identify the ideal anesthetic agent for cardiac EP study and catheter ablation of SVT. The primary objective was to compare the effects of anesthetic agents on cardiac electrophysiological parameters and arrhythmia inducibility. The secondary objective was to compare the patient, anesthesiologist, and cardiologist satisfaction scores with respect to the anesthetic agent used. Materials and Methods: Thirty adult patients with SVT for EP study and radiofrequency catheter ablation were administered either of the anesthetic agents: midazolam, fentanyl, propofol, ketamine, or sevoflurane titrated to produce conscious sedation corresponding to bispectral index (BIS) values between 71 and 90. Electrophysiological parameters were recorded before and after administering the anesthetic agent. Results: Arrhythmia could be induced in all patients. Although electrophysiological parameters remained stable with ketamine administration; higher values of the Richmond Agitation Sedation Scale score and BIS were recorded. Propofol and sevoflurane administration was associated with deviation in electrophysiological parameters more than fentanyl and midazolam. The highest values of patient, anesthesiologist, and cardiologist satisfaction scores were obtained in the fentanyl group and the lowest in the ketamine group (P < 0.002). Conclusion: In doses used to provide conscious sedation, fentanyl provided ideal conditions, and midazolam, propofol, sevoflurane, and ketamine provided satisfactory conditions for conducting EP study and catheter ablation for supraventricular tachyarrhythmias. The potential of propofol to impede cardiac conduction needs to be explored further. [ABSTRACT FROM AUTHOR]

These days, in vitro functional analysis of gene variants is becoming increasingly important for risk stratification of cardiac ion channelopathies. So far, such risk stratification has been applied to SCN5A, KCNQ1, and KCNH2 gene variants associated with Brugada syndrome and long QT syndrome types 1 and 2, respectively, but risk stratification of HCN4 gene variants related to sick sinus syndrome has not yet been performed. HCN4 is the gene responsible for the hyperpolarization-activated 'funny' current If, which is an important modulator of the spontaneous diastolic depolarization underlying the sinus node pacemaker activity. In the present study, we carried out a risk classification assay on those loss-of-function mutations in HCN4 for which in vivo as well as in vitro data have been published. We used the in vitro data to compute the charge carried by If (Qf) during the diastolic depolarization phase of a prerecorded human sinus node action potential waveform and assessed the extent to which this Qf predicts (1) the beating rate of the comprehensive Fabbri–Severi model of a human sinus node cell with mutation-induced changes in If and (2) the heart rate observed in patients carrying the associated mutation in HCN4. The beating rate of the model cell showed a very strong correlation with Qf from the simulated action potential clamp experiments (R2 = 0.95 under vagal tone). The clinically observed minimum or resting heart rates showed a strong correlation with Qf (R2 = 0.73 and R2 = 0.71, respectively). While a translational perspective remains to be seen, we conclude that action potential clamp on transfected cells, without the need for further voltage clamp experiments and data analysis to determine individual biophysical parameters of If, is a promising tool for risk stratification of sinus bradycardia due to loss-of-function mutations in HCN4. In combination with an If blocker, this tool may also prove useful when applied to human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from mutation carriers and non-carriers. [ABSTRACT FROM AUTHOR]

We investigated whether hypothermia and hyperthermia can alter the efficacy and potency of histamine at increasing the force of cardiac contractions in mice that overexpress the human H2 receptor only in their cardiac myocytes (labelled H2-TG). Contractile studies were performed in an organ bath on isolated, electrically driven (1 Hz) left atrial preparations and spontaneously beating right atrial preparations from H2-TG mice and wild-type (WT) littermate control mice. The basal beating rate in the right atrial preparations from H2-TG mice was lowered by hypothermia (23 °C) and elevated by hyperthermia (42 °C). Furthermore, the efficacy of histamine (0.01–100 µM) at exerting positive inotropic effects was more severely attenuated in the left and right H2-TG mouse atria under hypothermia and hyperthermia than under normothermia (37 °C). Similarly, the inotropic response to histamine was attenuated under hypothermia and hyperthermia in isolated electrically stimulated (1 Hz) right atrial preparations obtained from humans undergoing cardiac surgery. The phosphorylation state of phospholamban at serine 16 at 23 °C was inferior to that at 37 °C in left atrial preparations from H2-TG mice in the presence of 10 µM histamine. In contrast, in human atrial preparations, the phosphorylation state of phospholamban at serine 16 in the presence of 100 µM histamine was lower at 42 °C than at 37 °C. Finally, under hyperthermia, we recorded more and longer lasting arrhythmias in right atrial preparations from H2-TG mice than in those from WT mice. We conclude that the inotropic effects of histamine in H2-TG mice and in human atrial preparations, as well as the chronotropic effects of histamine in H2-TG mice, are temperature dependent. Furthermore, we observed that, even without stimulation of the H2 receptors by exogenous agonists, temperature elevation can increase arrhythmias in isolated right atrial preparations from H2-TG mice. We propose that H2 receptors play a role in hyperthermia-induced supraventricular arrhythmias in human patients. [ABSTRACT FROM AUTHOR]

Electrical impulses from cardiac pacemaker cardiomyocytes initiate cardiac contraction and blood pumping and maintain life. Abnormal electrical impulses bring patients with low heart rates to cardiac arrest. The current therapy is to implant electronic devices to generate backup electricity. However, complications inherent to electronic devices remain unbearable suffering. Therefore, cardiac biological pacing has been developed as a hardware-free alternative. The approaches to generating biological pacing have evolved recently using cell reprogramming technology to generate pacemaker cardiomyocytes in-vivo or in-vitro. Different from conventional methods by electrical re-engineering, reprogramming-based biological pacing recapitulates various phenotypes of de novo pacemaker cardiomyocytes and is more physiological, efficient, and easy for clinical implementation. This article reviews the present state of the art in reprogramming-based biological pacing. We begin with the rationale for this new approach and review its advances in creating a biological pacemaker to treat bradyarrhythmia. [ABSTRACT FROM AUTHOR]

This review provides an update on the morphology of the sinus venosus defect. It was earlier believed that a 'common wall' separated the right pulmonary veins from the superior caval vein. In the sinus venosus defects, this wall was absent. Current evidence shows that the superior rim of the oval fossa, rather than forming a second septum or representing a common wall, is an infolding between the walls of the caval veins and the right pulmonary veins. The sinus venosus defect is caused by the anomalous connection of one or more pulmonary veins to a systemic vein. However, the pulmonary vein(s) retain their left atrial connections, leading to a veno-venous bridge that allows interatrial shunting outside the oval fossa. True atrial septal defects are located within the oval fossa or in the anteo-inferior buttress, while sinus venosus defects, ostium defects and coronary sinus defects are morphologically distinct from them. [ABSTRACT FROM AUTHOR]

In rare cases, an HSV-Encephalitis can lead to sinus node dysfunction, as was the case in this 70-year old woman who suffered from recurrent syncopes. Diagnostic work-up showed sinus bradycardias and short-lasting sinus arrests, primarily considered to be of cardiac etiology. After development of fever and neurological alteration, an HSV1 encephalitis was diagnosed. As our research of the current literature showed, the connection between the two is not completely clear. The HSVtypical infestation of the insular cortex, which influences the autonomic nervous system, should be discussed by all means. However, due to cessation of arrhythmia after seizure-suppressing therapy, we suspected an epileptic cause in this case. This shows the importance of a thorough differential diagnostic evaluation. [ABSTRACT FROM AUTHOR]

Background: Cardiomyocytes derived from human iPS cells (hiPSCs) include cells showing SAN‐ and non‐SAN‐type spontaneous APs. Objectives: To examine whether the deep learning technology could identify hiPSC‐derived SAN‐like cells showing SAN‐type‐APs by their shape. Methods: We acquired phase‐contrast images for hiPSC‐derived SHOX2/HCN4 double‐positive SAN‐like and non‐SAN‐like cells and made a VGG16‐based CNN model to classify an input image as SAN‐like or non‐SAN‐like cell, compared to human discriminability. Results: All parameter values such as accuracy, recall, specificity, and precision obtained from the trained CNN model were higher than those of human classification. Conclusions: Deep learning technology could identify hiPSC‐derived SAN‐like cells with considerable accuracy. [ABSTRACT FROM AUTHOR]

Diabetic patients often have impaired heart rate (HR) control. HR is regulated both intrinsically within the sinoatrial node (SAN) and via neuronal input. Previously, we found lower ex vivo HR in type 2 diabetic rat hearts, suggesting impaired HR generation within the SAN. The major driver of pacemaking within the SAN is the activity of hyperpolarisation-activated cyclic nucleotide-gated 4 (HCN(4)) channels. This study aimed to investigate whether the lower intrinsic HR in the type 2 diabetic heart is due to changes in HCN4 function, protein expression and/ or distribution. The intrinsic HR response to HCN4 blockade was determined in isolated Langendorff-perfused hearts of Zucker type 2 Diabetic Fatty (ZDF) rats (DM) and their non-diabetic ZDF littermates (nDM). HCN4 protein expression and membrane localisation were determined using western blot and immunofluorescence, respectively. We found that the intrinsic HR was lower in DM compared to nDM hearts. The change in intrinsic HR in response to HCN4 blockade with ivabradine was diminished in DM hearts, which normalised the intrinsic HR between the groups. HCN4 protein expression was decreased in the SAN of DM compared to nDM controls with no change in the fraction of HCN4 localised to the membrane of SAN cardiomyocytes. The lower intrinsic HR in DM is likely due to decreased HCN4 expression and depressed HCN4 function. Our study provides a novel understanding into the intrinsic mechanisms underlying altered HR control in type 2 diabetes. [ABSTRACT FROM AUTHOR]

Sinoatrial node (SAN) beating interval variability (BIV) and the average beating interval (BI) are regulated by a coupled-clock system, driven by Ca2+-calmodulin activated adenylyl cyclase, cAMP, and downstream PKA signaling. Reduced responsiveness of the BI and BIV to submaximal, [X]50, β-adrenergic receptor (β-AR) stimulation, and phosphodiesterase inhibition (PDEI) have been documented in aged SAN tissue, whereas the maximal responses, [X]max, do not differ by age. To determine whether age-associated dysfunction in cAMP signaling leads to altered responsiveness of BI and BIV, we measured cAMP levels and BI in adult (2–4 months n = 27) and aged (22–26 months n = 25) C57/BL6 mouse SAN tissue in control and in response to β-AR or PDEI at X50 and [X]max. Both cAMP and average BI in adult SAN were reduced at X50, whereas cAMP and BI at Xmax did not differ by age. cAMP levels and average BI were correlated both within and between adult and aged SAN. BIV parameters in long- and short-range terms were correlated with cAMP levels for adult SAN. However, due to reduced cAMP within aged tissues at [X]50, these correlations were diminished in advanced age. Thus, cAMP level generated by the coupled clock mechanisms is tightly linked to average BI. Reduced cAMP level at X50 in aged SAN explains the reduced responsiveness of the BI and BIV to β-AR stimulation and PDEI. [ABSTRACT FROM AUTHOR]

Atrial fibrillation disrupts contraction of the atria, leading to stroke and heart failure. We deciphered how immune and stromal cells contribute to atrial fibrillation. Single-cell transcriptomes from human atria documented inflammatory monocyte and SPP1+ macrophage expansion in atrial fibrillation. Combining hypertension, obesity, and mitral valve regurgitation (HOMER) in mice elicited enlarged, fibrosed, and fibrillation-prone atria. Single-cell transcriptomes from HOMER mouse atria recapitulated cell composition and transcriptome changes observed in patients. Inhibiting monocyte migration reduced arrhythmia in Ccr2−∕− HOMER mice. Cell-cell interaction analysis identified SPP1 as a pleiotropic signal that promotes atrial fibrillation through cross-talk with local immune and stromal cells. Deleting Spp1 reduced atrial fibrillation in HOMER mice. These results identify SPP1+ macrophages as targets for immunotherapy in atrial fibrillation. [ABSTRACT FROM AUTHOR]

The synergy between Na+-K+ pumps, Na+-Ca2+ exchangers, membrane currents and the sarcoplasmic reticulum (SR) generates the coupled-clock system, which governs the spontaneous electrical activity of heart sinoatrial node cells (SANCs). Ca2+ mediates the degree of clock coupling via local Ca2+ release (LCR) from the SR and activation of cAMP/PKA signaling. Marinobufagenin (MBG) is a natural Na+-K+ pump inhibitor whose effect on SANCs has not been measured before. The following two hypotheses were tested to determine if and how MBG mediates between the Na+-K+ pump and spontaneous SAN activity: (i) MBG has a distinct effect on beat interval (BI) due to variable effects on LCR characteristics, and (ii) Ca2+ is an important mediator between MBG and SANC activity. Ca2+ transients were measured by confocal microscopy during application of increasing concentrations of MBG. To further support the hypothesis that Ca2+ mediates between MBG and SANC activity, Ca2+ was chelated by the addition of BAPTA. Dose response tests found that 100 nM MBG led to no change in BI in 6 SANCs (no BI change group), and to BI prolongation in 10 SANCs (BI change group). At the same concentration, the LCR period was prolonged in both groups, but more significantly in the BI change group. BAPTA-AM prolonged the BI in 12 SANCs. In the presence of BAPTA, 100 nM MBG had no effect on SANC BI or on the LCR period. In conclusion, the MBG effects on SANC function are mediated by the coupled clock system, and Ca2+ is an important regulator of these effects. [ABSTRACT FROM AUTHOR]

Sepsis has emerged as a global health burden associated with multiple organ dysfunction and 20% mortality rate in patients. Numerous clinical studies over the past two decades have correlated the disease severity and mortality in septic patients with impaired heart rate variability (HRV), as a consequence of impaired chronotropic response of sinoatrial node (SAN) pacemaker activity to vagal/parasympathetic stimulation. However, the molecular mechanism(s) downstream to parasympathetic inputs have not been investigated yet in sepsis, particularly in the SAN. Based on electrocardiography, fluorescence Ca2+ imaging, electrophysiology, and protein assays from organ to subcellular level, we report that impaired muscarinic receptor subtype 2-G protein-activated inwardly-rectifying potassium channel (M2R-GIRK) signaling in a lipopolysaccharide-induced proxy septic mouse model plays a critical role in SAN pacemaking and HRV. The parasympathetic responses to a muscarinic agonist, namely IKACh activation in SAN cells, reduction in Ca2+ mobilization of SAN tissues, lowering of heart rate and increase in HRV, were profoundly attenuated upon lipopolysaccharide-induced sepsis. These functional alterations manifested as a direct consequence of reduced expression of key ion-channel components (GIRK1, GIRK4, and M2R) in the mouse SAN tissues and cells, which was further evident in the human right atrial appendages of septic patients and likely not mediated by the common proinflammatory cytokines elevated in sepsis. [ABSTRACT FROM AUTHOR]

Background: Von Hippel-Lindau disease (VHL) is a rare multisystem neurocutaneous disorder. The abnormalities in the corpus callosum have been observed in patients with phacomatosis, but this has not been previously described in VHL. In this report, we present a unique case of VHL with corpus callosum agenesis. Case Report: A 7-year-old boy was referred to the hospital because of left flank pain and vomiting. The abdominal ultrasound revealed multiple small simple cysts in both kidneys and pancreas. A radiological suspicion of VHL was raised, and further imaging examinations were recommended. Brain magnetic resonance imaging demonstrated a parallel arrangement of the lateral ventricles, confirming the diagnosis of complete agenesis of the corpus callosum. Brain hemangioblastomas were not detected. Conclusion: Our case is the first to report a corpus callosum agenesis in a child with VHL, thus expanding the spectrum of neurocutaneous disorders associated with callosal anomalies. [ABSTRACT FROM AUTHOR]

Background: Ongoing research on Coronavirus Disease 2019 (COVID-19) infection has revealed that it is associated with serious damage to many organs, not only the lungs. This infection can also affect the cardiovascular system and lead to serious cardiac pathologies. In this article, we present a case of transient bradyarrhythmia in a patient who was diagnosed with COVID-19 and recovered spontaneously with treatment. Case Report: A 74-year-old male patient was hospitalized with symptomatic COVID-19 infection. At the time of hospitalization, the patient was asymptomatic from a cardiac standpoint, but was found to have sinoatrial exit block type 2-2, intermittent Mobitz type 2 atrioventricular block, and sinus bradycardia. The medical team decided to closely monitor the patient, who responded well to COVID-19 treatment and did not develop bradyarrhythmia as his symptoms improved. Conclusion: Patients with COVID-19 infection should be closely monitored for bradyarrhythmia. Permanent pacemaker should not be rushed in these patients. [ABSTRACT FROM AUTHOR]

Introduction: The advancement of artificial intelligence (AI) has aided clinicians in the interpretation of electrocardiograms (ECGs) serving as an essential tool to provide rapid triage and care. However, in some cases, AI can misinterpret an ECG and may mislead the interpreting physician. Therefore, we aimed to describe the rate of ECG misinterpretation and its potential clinical impact in patient's management. Methods: We performed a retrospective descriptive analysis of misinterpreted ECGs and its clinical impact from May 28, 2020 to May 9, 2021. An electrophysiologist screened ECGs with confirmed diagnosis of atrial fibrillation (AF), sinus tachycardia (ST), sinus bradycardia (SB), intraventricular conduction delay (IVCD), and premature atrial contraction (PAC) that were performed in the emergency department. We then classified the misinterpreted ECGs as wrongly diagnosed AF, ST, SB, IVCD, or PAC into the correct diagnosis and reviewed the misinterpreted ECGs and medical records to evaluate inappropriate use of antiarrhythmic drugs (AAD), beta‐blockers (BB), calcium channel blockers (CCB), anticoagulation, or resource utilization of cardiology and/or electrophysiology (EP) consultation. Results: A total of 4969 ECGs were screened with diagnoses of AF (2282), IVCD (296), PAC (972), SB (895), and ST (638). Among these, 101 ECGs (2.0%) were misinterpreted. Wrongly diagnosed AF (58.4%) was the most common followed by wrongly diagnosed PAC (14.9%), wrongly diagnosed ST (12.9%), wrongly diagnosed IVCD (7.9%), and wrongly diagnosed SB (6.0%). Patients with misinterpreted ECGs were aged 76.6 ± 11.6 years with male (52.5%) predominance and hypertension being the most prevalent (83.2%) comorbid condition. The misinterpretation of ECGs led to the inappropriate use of BB (19.8%), CCB (5.0%), AAD therapy (7.9%), anticoagulation (6.9%) in patients with wrongly diagnosed AF, as well as inappropriate resource utilization including cardiology (41.6%) and EP (8.9%) consultations. Conclusions: Misinterpretation of ECGs may lead to inappropriate medical therapies and increased resource utilization. Therefore, it is essential to encourage physicians to carefully examine AI interpreted ECG's, especially those interpreted as having AF. [ABSTRACT FROM AUTHOR]

Abnormalities in G-protein-gated inwardly rectifying potassium (GIRK) channels have been implicated in diseased states of the cardiovascular system; however, the role of GIRK4 (Kir3.4) in cardiac physiology and pathophysiology has yet to be completely understood. Within the heart, the KACh channel, consisting of two GIRK1 and two GIRK4 subunits, plays a major role in modulating the parasympathetic nervous system's influence on cardiac physiology. Being that GIRK4 is necessary for the functional KACh channel, KCNJ5, which encodes GIRK4, it presents as a therapeutic target for cardiovascular pathology. Human variants in KCNJ5 have been identified in familial hyperaldosteronism type III, long QT syndrome, atrial fibrillation, and sinus node dysfunction. Here, we explore the relevance of KCNJ5 in each of these diseases. Further, we address the limitations and complexities of discerning the role of KCNJ5 in cardiovascular pathophysiology, as identical human variants of KCNJ5 have been identified in several diseases with overlapping pathophysiology. [ABSTRACT FROM AUTHOR]

A structural flaw in the globin gene causes hemoglobinopathies, while a flaw in the globin chain's synthesis causes thalassemia. One of the most prevalent hemoglobinopathies worldwide is sickle cell disease. Any region of the body can be affected, but the spleen is one of the most often afflicted and the first organs to suffer damage in sickle cell anemia (SCA). Splenomegaly, splenic infarction, hypersplenism, and acute splenic sequestration are the most common manifestations. Usually enlarged in the first 10 years of life, it gradually atrophies and eventually requires an autosplenectomy. It is well established that splenic complications from SCA are linked to higher rates of morbidity and in some cases, even death. This case report discusses a unique presentation of a patient who has hypersplenism together with beta-thalassemia and sickle cell trait. This is a case report of the adult patient admitted in Dhiraj General Hospital, SBKS MIRC, Sumandeep Vidyapeeth, Waghodia, Piparia, Vadodara-391760. The high-performance liquid chromatography (HPLC) technique was used to diagnose thalassemia trait and sickle cell trait. The clinical, ultrasonographic and computed tomography scan methods have been used to identify splenomegaly. The patient had clinical symptoms suggestive of beta-thalassemia and sickle cell trait with hypersplenism. This was supported by his peripheral smear, ultrasonography, contrast-enhanced abdomen–pelvis, HPLC, and bone marrow biopsy. It is imperative that clinicians remain cognizant of this infrequent consequence. Hence, early diagnosis and genetic counseling are of great importance for improving the prognosis of these patients. [ABSTRACT FROM AUTHOR]

RationaleCardiac pacemaker cells (PCs) in the sinoatrial node (SAN) have a distinct gene expression program that allows them to fire automatically and initiate the heartbeat. Although critical SAN transcription factors, including Isl1 (Islet-1), Tbx3 (T-box transcription factor 3), and Shox2 (short-stature homeobox protein 2), have been identified, the cis-regulatory architecture that governs PC-specific gene expression is not understood, and discrete enhancers required for gene regulation in the SAN have not been identified.ObjectiveTo define the epigenetic profile of PCs using comparative ATAC-seq (assay for transposase-accessible chromatin with sequencing) and to identify novel enhancers involved in SAN gene regulation, development, and function.Methods and resultsWe used ATAC-seq on sorted neonatal mouse SAN to compare regions of accessible chromatin in PCs and right atrial cardiomyocytes. PC-enriched assay for transposase-accessible chromatin peaks, representing candidate SAN regulatory elements, were located near established SAN genes and were enriched for distinct sets of TF (transcription factor) binding sites. Among several novel SAN enhancers that were experimentally validated using transgenic mice, we identified a 2.9-kb regulatory element at the Isl1 locus that was active specifically in the cardiac inflow at embryonic day 8.5 and throughout later SAN development and maturation. Deletion of this enhancer from the genome of mice resulted in SAN hypoplasia and sinus arrhythmias. The mouse SAN enhancer also directed reporter activity to the inflow tract in developing zebrafish hearts, demonstrating deep conservation of its upstream regulatory network. Finally, single nucleotide polymorphisms in the human genome that occur near the region syntenic to the mouse enhancer exhibit significant associations with resting heart rate in human populations.Conclusions(1) PCs have distinct regions of accessible chromatin that correlate with their gene expression profile and contain novel SAN enhancers, (2) cis-regulation of Isl1 specifically in the SAN depends upon a conserved SAN enhancer that regulates PC development and SAN function, and (3) a corresponding human ISL1 enhancer may regulate human SAN function.

Directed cardiomyogenesis from human induced pluripotent stem cells (hiPSCs) has been greatly improved in the last decade but directed differentiation to pacemaking cardiomyocytes (CMs) remains incompletely understood. In this study, we demonstrated that inhibition of NODAL signaling by a specific NODAL inhibitor (SB431542) in the cardiac mesoderm differentiation stage downregulated PITX2c, a transcription factor that is known to inhibit the formation of the sinoatrial node in the left atrium during cardiac development. The resulting hiPSC-CMs were smaller in cell size, expressed higher pro-pacemaking transcription factors, TBX3 and TBX18, and exhibited pacemaking-like electrophysiological characteristics compared to control hiPSC-CMs differentiated from established Wnt-based protocol. The pacemaker-like subtype increased up to 2.4-fold in hiPSC-CMs differentiated with the addition of SB431542 relative to the control. Hence, Nodal inhibition in the cardiac mesoderm stage promoted pacemaker-like CM differentiation from hiPSCs. Improving the yield of human pacemaker-like CMs is a critical first step in the development of functional human cell-based biopacemakers.

Na/Ca exchange is the dominant calcium (Ca) efflux mechanism in cardiac myocytes. Although our knowledge of exchanger function (NCX1 in the heart) was originally established using biochemical and electrophysiological tools such as cardiac sarcolemmal vesicles and the giant patch technique [1-4], many advances in our understanding of the physiological/pathophysiological roles of NCX1 in the heart have been obtained using a suite of genetically modified mice. Early mouse studies focused on modification of expression levels of NCX1 in the ventricles, with transgenic overexpressors, global NCX1 knockout (KO) mice (which were embryonic lethal if homozygous), and finally ventricular-specific NCX1 KO [5-12]. We found, to our surprise, that ventricular cardiomyocytes lacking NCX1 can survive and function by engaging a clever set of adaptations to minimize Ca entry, while maintaining contractile function through an increase in excitation-contraction (EC) coupling gain [5,6,13]. Having studied ventricular NCX1 ablation in detail, we more recently focused on elucidating the role of NCX1 in the atria through altering NCX1 expression. Using a novel atrial-specific NCX1 KO mouse, we found unexpected changes in atrial cell morphology and calcium handling, together with dramatic alterations in the function of sinoatrial node (SAN) pacemaker activity. In this review, we will discuss these findings and their implications for cardiac disease.

Introduction: Dysfunction of the sinoatrial node (SAN) cells causes arrhythmias, and many patients require artificial cardiac pacemaker implantation. However, the mechanism of impaired SAN automaticity remains unknown, and the generation of human SAN cells in vitro may provide a platform for understanding the pathogenesis of SAN dysfunction. The short stature homeobox 2 (SHOX2) and hyperpolarization-activated cyclic nucleotide-gated cation channel 4 (HCN4) genes are specifically expressed in SAN cells and are important for SAN development and automaticity. In this study, we aimed to purify and characterize human SAN-like cells in vitro, using HCN4 and SHOX2 as SAN markers. Methods: We developed an HCN4-EGFP/SHOX2-mCherry dual reporter cell line derived from human induced pluripotent stem cells (hiPSCs), and HCN4 and SHOX2 gene expressions were visualized using the fluorescent proteins EGFP and mCherry, respectively. The dual reporter cell line was established using an HCN4-EGFP bacterial artificial chromosome-based semi-knock-in system and a CRISPR-Cas9-dependent knock-in system with a SHOX2-mCherry targeting vector. Flow cytometry, RT-PCR, and whole-cell patch-clamp analyses were performed to identify SAN-like cells. Results: Flow cytometry analysis and cell sorting isolated HCN4-EGFP single-positive (HCN4+/SHOX2-) and HCN4-EGFP/SHOX2-mCherry double-positive (HCN4+/SHOX2+) cells. RT-PCR analyses showed that SAN-related genes were enriched within the HCN4+/SHOX2+ cells. Further, electrophysiological analyses showed that approximately 70% of the HCN4+/SHOX2+ cells exhibited SAN-like electrophysiological characteristics, as defined by the action potential parameters of the maximum upstroke velocity and action potential duration. Conclusions: The HCN4-EGFP/SHOX2-mCherry dual reporter hiPSC system developed in this study enabled the enrichment of SAN-like cells within a mixed HCN4+/SHOX2+ population of differentiating cardiac cells. This novel cell line is useful for the further enrichment of human SAN-like cells. It may contribute to regenerative medicine, for example, biological pacemakers, as well as testing for cardiotoxic and chronotropic actions of novel drug candidates.

Coronary perforation (CP) is a rare complication of percutaneous coronary intervention (PCI) and can lead to pericardial tamponade. Prolonged balloon inflation is a reasonable treatment for CP, but there is no standard recommendation on the preferable choice between the balloon site for prolonged balloon inflation (ie, proximal and in situ of the perforation). We present a rare case of successful prolonged balloon inflation at the proximal site of the CP after the failure of balloon inflation at the site of perforation. The patient developed CP during balloon inflation post-stent, rapidly progressing to cardiac tamponade. In situ prolonged balloon inflation (3 times) failed to close the CP, but proximal inflation could manage the CP. The take-home message from this case is that balloon expansion at the proximal site could be better than in situ of perforation in patients with CP after PCI. [ABSTRACT FROM AUTHOR]

Even before the sinoatrial node (SAN) was discovered, cardiovascular science was engaged in an active investigation of when and why the heart would beat. After the electrochemical theory of bioelectric membrane potentials was formulated and the first action potentials were measured in contracting muscle cells, the field became divided: some investigators studied electrophysiology and ion channels, others studied muscle contraction. It later became known that changes in intracellular Ca2+ cause contraction. The pacemaking field was reunited by the coupled-clock theory of pacemaker cell function, which integrated intracellular Ca2+ cycling and transmembrane voltage into one rhythmogenic system. In this review, we will discuss recent discoveries that contextualize the coupled-clock system, first described in isolated SAN cells, into the complex world of SAN tissue: heterogeneous local Ca2+ releases, generated within SAN pacemaker cells and regulated by the other cell types within the SAN cytoarchitecture, variably co-localize and synchronize to give rise to relatively rhythmic impulses that emanate from the SAN to excite the heart. We will ultimately conceptualize the SAN as a brain-like structure, composed of intercommunicating meshworks of multiple types of pacemaker cells and interstitial cells, intertwined networks of nerves and glial cells and more. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'. [ABSTRACT FROM AUTHOR]

Although, for many decades, the day–night rhythm in resting heart rate has been attributed to the parasympathetic branch of the autonomic nervous system (high vagal tone during sleep), recently we have shown that there is a circadian clock in the cardiac pacemaker, the sinus node, and the day–night rhythm in heart rate involves an intrinsic rhythmic transcriptional remodelling of pacemaker ion channels, particularly Hcn4. We have now investigated the role of the sympathetic branch of the autonomic nervous system in this and shown it to have a non-canonical role. In mice, sustained long-term block of cardiac β-adrenergic receptors by propranolol administered in the drinking water abolished the day–night rhythm in pacemaking in the isolated sinus node. Concomitant with this, there was a loss of the normal day–night rhythm in many pacemaker ion channel transcripts. However, there was little or no change in the local circadian clock, indicating that the well-known day–night rhythm in sympathetic nerve activity is directly involved in pacemaker ion channel transcription. The day–night rhythm in pacemaking helps explain the occurrence of clinically significant bradyarrhythmias during sleep, and this study improves our understanding of this pathology. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'. [ABSTRACT FROM AUTHOR]

Patients with pulmonary arterial hypertension (PAH) have a high burden of arrhythmias, including arrhythmias arising from sinus node dysfunction, and the aim of this study was to investigate the effects of PAH on the sinus node. In the rat, PAH was induced by an injection of monocrotaline. Three weeks after injection, there was a decrease of the intrinsic heart rate (heart rate in the absence of autonomic tone) as well as the normal heart rate, evidence of sinus node dysfunction. In the sinus node of PAH rats, there was a significant downregulation of many ion channels and Ca2+-handling genes that could explain the dysfunction: HCN1 and HCN4 (responsible for pacemaker current, If), Cav1.2, Cav1.3 and Cav3.1 (responsible for L- and T-type Ca2+ currents, ICa,L and ICa,T), NCX1 (responsible for Na+–Ca2+ exchanger) and SERCA2 and RYR2 (Ca2+-handling molecules). In the sinus node of PAH rats, there was also a significant upregulation of many fibrosis genes that could also help explain the dysfunction: vimentin, collagen type 1, elastin, fibronectin and transforming growth factor β1. In summary, in PAH, there is a remodelling of ion channel, Ca2+-handling and fibrosis genes in the sinus node that is likely to be responsible for the sinus node dysfunction. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'. [ABSTRACT FROM AUTHOR]

P21-activated kinase 1 (Pak1) signalling plays a vital and overall protective role in the heart. However, the phenotypes of Pak1 deficiency in the cardiac atria have not been well explored. In this study, Pak1 cardiac-conditional knock-out (cKO) mice were studied under baseline and adrenergic challenge conditions. Pak1 cKO mice show atrial arrhythmias including atrial fibrillation (AF) in vivo, detected during anaesthetized electrocardiography without evidence of interstitial fibrosis upon Masson's trichrome staining. Optical mapping of left atrial preparations from Pak1 cKO mice revealed a higher incidence of Ca2+ and action potential alternans under isoprenaline challenge and differences in baseline action potential and calcium transient characteristics. Type-2 ryanodine receptor (RyR2) channels from Pak1 cKO hearts had a higher open probability than those from wild-type. Reverse transcription-quantitative polymerase chain reaction and Western blotting indicated that pCamkIIδ and RyR2 are highly phosphorylated at baseline in the atria of Pak1 cKO mice, while the expression of Slc8a2 and Slc8a3 as a Na+–Ca2+ exchanger, controlling the influx of Ca2+ from outside of the cell and efflux of Na+ from the cytoplasm, are augmented. Chromatin immunoprecipitation study showed that pCreb1 interacts with Slc8a2 and Slc8a3. Our study thus demonstrates that deficiency of Pak1 promotes atrial arrhythmogenesis under adrenergic stress, probably through post-translational and transcriptional modifications of key molecules that are critical to Ca2+ homeostasis. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'. [ABSTRACT FROM AUTHOR]

The atrioventricular node (AVN) is considered a "black box", and the functioning of its dual pathways remains controversial and not fully understood. In contrast to numerous clinical studies, there are only a few mathematical models of the node. In this paper, we present a compact, computationally lightweight multi-functional rabbit AVN model based on the Aliev-Panfilov twovariable cardiac cell model. The one-dimensional AVN model includes fast (FP) and slow (SP) pathways, primary pacemaking in the sinoatrial node, and subsidiary pacemaking in the SP. To obtain the direction-dependent conduction properties of the AVN, together with gradients of intercellular coupling and cell refractoriness, we implemented the asymmetry of coupling between model cells. We hypothesized that the asymmetry can reflect some effects related to the complexity of the real 3D structure of AVN. In addition, the model is accompanied by a visualization of electrical conduction in the AVN, revealing the interaction between SP and FP in the form of ladder diagrams. The AVN model demonstrates broad functionality, including normal sinus rhythm, AVN automaticity, filtering of high-rate atrial rhythms during atrial fibrillation and atrial flutter with Wenckebach periodicity, direction-dependent properties, and realistic anterograde and retrograde conduction curves in the control case and the cases of FP and SP ablation. To show the validity of the proposed model, we compare the simulation results with the available experimental data. Despite its simplicity, the proposed model can be used both as a stand-alone module and as a part of complex three-dimensional atrial or whole heart simulation systems, and can help to understand some puzzling functions of AVN. [ABSTRACT FROM AUTHOR]

As atrial fibrosis is the main feature of atrial structural remodeling, inhibiting atrial fibrosis is crucial to the prevention of atrial fibrillation (AF) progression. Research has shown the correlation between abnormal lipid metabolism and AF progression. However, the effect of specific lipids on atrial fibrosis remains unclear. In the present study, we applied ultra-high-performance lipidomics to analyze the lipid profiles in patients with AF and identify phosphatidylethanolamine (PE) as the differential lipid associated with AF. To detect the effect of the differential lipid on atrial fibrosis, we performed the intraperitoneal injection of Angiotensin II (Ang II) to mice to induce atrial fibrosis and supplemented PE in diets. We also treated atrial cells with PE to evaluate the cellular effect of PE. We found that PE supplementation aggravated atrial fibrosis and increased the expression of the fibrosis-related protein in vitro and in vivo. Moreover, we detected the effect of PE on the atrium. We found that PE increased oxidation products and regulated the expression of ferroptosis-related proteins, which could be alleviated by a ferroptosis inhibitor. PE increased peroxidation and mitochondrial damage in vitro, which promoted cardiomyocyte death induced by Ang II. Examination of protein expression in cardiomyocytes indicated that PE triggered ferroptosis and caused cell death to participate in myocardium fibrosis. In summary, our findings demonstrated the differential lipid profiles of AF patients and revealed the potential effect of PE on atrial remodelling, suggesting that inhibition of PE and ferroptosis might serve as a potential therapy to prevent AF progression. [ABSTRACT FROM AUTHOR]

Sinoatrial node (SAN) dysfunction (SND) and atrial arrhythmia frequently occur simultaneously with a hazard ratio of 4.2 for new onset atrial fibrillation (AF) in SND patients. In the atrial muscle attenuated activity of p21-activated kinase 1 (Pak1) increases the risk for AF by enhancing NADPH oxidase 2 dependent production of reactive oxygen species (ROS). However, the role of Pak1 dependent ROS regulation in SAN function has not yet been determined. We hypothesize that Pak1 activity maintains SAN activity by regulating the expression of the hyperpolarization activated cyclic nucleotide gated cation channel (HCN). To determine Pak1 dependent changes in heart rate (HR) regulation we quantified the intrinsic sinus rhythm in wild type (WT) and Pak1 deficient (Pak1−/−) mice of both sexes in vivo and in isolated Langendorff perfused hearts. Pak1−/− hearts displayed an attenuated HR in vivo after autonomic blockage and in isolated hearts. The contribution of the Ca2+ clock to pacemaker activity remained unchanged, but Ivabradine (3 μM), a blocker of HCN channels that are a membrane clock component, eliminated the differences in SAN activity between WT and Pak1−/− hearts. Reduced HCN4 expression was confirmed in Pak1−/− right atria. The reduced HCN activity in Pak1−/− could be rescued by class II HDAC inhibition (LMK235), ROS scavenging (TEMPOL) or attenuation of Extracellular Signal-Regulated Kinase (ERK) 1/2 activity (SCH772984). No sex specific differences in Pak1 dependent SAN regulation were determined. Our results establish Pak1 as a class II HDAC regulator and a potential therapeutic target to attenuate SAN bradycardia and AF susceptibility. [Display omitted] • The loss of p21-activated kinase (Pak1) results in sinoatrial node (SAN) bradycardia. • Pak1 regulates the SAN pacemaker's membrane clock component by attenuating class II HDAC4 activity. • During atrial fibrillation (AF), downregulation of Pak1 can contribute to SAN bradycardia through ERK1/2 activation. [ABSTRACT FROM AUTHOR]

Atrial fibrillation (AF) is the most common arrhythmia in adults, with a prevalence increasing with age. Current clinical management of AF is focused on tertiary prevention (i.e., treating the symptoms and sequelae) rather than addressing the underlying molecular pathophysiology. Robust animal models of AF, particularly those that do not require supraphysiologic stimuli to induce AF (i.e., showing spontaneous AF), enable studies that can uncover the underlying mechanisms of AF. Several mouse models of AF have been described to exhibit spontaneous AF, but pathophysiologic drivers of AF differ among models. Here, we describe relevant AF mechanisms and provide an overview of large and small animal models of AF. We then provide an in-depth review of the spontaneous mouse models of AF, highlighting the relevant AF mechanisms for each model. [ABSTRACT FROM AUTHOR]

Key Clinical Message: A 26‐year‐old male patient admitted to the hospital ward with experience of repetitive syncopes for a year. The patient was diagnosed with sick sinus syndrome. The aim of this clinical report is to highlight the variability of anatomical findings associated with polysplenia pattern. This case report presents a 26‐year‐old male patient who presented to the medical ward with a complaint of repeating blackouts for a year. The patient was then diagnosed with sick sinus syndrome, and further investigations revealed left isomerism, polysplenia, and no congenital heart defects. Holter monitoring, ultrasonography, electrocardiography, and computed tomography were used to confirm the diagnosis. The patient underwent DDDR pacemaker implantation for the treatment of SA node dysfunction. The report highlights the variability of anatomical findings associated with polysplenia pattern and the various types of heartbeat disruptions that may occur in the atrial appendages of the left side isomerism. [ABSTRACT FROM AUTHOR]

Background: Glucagon is thought to increase heart rate and contractility by stimulating glucagon receptors and increasing 3′,5′-cyclic adenosine monophosphate (cAMP) production in the myocardium. This has been confirmed in animal studies but not in the human heart. The cardiostimulatory effects of glucagon have been correlated with the degree of cardiac dysfunction, as well as with the enzymatic activity of phosphodiesterase (PDE), which hydrolyses cAMP. In this study, the presence of glucagon receptors in the human heart and the inotropic and chronotropic effects of glucagon in samples of failing and nonfailing (NF) human hearts were investigated. Methods: Concentration‒response curves for glucagon in the absence and presence of the PDE inhibitor IBMX were performed on samples obtained from the right (RA) and left atria (LA), the right (RV) and left ventricles (LV), and the sinoatrial nodes (SNs) of failing and NF human hearts. The expression of glucagon receptors was also investigated. Furthermore, the inotropic and chronotropic effects of glucagon were examined in rat hearts. Results: In tissues obtained from failing and NF human hearts, glucagon did not exert inotropic or chronotropic effects in the absence or presence of IBMX. IBMX (30 µM) induced a marked increase in contractility in NF hearts (RA: 83 ± 28% (n = 5), LA: 80 ± 20% (n = 5), RV: 75 ± 12% (n = 5), and LV: 40 ± 8% (n = 5), weaker inotropic responses in the ventricular myocardium of failing hearts (RV: 25 ± 10% (n = 5) and LV: 10 ± 5% (n = 5) and no inotropic responses in the atrial myocardium of failing hearts. IBMX (30 µM) increased the SN rate in failing and NF human hearts (27.4 ± 3.0 beats min−1, n = 10). In rat hearts, glucagon induced contractile and chronotropic responses, but only contractility was enhanced by 30 µM IBMX (maximal inotropic effect of glucagon 40 ± 8% vs. 75 ± 10%, in the absence or presence of IBMX, n = 5, P < 0.05; maximal chronotropic response 77.7 ± 6.4 beats min−1 vs. 73 ± 11 beats min−1, in the absence or presence of IBMX, n = 5, P > 0.05). Glucagon receptors were not detected in the human heart samples. Conclusions: Our results conflict with the view that glucagon induces inotropic and chronotropic effects and that glucagon receptors are expressed in the human heart. [ABSTRACT FROM AUTHOR]

Introduction: Myeloid differentiation protein 1 (MD1), a negative regulator of toll-like receptor 4 (TLR4), is widely expressed in the heart. Recent studies have shown that MD1 plays an important role in cardiac remodelling. However, the effects and potential mechanisms underlying MD1-mediated atrial remodelling in diabetic cardiomyopathy (DCM) remain unclear. Therefore, this study was designed to explore the role of MD1 in DCM-related atrial remodelling. Methods: MD1 knockout (MD1-KO) mice and wild-type (WT) littermates were injected with streptozotocin (STZ) to establish a diabetic mouse model. These mice were then used to evaluate MD1 expression and its effects on atrial remodelling in vivo. Results: MD1 expression was significantly decreased in STZ-induced diabetic mice. The loss of MD1 aggravated atrial fibrosis, inflammation, and apoptosis in DCM mice and promoted atrial remodelling. MD1-KO diabetic mice also showed higher susceptibility to atrial fibrillation (AF) and worse cardiac function. Mechanistically, the deletion of MD1 promoted the activation of the TLR4/NF-κB signalling pathway, resulting in atrial remodelling in DCM mice via increased p65 phosphorylation. Conclusions: The deletion of MD1 plays an important role in inflammatory and apoptotic atrial remodelling and increases susceptibility to AF in DCM mice, providing a new target for the preventive treatment of DCM-related atrial remodelling. [ABSTRACT FROM AUTHOR]

For post-menopausal women, stroke is complicated by the variable effects of estrogen therapy and the age-related therapeutic consequences involved. Estrogen therapy has been shown to have an age-dimorphic effect, which is neuroprotective in young females, but non-neuroprotective, even neurotoxic in acyclic females. We hypothesized that arterial baroreflex (ABR) and its downstream acetylcholine-a7 nicotinic acetylcholine receptor (a7nAChR) anti-inflammatory pathways are involved in estrogen efficacy toward cerebral ischemic damage. Our data showed that estrogen supplements contributed to ABR improvement and neuroprotection in adult, not aged, ovariectomized (OVX) rats. In adult rats, OVX-induced estrogen deficiency aggravated middle cerebral artery occlusion (MCAO), which induced brain infarction and reduced ABR function, with decreased a7nAChR expression of the brain and exaggerated inflammation following MCAO; these effects were significantly prevented by supplementation with estrogen. ABR impairment by sinoaortic denervation partly attenuated the estrogen effect on baroreflex sensitivity (BRS) and ischemic damage in adult rats, as well as a7nAChR expression and inflammatory response. These data suggested that ABR and acetylcholine-a7nAChR anti-inflammatory pathways are involved in the neuroprotection of estrogen in adult OVX rats. In contrast, aged rats exhibited more severe ischemic damage and inflammatory response than adult rats, as well as poorer baroreflex function and lower a7nAChR expression. Estrogen supplements did not improve BRS or confer neuroprotection in aged rats without affecting brain a7nAChR and post-ischemic inflammation. Most importantly, ketanserin restored ABR function and significantly postponed the onset of stroke in aged female strokeprone spontaneously hypertensive rats, whereas estrogen treatment failed to delay the development of stroke. Our findings reveal that estrogen is protective against ischemic stroke (IS) in adult female rats and that ABR played a role in this beneficial action. Dysfunction of ABR and unresponsiveness to estrogen in aged female rats may contribute to a reduced estrogen efficacy against cerebral ischemia. [ABSTRACT FROM AUTHOR]

Atrial fibrillation (AF) is the commonest cardiac arrhythmia, affecting 3 million people in the USA and 8 million in the EU (according to the European Society of Cardiology). So, why is it that even with the best medical care, around a third of the patients are treatment resistant. Extensive research of its etiology showed that AF and its mechanisms are still debatable. Some of the AF origins are ascribed to functional and ionic heterogeneities of the heart tissue and possibly to additional triggering agents. But, have all AF origins been detected? Are all accepted origins, in fact, arrhythmogenic? In order to study these questions and specifically to check our new idea of intermittency as an arrhythmogenesis agent, we chose to employ a mathematical model which was as simple as possible, but which could still be used to observe the basic network processes of AF development. At this point we were not interested in the detailed ionic propagations nor in the actual shapes of the induced action potentials (APs) during the AF outbreaks. The model was checked by its ability to exactly recapture the basic AF developmental stages known from experimental cardiac observations and from more elaborate mathematical models. We use a simple cellular automata 2D mathematical model of N × N matrices to elucidate the field processes leading to AF in a tissue riddled with randomly distributed heterogeneities of different types, under sinus node operation, simulated by an initial line of briefly stimulated cells inducing a propagating wave, and with or without an additional active ectopic action potential pulse, in turn simulated by a transitory operation of a specific cell. Arrhythmogenic contributions, of three different types of local heterogeneities in myocytes and their collaborations, in inducing AF are examined. These are: a heterogeneity created by diffuse fibrosis, a heterogeneity created by myocytes having different refractory periods, and a new heterogeneity type, created by intermittent operation of some myocytes. The developmental stages (target waves and spirals) and the different probabilities of AF occurring under each condition, are shown. This model was established as being capable of reproducing the known AF origins and their basic development stages, and in addition has shown: (1) That diffuse fibrosis on its own is not arrhythmogenic but in combination with other arrhythmogenic agents it can either enhance or limit AF. (2) In general, combinations of heterogeneities can act synergistically, and, most importantly, (3) The new type of intermittency heterogeneity proves to be extremely arrhythmogenic. Both the intermittency risk and the fibrosis role in AF generation were established. Knowledge of the character of these arrhythmogenesis agents can be of real importance in AF treatment. [ABSTRACT FROM AUTHOR]

Bradyarrhythmias are adequately treated with pacemakers. There are different pacing modes (single-chamber, dual-chamber, cardiac resynchronisation therapy [CRT] and conduction system pacing [CSP]) and a choice between leadless or transvenous pacemakers. The expected pacing need is important for determining optimal pacing mode and device type. This study aimed to evaluate atrial pacing (AP) and ventricular pacing (VP) percentages over time for the most common pacing indications. Included patients were aged ≥18 years with a dual-chamber rate-modulated [DDD(R)] pacemaker implantation and ≥1 year of follow-up at a tertiary centre between January 2008 and January 2020. Baseline characteristics, AP and VP at yearly follow-up visits up to 6 years after implantation were retrieved from the medical records. A total of 381 patients were included. Primary pacing indications were incomplete atrioventricular block (AVB) in 85 (22%), complete AVB in 156 (41%) and sinus node dysfunction (SND) in 140 (37%) patients. Mean age at implantation was 71±14, 69±17 and 68±14 years, respectively (p=0.23). Median follow-up was 42 months (25–68 months). Overall, AP was highest in SND with median 37% (7%–75%) versus 7% (1%–26%) in incomplete AVB and 3% (1%–16%) in complete AVB (p<0.001); VP was highest in complete AVB with median 98% (43%–100%) versus 44% (7%–94%) in incomplete AVB and 3% (1%–14%) in SND (p<0.001). Ventricular pacing significantly increased over time in patients with incomplete AVB and SND (both p=0.001). These results confirm the pathophysiology of different pacing indications, causing clear differences in pacing need and expected battery longevity. They may help guide optimal pacing mode and suitability for leadless or physiological pacing. [ABSTRACT FROM AUTHOR]

Mitochondrial oxidative stress has been implicated in aging and several cardiovascular diseases, including heart failure and cardiomyopathy, ventricular tachycardia, and atrial fibrillation. The role of mitochondrial oxidative stress in bradyarrhythmia is less clear. Mice with a germline deletion of Ndufs4 subunit respiratory complex I develop severe mitochondrial encephalomyopathy resembling Leigh Syndrome (LS). Several types of cardiac bradyarrhythmia are present in LS mice, including a frequent sinus node dysfunction and episodic atrioventricular (AV) block. Treatment with the mitochondrial antioxidant Mitotempo or mitochondrial protective peptide SS31 significantly ameliorated the bradyarrhythmia and extended the lifespan of LS mice. Using an ex vivo Langendorff perfused heart with live confocal imaging of mitochondrial and total cellular reactive oxygen species (ROS), we showed increased ROS in the LS heart, which was potentiated by ischemia-reperfusion. A simultaneous ECG recording showed a sinus node dysfunction and AV block concurrent with the severity of the oxidative stress. Treatment with Mitotempo abolished ROS and restored the sinus rhythm. Our study reveals robust evidence of the direct mechanistic roles of mitochondrial and total ROS in bradyarrhythmia in the setting of LS mitochondrial cardiomyopathy. Our study also supports the potential clinical application of mitochondrial-targeted antioxidants or SS31 for the treatment of LS patients. [ABSTRACT FROM AUTHOR]

Leadless pacemakers have been developed with key advantages over traditional transvenous pacemakers by substantially mitigating the risks of device infection and lead related complications, and providing an alternative pacing strategy in patients with barriers to superior venous access. The Medtronic Micra leadless pacing system is designed for implantation through a femoral venous approach across the tricuspid valve, via Nitinol tine fixation into the trabeculated subpulmonic right ventricle. Patients with surgically corrected dextro-transposition of the great arteries (d-TGA) have an increased risk of pacing requirement. There is limited published experience of implantation of leadless Micra pacemakers in this population, with key challenges relating to trans-baffle access, and deployment of the device into the less trabeculated subpulmonic left ventricle. Here we describe a case report of leadless Micra implantation in a 49 year old male with d-TGA and Senning procedure in childhood, who required pacing for symptomatic sinus node disease, with anatomic barriers to transvenous pacing. Micra implantation was successfully performed following careful consideration of patient anatomy, including the utilisation of 3D modelling to guide the implantation procedure. [ABSTRACT FROM AUTHOR]

The sinoatrial node (SAN) and subsidiary pacemakers in the cardiac conduction system generate spontaneous electrical activity which is indispensable for electrical and therefore contractile function of the heart. The hyperpolarisation-activated cyclic nucleotide-gated channel HCN4 is responsible for genesis of the pacemaker “funny” current during diastolic depolarisation. S-palmitoylation, the reversible conjugation of the fatty acid palmitate to protein cysteine sulfhydryls, regulates the activity of key cardiac Na+ and Ca2+ handling proteins, influencing their membrane microdomain localisation and function. We investigated HCN4 palmitoylation and its functional consequences in engineered human embryonic kidney 293T cells as well as endogenous HCN4 in neonatal rat ventricular myocytes. HCN4 was palmitoylated in all experimental systems investigated. We mapped the HCN4 palmitoylation sites to a pair of cysteines in the HCN4 intracellular amino terminus. A double cysteine-to-alanine mutation CC93A/179AA of full length HCN4 caused a ~67% reduction in palmitoylation in comparison to wild type HCN4. We used whole-cell patch clamp to evaluate HCN4 current (IHCN4) in stably transfected 293T cells. Removal of the two N-terminal palmitoylation sites did not significantly alter half maximal activation voltage of IHCN4 or the activation slope factor. IHCN4 was significantly larger in cells expressing wild type compared to non-palmitoylated HCN4 across a range of voltages. Phylogenetic analysis revealed that although cysteine 93 is widely conserved across all classes of HCN4 vertebrate orthologs, conservation of cysteine 179 is restricted to placental mammals. Collectively, we provide evidence for functional regulation of HCN4 via palmitoylation of its amino terminus in vertebrates. We suggest that by recruiting the amino terminus to the bilayer, palmitoylation enhances the magnitude of HCN4-mediated currents, but does not significantly affect the kinetics. [ABSTRACT FROM AUTHOR]

The pacemaker activity of the sinoatrial node (SAN) has been studied extensively in animal species but is virtually unexplored in humans. Here we assess the role of the slowly activating component of the delayed rectifier K+ current (IKs) in human SAN pacemaker activity and its dependence on heart rate and β-adrenergic stimulation. HEK-293 cells were transiently transfected with wild-type KCNQ1 and KCNE1 cDNA, encoding the α- and β-subunits of the IKs channel, respectively. KCNQ1/KCNE1 currents were recorded both during a traditional voltage clamp and during an action potential (AP) clamp with human SAN-like APs. Forskolin (10 µmol/L) was used to increase the intracellular cAMP level, thus mimicking β-adrenergic stimulation. The experimentally observed effects were evaluated in the Fabbri–Severi computer model of an isolated human SAN cell. Transfected HEK-293 cells displayed large IKs-like outward currents in response to depolarizing voltage clamp steps. Forskolin significantly increased the current density and significantly shifted the half-maximal activation voltage towards more negative potentials. Furthermore, forskolin significantly accelerated activation without affecting the rate of deactivation. During an AP clamp, the KCNQ1/KCNE1 current was substantial during the AP phase, but relatively small during diastolic depolarization. In the presence of forskolin, the KCNQ1/KCNE1 current during both the AP phase and diastolic depolarization increased, resulting in a clearly active KCNQ1/KCNE1 current during diastolic depolarization, particularly at shorter cycle lengths. Computer simulations demonstrated that IKs reduces the intrinsic beating rate through its slowing effect on diastolic depolarization at all levels of autonomic tone and that gain-of-function mutations in KCNQ1 may exert a marked bradycardic effect during vagal tone. In conclusion, IKs is active during human SAN pacemaker activity and has a strong dependence on heart rate and cAMP level, with a prominent role at all levels of autonomic tone. [ABSTRACT FROM AUTHOR]

Muscle metaboreflex activation during hypercapnia leads to enhanced pressive effects that are poorly understood while autonomic responses including baroreflex function are not documented. Thus, we assessed heart rate variability (HRV) that is partly due to autonomic influences on sinus node with linear tools (spectral analysis of instantaneous heart period), baroreflex set point and sensitivity with the heart period–arterial pressure transfer function and sequences methods, and system coupling through the complexity of RR interval dynamics with nonlinear tools (Poincaré plots and approximate entropy (ApEn)). We studied ten healthy young men at rest and then during muscle metaboreflex activation (MMA, postexercise muscle ischemia) and hypercapnia (HCA, PetCO2 = + 10 mmHg from baseline) separately and combined (MMA + HCA). The strongest pressive responses were observed during MMA + HCA, while baroreflex sensitivity was similarly lowered in the three experimental conditions. HRV was significantly different in MMA + HCA compared to MMA and HCA separately, with the lowest total power spectrum (p < 0.05), including very low frequency (p < 0.05), low frequency (p < 0.05), and high frequency (tendency) power spectra decreases, and the lowest Poincaré plot short-term variability index (SD1): SD1 = 36.2 ms (MMA + HCA) vs. SD1 = 43.1 ms (MMA, p < 0.05) and SD1 = 46.1 ms (HCA, p < 0.05). Moreover, RR interval dynamic complexity was significantly increased only in the MMA + HCA condition (ApEn increased from 1.04 ± 0.04, 1.07 ± 0.02, and 1.05 ± 0.03 to 1.10 ± 0.03, 1.13 ± 0.04, and 1.17 ± 0.03 in MMA, HCA, and MMA + HCA conditions, respectively; p < 0.01). These results suggest that in healthy young men, muscle metaboreflex activation during hypercapnia leads to interactions that reduce parasympathetic influence on the sinus node activity but complexify its dynamics. [ABSTRACT FROM AUTHOR]

Rosai-Dorfman disease is characterized by dilated lymph node sinuses filled with lymphocytes, plasma cells, and histiocytes. Many of these histiocytes classically exhibit emperipolesis of lymphocytes and plasma cells. Abundant immunoglobulin G4+ plasma cells occur in some cases, and a potential relationship with immunoglobulin G4-related disease has been suggested. Here, we report 3 cases of immunoglobulin G4-associated Rosai-Dorfman disease. Immunoglobulin G4-related disease was suspected based on immunoglobulin G4+ plasma cell infiltration, but the final diagnosis was immunoglobulin G4-associated Rosai-Dorfman disease. At present, the evidence does not support a link between immunoglobulin G4-associated Rosai-Dorfman disease and immunoglobulin G4-related disease, and one condition should not be considered part of the spectrum of the other. We believe it is of paramount importance to increase the awareness of immunoglobulin G4-associated Rosai-Dorfman disease for pathologists who interpret the biopsies and clinicians who integrate the diagnosis and treat such patients to not overdiagnose immunoglobulin G4-related disease. [ABSTRACT FROM AUTHOR]

Physical inactivity increases cardiometabolic risk through a variety of mechanisms, among which alterations of immunological, metabolic, and autonomic control systems may play a pivotal role. Physical inactivity is frequently associated with other factors that may further worsen prognosis. The association between physical inactivity and hypoxia is particularly interesting and characterizes several conditions—whether physiological (e.g., residing or trekking at high altitude and space flights) or pathological (e.g., chronic cardiopulmonary diseases and COVID-19). In this randomized intervention study, we investigated the combined effects of physical inactivity and hypoxia on autonomic control in eleven healthy and physically active male volunteers, both at baseline (ambulatory) conditions and, in a randomized order, hypoxic ambulatory, hypoxic bedrest, and normoxic bedrest (i.e., a simple experimental model of physical inactivity). Autoregressive spectral analysis of cardiovascular variabilities was employed to assess cardiac autonomic control. Notably, we found hypoxia to be associated with an impairment of cardiac autonomic control, especially when combined with bedrest. In particular, we observed an impairment of indices of baroreflex control, a reduction in the marker of prevalent vagal control to the SA node, and an increase in the marker of sympathetic control to vasculature. [ABSTRACT FROM AUTHOR]

Background: Incidence of cardiac conduction disorders are escalating in the current era especially due to enhanced life expectancy in the geriatric population and better standards of medical care and coronary revascularization. Aims and Objectives: This study aimed to analyse the demographical aspect and temporal trends of permanent pacemaker (PPM) therapy in patients aged above 60 years of age from an observational 2-year retrospective data from a single-center tertiary care academic hospital. Results: Males consisted of more than two-third of the patients and complete atrioventricular block was the most common conduction pathology. Fascicular and Bundle branch blocks appeared to have a male preponderance, whereas sinus node dysfunction was found to have statistically significant association with the female cohort. Most of the patients were implanted out of admission from cardiac emergency and single chamber ventricle paced and sensed, inhibition response with rate adaptation (VVIR) mode was the predominant modality of pacing management, not found to be influenced by the age or sex of the patients. However, there was a statistical correlation noted of utilization of dual chamber Dual paced and sensed, dual inhibition with rate adaptation (DDDR) mode in patients with sick sinus pathology. Conclusion: Implantation of PPM is on the rise as a modality of bradyarrhythmia treatment in the increasing proportion of geriatric population with advanced life expectancy. [ABSTRACT FROM AUTHOR]

Coronary artery aneurysms (CAAs) occur when an artery dilates 1.5 times the reference vessel. They occur most commonly because of atherosclerosis. CAAs are a rare phenomenon, and it is even rarer to find a giant CAA, which is roughly defined as a size 400% above the reference vessel. Giant CAAs are commonly found in the right coronary artery. The sinoatrial nodal artery (SNA) is among the least common sites for CAA involvement. Sometimes, communication exists between the aneurysm and a chamber of the heart or a great vessel. The consequences of the fistula depend on its size. Because of the rarity of the condition, guidelines are not well developed. However, small CAAs can be managed conservatively, whereas giant CAAs require resection, ligation, and bypass grafting. CAAs have a predilection for males and the elderly. We describe a 40-yearold South Asian woman presenting with mild dyspnea on exertion of 1 year's duration. Echocardiography showed a 60×60 mm cystic sac, subsequently confirmed by computerized tomography, which showed 3 large aneurysms (70×61 mm) and 3 small aneurysms in the SNA. Coronary angiography illustrated that the SNA branched off the left main coronary artery, and the aneurysm communicated with the right coronary artery. The aneurysm was partially resected and plicated. [ABSTRACT FROM AUTHOR]

Atrial fibrillation (AF) is the most prevalent arrhythmia, often caused by myocardial ischemia/infarction (MI). Men have a 1.5× higher prevalence of AF, whereas women show a higher risk for new onset AF after MI. However, the underlying mechanisms of how sex affects AF pathophysiology are largely unknown. In 72 pigs with/without ischemic heart failure (IHF) we investigated the impact of sex on ischemia-induced proarrhythmic atrial remodeling and the susceptibility for AF. Electrocardiogram (ECG) and electrophysiological studies were conducted to assess electrical remodeling; histological analyses were performed to assess atrial fibrosis in male and female pigs. IHF pigs of both sexes showed a significantly increased vulnerability for AF, but in male pigs more and longer episodes were observed. Unchanged conduction properties but enhanced left atrial fibrosis indicated structural rather than electrical remodeling underlying AF susceptibility. Sex differences were only observed in controls with female pigs showing an increased intrinsic heart rate, a prolonged QRS interval and a prolonged sinus node recovery time. In sum, susceptibility for AF is significantly increased both in male and female pigs with ischemic heart failure. Differences between males and females are moderate, including more and longer AF episodes in male pigs and sinus node dysfunction in female pigs. [ABSTRACT FROM AUTHOR]